MAI utilizes a variety of analytical tools and techniques to identify fatigue fractures and their root cause. These include stereo microscopy, high magnification optical microscopy and scanning electron microscopy (SEM).

Scanning Electron Microscope

Fatigue fractures exhibit distinctive features, called striations, when viewed at high magnification using a scanning electron microscope. Striations appear as relatively evenly spaced parallel lines. Each striation is actually a microscopic crack that results from a single load, or stress, cycle on the effected part. Repetition of these cycles produces an advancing progression of microscopic cracks as shown in the SEM image above (1) of a fatigue fracture in a hydraulic valve body. This process of repeated cracking is characterized by the term, “fatigue crack propagation”.

Fatigue Crack1   Fatigue Crack2

The appearance, or morphology, of fatigue fracture striations varies depending on the magnitude and frequency of the applied load and the physical characteristics of the affected component such as its hardness, microstructure and the chemical composition of the alloy. These SEM images illustrate fatigue striations in an aluminum valve body (2) and an alloy steel high pressure hydraulic cylinder (3).

fatigue striations aluminum

In some cases, the root cause of a fatigue failure can only be determined by an analysis of the internal characteristics of a component at the crack location. In this example (4), a metallographic cross section through a fatigue crack revealed decarburization (dark phase at arrow) at the surface of a steering arm due to faulty heat treating. This carbon depleted layer has significantly reduced hardness and strength, as well as residual tensile stress, conditions highly conducive to fatigue crack initiation.

decarburization1   decarburization2

Other types of internal defects which act as initiation sites for fatigue are sometimes apparent on the fracture surface. Examination of this brake return spring by SEM (5) revealed fracture features which radiate from a single initiation point. Viewed at higher magnification (6), this initiation point exhibits a void containing a non-metallic inclusion which acted as a stress concentration.

An effective failure analyses must provide specific answers to three critical questions when evaluating a fatigue failure. They are: 1. how did it fail? 2. Why did it fail? and 3. What will prevent future failures? The accurate identification of the failure mode – How? – is the critical first in identifying “Why”. With these two questions resolved, the objective of the failure analysis can now be accomplished; a specific plan of action to prevent future failures.

 

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Case Study: Metallurgical Failure Analysis of Cracked Turbine Blades

INTRODUCTIONBLADES

This failure analysis case study of cracked turbine blades is presented in a condensed version of MAI’s client report format. The report and analysis from which this case study was taken evaluated multiple blades and significantly more data than is presented here. Along with the analytical data, our reports include specific identification of how and why the failure occurred and practical options to prevent similar failures. These answers reduce our clients’ costs and liability exposure, and enhance the quality of their products. Images and data tables are located after the text.

BACKGROUND

Blades from a 660 megawatt steam turbine from an electric power utility were submitted to determine the mode and contributing factors to cracks that formed in service. The turbine operates at a main steam pressure of 2400 psig and a temperature of 1000°F. This unit was reportedly exposed to a contaminated water chemistry event in 1985, however, no cracks were found during an in-situ inspection in 1993. A recent inspection, however, revealed numerous cracks in the blade roots at the attachment to the hub. The operating temperature at the crack locations is approximately 250°F, which is slightly above the steam saturation temperature. The unit has reportedly been cycled through approximately 300 start/stops in its service history.

VISUAL EXAMINATION

Visual examination is often taken for granted. That’s a mistake. A thorough and thoughtful visual examination lays the foundation for an effective failure analysis, identifying critical features and aspects of the failed component.

Typical cracks exhibited by the blades were opened, which revealed dark prior cracks as shown in Figures 1. Examination of these opened prior cracks by optical stereomicroscopy revealed granular macroscopic features, but no evident defects that could have contributed to the failures. The laboratory generated fractures produced during opening of these prior cracks exhibit fibrous macroscopic features that are consistent with ductile single cycle overloads. Corrosion pits are present on the blade surfaces adjacent to these opened cracks.
MECHANICAL PROPERTIES

Tensile testing of bars machined parallel to the lengths of the blades indicates properties that are in conformance with Class 7 material per ASTM A470-65, “Standard Specification for Vacuum-Treated Carbon and Alloy Steel Forgings for Turbine Rotors and Shaft”. No abnormalities are present in the tensile properties of the blades that could have contributed to the cracking at the roots.

Charpy V-notch impact samples were also machined parallel to the length of the blades with the notches in the radial direction. The results of these impact tests are shown in Table 1. The impact properties are in conformance with the 40 ft.-lbs. minimum room temperature impact energy and the +50 °F maximum FATT specified in ASTM A470 for Class 7 material. No abnormalities are present in the impact properties of these two samples that could have contributed to the cracking at the dovetail hooks.

Chemical analysis performed prior to this investigation confirmed that the blades are made from the specified material.

SCANNING ELECTRON MICROSCOPY

The opened crack shown in Figure x was examined using a scanning electron microscope (SEM) equipped with an Energy Dispersive Spectrometer (EDS). EDS spectra are quantified, however, because EDS is a surface analysis technique, oxidation, contaminants and non-homogeneous surface composition may result in variations from bulk chemical analysis results.
EDS analysis of deposits in the dark discoloration on the opened crack revealed detectable amounts of sulfur, chlorine, chromium, manganese, iron, and nickel as shown in Figure 2. Light element EDS of these deposits revealed substantial oxygen and small amounts of carbon and potassium are also present as shown in Figure 3. The chromium, manganese, iron, and nickel are present in the rotor material. The remaining elements are present in the deposit. The substantial oxygen is consistent with an oxidation or corrosion product. The carbon indicates that some organic material is present, which could be a carbonate from the water. The chlorine and potassium are consistent with chloride salts. The source of the sulfur is not clearly evident from this investigation.

The dark deposits on the opened cracks were removed by ultrasonic cleaning in a laboratory grade detergent solution and the opened crack was examined by SEM. No material discontinuities are present on the opened crack. This crack occurred due to intergranular rupture as shown at higher magnifications in Figures 4. These features are consistent with stress corrosion cracking of an alloy steel.

Stress corrosion occurs when a susceptible material is exposed to a specific environment while subjected to sustained static tensile stress. Carbon and alloy steels are susceptible to stress corrosion cracking if exposed to a caustic environment, nitrates, dry ammonia containing carbon dioxide or oxygen, carbon dioxide and carbonate solutions, sulfuric acid, ferric chloride, and other environments. The sustained static tensile stress is due to the radial stresses imposed on the rotor by the mating rotating blades. The laboratory generated fracture produced during opening of this prior crack occurred due to microvoid coalescence as shown in Figure 5. This failure mode is indicative of a ductile single cycle overload, which is consistent with the method used to open the prior crack. This also indicates that the material is not inherently embrittled, which is consistent with the results of the Charpy V-notch impact testing, and that all four cracks occurred due to stress corrosion from exposure to a caustic environment.

METALLOGRAPHY

Transverse metallographic sections were prepared through unopened portions of the crack that was examined by scanning electron microscopy. A branching crack extends from the radius of the root as shown in Figure 6. The branching cracks are intergranular, which is consistent with the results of scanning electron microscopy. This indicates that the crack occurred due to intergranular stress corrosion cracking. Corrosive pitting is also evident on the surface and a small secondary crack extends from this pitting indicating that they are a contributory factor to the cracking. The microstructure adjacent to this crack consists of coarse grained tempered martensite, indicating that this rotor was through hardened by a quench and temper heat treatment as shown in Figure 7. No microstructural abnormalities are present that could have contributed to the cracking.
SUMMARY AND CONCLUSIONS

This investigation indicates that the tensile properties of the blade is consistent with Class 7 material per ASTM A470-65, “Standard Specification for Vacuum-Treated Carbon and Alloy Steel Forgings for Turbine Rotors and Shafts”. The room temperature Charpy V-notch impact energy and the fracture appearance transition temperature (FATT) are also in conformance with this material class. The microstructure of the blade indicates that it was hardened by quenching and tempering. No abnormalities are present in the tensile or impact properties, or in the microstructure, of this blade that could have contributed to the cracking in the root area.
The crack is intergranular, which is consistent with stress corrosion cracking of an alloy steel. Stress corrosion occurs when a susceptible material is exposed to a specific environment while subjected to sustained static tensile stress. Carbon and alloy steels are susceptible to stress corrosion cracking if exposed to a caustic environment, nitrates, dry ammonia containing carbon dioxide or oxygen, carbon dioxide and carbonate solutions, sulfuric acid, and ferric chloride, and other environments. The reported contaminated water chemistry event that occurred shortly after this unit was placed in service in 1985 may have had a contributory effect on the failures. However, because no cracking was detected for a significant period of time after this event, the concentration of caustic compounds due to possible alternate wetting and drying of the steam is the more probable source of corrosion that contributed to the failure. The substantial oxygen identified by Energy Dispersive Spectroscopy is consistent with a corrosion product, and carbon and some of the oxygen are consistent with carbonates from the steam. The chlorine and potassium are characteristic of chloride salts. The source of the sulfur is not clearly evident from this investigation, but could be associated with chemicals added to the water to control its chemistry.

It is recommended that the water chemistry used in this steam turbine be reviewed to identify any abnormalities that could have contributed to the stress corrosion cracking of the blade.

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Springs are one of the few engineered components that are intentionally designed to deform. As they absorb, store and release energy in service they must be able to deform and then return to their original shape over thousands, millions, or even tens of millions of cycles. These service conditions make springs extremely susceptible to fatigue failures, often initiating from relatively small nicks, scratches or seams that act as stress concentrations.

As this case study shows, however, there are many other more subtle conditions that can result in spring failures by other fracture modes.

INTRODUCTIONMetallurgical-Spring-Failure

This case study of a failed torsion spring is presented in the format used in MAI Failure Analysis reports, though in a condensed form for reasons of space. Images and data tables are located after the text. MAI reports include exact identification of “how” and “why” the failure occurred, and specific recommendations to prevent future failures, along with the data to support those conclusions. The answers to these questions reduce our clients’ costs and liability exposure, and enhance the quality of their products.

Comments offered as context and explanation in this case study are in italics.

BACKGROUND

A fractured torsion spring was submitted for evaluation of the material, heat treatment, and root cause of the failure. It was reported that no failures of this component had occurred after several years of production before three recent failures were reported in service. Four of these springs are used to provide a counter force which allows one person to lift an 1100 pound cover on an aircraft ground support (fuel storage) vault.

The springs are specified to be made of SAE 5160H medium carbon alloy steel, hot-formed and are then oil quenched. They are then specified to be tempered at 750 to 800 ºF to a hardness of approximately 51 to 52 Rockwell C. After heat treatment the springs are shot peened for increased fatigue resistance.

VISUAL EXAMINATION

Visual examination is often taken for granted. That’s a mistake. A thorough and thoughtful visual examination lays the foundation for an effective failure analysis, identifying critical features and aspects of the failed component.

The spring fractured approximately seven coils from one end, and approximately 2-1/2 coils from the opposite end as shown in Figure 1. No localized “necking” or other macroscopic indications of ductility are present adjacent to the fracture. No substantial damage or corrosion is present on the wire surface adjacent to the fracture that could have acted as a stress concentration and contributed to the failure. Some corrosion is present on the fracture surface that occurred after the failure as shown in Figure 2. The fracture exhibits granular macroscopic features that are consistent with a brittle single cycle overload. Examination of the fracture surfaces by optical stereomicroscopy at magnifications up to approximately 40X did not reveal any discontinuities that could have contributed to the failure.

CHEMICAL AND MECHANICAL PROPERTIES

Chemical and mechanical properties are typically presented under separate headings but are combined in this case study for the sake of brevity. Determining that the appropriate material was used is an essential baseline to the analysis.

Metallurgical-Spring-Failure-Cross-SectionThe chemical analysis of this spring conforms to SAE 5160H medium carbon alloy steel. The sulfur content is elevated (0.042%), but is within the standard product analysis tolerance. This elevated sulfur would decrease the ductility and toughness in the radial direction, and would increase the likelihood of longitudinal or angular cracking, however, it is not a contributory factor to the failure. The spring material is deoxidized with silicon and vanadium, indicating that it was produced to a fine-grained melting practice, probably by a continuous (strand) casting process. This spring contains an elevated amount of arsenic (0.014%), which can contribute to temper embrittlement and is a significant contributory factor to the failure.

The surface hardness of this spring (46 Rockwell C) in slightly lower than the core hardness (48 Rockwell C), which is indicative of some carbon depletion. The hardness is lower than the 51 to 52 Rockwell C specified on the part drawing. This contributed to the failure by decreasing the tensile properties and increasing the likelihood of overload failures in service. The chemical analyses and hardness indicate that the spring was tempered at approximately 800 ºF, which is towards the high end of the range specified on the part drawing. This tempering temperature is also towards the high end of the range at which tempered martensite (blue) brittleness can occur, and is at the low end of the range at which temper embrittlement can occur.

A standard round tensile test bar was machined from one of the straight tang ends of the spring. The tensile strength is lower than the approximately 232,000 psi expected from a steel having a hardness of 48 Rockwell C. The yield to tensile strength ratio is very high and indicates that this spring was properly quenched. The tensile test bar fractured outside of the gage length, precluding accurate measurement of elongation. The reduction in area, however, is consistent with the measured tensile properties. The tensile test fracture exhibits substantial ductile shear indicating that the spring material has inherently good ductility.

Standard Charpy V-notch impact test samples were machined from the tang at the opposite end of the spring. The Charpy V-notch impact properties are low, which is consistent with temper embrittlement. The impact test bar fractures exhibit primarily granular features that are consistent with brittle fracture. Only very small amounts of ductile fracture are present at the edges of the test bar. This indicates that the spring has inherently low toughness at room temperature.

SCANNING ELECTRON MICROSCOPY

The corrosion products were removed from the fracture by ultrasonic cleaning in an inhibited hydrochloric acid solution and the fracture was examined using a scanning electron microscope. Some mechanical damage is present as shown at higher magnification in Figure 3. The remaining fracture features in this area indicate that the failure is the result of intergranular rupture that is indicative of embrittlement of the spring material as shown in Figures 4 and 5. No evidence of hydrogen embrittlement are present. Similar intergranular rupture is also present at the mid-radius and center of the fracture surface along with small areas of transgranular cleavage, which is the normal brittle single cycle failure mode in this material.

The tensile test bar fracture and a typical Charpy V-notch impact test bar fracture were also examined by scanning electron microscopy. The flat portion of the tensile test bar fracture occurred due to brittle transgranular cleavage as shown in Figure 6. No intergranular rupture features are present in this area of the fracture. The angled areas of the fracture occurred due to microvoid coalescence that is consistent with a ductile shear overload. This indicates that this spring did not fail due to a slowly applied single cycle overload.

The Charpy V-notch impact test bar fracture exhibits substantial amounts of intergranular rupture as shown in Figure 7. This indicates that this spring fractured due to temper embrittlement from a relatively low impact load in service. These results indicate that the elevated arsenic in the spring material is a primary contributing factor by increasing the likelihood of temper embrittlement.

METALLOGRAPHY

Metallography is the examination of the microstructure of metal alloys, typically using an optical microscope. Because of the limited depth of focus available with an optical microscope, metallography samples are polished to a flat plane. They are then etched with acid to expose their grain structure. Variations in grain structure reveal the processing “history” of the metal. Some service applications, such as high temperature exposure, also introduce microstructural indications.

A longitudinal metallographic section was prepared through the fracture shown in Figure 4. Some oxides are present at the coil OD, indicating that this spring was not chemically cleaned prior to painting. No excessive nonmetallic inclusions are present that could have contributed to the failure, despite the elevated sulfur content of the spring material. The microstructure adjacent to the coil OD consists of uniform tempered martensite that is consistent with the specified quench and temper heat treatment as shown in Figure 8. Only slight amounts of grain boundary ferrite extend from the wire surface, which is indicative of slight amounts of carbon depletion. This is not a contributory factor to this failure.

A transverse metallographic section was also prepared adjacent to the fracture. No seams or other material discontinuities extend from the wire surface that could have contributed to the failure. No plastic deformation or microstructural alterations that could be indicative of the specified shot peening is evident adjacent to the wire surface. No abnormalities are present in the microstructure of this spring that could have contributed to the failure.

SUMMARY and CONCLUSIONS

This investigation indicates that the fractured spring submitted for this investigation is made of the specified SAE 5160H medium carbon alloy steel. The spring is deoxidized with silicon and vanadium indicating that it was produced to fine-grained melting practices, which normally results in optimum mechanical properties after heat treatment. The use of vanadium, rather than the more common aluminum deoxidizing agent indicates that it was produced by a continuous (strand) casting process, rather than an ingot casting process. Depending upon the starting billet size, this could result in decreased hot working to the final wire size, which can result in increased chemical segregation and decreased mechanical properties. However, there are no indications in the microstructures of these two springs, however, that chemical segregation is a contributory factor to the failures.

The spring contains elevated amounts of arsenic that is not specified in SAE 5160H and can contribute to temper embrittlement. The elevated arsenic in this spring is a significant contributory factor to the failure.

The spring was through hardened by quenching and tempering to a hardness that is below the value specified on the part drawing. This lower hardness is indicative of decreased strength, which could increase the likelihood of failures due to overloading in service. The tensile strength of this spring is somewhat lower than expected from the measured core hardness, however, the tensile test bar exhibits good ductility and no intergranular rupture is present on the tensile test bar fracture. This indicates that this spring did not fail due to a slowly applied single cycle overload. The high yield to tensile strength ratio indicates that this spring was properly quenched.

The room temperature Charpy V-notch impact energy is relatively low, and a typical impact test bar fracture exhibits substantial amounts of intergranular rupture. This intergranular rupture is indicative of temper embrittlement due to the elevated arsenic content. Temper embrittlement occurs when asusceptible steel is heated to approximately 700 to 1070 ºF or is slowly cooled through this temperature range. The hardness is indicative of tempering within this embrittling range. Therefore, the combination of the arsenic in the steel and the tempering temperature required to obtain the measured hardness is the cause of the temper embrittlement that resulted in the failure of this spring.

No features that could be indicative of the specified shot peening is evident on this spring. However, shot peening only improves the low stress, high cycle fatigue resistance and does not increase the resistance to overload failures.

The spring was subjected to a relatively low impact load in service, which could be related to the opening or closing of the vault cover. Due to the temper embrittlement, however, a high impact load was not required to cause the failure.

The prevent fractures from these causes in future spring production:

• Specify the use of steels with low levels (below approximately 0.010% and preferably below 0.005%) of phosphorus, tin, arsenic and antimony.

• Use as low a tempering temperature as possible while maintaining the specified hardness. Preferably, this tempering temperature should be close to 700 ºF, which is the approximate lower limit where temper embrittlement can occur, and is the approximate upper limit where tempered martensite (blue) brittleness can occur.

• The two fractured springs examined to date have hardnesses that are lower than desired, which is indicative of higher tempering temperatures that increase the likelihood of temper embrittlement. This may require the development of a proprietary material and heat treatment specification.

• Consideration could also be given to changing the spring material to one that is less susceptible to temper embrittlement. Steels containing nickel and molybdenum are less susceptible to this failure mechanism, so consideration could be given to changing the spring material to SAE 4161, SAE 4340, SAE 8655, or SAE 8740. These materials, however, would be more expensive than the currently specified SAE 5160H and would not be as readily available.

• The design and operation of the vault covers should also be reviewed to determine if changes can be made to decrease the likelihood of impact loading.

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INTRODUCTION
Ruptured TankThis case study of two failed pressurized water tanks is presented in the format used in MAI Failure Analysis reports. Images and data tables are located after the text. Along with the analytical data, MAI reports include specific identification of the “how”, “why” and “prevention” of a product or component failure. “How” is only the beginning of the analysis. The critical questions are why the failure occurred and what will prevent further failures. The answers to these questions are what reduce our clients’ costs and liability exposure, and enhance the quality of their products.

For reasons of space, the failure analysis report on which this case study is based has been condensed and the number of images and tables have been reduced. Comments offered as context and explanation in this case study are in italics.

BACKGROUND
Two pressure tanks, used in an industrial application, were submitted for evaluation of the materials from which they were manufactured and to determine the mode and contributing factors to ruptures that occurred in service. One tank, identified as Unit 1, exhibited a fracture adjacent to the longitudinal weld seam. The second tank, identified as Unit 2, had ruptured along the entire longitudinal weld seam with substantial deformation. The rupture also extended into both the upper and lower heads. No information was provided by our client on the specified material or service history of these tanks.

VISUAL EXAMINATION
Ruptured Tank2 Visual examination is often taken for granted. That’s a mistake. A thorough and thoughtful visual examination lays the foundation for an effective failure analysis, identifying critical features and aspects of the failed component.

An approximately 13-1/2″ long fracture is present adjacent to the bottom portion of the Unit 1 tank that extends along one toe of the longitudinal weld seam in the shell as shown in Figure 1. A slight outward bulge is present adjacent to the fracture that is consistent with plastic deformation of the tank during the failure sequence. A repair weld extends from the upper portion of the fracture, indicating that this tank had cracked previously. Corrosion deposits are present on both the outer and inner surfaces of the tank. No prior cracks or welding discontinuities are present, and no clearly defined fracture origin is evident.

The Unit 2 tank ruptured for the entire length of the longitudinal weld seam and extends into both the top and bottom heads, with substantial plastic deformation of the shell as shown in Figures 2. No repair welds are evident and the outer and inner surfaces exhibit corrosion deposits. The fracture in this tank also extends along one toe of the longitudinal weld seam. Heavy corrosion deposits are present on the ID surface. No indications of any prior cracks or welding discontinuities are present, and no clearly defined fracture origin is evident.

Chemical and mechanical properties are typically presented under separate headings but are combined in this case study for the sake of brevity. Determining that the appropriate material was used is an essential baseline to the analysis.

CHEMICAL AND MECHANICAL PROPERTIES
The chemical analyses of the ruptured shells of both tanks are within product analysis tolerances for Material Designation SS Grade 55 structural carbon steel as shown in Table 1. No information was provided on the specified material for these tanks, however, no abnormalities are present in the chemical analyses that could have contributed to the failures.

Standard flat tensile test bars were prepared from each shell adjacent to rupture. The tensile properties of both shells are in conformance with Material Designation SS Grade 55 structural steel as shown in Table 2. The Rockwell hardness of both shells are similar and are consistent with the measured tensile properties. No abnormalities are present in the tensile properties that could have contributed to the failures.

Examination of the fracture using a scanning electron microscope (SEM) is essential to identify the mode (“how”) of fracture, the origin (if a specific origin location is present), and any microscopic defects that may have contributed to the failure. The mode (how) must be compatible with the cause (why), as well as the material characteristics and conditions under which the failure occurred. If it is not, the analyst must re-examine his or her concept of the cause.

Energy Dispersive Spectroscopy (EDS) is a semi-quantitative chemical analysis technique used in conjunction with SEM to non-destructively analyze small amounts of material such as corrosion deposits.

SCANNING ELECTRON MICROSCOPY
EDS analysis of corrosion deposits on the Units 1 and 2 tanks revealed detectable amounts of aluminum, silicon, sulfur, chlorine, calcium, manganese, iron, and copper as shown in Figures 3 and 4. The manganese and iron are from the tank material. The aluminum, silicon, and calcium are consistent with dirt or soil, however, the calcium is also consistent with calcium carbonate present in hard water. The copper is consistent with corrosion to a copper alloy component in the system. The sulfur and chlorine contributed to the corrosion of the tanks, however, their source is not clearly evident from the currently available background information.

Area 3 2000XSEM examination of the fractures in the Units 1 tank revealed substantial thinning of the specified wall thickness due to corrosion. Fracture features at these locations consist of microvoid coalescence as shown in Figures 5 through 7. This fracture morphology is indicative of a ductile single cycle overload failure. The failure occurred when corrosion from the tank ID reduced the tank wall cross section to the point at which it could no longer sustain the applied load from the internal pressurization of this tank. No features that could be indicative of any welding defects or prior progressive cracking, such as fatigue cracks, are present on the fractures.

The longitudinal seam weld from the Unit 2 tank shell exhibits substantial reinforcement welds on both the OD and ID surfaces. Corrosion is evident on the weld reinforcement on the ID surface. The fracture surface adjacent to the ID occurred due to microvoid coalescence as shown in Figures 8 to 10. These results indicate that this fracture also occurred due to a single cycle ductile overload. No features that could be indicative of any welding discontinuities or prior fatigue cracks are evident on this rupture surface.

Metallography is the examination of the microstructure of metal alloys, typically using an optical microscope. Because of the limited depth of focus available with an optical microscope, metallography samples are polished to a flat plane. They are then etched with acid to expose their grain structure. Variations in grain structure reveal the processing “history” of the metal. Some service applications, such as high temperature exposure, also introduce microstructural indications.

METALLOGRAPHY
A transverse metallographic section was prepared through the fracture at the longitudinal seam weld of the Unit 1 tank adjacent to the area examined by scanning electron microscopy. The weld was made in a single pass as shown in Figure 11. Localized corrosion at the weld fusion line extends from the ID to within approximately 0.035″ of the OD surface at the fracture location as shown in Figure 12. The remaining tank wall thickness adjacent to this fracture is approximately 0.80″ and, therefore, the localized corrosion reduced the wall thickness by approximately 55% at the fracture location.

A transverse metallographic section was also prepared through the fracture of the Unit 2 tank adjacent to the area examined by scanning electron microscopy. This weld was made in multiple passes and substantial reinforcement is present along both the OD and ID surfaces as shown in Figure 13. The fracture extends along the weld fusion line, however, no significant localized corrosion is evident at the ID surface as shown in Figure 14. The remaining wall thickness adjacent to this fracture is approximately 0.088″. No localized corrosion is present at the ID surface adjacent to the opposite side of this weld. The remaining wall thickness at this location is approximately 0.099″. This weld reinforcement acted as a geometric stress concentration and decreased the amount of internal pressure required to rupture the tank, however, it is not the sole or primary cause of the failure.

Microhardness testing uses a very small pointed diamond to make an indentation in a test sample. Various loads are applied (for example 500 grams) depending on the tested materials characteristics, to press the diamond into the sample. The softer the sample, the deeper and wider the indentation is. Measurements of the width of the indentation are converted to hardness numbers. Because the indentation is very small the hardness of small features, such as weld heat affected zones and fusion lines, can be measured.

MICROHARDNESS
Microhardness measurements were made in the weld metal, heat affected zone, and base metal of the Unit 1 and 2 tanks. The weld metal is slightly harder than the base metal as shown in Table 3. This is typical of a low carbon steel weld and is indicative of some overmatching of the weld metal strength. The average heat affected zone hardness is higher than the weld metal hardness. None of the hardnesses are sufficiently high to reduce the toughness or ductility of the tanks. No abnormalities are present in the weld metal, heat affected zone, or base metal hardnesses that could have contributed to the failures.

Summary and Conclusions specifically identify the cause of the failure. His section also provides recommendations to prevent failures in the future and identifies any additional manufacturing or service history that could be provided by the client to further evaluate their prevention options.

SUMMARY & CONCLUSIONS
Both tanks are made of carbon steel with chemical compositions and tensile properties that are in conformance with Material Designation SS Grade 55 of ASTM A1011. No abnormalities are present in the chemical compositions that could have contributed to the failures. No abnormalities are present in the tensile properties, hardnesses, or microstructures of the tank shells that could have contributed to the failures.

Unit 1
The Unit 1 tank fractured along the longitudinal seam weld. A repair weld is present adjacent to the fracture at approximately the mid-length of the tank, indicating that it had leaked previously and had been repaired. Substantial localized corrosion is present along the weld fusion lines on the ID surface. This localized corrosion is consistent with galvanic corrosion that occurred due to slight electrochemical differences between the weld metal and the heat affected zone due to variations in their chemical compositions and microstructures. Additional uniform corrosion on the tank ID also resulted in a generalized thinning of the wall in the vicinity of the longitudinal weld seam.

Energy Dispersive Spectroscopy (EDS) of the corrosion deposits revealed elements consistent with dirt or soil, hard water scale, and ferrous and copper based alloy corrosion products. Sulfur and chlorine, which are corrosive to carbon steel, were detected and these elements accelerated the progression of corrosion.

The general corrosion decreased the wall thickness adjacent to the weld to approximately 0.80″. Additional localized galvanic corrosion at the fracture location further decreased the wall thickness to approximately 0.035″. If the tank was pressurized to the rated working pressure of 125 psi at the time of failure, rupture would occur when the material thickness was decreased to approximately 0.024″ based upon the material tensile strength. However, the notch affect produced by the localized galvanic corrosion increased the wall thickness at which failure would occur to the measured 0.035”.

The results of this investigation indicate that this tank failed by a ductile single cycle overload due to the localized corrosion of the weld fusion lines from the ID surface. This localized corrosion reduced the tank wall thickness to the point at which it could no longer sustain the applied load of pressurization of the tank. No weld discontinuities or prior cracking due to fatigue are present.

It is recommended that the chemistry of the water used in this tank be evaluated to confirm that it is the source of the chlorine and sulfur, identified by EDS analysis, which acted as accelerants in the aggressive corrosion of this tank. Once the source of these elements is identified, effective filtering or other means of remediation can be recommended. Other tanks exposed to the same water conditions should be evaluated for general corrosion by ultrasonic thickness gaging, and by either angle beam ultrasonic inspection or radiography to detect the localized corrosion along the weld fusion lines.

Changing the tank shell materials to a more corrosion resistant alloy is unlikely to be economically feasible, however, MAI can provide recommendations if this option is considered.

Unit 2
No localized corrosion is present at the longitudinal seam weld fusion lines of the Unit 2 tank, although slight general corrosion is present. The remaining wall thickness of this tank is approximately 0.10″, indicating little, or no, significant wall reduction occurred.

This tank also fractured along the longitudinal seam weld fusion line. No weld discontinuities or prior cracking due to fatigue are evident. The longitudinal weld exhibits substantial reinforcement on both the OD and ID, and was made in multiple passes. There is no indication of any repair welding, indicating that this is the original seam weld produced during the tanks’ manufacture.

The fracture occurred due to a ductile single cycle overload. The substantial weld reinforcement resulted in a high geometric stress concentration that contributed to the failure. Excluding the geometric stress concentration of the weld reinforcement, this tank would rupture at an internal pressure of 534 psi. However, the geometric stress concentration of the weld reinforcement significantly reduced the internal pressure required to rupture this tank. These results indicate that this tank failed due to over-pressurization This over-pressurization also resulted in the substantial deformation of this tank during the failure sequence.

It is recommended that the service history of this tank be reviewed to identify the cause of the over-pressurization that resulted in this failure.

MAI utilizes a variety of analytical tools and techniques to identify fatigue fractures and their root cause on a microscopic scale. These include stereo microscopy, high magnification optical microscopy and scanning electron microscopy (SEM).

1-failure-analysis-of-fatigue

Fatigue fractures exhibit distinct features, called striations, when viewed at high magnification using a scanning electron microscope. Striations appear as relatively evenly spaced parallel lines. Each striation is actually a shallow crack that resulted from a single load, or stress, cycle. Repetition of these cycles produces an advancing progression of shallow cracks as shown above in this fatigue fracture in a hydraulic valve body. This process is characterized by the term, “fatigue crack propagation”.

2-failure-analysis-of-fatigue

3-failure-analysis-of-fatigue

The appearance, or morphology, of fatigue fracture striations varies depending on the magnitude and frequency of the applied load and the physical characteristics of the affected component such as hardness, microstructure and chemical composition of the alloy. These SEM images illustrate fatigue striations in an aluminum valve body (above-top) and an alloy steel high pressure hydraulic cylinder (above-bottom).

4-failure-analysis-of-fatigue

In some cases, the root cause of a fatigue failure can only be discovered by an analysis of internal characteristics of a component at the crack location. In this example, a metallographic cross section revealed decarburization (dark phase at arrow) of the surface of a steering arm due to faulty heat treating. This carbon depleted layer has significantly reduced hardness and strength, as well as residual tensile stress, conditions highly conducive to fatigue crack initiation.

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Other types of internal defects which act as initiation sites for fatigue are apparent on the fracture surface. Examination of this brake return spring by SEM revealed fracture features which radiate from a single initiation point. Viewed at higher magnification, this initiation point exhibits a void containing a non-metallic inclusion which acted as a stress concentration.

As with all failure analyses, the analyst must provide specific answers to three critical questions when evaluating a fatigue failure. They are: 1. how did it fail? 2. Why did it fail? and 3. What will prevent future failures? The accurate identification of the failure mode – How? – is a critical step in the failure analysis sequence. But that identification is only the first objective. Why the failure occurred and how to achieve prevention must now be addressed to complete a root cause failure analysis.

MAI utilizes variety of analytical tools and techniques to identify fatigue fractures and their root cause. These include macroscopic examination, microstructural analysis, hardness testing, chemical analysis, microprobe chemical analysis and scanning electron microscopy (SEM).

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There are three stages in the life of a fatigue failure:

  1. Initiation
  2. Crack Growth (propagation)
  3. Final Fracture

These stages are illustrated in the SEM image of a fractured rectangular section wire above. The Initiation is indicated by the large red arrow at the lower left. The area of progressive Crack Growth extends from this arrow to the line indicated by the three smaller red arrows. Final Fracture, the point at which the remaining intact cross section of the wire could not sustain the next cyclic stress load – “the straw that breaks the camel’s back” – and complete fracture occurrs, is the light area above the three arrows. This fracture is an example of bending fatigue (in one direction) initiating from a single point of origin.

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The fractured crane lifting hook shown above is an example of reverse bending fatigue (back and forth in opposing directions). In this case, the major bending stress was applied from the side of the fracture oriented to the bottom in this photo, and a lower magnitude cyclic stress was applied from the top. The darker gray area indicates final fracture in a single stress cycle. The thin horizontal band at mid fracture indicates a significant “jump” in the fracture progression that occurred in the cycle proceeding final fracture which almost, but not quite, resulted in complete fracture. This fatigue fracture initiated from multiple origins. Multiple origins are indicated by the steps, or “ratchet marks”, at the outer diameter of the fracture indicated by the arrows. Ratchet marks occur when multiple cracks initiate at slightly different planes on a component’s surface. As these multiple cracks progress into the component, they eventually join into a single fracture plane as show above.

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Ratchet marks resulting from multiple fatigue origin locations are shown at high magnification in these images taken on our Scanning Electron Microscope (SEM). Fatigue cracking penetrated only a short distance into this automotive suspension component before it failed completely in a single load cycle. As a result, the multiple origin fatigue cracks never progressed far enough to coalesce into a single fracture plane. Several of the individual origin sites are indicated by arrows. Ratchet marks are not exclusive to fatigue fractures. Other fracture modes can also produce these macro-features.

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The diagonal bands exhibited by the fatigue fracture of this compressor connecting rod are commonly called “arrest lines”. These indicate a change in the frequency of cyclic stresses, such as “stop-start” sequences, changes in RPM, or variations in load. The initiation site at the crankshaft journal bore (arrow) is heavily damaged. This is not uncommon in fatigue failures. As the first location to crack, the initiation site is exposed to potential relative movement of the two sides of the crack during propagation up to the point of final fracture.. This presents a significant challenge to the analyst in determining the root cause of fatigue cracking.

As with all failure analyses, the analyst must provide specific answers to three critical questions when evaluating a fatigue failure. They are:

  1. How did it fail?
  2. Why did it fail?
  3. What will prevent future failures?

The accurate identification of the failure mode – How? – is a critical step in the failure analysis sequence. But that identification is only the first objective. Why the failure occurred and how to achieve prevention must now be addressed to complete a root cause failure analysis.

In Failure Analysis of Fatigue – Part 7 we will discuss other examples of fatigue failures and microscopic features that are characteristic of fatigue.

The term metallography has several definitions. In the strictest sense, metallography is the study of the structure of metals and metal alloys, typically using magnification by optical or scanning electron electron microscopy. A second widely used definition of metallography is the technique and process of preparing metal samples to reveal and display their internal structure, or, microstructure. For large components a smaller piece, or sample, is cut from the component for preparation. Selecting the appropriate area of the larger part for sampling can be critical relative to the objective of the evaluation since many components do not exhibit a uniform microstructure. The sample is then encapsulated in thermosetting plastic or cold cure epoxy, called a mount or micro, that is typically between 1 inch and 2 inch in diameter. The type of mounting material used depends on the characteristics of the sample configuration and on what aspects of the sample are of interest.

The mount holds the sample in the desired orientation and makes it easier to handle for the next steps in the process – grinding and polishing. Grinding is done using progressively finer grits of abrasives, and must be done carefully to avoid smearing of the metal which obscures (or distorts) its internal structure. Following grinding, the mounted sample is polished to a mirror finish, typically with fine diamond particles suspended in a light oil, and then with even finer aluminum oxide particles (alumina) suspended in filtered water. A great deal of information can be learned by examining the mount in this condition, and photomicrographs are often recorded to document the sample in this “as-polished” condition. However, this process is usually followed by etching of the sample with an acid. Etching is done using a wide variety of acids or combinations of acids depending on the material that is being etched. Etching reveals a vast amount of information about the sample’s microstructure, and through interpretation of that microstructure, the “history” of the material, such as how it was heat treated; what temperatures it was exposed to in service; whether or not it was properly forged, machined, or plated; whether it was exposed to corrosive environments; and other valuable information. This “history,” as revealed by examination on an optical metallurgical microscope (see metallograph), and/or a scanning electron microscope and photographically documented.

Confronting Fatigue – Attack and Defense

From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment. In this part, we discuss a few of the many factors that can initiate fatigue failure in the service environment.

The Service Environment

Once a product leaves the factory you, the manufacturer, have lost control of the many factors that can initiate a fatigue failure once it is placed in service. Abuse and inadequate maintenance are leading preliminaries of failure by fatigue, as well as other failure modes. Failures of components or assemblies “up stream” from your product may introduce higher loads than the product or component was designed to sustain. Harsh service environments, such as road salts or ocean front installations may instigate corrosive attack, with corrosion pits providing a fatigue initiation sight. Analysis and identification of the root cause of fatigue failures in service is critical to educating your customer in the appropriate use and maintenance of your product and getting them back on track as a satisfied customer.

stacking-chair-failure-analysisIdentifying the root cause of service environment initiated fatigue failures can be challenging, and sometimes obscure, as the following example illustrates. Some years ago, we provided analytical support on a lawsuit which was filed after a person sustained a back injury when the metal leg of a “stacking chair” fractured. Stacking chairs are the type of institutional chairs you often see in school auditoriums and other public buildings and are designed to be stacked, one upon the other, for more compact storage when not in use. This particular chair came from a college in Ohio. Our analysis proved that low stress, high cycle fatigue was the failure mode. In other words, low magnitude stresses applied at high frequencies, in this case over a million cycles.

The chair had been in use for a relatively brief time, and even if it had seen longer service, it seemed unlikely that it could have been subjected to the number of load cycles indicated by the fracture morphology. This presented something of a mystery, as the failure mode was indisputable. Investigation of the service environment revealed that the chairs were used sporadically and when not in use, were stacked in a storeroom. The college staff was methodical in setting up the chairs in orderly rows in an adjacent auditorium, then stacking them from the same end of the same rows when they were no longer required, with the same chair ending up on the bottom of the stack before going back into storage.  The stack was higher than the maximum specified by the manufacturer, providing a load in excess of the design limit. A survey of the area revealed that the storeroom was immediately above the main HVAC installation, the final and key piece of the puzzle. Vibration from the HVAC system, transmitted through the storeroom floor, and loads from the weight of chairs stacked in excess of the design limit provided the stresses required to initiate the fatigue crack. Once the crack grew to the point at which the remaining intact tubular leg could no longer sustain the load of a sitting person, final fracture occurred.

As with all failure analyses, the analyst must provide specific answers to three critical questions when evaluating a fatigue failure.  They are: 1. How did it fail?  2. Why did it fail?  and  3. What will prevent future failures?  If you have commissioned a failure analysis, and all three of these questions are not answered, all you have paid for is some interesting pictures and a possible lawsuit when your product fails again.

In Failure Analysis of Fatigue – Part 6 we will discuss some examples of fatigue failures and features that are characteristic of fatigue fracture.

turbine-failure-analysisConfronting Fatigue – Attack and Defense

From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment. In this part, we discuss aspects of the manufacturing process to monitor in order to prevent fatigue.

The Manufacturing Process

Manufacturing processes are a rich, though unintended, source of stress concentrations from which fatigue cracks can initiate. The list is almost endless, and includes rough machined surfaces from dull tooling or excessive feeds and speeds, burrs from cutting or drilling operations, and insufficient chamfers or corner radiuses. Mechanical fasteners – bolts, screws, studs, and rivets- are highly prone to fatigue failure. Prominent among these are thread laps, folds or seems, that are formed when the threads are cut into the fastener. Threads formed by rolling are much less susceptible to laps and consequent fatigue failure. Whether threads are cut or rolled, however, insufficient tightening torque during the assembly stage of the manufacturing process is probably the number one source of fatigue failure in fasteners.

Welds, even when technically faultless, provide geometric stress concentrations. Defective welds and welding procedures may result in porosity and high hardness heat affected zones from which fatigue can initiate. Similarly, braze and solder joints, by their very nature, typically produce a geometric configuration that can potentially invite fatigue initiation. Fatigue susceptibility resulting from these joining processes can be significantly mitigated by careful consideration in the design stage, but design quality cannot compensate for weld and joining defects such as undercut, porosity and slag inclusions.

Care in manufacturing and a good quality control program will avert many of these potential sources of fatigue initiation. However, despite the best quality control program, the manufacturer is often at the mercy of their raw material supplier. These suppliers may open the door to fatigue failure through castings which contain excessive porosity or  microstructural defects, mill products which are work hardened, forgings with undetected laps or seams, or gross non-metallic inclusions in any of these products. Appropriate specifications on outsourced stock and components are vital in guaranteeing their quality, but as with so many aspects of production, they are a compromise.  Loose specs solicit low cost bids, but a potentially high percentage of defective product, while tight specs limit the number of vendors capable of meeting them and drive costs higher, cutting into profits.

In Failure Analysis of Fatigue – Part 5 we will discuss fatigue prevention in service, the longest period of exposure in a component’s life.

failure-analysis-of-fatigueConfronting Fatigue – Attack and Defense

From a practical standpoint, fatigue failures present a danger to you, the manufacturer, at three points in a components life. These are the design stage, the manufacturing process, and the service environment.

Design

The designer or design engineer is the first line of defense against fatigue failure. He or she can’t prevent failures originating in the manufacturing process or service environment, but the designer lays the foundation of prevention.

In an ideal world, each design would be subjected to extensive stress calculations and fatigue testing.In the real world this is rarely cost effective for non-critical components. Instead, accepted and “proven” parameters are applied. These typically include safety margins which are more than adequate. Typically, but not always. And even a common “off the shelf” fastener can take complex products out of service if it fails.

A working understanding of material strengths and properties by the designer is optimal. Unfortunately, that is a relatively rare combination of expertise. And although material strength and property data is widely available, the effective application of this information is sometimes outside the experience of the designer.

The job of the designer becomes even more challenging when the many potential variables inherent in the manufacturing process are considered. Leaving aside the production of the raw material at the mill – the bar, plate and sheet – manufacture of the designer’s envisioned component may include a host of processes that he or would ideally be familiar with through which the seeds of fatigue failure could potentially be introduced.

Computer Aided Design (CAD), Finite Element Analysis (FEA) and a variety of other computer driven design and predictive technologies can greatly enhance the fatigue resistance of a component at the design stage. But they cannot prevent fatigue failures. That’s because the next two threats of fatigue failure are beyond the designer’s control.

In Failure Analysis of Fatigue – Part 4 we will discuss fatigue prevention at the manufacturing stage of a component’s life.

failure-analysis-of-fatigue2Fatigue in the “Real World”

In the “real world” fatigue usually – not always, but usually – initiates at a location that acts as a stress concentration, or “stress riser”. A component is most resistant to fracture when the stress is evenly distributed over it. A stress riser disrupts this even distribution and concentrates the stress at a geometric feature or reduction in the component’s area. Typical stress risers include holes, slots, corners and radii, rough surface finish, welds, corrosion pits, cracks and microstructural defects such as inclusions.

The exception to “usually” – the cases where fatigue fractures initiate from component surfaces that are free of stress risers – typically result from one of two causes; under-design of the component, or abusive service conditions.

Just as all materials have an ultimate tensile strength, they also have a fatigue strength, sometimes called the fatigue limit or endurance limit. Once a component is subjected to cyclic stresses that exceed this limit, fatigue fracture occurs, even though no stress riser is present.

Fatigue failures of this type are less common than fatigue failures initiating from stress risers. Usually components are intentionally over-designed to deal with stresses several times greater than those they would be subjected to in service as a safety margin.

Fatigue Crack Initiation – The Critical Event

If the initiation stage can be prevented, fatigue fracture will not occur. It sounds obvious and simple.  It’s not. Initiation is the most complex stage of fatigue fracture. A low magnitude load, which would have no effect whatsoever on a component in a single application, can be devastating when repeatedly applied in thousands or millions of cycles. The cumulative effect of these cyclic loads are microscopic “shifts” in the material’s structure which ultimately produce a “dislocation” – at this scale it is too small to be called a crack – and the focal point of stress concentration is born. Vibration harmonics, dampening of the system and the service environment further complicate the issue. Collectively, these affects become difficult to predict in the design stage.

In Failure Analysis of Fatigue – Part 3 we will discuss fatigue prevention at the design stage of a component’s life, with following entries focused on the manufacturing and service environments and their relationship to fatigue failure.

Train-3On May 11th, 1842 the first major railroad disaster in history set off a chain of events which led to the discovery of the phenomenon that we now know as fatigue failure.

The Paris – Versailles Express, hurtling down the tracks at the then astounding speed of 50 miles per hour, exploded in flames when the drive axle on the lead locomotive broke, digging its front end into the railbed. The second locomotive in the tandem drive set smashed into the firebox of the lead engine along with the first three cars, killing 57 passengers outright and injuring over a hundred more.  It was the 1800’s equivalent of a jumbo jet crash, and the great scientific minds of the day focused their collective wisdom on perhaps the first major failure analysis in history. The result of their decade long investigation produced the beginnings of our understanding of fatigue.

Fatigue is the most common type of fracture in engineered components. Fatigue fractures are also particularly dangerous because they can occur under normal service conditions, with no warning that a progressively growing crack is developing until the final catastrophic failure. The component, whether it’s the outer aluminum skin of a commercial jet or a simple tubular chair leg, often appears to be perfectly sound with no visible distortion to warn of impending failure.

A technical understanding of fatigue requires a comprehensive knowledge of metallurgy, physics, and phenomena like plastic deformation, slip planes and dislocation theory. In fact, there are several competing theories on exactly what happens at a microscopic level when a fatigue crack initiates. But a practical understanding of the process is extremely beneficial and has direct application to its prevention in service and the manufacturing environment.

To the non-technically inclined, the term “fatigue” suggests this type of failure is related to the age of a component; that the material is “tired”. In fact, fatigue fracture can occur within hours of a component going into service or, conversely, even highly stressed components can operate for decades with no fatigue cracking or failure.

Fatigue fractures result from repeated, or cyclic, stresses. These stresses can take a variety of forms, such as bending (in one direction), reverse bending (back and forth in two directions), torsion (twisting in one or more axis) and rotation. Regardless of this variation in form, the stress on the component at the initiation point of a fatigue fracture is always tensile stress. In other words, the point of origin at which the fracture begins is being “stretched apart”, or pulled in opposite directions. To illustrate this, visualize a tube which is being repeatedly bent in one direction. The side of the tube that is concave when it is bent is being compressed.  The side of the tube which is convex is being “stretched”, or subjected to a tensile stress. This is the side on which a fatigue crack will initiate.

Fatigue cracks initiate at stresses below the tensile strength of the material. Tensile strength is the stress, or load, at which a material breaks when pulled in opposite directions. Each metal alloy has a specific tensile strength, expressed as a numerical value, varying somewhat depending on heat treatment and other processing operations. These values are widely available in engineering reference manuals, typically expressed as pounds per square inch in American references. The fact that fatigue cracks can initiate at stress levels below the tensile strength of a material is difficult to explain. Theories on why this occurs suggest physical and structural changes at the microscopic (0.0001” or less) area of crack initiation.

Fatigue is a progressive fracture mechanism.  Once a fatigue crack initiates, it advances further into the component with each stress cycle. This crack growth process continues as long as the component is subjected to cyclic stress. Depending on the magnitude and frequency of the stresses, the crack may grow over time frames ranging from hours to years. Eventually, the crack advances to a point where the remaining intact cross section of the component cannot sustain the next cyclic stress load – “the straw that breaks the camel’s back” – and complete fracture of the component occurs.

In Failure Analysis of Fatigue – Part 2 we will discuss strategies to prevent fatigue initiation that can be implemented at the design and manufacturing stages of a components life.

Fracture Toughness

The term fracture toughness is used in several ways. When used generically, it refers to the resistance of a crack to grow under stress. This definition is often used to describe the results of fracture toughness tests such as the Charpy impact test. Using the more rigorous definition, fracture toughness refers to strictly defined, mathematically determined results obtained from carefully pre-cracked test specimens that are then failed by the application of a load or force. Fracture toughness results, based on this more rigorous definition, are less common due to the relatively high costs of preparing and testing these specimens. In either case, fracture toughness measurements estimate the stress and the flaw, or crack, size that a structure will tolerate before it fails.

Carbonitriding

A heat treating process that increases the surface hardness to a part by immersing it in a carbon and nitrogen-rich atmosphere at elevated temperatures. This results in the diffusion of carbon and nitrogen into the surface of the part. The depth of diffusion depends on the temperature and the amount of time that the part is held at that temperature. The amount of carbon and nitrogen entering the part can be controlled by adjusting the amount of carbon and nitrogen (called potential) in the furnace atmosphere. Carbonitriding produces a shallow high hardness surface layer (also called case hardness) and is commonly performed on parts that are thin or have relatively small cross sections and require enhanced wear resistance in service, such as self-tapping screws.

An improperly adjusted carbonitriding atmosphere can result in alterations to a material’s microstructure that can actually decrease its surface hardness. Improper carbonitriding can also produce sub-surface voids or holes in parts. This can significantly reduce fatigue strength.

Rapid, efficient and expert failure analysis and materials engineering is not achieved by inertia. As with your company, our progress and professional growth are a journey that does not have an end.

As a part of that journey, MAI’s Metro Milwaukee offices began our relocation to larger and more efficient facilities on February 28th. Our increased square footage and optimized layout will expedite the flow of testing and analysis, expand engineering capacity, and offer networked audio/visual capable meeting facilities to our clients for project planning and review, and for multi-party witnessed testing. Both in-person and over-the internet meetings can be scheduled and accommodated.

To provide our client’s with uninterrupted service, our move will take place in planned stages over the month of March. Stage One, the relocation of engineering and administration, was completed on Saturday, March 1st.

Our scanning electron microscopy lab will move to our new location on March 11th, with sample sectioning, preparation and testing following shortly after.

Sample shipping to MAI’s New Location

Please route delivery by UPS, FedEx, US Mail and all other modes to our new address at:

MAI Metallurgical Associates Inc.
20900 Swenson Drive
Suite 800
Waukesha, WI 53186

We will, of course, have forwarding service in place for deliveries that are sent to our previous address.

MAI provides expert failure analysis, manufacturing process problem solving, and materials evaluation. Our professional engineers bring knowledge, experience, and commitment to the analysis and prevention of failures in engineered components and systems, optimization of manufacturing processes, reverse engineering, research and development, and materials characterization. We serve a comprehensive range of industries including power generation, transportation, food and pharmaceutical, automotive, petro-chemical, medical devices, aerospace, construction, and consumer products.

MAI also serves the insurance industry and the legal profession with failure analysis, professional engineering services, and expert witness testimony.

We look forward to serving you from our new offices and laboratory more efficiently than ever.

Rob Hutchinson
Managing Director

 

A LITTLE HISTORY

Hydrogen embrittlement is a relatively recent phenomenon. With a few exceptions, failures by this mode did not occur prior to the middle of the last century. In a sense, the genesis of hydrogen embrittlement was the jet engine.

In the late 1940’s a revolution was underway in aviation. Jet propulsion was rapidly replacing the old piston engine driven propeller technology and aircraft performance began to exceed levels that had been considered physically impossible just ten years earlier. The dramatic increase in power provided by jet propulsion demanded airframes that could withstand the resulting higher loading. That increase in performance, and aviations never-ending quest for weight reduction, only added to the demands placed on existing materials. The result was a push for higher strength alloys from which stronger and lighter components could be made.

images[7]Low alloy steels such as 4130 had been used in aviation in the past. However, these materials were typically used in the normalized heat treated condition, at tensile strengths in the 90,000 to 120,000 psi range – well below levels susceptible to hydrogen embrittlement. In response to demands for more strength, “radical” heat treatments resulting in tensile strengths approaching 200,000 psi were applied to 4130 and other “anemic” low alloy steels. Some of the first hydrogen embrittlement failures appeared in this material, though the cause was not initially recognized.

Enhanced low alloy steels, such as 4140 and 4340 were used in response to these failures, and the cycle was repeated, with the demand for more performance from smaller components resulting in processing to ever higher tensile strength levels.

One of the unfortunate consequences of increasing the strength of low alloy steels is a corresponding reduction in corrosion resistance. To combat increased corrosion in service, a variety of electroplated coatings, such as chromium, nickel and cadmium, were applied.

With a potent source of hydrogen now available from the plating baths used to protect the new high strength alloys, a dramatic increase in hydrogen embrittlement failures occurred in both the aerospace industry and other industries to which the new materials technology had filtered down. Once hydrogen had been identified as the Achilles heel of these materials, the prevention strategies described in part four of this series were developed.

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Grade 8 fastener bolt hydrogen embrittlement failure

In the first three parts of this series we discussed the physical and metallurgical aspects of hydrogen embrittlement. An understanding of how the phenomenon occurs is the foundation of the ultimate question – how can hydrogen embrittlement be prevented.

PREVENTION

The two critical points at which most hydrogen embrittlement failures can be prevented are at the design stage and during the manufacturing process. Designers with limited materials engineering exposure may not realize the implications of the materials and manufacturing processes they call for in their drawing specifications. As with other failure types, hydrogen embrittlement failures can inadvertently be “designed into” a part.

On the manufacturing side, avoiding the use of reducing acids where possible removes an abundant source of hydrogen from potential exposure to a part. Pickling, etching and electroplating are common manufacturing processes in which acid exposure occurs. Because it is so widely used, electroplating requires particular attention. Minimizing plating time and maximizing current density will generally reduce the volume of absorbed hydrogen. However, consideration of electroless plating or vapor deposition as an alternative eliminates the possibility of hydrogen absorption from the coating process altogether.

Protecting components that will be heat treated or welded from corrosion, or cleaning them prior to these processes will avoid hydrogen introduction by these routes. It is also critical that welding rods are stored in a manner which prevents the absorption of moisture into their flux coating. Any process associated with elevated temperatures will also increase the mobility and absorption of hydrogen, increasing the potential for hydrogen embrittlement.

HYDROGEN MANAGEMENT

Despite the most stringent precautions, processing requirements will sometimes introduce hydrogen into parts that are at or above the tensile strength and hardness thresholds at which hydrogen embrittlement can occur. Fortunately, there is a procedure that will effectively remove absorbed hydrogen. This is an oven heating process, referred to as “baking”, that is performed within the following parameters:

  1. Parts must be baked within 4 hours of hydrogen exposure. The sooner, the better.
  2. Parts must be baked at a minimum of 400º F.
  3. Parts must be held at 400º F for a minimum of 4 hours.  Longer may be required depending on part size.

To be effective, these time and temperature parameters must be strictly followed. Short-cuts or delays will dramatically reduce the effectiveness of baking in removing absorbed hydrogen. For example, twice as much hydrogen will be baked out at 400º F versus 350ºF, and doubling the bake time doubles the amount of hydrogen that is baked out.

The sooner baking begins after exposure to hydrogen, the better. The 4 hour “window” is a maximum. Note that baking must be performed within 4 hours of each hydrogen exposure. For example, within 4 hours of pickling, AND within 4 hours of subsequent plating. No amount of baking will salvage embrittled parts if these time and temperature parameters have not been met.

A final word of caution. A 30 year analysis of hydrogen embrittlement failures in the aircraft industry found that over 70% resulted from improper baking procedures.

In the next, and final, part of the series we’ll talk briefly about the “history” of hydrogen embrittlement, and why it’s a relatively recent phenomenon.

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Acid cleaning of this transmission input shaft during manufacture resulted in failure by hydrogen embrittlement.

In our last post we discussed the metallurgical aspects of hydrogen embrittlement – what actually occurs that results in hydrogen absorption in metals and how it affects their material properties. In this post we will look at the potential sources of hydrogen, what materials are susceptible to hydrogen embrittlement, and why.

HYDROGEN SOURCES

One of the challenges in predicting and preventing hydrogen embrittlement is the wide range of available sources of hydrogen, in both the manufacturing and the service environments.

Thermal dissociation of hydrogen from water is a prime source of hydrogen in manufacturing processes. This can occur in the initial steel making process, as well as subsequent casting or forging operations. Hydrogen can also be absorbed during grinding, abrasive blasting or tumbling, soldering, brazing and welding. At these stages, hydrogen can be dissociated directly from high dew point atmosphere, or from water absorbed in process related media, such as welding electrode flux coatings, abrasive grinding wheels and fine absorbent “dust” in blasting and tumbling media.

Acid cleaning or pickling, and electro-plating, however, are the most common source of hydrogen in manufacturing. Hydrogen containing acids used in these operations invariably infuse susceptible parts with atomic hydrogen.

Service related sources of hydrogen include incidental contact with hydrogen containing acids or cleaning solutions, or absorption from hydrogen containing product that is being processed, such as chemicals, food, or even waste water.

The most common source of hydrogen in service by far, however, is corrosion. Corrosion can also act as a source of hydrogen in the manufacturing process as well. Rusted ingots and scrap used in casting melts, welding on parts that have corroded, and heat treating corroded parts, are potential sources of absorbed hydrogen, particularly when exposed to elevated temperatures which increase the mobility of hydrogen atoms.

SUSCEPTIBLE MATERIALS

High strength steels with tensile strengths above 130,000 psi and a hardness of Rockwell C35 or greater are the most prone to hydrogen embrittlement. Steels below these tensile and hardness levels are generally immune to hydrogen embrittlement. Why?

Increased hardness, most commonly by heat treating, is accompanied by a corresponding decrease in ductility. In simple terms, ductility is the ability of a material to deform under stress rather than crack or fracture. When hydrogen atoms combine into molecules in a steel that exceeds the tensile strength and hardness thresholds, the steel cracks under the stress increase. But if the tensile and hardness levels are below the critical threshold, the higher degree of ductility allows the steel to deform, absorbing and redistributing the stress increase, rather than cracking.

Susceptibility to hydrogen embrittlement increases in alloy steels with heat treatment to higher strength. The strength/susceptibility relationship, in fact, approaches exponential levels. In other words, doubling the strength through heat treating, quadruples the steel’s susceptibility to hydrogen embrittlement.

Identifying and sorting embrittled parts from good components before they fail is virtually impossible. The detection limit of hydrogen by chemical analysis is generally well above the 5 to 10 ppm level at which embrittlement has been shown to occur. Even if such detection capability was available, hydrogen tends to concentrate at specific locations within the part. This leaves the majority of the part with “low” or undetectable hydrogen levels. Chemical analysis of parts after failure, to determine if hydrogen embrittlement was the cause, is also not viable since the hydrogen diffuses from the part after fracture.

Since most hydrogen embrittlement results from hydrogen absorbed during the manufacturing process, parts which are “batch” processed are usually either all embrittled or all “good”.  The failure of one part from a “batch”, therefore, is usually a good indication that others from the same batch will fail.

Expanded grain boundaries produced during a hydrogen embrittlement failure are clearly visible in this image taken using a scanning electron microscope.

Expanded grain boundaries produced during a hydrogen embrittlement failure are clearly visible in this image taken using a scanning electron microscope.

THE METALLURGICAL PHENOMENON

In our last post on Hydrogen Embrittlement and its effects on high strength steel we discussed the characteristics of hydrogen embrittlement failures and some of the types of material that are affected. In this post, we will describe how hydrogen embrittlement occurs, from a microscopic and metallurgical perspective.

Hydrogen atoms are the smallest of any element. So small, that they easily travel between iron atoms in steel and other ferrous alloys. Structurally, metals are composed of multi-atom crystals, or grains, that are analogous to the cells that make up biological organisms. Typically, the grains in metals are microscopic and the space between the grains, called the grain boundaries, is virtually immeasurable. But in comparison to the size of a hydrogen atom, the grain boundaries are gapping canyons.

Under unfavorable conditions individual hydrogen atoms can enter these grain boundaries and move, or diffuse, into a metal component. Once absorbed in this manner, hydrogen atoms are attracted to microscopic crystal defects, or misalignments, where there is slightly more space between grains. They are also attracted to areas under tensile stresses that result in a very slight increase in the space between grains from the opposing “pull” of the stress. A typical example of this condition is a bolt that has been tightened.

As more hydrogen atoms accumulate at these areas, they combine to form hydrogen molecules. Although composed of two hydrogen atoms, a hydrogen molecule (H2) is significantly larger than two individual hydrogen atoms. This produces pressure between grains, expanding the size of the defect or grain boundary interface, attracting more hydrogen atoms and accelerating the process.

This cycle produces a raising tensile stress inside the component which eventually results in a micro-crack. These micro-cracks grow rapidly and simultaneously at numerous locations within the part, reducing the actual intact load bearing cross section by as much as 10-20%.

In order for hydrogen embrittlement to occur, three conditions must coincide:

  1. The part must have a tensile strength in excess off approximately 130,000 psi.  This generally corresponds to a hardness of Rockwell C 35.
  2. The part must be in contact with a source of hydrogen. This may occur during manufacture, in service, or both.
  3. The part must be subjected to a tensile stress.

This last condition can be deceptive because parts do not need to be assembled or in service to be under tensile stress.  Residual internal stresses from casting, forging, welding and other manufacturing processes are significant and, in fact, are probably the root cause of most hydrogen embrittlement failures. Heat treating to raise strength levels above 130,000 psi induces substantial levels of residual stress. The disturbing phenomenon of “shelf popping”, unassembled parts cracking in storage or inventory with an audible “pop”, results from hydrogen embrittlement resulting from residual stress.

Since the hydrogen is absorbed through the grain boundaries, hydrogen embrittlement cracking is primarily intergranular (fracture at the grain boundary) rather than transgranular (fracture through the grains) as in some other forms of brittle cracking.

In our next post, we will discuss potential sources of atomic hydrogen and conditions that make components susceptible to hydrogen embrittlement.

Hydrogen-enbrittlement-2000X

Scanning electron microscopy of the fracture is an essential element of the failure analysis of this Grade 8 High Strength fastener.

THE PHENOMENON

Sudden brittle fractures in high strength steels resulting from hydrogen embrittlement represent a dangerous threat to industry. Not only are there the usual issues of cost such as warranty claims, but in cases of personal injury or property damage, liability points clearly and directly at the manufacturer. This is because hydrogen embrittlement is usually the result of deficient procedures in the manufacturing process.

We’ll get into the why and how of hydrogen embrittlement in the next posting in two weeks. For now though, let’s just discuss some of the characteristics of this type of failure. Perhaps you’ll recognize some of these from fractures you’ve encountered, but didn’t realize at the time that hydrogen was the cause.

Hydrogen embrittlement reduces ductility, often to the point where metals behave like ceramics. Consequently, their resistance to fatigue fracture, their fatigue strength, is significantly reduced as well. Fracture toughness, the ability of a metal to resist fracture growth when a small crack is present, is also dramatically reduced. Brittle fracture due to hydrogen embrittlement occurs without any visible distortion or other warning signs and can happen within hours of manufacture or after years in service. Hydrogen embrittlement failures have even been observed in unassembled parts in inventory, a phenomenon known as “shelf popping”.

Generally, the higher the strength of the steel, the more at risk it is to hydrogen embrittlement and the more vulnerable it is to lower levels of hydrogen. Embrittlement at levels of 10 parts per million and less are not uncommon. Some research suggests this relationship is exponential. In other words, doubling the strength of the steel, quadruples its susceptibility to hydrogen embrittlement.

Although hydrogen embrittlement occurs in many different metal alloys, high strength steel appears to be the most sensitive, is the most widely used, and accounts for the largest number of hydrogen embrittlement failures. A professionally conducted failure analysis can definitely recognize hydrogen embrittlement when present, what caused it, and how to prevent it.

In our next post we will discuss the phenomenon of hydrogen embrittlement from a metallurgical perspective – what actually occurs, on a microscopic scale that causes hydrogen embrittlement?

scanning-electron-microscopy

At MAI, our mission is focused; We solve materials engineering problems with expert failure analysis, manufacturing process problem solutions, and materials characterization. Your requirement for practical, cost effective and rapid response is paramount. We continually search for opportunities, in both technology and expertise, to meet and exceed that requirement.

Our recent purchase of the leading edge in scanning electron microscopy (SEM), the Tescan VEGA3-XMU, increases both our capabilities and capacity.  The VEGA3 features the latest detector technology and imaging software, enabling MAI engineers to fully and rapidly identify and communicate vital root cause evidence to our client’s.

With the largest sample chamber available, and multiple sample stations, a larger number of samples can be analyzed during an evaluation. This enhances both the speed and level of detail at which results can be delivered to our clients.

sem-chamber

MAI engineers combine over 40 years of SEM experience and thousands of analyses to provide our clients with the absolute maximum benefit SEM offers to solve their materials engineering challenges.

Scanning Electron Microscopy is an essential tool in the failure analyses we perform for clients in manufacturing, as well as the litigation and insurance claim investigations we support. SEM also provides critical surface data to customers who develop and manufacture demanding products such as medical implants, stationary and aerospace turbine blades and components, and pharmaceutical and food processing installations.

The Tescan VEGA3-XMU is another step in our commitment to analytical excellence and the success of the clients we serve.

WEAR-PARTICLE-ANALYSIS-IMAGE-2

Wear particle analysis can be used for both wear analysis and contaminant analysis.

MAI engineers have developed an analytical process that identifies specific components which are failing by wear – before they fail. And while the machines or systems they are part of are still in service.

This revolutionary approach identifies the alloy group of wear particles, the number of particles of each alloy type, their size, and their shape. Particles ranging in size from millimeters to sub-micron can be analyzed. Using this data, specific parts or components that are wearing more rapidly than predicted, can be identified.

Both metal and non-metal wear particles are characterized. As a result, failing organic components such as polymer, elastomer or ceramic seals can be identified, as well as metal parts such as bearings, bushings, pistons, cylinders and any other part functioning in relative movement to another.

The immediate benefit of wear particle analysis is the opportunity to replace prematurely wearing parts by a planned process that minimizes or eliminates down time. Replacement of wearing parts may also be coordinated with scheduled maintenance, depending on the rate of wear.

Long term benefits include the identification of improvement opportunities through redesign or specification revisions for longer service life. Parts that do not conform to current specification, and may have slipped through your quality system, may also be identified before they fail.

wear-particle-analysis-1

Wear failure can be avoided in brearing failure, bushing failure, pisron failure, cylinder failure and seal failure by wear particle analysis.

Wear particle analysis also provides the value of confirmation that no unexpected wear is occurring and the machine or system is operating efficiently. Analyses at defined intervals are also an effective tool for monitoring the long term performance of optimized systems and machines.

Wear particle analysis is ideally suited to evaluating wear in fluid systems such as lubrication systems, hydraulics, heat exchangers, and fluid product processing.

Wear particle analysis gives you the advantage of identifying wear failures before they occur. MAI engineers will provide all the guidance you need to collect samples for wear particle analysis from systems in the field or pre-production development test programs.

Photo – “Sparks” generated during arc welding are each a small droplet of molten metal that solidifies before hitting the ground. This image, taken using a scanning electron microscope, shows solidified spatter magnified several hundred times.

There are times when you just can’t get out of the way of Murphy’s Law. A manufacturer of industrial transmissions contacted us to determine the cause of a batch of failures their normally bulletproof products experienced. These failures were unprecedented in this mature designs history.

The gears, shafts, and bearings were well and truly destroyed, and a great deal of debris from that destruction had collected in the sump of the transmission cases. Careful examination of the debris using one of our scanning electron microscopes (SEM) revealed something both interesting and unusual. A high percentage of the debris was consistent is size and was perfectly spherical.

We have a great deal of experience in examining particles and identifying their source. They cause a lot of failures. A raindrop is perfectly spherical until it hits the ground. So is the spatter that is produced in arc welding. But being a rapidly solidifying liquid, molten arc spatter cools sufficiently to retain its spherical shape by the time it reaches the floor.

Enough weld spatter particles had collected in the cases to destroy the transmission in service, but not enough to be discovered during assembly. It doesn’t take much.

There was another problem however. No welding was done at this manufacturing facility and all parts were scrupulously cleaned prior to assembly.

Discussions between the manufacturer and its customers regarding the source of the weld spatter started to get a little heated. Then someone remembered that an area of the shop had been expanded to accommodate additional machinery. And that some structural welding had been done during that expansion. Near a segment of the assembly line.

The manufacturing dates of the affected transmissions were quickly corroborated with the time during which the expansion was underway. Our analysis of the weld spatter matched the composition of the weld filler metal in the structural welds, and the source of the weld spatter was finally identified.

Photo – The delicate features on this fractured ball stud can be easily damaged by contact, or degraded by corrosion. They indicate a ductile fracture through and steel with a high manganese sulfide content, such as a free machining steel. The actual area shown in this field is approximately one thousandth of an inch square.

That component fracture that you were just called or emailed about may not seem like an asset at the moment. But if it’s important for your company to know what caused it, that fracture is one of your best allies. Depending on what an analysis determines the root cause to be, that fracture may be a key piece of evidence. Preserving it in as close to “as is” condition as possible is important.

A fracture is only the end result of a sequence of events that went wrong. There are a lot of events throughout a components life that can go wrong and end in fracture. Some of these are defective stock from a vendor, defects introduced in forming or machining, errors in heat treating or other processes, or customer abuse, to name just a few.

The root cause of many of these is apparent from analyses of aspects of the component other than the fracture. But regardless of the root cause, it’s always good to identify how the fracture finally occurred. Was it progressive over a long period of time and can other components be examined for signs of impending fracture? Can you take actions like weld repair or reinforcement and safely save the cost of replacement? Did it occur instantaneously, without warning? Instantaneous fractures are particularly dangerous.

How a fracture finally occurred is preserved in the microscopic features of the fracture surface. These features are so small that even powerful optical microscopes cannot identify them with certainty. That requires the extraordinary magnification and depth of focus that only a scanning electron microscope (SEM) can provide.

These microscopic fracture features can be extremely delicate and prone to damage. To preserve them for analysis, we suggest the following:

  1. Never fit the two halves of a fracture back together. Even light contact of the mating fracture surfaces can cause extensive damage microscopically.
  2. Protect the fracture from corrosion. It’s going to start oxidizing the moment the fracture halves separate, but any steps that can protect it from rain, snow, road salts, and other corrosion accelerators will help.
  3. Coating the fracture with oil or rust inhibitors can be a problem, since these typically contain additives that can mask evidence of contamination in processing or service that may have been the root cause of, or a contributing factor to, the fracture. If exposure to the atmosphere can’t be avoided, use a non-detergent motor oil to coat and protect the fracture. Use new oil. Used oil is typically contaminated with sulfur.
  4. Protect the fracture from high temperatures such as torch cutting or welding. These can result in the rapid conversion of the actual fracture features to a high temperature oxide.
  5. Never clean the fracture or remove corrosion from it for us. We use highly controlled cleaning procedures that have been developed to clean or remove rust with the absolute minimum of damage to the microscopic fracture features. We once had a helpful client sand blast a fracture to clean it up before sending it to us for analysis. That part was the subject of a lawsuit. The judge was not impressed.

We realize that the steps listed above are not always practical in “the real world”. We’ve had fractures submitted for analysis that weighed up to 8 tons and failed at the top of a South American mountain or in an Australian desert. Protecting life, limb and property is always a higher priority, but when in doubt after those priorities have been taken care of, give us a call or send an email.

We’ve learned to work around the problems that can result from damaged fracture surfaces. They’re actually the rule on the samples we receive for analysis, rather than the exception. But any of the steps listed above that you can take will help. And that will make that fracture surface a valuable ally in preventing future fractures and loss.

Photo – Elemental map of a wear particle filtered from a hydraulic system. Blue indicates the particle is an iron based alloy, such as a low alloy steel. The light red indicates the particle scraped against a zinc, or zinc plated, component during operation of the hydraulic system.

Instrumentation to perform chemical analyses of small, and even microscopic, amounts of material has been around for awhile. A highly practical form of this instrumentation is EDS, or energy dispersive spectroscopy. This technology is used in conjunction with the high magnification capabilities of a scanning electron microscope (SEM) to identify features and particles as small as one-fifty millionths of an inch in size for analysis.

Elemental mapping, using EDS generated data, has also been available for some time. Using this technique, “dot maps” are generated that show the location of analyst selected elements in a field of view collected on the SEM.

While valuable to obtain data available by no other practical means, these maps had their limitations. They were not sensitive to very small relative amounts of elements. A lot of background “noise” was also generated that degraded the resolution of the maps. In other words concentrations of a particular element appeared to have “fuzzy” edges or, in the case of microscopic particles, could not be distinctly resolved from random background indications.

Worst of all, they require huge amounts of data and as a result, the collection time needed to minimize these resolution short comings runs into hours or even the better part of a 24 hour day. These limitations make routine mapping, and the valuable data it could provide, economically unfeasible for many clients. It is also difficult for laboratories to delegate high demand SEM time to single analyses.

MAI recently resolved these disadvantages with our acquisition of the newest cutting edge EDS technology available. This instrumentation collects data 100 to 200 times faster than previously available instruments. It is also sensitive to extremely small relative amounts of a given element, and provides photographic levels of resolution.

In addition to these elemental mapping capabilities, this instrument combination can generate size, area, aspect ration, phase percentage and other data, opening new doors to materials engineering solutions. These include advanced wear failure particle analysis of filtered particulate, but the applications only begin there.

We would be happy to demonstrate this revolutionary technology for you and your company. Please call or email to schedule a time, or online demonstration, at your convenience.

FOR IMMEDIATE RELEASE:

Failure Analysis Engineering Firm, Metallurgical Associates, Plans Atlanta Area Expansion

Metallurgical Associates, a leading engineering firm providing Material Testing and Failure Analysis, today announced plans for a new office in the Atlanta area to better serve manufacturing and industrial clients in the Southern United States.

Atlanta, GA May 15, 2012 – Metallurgical Associates (MAI) today announced that it has secured a location in the Atlanta suburb of Acworth for its new Southern Office. The new Atlanta-area office will allow MAI’s engineers to better serve their clients by providing immediate response to manufacturing interruptions when parts fail.

Rob Hutchinson, Managing Director of MAI, stated “MAI has experienced significant growth by working with clients to provide practical cost effective solutions to their unique failure analysis and manufacturing process challenges. Our new Atlanta-area office will allow us to work closely with our many clients in the Southern US”.

MAI provides materials engineering and analytical services to clients throughout the United States and North America, as well as Europe, the Middle East, the Far East and Central America. MAI South will provide a presence and accelerated response for our clients in Georgia, North Carolina, South Carolina, Alabama, Tennessee, Mississippi, Louisiana and Florida.

Contact details, as well as the official opening date, will be announced shortly. In the interim, please contact Rob by phone or email with any questions or testing requirements that MAI South can assist you with.

Metallurgical Associates
Rob Hutchinson
Managing Director
262 -798-8098

or visit www.metassoc.com

Metallurgical Associates is a Testing Laboratory that provides Engineering services including expert Failure Analysis, Manufacturing Process Problem Solving and Improvement, and Metals & Materials Testing, Analysis and Engineering.

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The deterioration of a material by chemical or electro-chemical interaction with its environment. Corrosion can take on many forms depending on the type of material which is corroded, the stresses the material is subjected to in service while corrosion is occurring, and the environment to which the material is exposed. Examples of the various types of corrosion include uniform (general) corrosion, pitting corrosion (small concentrated corrosion), intergranular corrosion (at the microscopic crystal boundaries of a material), and selective leaching (corrosion of only one element in a multi- element alloy).

A simple formability test in which a strip of metal is bent over a mandrel of specified radius.  The bend is then examined for cracks or tears. If present, these cracks or tears indicate a failure if they are greater than a specified length. Bend testing is performed on plate or sheet metal which is manufactured by passing the metal between rollers until the desired thickness is attained. The orientation of the bend test relative to this rolling direction produces quite different results, as rolled materials bend more easily across the rolling direction than parallel to it. Bend testing can be used to predict a material’s suitability to similar bending processes in manufacturing applications. However, it is not a good predictor of a material’s suitability for three dimensional forming processes, such as drawing operations used to form cup-shaped parts, or other three dimensional shapes.

Welds are also tested using a similar procedure called a guided bend test. This test uses one of several types of fixtures to bend the welded test coupon to determine the ductility and integrity of the weld. This test is specified for welder qualification and welding procedure requirements under ASME IX, EN 287 and 288, and ISO 15614 Part 1.

Adhesive wear occurs at the interface between two sliding surfaces. A shaft rotating in a bushing is a good example of two such surfaces. If there is insufficient lubrication between the shaft and the bushing, the resulting friction will cause a buildup of heat. This can lead to elevated temperatures at relatively small localized areas, which are high enough to melt the shaft, the bushing, or both. When this occurs, a microscopic weld is momentarily formed between the shaft and the bushing. These “micro welds” are broken within a fraction of a second by the continued rotation of the shaft, tearing a microscopic piece of metal from either or both parts. This process may occur at hundreds or even thousands of locations, with each revolution of the shaft tearing more and more metal from the parts until they fail.

A test procedure that determines the tensile strength and tensile properties of a material. To perform this test, a bar is machined from the material to be tested. The bar can be machined from an actual part or component or it can be made from stock that will be used to manufacture a component. Test bars vary in size but are generally about six inches in length or smaller, depending on the amount of material available. Round cross section test bars are use to test castings, forgings, wrought bars, and other three dimensional shapes. Flat cross section test bars are used to test plate, sheet, and strip materials.

machined Round TensileMachined Round Tensile Specimen

Hydraulic Tensile Test MachineTypical Hydraulic Tensile Test Machine

To perform the test, the bar is placed in the test fixture, with clamps securing each end. Using mechanical or hydraulic force, the bar is then “stretched” or pulled and the “stretching” response of the bar is recorded. The test is usually continued, with the amount of force increased, until the bar breaks. The tensile strength of chain, wire, and wire rope can also be determined by tensile testing as well as the tensile strengths of plastics, rope, and other materials. In addition to tensile strength, tensile testing can determine properties such as yield strength, elongation, and reduction in area.

Stress corrosion cracking (SCC) results from a combination of tensile stress and corrosion. Initiation of stress corrosion cracks usually begins at a small surface corrosion pit (see pitting corrosion) that is subjected to tensile stress. The tensile stress “stretches” the opposite sides of the pit apart which exposes new material at the bottom of the pit to further corrosion. As this “corrosion – tensile stress cycle” continues, the resulting separation grows into a crack which penetrates further and further into the part until complete fracture occurs.

The tensile stress that contributes to SCC is typically significantly lower than that required to produce a tensile fracture, however, the continuing corrosion process weakens the metal at the advancing crack front to the point at which it fractures under this tensile stress.

Naturally, a corrosive environment is required for stress corrosion cracking to occur. This environment may be extremely subtle and can range from mildly acidic rain to the highly concentrated chloride road salts. Different types of material are affected differently by various environments. Stress corrosion cracking occurs in carbon, alloy and stainless steels, from exposure to chlorides. Copper alloys such as brass and bronze are susceptible to SCC in chloride or ammonia environments.

Stress Corrosion Cracking is easily mistaken for other failure modes Analysis of SCC should be performed by engineers with Failure Analysis experience in this fracture mechanism.

scanning electron microscopeA Scanning Electron Microscope (SEM) offers several major advantages over the more common and familial optical microscope. (1) The SEM has higher magnification capabilities than an optical microscope (100,000X compared to 1000X for an optical microscope). (2) The SEM can obtain in-focus images of rough samples which have a large variation in vertical height. In other words, the SEM can focus on both the “peaks” and “valleys” of a rough fracture surface at the same time while an optical microscope can only focus on the “peaks” or “valleys”. This large depth of focus (300 times deeper than an optical microscope) is one of the SEM’s greatest assets. Since it provides 3-D like images of fractures, it allows the analyst to visually identify the fracture type and origin, a critical step in any failure analysis. (3) The illumination source for SEM, a beam of high energy electrons, causes the sample to emit low level x-rays. These x-rays can be used to perform chemical analyses of the sample corresponding to the area viewed on the SEM. By increasing the magnification and thereby illuminating a smaller and smaller area, pinpoint chemical analyses of microscopic features and particles can be performed. This last feature is discussed further under the entry for Energy Dispersive Spectroscopy.

An optical microscope uses light to illuminate a sample for examination. A scanning electron microscope uses a beam of electrons. Sophisticated electronic circuitry is utilized to generate a stable electron beam which is then focused on the sample with electro-magnetic lenses. Additional circuitry transfers the focused image of the sample to a monitor for viewing.  Images are then collected and saved in digital format. The beam and sample are under extreme vacuum during the examination process.

scanning electron microscope imageThe AMRAY 1830i Scanning Electron Microscope is shown at the top of the page (right). The image to the immediate right, while not a typical materials engineering subject, demonstrates the high magnification and depth of focus capabilities of the SEM. The object at the upper left (A) is a human red blood cell. A white blood cell is shown at left center (B). The smaller spherical object at lower left is a bacteria. Magnification of this image is 20,000X.

A hardness testing technique in which an indenter is pressed into a test sample by a weight or load. The indenter contacts the surface of the test sample upon the application of a light pre-load, called the minor load. This “sets” the indenter in the sample and determines the starting point of indenter penetration. A heavier major load is then applied. This pushes the indenter into the test sample. The indenter is then withdrawn and the distance to which it has penetrated is measured and used to calculate a Rockwell hardness number. The shape of the indenter and the amount of weight applied as the major load varies depending on the material which is being tested. The indenter may be a cone-shaped diamond or a 1/16″ diameter metal ball, and the major load may range from 15 to 150 kilograms. These variations are identified by a letter designation, for example, Rockwell A, B, C, etc. or a number/letter combination such as 15N, 15T, or 30N.

The automotive industry takes durability testing seriously. They cannot afford to do otherwise. If defective parts enter the assembly stream undetected, a lot of vehicles may be built and sold before the ultimate durability testers, car buyers, expose the problem.

automible connecting rod bolts
Engines undergo extensive dynamometer testing lasting weeks or even months. This testing continues in order to identify further reliability and performance improvements, even after an engine design has gone into production. In a recent extended dyno testing program, an engine manufacturer in Michigan encountered a rash of connecting rod bolt failures in multiple test engines as they reached the equivalent of 30,000 miles in dyno time. Engine production immediately stopped while an urgent investigation was implemented. Thread “laps” – seams in the bolt threads resulting from defective manufacturing – were identified as the cause of the fractures. The bolt vendor brought the manufacturing process back to specified parameters, eliminating the lap defect. A new lot of bolts was quickly brought in, tested to confirm that no defects were present, and engine production resumed.

lap defect connecting rod bolt
A major problem remained, however. The auto manufacturer had no idea how many defective lots of connecting rod bolts had been introduced into the system and assembled into engines prior to their discovery during dynamometer testing. To further complicate the issue, it appeared that only one in every 200 or 300 bolts was defective. Each V-6 engine contained twelve connecting rod bolts. This meant that on average, between one-in-16 to one-in-25 previously installed engines could have been assembled with a defective bolt. Depending on how far back the problem went, cars with defective engines could be waiting delivery to dealerships, at dealerships awaiting sale or sold to customers and on the road. Fortunately, the manufacturer had set aside a percentage of each lot of connecting rod bolts for possible “post assembly analysis” in case just such a situation occurred. Unfortunately, the set asides from previously assembled lots constituted just over 5000 bolts.

connecting rod thread cross section
Immediate analysis of these bolts and identification of when defective bolts first entered the supply stream was required. The manufacturer didn’t have the staff to perform these analyses in the required time, and called Metallurgical Associates Inc. We received the bolts at our Milwaukee materials analysis lab at 4PM on a Friday afternoon. Our metallurgical engineers and technicians went into immediate “round the clock” shifts, visually examining each of the 5000 bolts by stereomicroscopy to sort potentially defective bolts. These were then examined by Scanning Electron Microscopy to confirm the presence of thread laps. Bolts with confirmed laps were then sectioned, mounted, polished, and metallographically examined to document the extent and depth of the thread laps to critical “pass/fail” criteria. By Monday morning at 8AM, 72 hours after receiving the first lot, all 5000 bolts had been examined, the percentage of defects in each lot calculated, and the date on which the first defective lot had entered the production stream, identified.

With the test results provided by Metallurgical Associates Inc. materials analysis engineers, the auto manufacturer was able to quickly identify the production dates of engines with potentially defective connecting rod bolts. Several thousand had already been installed in cars and had left the assembly plants. These were diverted from the supply chain before delivery to dealerships and retro-fitted with replacement bolts, an expensive and time consuming process. This expense, however, was a fraction of the cost of a nation wide recall both financially and to the manufacturer’s public image.

Residual stresses are internal forces contained within a part after the original source of those stresses has been removed. Typical sources of stress include loads applied in deforming operations such as bending, forging or extruding, and temperature gradients such as those encountered in casting and welding. When a metal part is permanently deformed, as in bending or forging, these residual stresses are deposited into the part. Similarly, expansion and contraction from temperatures encountered in casting and welding also deposit residual stresses. If no further change is made to the part, these residual stresses may simply remain contained within the part with little or no affect. However, if a section of the part is machined away or if the part is heated, these stresses can be re-distributed in a manner which will cause the part to distort. This distortion can result in misaligned bores, threaded holes, and bearing surfaces.

Residual stresses are cumulative. In other words, if a beam has the capacity to carry a load of 1000 pounds and contains residual stresses in the same orientation as that load carrying capacity of 100 pounds, then any load applied to the beam that exceeds 900 pounds will overload (100 pounds residual + 900 pound load = 1000 pounds) the beam’s carrying capacity and cause it to fail. This is an obvious factor to consider in the design and manufacture of load carrying parts.

The microstructure of a material determines its properties. The understanding and modification of microstructure is, in many respects, the foundation of materials analysis and engineering. Like all matter, metals are composed of atoms. These atoms combine in small clusters which are called crystals. Groups of crystals combine to form grains. The size, shape, orientation and combination of the grains with other compositional elements in metals make up their microstructure.

These factors also govern their physical properties such as tensile strength, fatigue strength, hardness, brittleness, corrosion resistance, machinability, weldability and many other critical characteristics. These characteristics can be modified and selectively optimized by a variety of processes including heat treating, alloying, cold working and others.

A classic example of microstructure modification to dramatically enhance a materials properties is the conversion of gray iron to ductile iron. Gray iron is the most common form of cast iron. It is inexpensive, reasonably strong and hard. But it is also very brittle. If bent or stretched, it easily breaks. This lack of “give”, defined as low ductility and quantified by the amount a material will elongate or stretch before breaking, required the substitution of more expensive steel in components subjected to bending or stretching (tensile) loads in service.

microstructure

microstructure

The microstructure of gray iron (above left) consists of laminations of carbide and iron called Pearlite (A), relatively pure iron called Ferrite (B), and a high level of carbon in the form of graphite flakes (C).  In the late 1940’s a new form of cast iron was developed called Ductile Iron. A measured amount of magnesium is added to the molten iron minutes before it is poured. This produces the microstructure shown above at the right, which also contains Pearlite (D) and Ferrite (E), but with the graphite converted from flakes to spheres (F).

This microstructural change dramatically improves Ductile Irons ability to accept bending and tensile loads compared to Gray Iron. While Gray Iron will elongate only 0.06% before breaking, Ductile Iron will elongate 18%.

Hydrogen embrittlement fractures occur when a metal absorbs hydrogen from an external source. There are numerous potential sources of hydrogen in both the manufacturing process and service environment.  These include moist corrosion, arc welding with damp electrodes, acid pickling or cleaning solutions, and electroplating baths containing hydrochloric acid. In order for a hydrogen embrittlement fracture to occur, a part which has absorbed hydrogen must be subjected to tensile stress. Within a relatively short period of time (usually 48 hours or less) from the first application of this stress, fracture will occur. The mechanism by which hydrogen embrittlement fracture occurs is relatively simple. Individual hydrogen atoms, which even by atomic standards are extremely small, diffuse into the metal at the grain boundaries, which are inherent to metallic microstructure.  When the part is stressed as occurs, for example when a bolt is tightened, the microscopic gaps between the grains widen slightly. When this occurs, the hydrogen atoms become mobile, moving along the grain boundaries, and when two atoms meet they combine to form a hydrogen molecule. The amount of volume that a single hydrogen molecule occupies is many times greater than that of two individual hydrogen atoms. This increased volume results in pressure between adjacent grains which literally “pushes” the grains apart, resulting in fracture.

Hydrogen embrittlement typically occurs in relatively high strength materials with a hardness of Rockwell HRC 32 or greater.  A frequently encountered example might be a plated high strength bolt which has absorbed hydrogen in the electroplating process.

Hydrogen embrittlement fractures are very similar in appearance to intergrannular fractures resulting from other causes.  Specific microscopic features, however, differentiate this failure mode when the fracture is examined using a scanning electron microscope.  Identification of these features by an experienced materials engineer is critical to an accurate finding of Hydrogen Embrittlement in the course of a failure analysis.

Fatigue is the most common type of fracture in engineered components. Fatigue fractures are also particularly dangerous because they can occur under normal service conditions, with no warning that a progressively growing crack is developing until the final catastrophic failure. The component, whether it’s the outer aluminum skin of a commercial jet or a simple tubular chair leg, often appears to be perfectly sound with no visible distortion to warn of impending failure.

A technical understanding of fatigue requires a comprehensive knowledge of metallurgy, physics, and phenomena like plastic deformation, slip planes and dislocation theory. In fact, there are several competing theories on exactly what happens at a microscopic level when a fatigue crack initiates. But a practical understanding of the process is extremely beneficial and has direct application to its prevention, and the manufacturing environment, as discussed below.

To the non-technically inclined, the term “fatigue” suggests this type of failure is related to the age of a component, that the material is “tired”. In fact, fatigue fracture can occur within hours of a component going into service. Conversely, even large, highly stressed components can operate for decades with no fatigue cracking or failure.

Fatigue fractures result from repeated, or cyclic, stresses. These stresses can take a variety of forms, such as bending (in one direction), reverse bending (back and forth in two directions), torsion (twisting in one or more axis) and rotation. Regardless of the variation in direction, the stress on the component at the point of fatigue fracture is always tensile stress, in which the fracture initiation site is being “stretched”, or pulled in opposite directions. To illustrate this, visualize a tube which is being repeatedly bent in one direction. The side of the tube that is concave when it is bent is being compressed. The side of the tube which is convex is being “stretched”, or subjected to a tensile stress. This is the side on which a fatigue crack will initiate.

Fatigue cracks initiate at stresses below the tensile strength of the material. Tensile strength is the stress, or load, at which a material breaks when pulled in two opposing directions. This load is a specific value for each metal alloy, varying somewhat depending on heat treating and other processing operations. These values are widely available in engineering reference manuals, typically expressed as pounds per square inch in American references. The fact that fatigue cracks can occur at stress levels below the tensile strength of a material is difficult to explain. Theories on this focus on physical and structural changes at the microscopic (0.0001″ or less) area of crack initiation.

Fatigue is a progressive fracture mechanism. Once a fatigue crack initiates, it is driven further into the component with each stress cycle. This crack growth process continues as long as the component is subjected to cyclic stress. Depending on the magnitude and frequency of these stresses, the crack may grow over time ranging from hours to years. Eventually, the crack advances to a point where the remaining intact cross section of the component can not sustain the next cyclic stress load – “the straw that breaks the camels back” – and complete fracture of the component occurs.

In the “real world” fatigue usually – that’s usually, not always – initiates at a location that acts as a stress concentration, or focal point, to the stresses imposed on a component. Stress concentrations take a wide variety of forms. They include geometric features (such as holes, slots, corners and radii), rough areas of surface finish, welds, corrosion pits, and microstructural defects such as inclusions.

The exception to “usually”, the cases where fatigue fractures initiate from component surfaces that are free of stress concentrations, typically result from one of two causes; under-design of the component, or abusive service conditions. Just as all materials have an ultimate tensile strength, they also have a fatigue strength, sometimes called the fatigue limit or endurance limit. Once a component is subjected to cyclic stresses that exceed this limit, fatigue fracture occurs. Fatigue failures of this type are less common than fatigue failures initiating from stress concentrations. Usually components are intentionally over-designed to deal with stresses several times greater than what they would be subjected to in service as a safety margin.

Fatigue crack initiation is the critical factor in fatigue fractures. If the initiation stage can be prevented, fatigue fracture will not occur. It sounds so obvious and simple. It’s not. As noted above, initiation is the most complex stage of fatigue fracture. A low magnitude load, which would have no effect whatsoever on a component in a single application, can be devastating when repeatedly applied as thousands or millions of cycles. The cumulative effect of these cyclic loads are microscopic “shifts” in the material’s structure which ultimately produce a “dislocation” – at this scale it is too small to be called a crack – and the focal point of stress concentration is born. Corners, holes, rough surface finish, welds and other features only accelerate the process. To further complicate the issue, vibration harmonics, dampening of the system, and the environment in which the component functions add a large unknown factor. Collectively, these affects become difficult to predict.

A comprehensive failure analysis, performed by experienced metallurgical or materials engineers is crucial to identifying the true root cause of the initiation of fatigue fractures. To be of value, the failure analysis must identify the cause of initiation and practical cost effective options that will prevent future fatigue failures.

A chemical analysis technique often used to analyze samples, or features on samples, which are too small for other types of analysis.

scanning electron microscopeScanning Electron Microscope (SEM) equipped with an Energy Dispersive Spectrometer (above).

red Blood Cells Energy Dispersive Spectrum imageRed blood cells magnified 20,000 times by the SEM.

EDS identifies the elements present in a sample and determines their relative percentages. Amounts as low as 1/10th of one percent can be detected. EDS is a “Mass Spectroscopy” technique which identifies all the detectable elements present in a sample, rather than only specific elements requested by the analyst as is common in many other chemical analysis techniques. (see Scanning Electron Microscope)

sem Complex Inclusion-EDSSEM image of a complex inclusion magnified 500 times.

eds Analysis Inclusion EDSEDS analysis of this inclusion (right) revealed the presence of a broad range of elements.

brinell hardness testA hardness test in which a ball shaped indenter (normally 10 mm in diameter) is pressed into the material to be tested by a specified load ranging from 500 to 3000 kilograms. The diameter of the resulting indentation in the tested material is measured and this dimension is converted to a Brinell Hardness number using a mathematical formula or reference table. Brinell Hardness indentations are relatively large, making this an ideal technique for measuring the hardness of materials which are not uniform or homogeneous such as cast iron. This large indentation, however, along with the high load used, makes Brinell Hardness Testing impractical for small or thin materials or parts on which the resulting indentation is unacceptable for cosmetic or dimensional reasons.

The primary objective of the annealing heat treating process is to soften a part or component. Annealing is performed by heating to a specific temperature, holding at that temperature for a specific time, then slowly cooling to room, or ambient, temperature. Temperature and time vary depending on the metal or alloy.

Many industrial metals become hard and brittle when their form or shape is changed during the manufacturing process. Examples of processes that harden and embrittle metals include drawing wire to smaller diameter, forging parts to desired shapes by hammering, and stamping sheet metal to specific shapes. Periodic annealing prevents tearing or fracture of the part in the manufacturing process by returning it to a softened condition. Once annealed, continued shaping processes can be applied to the part.

Annealing is also used to relieve internal stresses that result from manufacturing processes (stress relieve) and to refine microstructure to obtain beneficial material properties.

Abrasive wear is a cutting process. This may seem counter-intuitive since most perceptions of cutting require a tool, such as a scissors, chisel, or machine tool. Abrasive wear, however, is cutting on a microscopic scale. The “tool” is either contaminant particles from an outside source, or a mating component.

Wear of mining or excavation machinery shovels and buckets from ore, rock or gravel is an example of Two-Body Abrasive Wear – the two bodies being the shovel and the geological material.

Hard contaminant particles between two sliding or rolling components produces Three-Body Abrasive Wear – the three bodies being each of the two sliding or rolling components, and the hard particles. Typical examples of Three Body Abrasive Wear are bearings, bushings or pistons which have been contaminated by sand, corrosion product (rust particles), or wear particles resulting from small fractures in these components. Hard particles can also originate from within one or both of the components in the form of carbides in their microstructure, or glass reinforcing fibers in plastics. Three Body Abrasive Wear typically accelerates rapidly since more particles are generated from the sliding or rolling components which are additive to the outside contaminant particles.

To further enhance service to our clients, both current and future, Metallurgical Associates has introduced a major revision to our web site at www.metassoc.comMAI specializes in failure analysis and manufacturing process problem solving. However, we also provide targeted individual tests and analyses to client requirements. These range from straightforward chemical analyses, hardness or micro-hardness testing and tensile testing to more in depth heat treatment evaluations, reverse engineering and non-destructive testing.

The new site offers an expanded listing of our services along with detailed descriptions and examples of their application. Links to services are distributed throughout the site, saving time when looking for the test or analysis that meets your specific requirements.

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At MAI, our “product” is your company’s enhanced quality and increased profitability. Our new web site is intended to improve that “product” and, as always, we welcome your view and suggestions.