Case Study: Metallurgical Failure Analysis of Two Ruptured Pressure Tanks

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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.