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.
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.
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:
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.
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.
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.
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.
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:
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.
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?
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.