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.