Failure Analysis

Hydrogen Embrittlement – Part 2

Posted on 06. Dec, 2010 by Rob in Failure Analysis

Hydrogen Embrittlement – Part 2
High Strength Steels Achilles Heel

The Metallurgical Phenomenon

Hydrogen atoms are the smallest of any element. So small, that they easily travel between iron atoms. The boundaries between crystals, or grains, which are the structure of metals, are gapping canyons in relative size to hydrogen atoms. Once absorbed, 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 stress that cause a very slight increase in the space between grains from the opposing “pull” of the stress.

As more hydrogen atoms accumulate at these areas, they combine to form relatively very large hydrogen molecules (H₂) which raises internal pressure, expands the size of the defect or grain boundary interface and attracts still more hydrogen atoms, 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 associated with residual stress.

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

Hydrogen Sources

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

Sources from manufacturing include the original steel making process, subsequent casting or forging, grinding operations, soldering and brazing fluxes, blasting and tumbling media, welding electrodes, acid cleaning or pickling and electro-plating, etc.

Service related sources of hydrogen may include incidental contact with acids or hydrogen containing cleaning solutions, or absorption from hydrogen containing product by equipment used in its processing. The most common source in service by far, however, is corrosion. Corrosion can also act as a source of hydrogen in the manufacturing process. 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.

In Part 3, we will discuss the conditions that render steels susceptible to hydrogen embrittlement, and how to prevent its occurrence in both the manufacturing and service environments.

Part 4 will briefly discuss the analysis of hydrogen embrittlement failures and illustrate several “infamous” and unusual failures.

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