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