How to Assess Hydrogen Hardness of Metals

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Hydrogen embrittlement (HE) is the loss of toughness and ductility caused by the introduction and diffusion of atomic hydrogen into a metal lattice. It causes brittle failures below the anticipated proof or yield stress and is typically associated with a reduction in load-bearing capacity. HE can occur in most engineering alloys, although some are less susceptible than others.

Various factors can affect the susceptibility of metals to HE, including the crystal structure and the distribution of impurities, micro-voids and inclusions. The resistance of a material to HE can also be improved by micro-structure modifications such as grain refinement, work hardening and the presence or absence of a martensite phase.

In tensile tests on 304 ASS, pre-existing a’-martensite enhanced the strength and resistance to HE of the specimens. When the steel was hydrogen charged, nanoindentation experiments showed that a’-martensite reduced the permeability of the sample to hydrogen and increased its diffusivity. Hydrogen penetration and diffusion into the a’-martensite lattice was impeded, and the fracture mode of the specimens became transgranular cracking along twin boundaries.

A new method has been developed for assessing the susceptibility of plated or coated fasteners to HE that can be used in conjunction with the current ASTM F1624 test for process control verification. The test measures the ability of a sample to resist scratching by a tungsten carbide indenter, and has been found to be a good indicator of the likelihood of a fastener failing due to HE.