hydrogen hardness is the resistance of a metal to hydrogen embrittlement. It is well known that hydrogen has a negative effect on the ductility, fracture toughness and fatigue properties of metals due to its promotion of brittle hydride formations. However, surprisingly, the same metals that can be embrittled by hydrogen can also be made more resistant to it by charging the material with supersaturated hydrogen.

The reason why some steels are more susceptible to hydrogen embrittlement than others is that the brittle failure mode of the latter mainly occurs at low temperature, as atomic hydrogen tends to form hydrides with the parent metal. Hydrides are more brittle than the parent metal, and they are prone to crack propagation, causing embrittlement.

In contrast, some metallic materials — including carbon and low alloy steels — are not affected by hydrogen in this way. This is because atomic hydrogen has difficulty forming hydrides with the parent metal at room temperature, or at temperatures lower than that of the gas they are transported in.

The authors reported that hex bolts from a ship gearbox that failed during cruising were found to have been subjected to brittle failure, which was attributed to the ingress of atomic hydrogen caused by corrosion-induced stress cracking (HIC) in the presence of H2S. The hex bolts were plated with low-alloy steels having a hardness exceeding 30 HRC, and the failure investigation showed that the hex bolts exhibited ductility failure mode and contained hydrogen in a quantity of up to 1.5 ppm. The authors found that the hydrogen concentration of the hex bolts was closely dependent on mineral-water hardness, temperature and container materials. The highest concentration of hydrogen was observed in water with a high hardness level, while the lowest concentration of hydrogen was found in the case of commercially available purified water of very low hardness.

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.