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hydrogen hardness (HE) is the term commonly used to describe a range of different phenomena that occur when atomic hydrogen interacts with steels. These phenomena are likely to be caused by very different mechanisms, but they are aggregated under the single label because the macroscopic manifestation of their effect is a decrease in mechanical properties.

When it occurs in structural materials, HE can result in sudden catastrophic failure of the structure at a fraction of the expected proof or yield strength. In less dangerous applications, HE can cause fatigue cracking that initiates at a surface flaw or defect and grows until the critical crack length is reached, resulting in a reduction of load-bearing capacity below the anticipated proof or yield stress.

In both cases, HE is caused by the interaction of hydrogen with the crystal structure of the metal. Therefore, it is important to understand how the metallurgical and physical properties of a material affect its HE susceptibility.

Much work has been done to catalog the HE susceptibility of metals and alloys, as well as to develop welding procedures that reduce HE in weldments. The emphasis has now shifted to address the ways that environmental exposures can create HE and cracking.

The physics and metallurgy of HE is complex, but the underlying causes can be broadly divided into two categories: internal hydrogen embrittlement (IHE) and environmental hydrogen embrittlement (EHE). IHE is primarily caused by internal hydrogen generated during manufacturing processes such as steelmaking, heat treatment, and surface finishing. EHE is primarily caused by hydrogen generated in the service environment, but can also be the result of welding and other chemical processing operations.