SAE USCAR 5-5:2019

This standard outlines the conditions that enhance the risk of
hydrogen embrittlement of steel and defines the relief procedures
required to minimize the risk of hydrogen embrittlement. It is
intended to control the process.

Hydrogen embrittlement of steel, which can cause brittle
fractures under stress, occurs as a result of the absorption of
hydrogen during cleaning, phosphate coating and plating processes.
The susceptibility to hydrogen embrittlement increases with
increasing stress (internal or externally applied stress) and
increasing material strength.

Hardness readings in this specification are in Vickers scale.
SAE J417 should be referred to for conversion to other scales.