Exam API-571 topic 1 question 89 discussion

Hydrogen stress cracking is a form of environmental cracking that can occur in high-strength low-alloy (HSLA) steels and carbon steels with highly localized zones of high hardness in the weld metal and HAZ, as the result of exposure to aqueous HF acid environments.

Carbon steels and low-alloy steels are of prime concern, particularly older steels. 400 series SS are also susceptible even if not embrittled. In addition, materials susceptible to an embrittling mechanism such as sigma phase embrittlement (3.56), 885 °F (475 °C) embrittlement (3.1), temper embrittlement (3.63), strain-aging embrittlement (3.60), or titanium hydriding (3.66) pose a risk for brittle fracture if they are sufficiently embrittled.