API 571 Certification (Corrosion & Materials) Practice Test 2026 - Free Practice Questions and Study Guide

Hydrogen-induced cracking (HIC) occurs when hydrogen atoms diffuse into the microstructure of a material and lead to the formation of cracks. The material's resistance to this form of cracking is influenced by its composition, microstructure, and treatment processes.

PWHT (Post Weld Heat Treatment) helps relieve residual stresses and results in a more homogenous microstructure, which can significantly enhance the material's toughness and ductility. These properties make PWHT'd carbon steels less susceptible to HIC because the treatment mitigates the effects of hydrogen diffusion into grain boundaries, thus reducing the potential for cracking.

In contrast, non-PWHT'd carbon steels are more vulnerable to HIC due to the higher residual stresses and potential microstructural inhomogeneities. C-0.5Mo steels and 5Cr-0.5Mo alloys have improved resistance to HIC compared to non-PWHT'd carbon steels, but they may not provide the same level of resistance as PWHT'd carbon steels.

Therefore, the correct answer is that PWHT'd carbon steels have the highest resistance in the context of hydrogen-induced cracking due to their improved microstructural integrity and reduction in residual stresses and brittleness.