Differential expansion in bimetallic welds can give rise to:

API RP 571 under Thermal Fatigue highlights:

“Differential thermal expansion between dissimilar materials (e.g., carbon steel and stainless steel) in welds causes high cyclic stresses that can result in thermal fatigue cracking.”

“These stresses typically occur during transient thermal cycling such as startup/shutdown operations.”

(Reference: API RP 571, Section 4.2.1.5 – Thermal Fatigue)

Thus, the correct mechanism resulting from differential expansion in bimetallic welds is thermal fatigue, making option B correct.