Cracking of Non-PWHT’d Carbon Steel Operating at Conditions Immediately Below the Nelson Curve

Cracking was observed in non-PWHT’d carbon steel piping and vessels operating at conditions immediately below the “Nelson” curve. This curve provides a threshold limit for high temperature hydrogen attack as a function of the equipment operating temperature and hydrogen partial pressure. This curve is based on industry experience with steel equipment operating for many years in high temperature high pressure hydrogen service. Our investigation indicated that cracking occurred in 2 stages. Stage 1 or the initial stage of cracking appeared to occur as a result of intergranular “hydrogen assisted” cracking very similar to high temperature hydrogen attack. It appears that Stage 1 cracking is driven by the combined effects of residual welding stresses and “methane pressure” stresses from the decomposition of carbides. The circumstances of the observed cracking indicate that, unlike high temperature hydrogen attack, it occurred over a relatively short period of time after an operating change to a higher hydrogen partial pressure. Stage 2 cracking results from sulfide scale packing the crack during high temperature operation in a sulfidizing environment and causes the Stage 1 cracks to further propagate through wall. Once the sulfide scale filled crack cools down during a shutdown, the surrounding metal contracts around the scale and causes a high load on the crack tip which promotes further crack propagation. The presence of dissolved hydrogen in the steel further promotes Stage 2 crack propagation during shutdown periods.