Postweld heat treatment cracking in chromium molybdenum steels

The postweld heat treatment (PWHT) cracking susceptibility of standard and modified 2 1/4 Cr-1Mo and 3Cr-1 and 1 1/2 Mo steels has been investigated. The modified steels tested include three heats of 3Cr-1 1/2 Mo-0.1V, a heat of 3Cr-1 1/2 Mo- 1/2 Ni and a heat each of V-Ti-B modified 2 1/4 Cr-1Mo and 3Cr-1Mo. Tests used to evaluate PWHT cracking susceptibility were Gleeble simulation and a 'C-Ring' test adapted from that described in ASTM G-38 (used to evaluate stress corrosion cracking susceptibility). Transmission electron microscopy of carbide extraction replicas leads to the general conclusion that elements (such as V, Si) that contribute to the stability of coherent matrix precipitates of the MC, M₂C or M₄C₃ type lead to enhanced susceptibility of PWHT cracking. The presence of elements such as nickel that will drive the precipitation process faster towards equilibrium, reduces susceptibility to PWHT cracking.