Effect of Postweld Heat Treatment on the Sulfide Stress Cracking of Dissimilar Welds of Nickel-Based Alloy 625 on Steels

Alloy 625 overlay on 2.25Cr-1Mo (F22) Steel or AISI 8630 is used in oil and gas industry for corrosion protection. In some cases, F22/625 or 8630/625 interface is possibly exposed to the H2S-containing production fluid and there is a risk of sulfide stress cracking (SSC). The postweld heat treatment (PWHT) is used to reduce the cracking susceptibility of the F22/625 or 8630/625 dissimilar metal weld (DMW). The effect of PWHT on SSC resistance was investigated using four-point bend tests in a pH buffered brine solution with 8.2 wt% H2S. PWHT was found to improve the SSC resistance. The F22/625 DMW was more resistant to SSC than the 8630/625 DMW. The SSC of DMW between Alloy 625 and steel can be understood as a combination of stress corrosion cracking and hydrogen assisted cracking. It proved that the hardness requirement 250 Vickers hardness number (VHN) mandated in NACE Standard MR0175/ISO 15156 is overly conservative and the hardness can be increased at least to 275 VHN to 280 VHN.