Postweld Heat Treatment to Avoid Intergranular Stress Corrosion Cracking of Supermartensitic Stainless Steels

Abstract

Supermartensitic stainless steels (SMSS) are attractive materials for flowlines transporting produced fluids with high levels of CO2 and low levels of H2S. However, recent cracking of lean grade material in service and both lean and high-alloy grades during qualification testing have revealed sensitivity to intergranular stress corrosion cracking (IGSCC) at some girth welds, although all flowlines in high alloy SMSS have apparently had no such problems in service. One potential solution is to use a brief postweld heat treatment, typically at around 630–650 °C for five minutes, which has been shown to overcome susceptibility to IGSCC in laboratory tests. The paper considers existing information on the effects of brief PWHT on welded SMSS, presents additional data for a range of pipes and weld types, and discusses the likely mechanism by which PWHT may be effective in preventing IGSCC. It is concluded that a microstructural effect is probably dominant. Based on this preliminary conclusion and a consideration of the potential detrimental effects of an inappropriate PWHT cycle, the necessary control of the PWHT process is addressed and recommendations are made with respect to application of PWHT, highlighting best practice based on current knowledge.