Effect of long-time service on microstructure and properties of weld HAZ of 316L stainless steel

Metallographic analysis was used to investigate the microstructure of heat-affected zone (HAZ) of 316L stainless steel weld in which stress corrosion failure occurred after 15-year service. The precipitation behavior of Cr-rich phase in HAZ and its effect on the corrosion resistance of welded joints were discussed. Comparison test of stress corrosion susceptibility was conducted on welded joints before and after service respectively. The results showed that in HAZ the main precipitates inside HAZ were Cr-rich carbides and σ phase after long-time service. On average, SCC occurred in HAZ after 54 hours etching of the sample after long-time service, but for the raw sample, SCC would not occur even after 200 hours etching. It is suggested that Cr-rich carbides and σ phase nucleated inside HAZ during welding and then is coarsened continuously during the operation service due to the effect of residual stress, leading to prominently increasing stress corrosion susceptibility of welded joints.