Effect of Post-weld Heat Treatment on Microstructure and Mechanical Properties of Dissimilar Metal Weld Used in Power Plants

High base metal dilution (50 pct) dissimilar metal weld between A508 low-alloy steel and 309L clad layer was fabricated to investigate the effect of extended post-weld heat treatment (PWHT) on microstructure and mechanical properties. Extended PWHT at 607 °C caused significant carbon migration and microstructural change across the fusion boundary. Following PWHT at 607 °C for 20 hours, the carbon-enriched zone exhibited little to no reduction in hardness although 20 pct hardness reduction occurred in the remainder of the first butter layer. This is because tempering of the martensite in the carbon-enriched zone was entirely offset by the formation of a high density of chromium carbides. In contrast, following PWHT at 607 °C for 20 hours the average hardness of the carbon-depleted zone decreased from 241 HV to 169 HV due to the elimination of martensite, ferrite grain coarsening and carbon depletion. In addition, the extended PWHT significantly enhanced the mismatch in mechanical properties between the carbon-enriched zone (351 HV) and the adjacent carbon-depleted zone (169 HV). The formation of chromium carbides (less than 200 nm in size) in the carbon-enriched zone reduced the carbon concentration in the matrix, generating a continued driving force for carbon migration from A508 steel to the weld metal during PWHT.