Microstructures in HAZ after Heat Treatment of Carbon Steel and Austenitic Stainless Steel Welds

The influence of post weld heat treatments (PWHT) at 400°C, 600°C, 900°C on microstructures in heat affected zone (HAZ) of dissimilar welds between carbon steel and austenitic stainless steel was studied. As-welded condition, the fully Martensitic layer along the fusion line, Widmanstatten Ferrite, Bainite, Pearlite phases in the HAZ of carbon side and the fully austenitic zone in the weld metal can be observed. After PWHT, the microstructures of these zones were dramatically modified as a result of carbon diffusion from the carbon steel toward the weld metal. Decarburization of the base metal led to the formation of a zone with large Ferrite grains. Bainite or fine Pearlite were formed by carbon diffused to both the interfacial Martensite and the purely Austenite zone. The lowest hardness value in the decarburization zone was 92HV on average after PWHT at 900°C and the peak hardness value that was documented in the carburize zone with 366HV at 600°C. Carbides precipitation (M23C6, M7C3) were found in both the HAZ of carbon steel and austenitic stainless steel.