Advanced functionally graded joining of ferritic low alloy steel to austenitic stainless steel

The joining between ferritic SA 508 low alloy steel and austenitic 316L stainless steel requires a Ni-based transition alloy to minimize the thermal expansion mismatch and the metallurgical changes during post weld heat treatment. A bimetallic joint design utilizing functionally graded transition from SA508 and 316L was proposed to minimize the use of Ni alloy in this dissimilar metal joint and improve the mechanical properties. Two modalities of functionally graded joint, including powder metallurgy hot isostatic pressing (PM-HIP) and laser directed energy deposition (DED), were investigated. Guided by thermodynamic simulation, a variety of joint configurations are designed and evaluated. The key design criteria is to minimize the intergranular carbide formation near the bimetallic metal interface due to the inter-diffusion during steel heat treatment. The presence of these carbides reduces the interfacial toughness and also leads to the high sensitivity to corrosion. Laser DED functionally graded joint was demonstrated to significantly improve the interfacial toughness and eliminate the intergranular carbide formation. The process-microstructure-property relationship of PM-HIP and laser DED functionally graded joints will be discussed in the talk.