Solidification modeling and solid-state transformations in high-energy density stainless steel welds

The solidification and solid-state transformations which occur during the high-energy density (HED) welding of austenitic stainless steel were studied. Comparisons were made between structures observed in gas tungsten arc (GTA) welds and those of electron beam (EB) and laser welds using Fe-Ni-Cr ternary alloys with Cr/Ni ratios ranging from 1.5 to 1.85. Weld solidification and microsegregation was modeled using a finite difference analysis and compared with experimental results. These calculations were also used to help interpret the origin of the observed microstructures. Calculations showed that little solid-state diffusion occurs during the solidification and cooling of primary austenite solidified welds, whereas structures which solidify as ferrite may become almost completely homogenized as a result of diffusion. A change in solidification mode from primary austenite to primary ferrite was found to occur at higher Cr/Ni ratios with the HED welds than with GTA welds and is attributed to dendrite tip undercooling. A nearly segregation-free, single-phase austenite structure which appears to be unique to the rapid solidification velocities and cooling rates of HED welds was also observed. It is suggested that this structure is a product of ferrite solidification which transforms to austenite via a massive transformation.