Solidification of plasma TIG-welded N-alloyed austenitic CrMnNi stainless steel

In this study, the type of solidification of an austenitic stainless steel during plasma tungsten inert gas (TIG) welding was investigated. The stainless steel Fe-17Cr-7Mn-4Ni-(0.12–0.19)N, concentration in wt.%, was studied in hot-rolled condition with a δ-ferrite fraction less than 5 vol.%. The investigated steel could be successfully welded without filler metal. The steel had an increased Mn content and a lowered Ni content to reduce alloying costs. To verify the solidification type after welding, microstructural investigations are performed using light optical microscopy, scanning electron microscopy, and hardness measurements. Moreover, the microstructure was characterized by scanning electron microscope using electron backscatter diffraction (EBSD). Based on the EBSD measurements, it was possible to elucidate the solidification process. In the present alloy system, the primary solidification of the weld seam according to Scheil-Gulliver model was calculated, taking the fast diffusing N into account. The Scheil-Gulliver model predicted primary ferritic (body-centered cubic, BCC) solidification. After solidification of approx. 84% solid phase, the austenite (face-centered cubic, FCC) is formed via eutectic reaction and the residual melt solidified as austenite. The theoretically determined solidification type agrees with the experimentally observed microstructure. In addition, susceptibility to solidification cracking was investigated, for which no evidence was found in the studied microstructure.