Thermal and diffusion conditions for delta ferrite formation in partially melted and heat-affected zones of multipass SAW joint performed in martensitic stainless steel 12Cr–1Mo

Abstract

It is widely known the harmful delta ferrite effect on the mechanical properties of martensitic stainless steel (MSS), especially in the partially melted (PMZ) and heat-affected (HAZ) regions brought about the welding thermal cycle. Therefore, this research studied the cooling and diffusion conditions responsible for delta ferrite formation, distribution, and morphology. A finite element numerical model helped to estimate the welding thermal cycle and then was correlated with delta ferrite formation. The numerical analysis considered the application of the submerged arc welding (SAW) process in multipass mode to obtain a coupon test fabricated of MSS 12Cr–1Mo. Heat diffusion and convection processes were strongly related to ferritic phase formation in the PMZ, which affected heat dissipation. Meanwhile, solid-state diffusion of chemical species C, Cr and Mo was analyzed in the HAZ to quantify its effect on delta ferrite grains formation derived from the carbide dissolution process. There was a significant fall in mechanical strength and corrosion resistance (pitting formation) in the HAZ, where some vermicular delta ferrite chains were detected.