The non-equilibrium cooling of 304 SS by TIG welding, making the formation of dendritic δ-ferrite in fusion zone (FZ), whereas the lath δ-ferrite formed in the heat affected zone (HAZ) that accompanied by high proportion of high-angle grain boundaries and precipitation hardening. Consequently, welded joint has higher strength and hardness compares to the base metal (BM), and fracture exhibits ductile intergranular. The slower cooling rate in HAZ promotes precipitation strengthening and refinement of δ-ferrite, inducing microhardness gradient distributed as HAZ > FZ > BM and effectively improve tensile strength. However, electrochemical measurements reveal FZ and HAZ are more susceptible to corrosion than the BM. Cr segregation and depletion surrounded δ-ferrite phase, lowering protection film formation on welding bead of γ-austenite.
Ho et al. (Fri,) studied this question.