Effect of nitrogen content on microstructure, mechanical properties, and corrosion behaviour of coarse-grained heat-affected zone of nitrogen-containing austenitic stainless steel

The microstructures and properties of austenitic stainless steel with varying nitrogen content after welding thermal simulation were investigated. The results indicate that the nitrogen fraction has a considerable influence on the phase composition and properties. Low nitrogen fraction leads to formation of δ-ferrite. After a welding thermal cycle, the number of annealing twins of high nitrogen-containing steel decreases, M${}_{23}$C${}_6$ precipitates, and fine M${}_{23}$C${}_6$ are observed. For low nitrogen-containing steel, large-size M${}_{23}$C${}_6$ and elongated δ-ferrite are formed, which deteriorate the ductility. Meanwhile, equilibrium phase calculations also reveal the inhibiting effect of nitrogen on M${}_{23}$C${}_6$ and δ-ferrite. Furthermore, susceptibility to intergranular attack by 10 % oxalic acid solution was detected, and the synergistic effect of coherent twin boundaries, M${}_{23}$C${}_6$, and δ-ferrite leads to low resistance to corrosion for low nitrogen-containing steel.