Study of Weldability of High Nitrogen Stainless Steel

Abstract

The microstructure and mechanical properties of weld joints of high nitrogen stainless steel are studied. Thermal simulation, gas tungsten arc welding, gas metal arc welding and laser welding were conducted. The main results are summarized as follows: (1) thermal simulation results indicate that the microstructure of the heat-affected zone consists of austenite and a small amount of δ-ferrite. Cr23C6 occurs on the grain boundaries, and Cr2N does not exist in the HAZ. The hardness of HAZ is higher than that of the base metal, indicating no softening in the HAZ under appropriate welding conditions. The impact toughness of coarse-grained heat-affected zone is improved at first and then decreased with the increase of the cooling rate, whereas two brittle zones exist in the HAZ. (2) For gas metal arc welding and laser welding, the N-content of the weld metal increases as the N2 fraction in the Ar + N2 shielding gas is increased. The nitrogen pore can be avoided when the N2 fraction of the shielding gas is lower than a critical amount. For laser welding, higher heat input and more N2 in shielding gas decrease the porosity in the weld metal. (3) The microstructure in the weld metal of laser welding is austenite and δ-ferrite. The size of δ-ferrite increases with increasing heat input. The hardness of weld metal increases with decreasing heat input and increasing N2 amount in the shielding gas. The toughness increases when the heat input decreases, whereas the composition of shielding gas shows no influence. (4) Cr-Mn-Ni-N welding wire is suitable for the welding of high nitrogen stainless steel. The microstructure in the weld metal is austenite and δ-ferrite. The strength of the weld metal obviously increases with the addition of some N2 into the shielding gas. The weld metal has good toughness. But the HAZ of weld joint with multi-pass welding shows low toughness.