Y. Peng, L. Zhao, Z. Tian, H. Xiao, C. Ma, Central Iron & Steel Research Institute, Beijing, China
To study the nitrogen porosity tendency in weld metal of high nitrogen steel, the steel was remelted by gas tungsten arc welding (GTAW), and the influence of the shielding gas compositions and heat input on the nitrogen content and porosity of the weld metal was investigated. Nitrogen content of weld metal increases with higher nitrogen gas proportion in the Ar+N2 shielding gas. The nitrogen pore can be avoided when the nitrogen gas content of the shielding gas is lower than a critical value. The microstructure of weld metal is analyzed. The microstructure and mechanical properties of HAZ were investigated by using thermal simulation method. Experiment results indicate that the microstructure in HAZ is composed of austenite and δ-ferrite. Carbide precipitation (Cr23C6) occurs in the HAZ where the peak temperature of welding thermal cycle is about 800oC, and nitride (Cr2N) is not found. The hardness of HAZ is higher than that of the base metal and there is no softened zone. There are two brittle zones exist in the HAZ. One is around the 1350oC high temperature zone because of its coarse grain size and more ferrite content, and another is around the 800oC zone because of Cr23C6 precipitation. Using a Cr-Mn-Ni-N (containing 0.2% nitrogen) wire as filler metal to weld the steel by GTAW method, the tensile strength of weld metal is close to that of base metal, and the welded joint has high toughness.
Summary: High nitrogen austenite stainless steel containing over 0.4wt% nitrogen is a kind of developing, economical and environment friendly steel, which has high strength, high ductility and high toughness. The engineering application of the steel needs to obtain welded joint with good comprehensive properties. The weldability of the steel is studied.
