Precipitation characteristics and tensile properties of high-nitrogen chromium-manganese steel fabricated by wire and arc additive manufacturing with isothermal post-heat treatment
Xiaoyong Zhang, Yong Peng, Yong Huang, Kehong Wang, Jian Kong, Zhonghua Du, Tongli Wu, Ming Zhou
Abstract
Wire and arc additive manufacturing (WAAM) technology is a promising method for fabricating high-performance protective materials and structural parts from high-nitrogen steel (HNS). However, owing to the complicated thermal fields generated by the process, it may produce some undesirable precipitates, such as the intermetallic σ phase or chromate nitrides, especially when the fabricated part is large. This study focused on the precipitation features of WAAM-produced high-nitrogen Cr-Mn steel containing 0.7 wt% nitrogen. When the isothermal post-heat treatment was conducted at approximately 900 °C, the σ phase dominated the original inter-dendritic ferrite (Inter-F) region. And, there were some secondary austenite precipitates (γ2) along with a small amount of Cr2N. The oxide inclusions (especially the MnO particles) provided excellent nucleation sites for the σ phase in the Inter-F region or at the Inter-F and γ matrix interface due to the high interface energy and lattice matching index. Further, when aging above 1000 °C, no σ phase was found, and the decomposition of the Inter-F to γ matrix was dominated by element diffusion. With the precipitation of the σ phase, a typical brittle fracture was observed in the samples aged at low temperatures of 800 °C and 900 °C.