Effect of simulated welding thermal cycles on microstructure and mechanical properties of coarse-grain heat-affected zone of high nitrogen austenitic stainless steel

Abstract

The microstructural evolution, strength, ductility change, and precipitates in the base metal and coarse-grain heat-affected zone (CGHAZ) of high nitrogen austenitic stainless steel with different thermal cycles were investigated. The research results indicate that the grain size of austenite and the precipitates reduced while the strength improved with increasing cooling rate. Furthermore, the shape and formation location of the precipitates that were proved to be M23C6 and Cr2N by transmission electron microscopy (TEM) varied with the cooling rate. M23C6 precipitated from the catenary to the round type and from the grain boundary to the grain interior with increasing cooling rate, causing the transformation of the fracture mode from an intergranular fracture to a transcrystalline fracture. Cr2N precipitated cellularly and was detrimental to the mechanical properties. The low cooling rate (<5 °C/s) deteriorated the strength and ductility owing to the mass of intermetallic compounds, while the properties of the simulated specimens above 5 °C/s were better than those of the base metal. However, the ductility was the best at 5 °C/s and the fracture surface consisted of many deep dimples. Meanwhile, the grains deformed and increased as the cooling rate changed from 1 °C/s to 30 °C/s according to electron backscatter diffraction (EBSD). Further, the results of Taylor factor are consistent with the variation in ductility.