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 M 23 C 6 and Cr 2 N by transmission electron microscopy (TEM) varied with the cooling rate. M 23 C 6 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. Cr 2 N 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. Highlights • The improved strengths are obtained with the increase of cooling rate. • Change of precipitates with cooling rate is quantitatively analyzed. • The mode between microstructure and mechanical property is established. • The fracture behavior and strengthening mechanism are studied. • Taylor factor can predict the change of the elongation and explained ductility change. [ABSTRACT FROM AUTHOR]