Microstructure and Impact Toughness of the Intercritically Reheated Coarse-grained Heat Affected Zone of 13MnNiMoR Steel

Abstract

The microstructure and impact toughness of intercritically reheated coarse-grained heat affected zone (ICCGHAZ) of 13MnNiMoR and the effect of post-welding heat treatment (PWHT) process were investigated by means of welding thermal simulation test. The result shows that after the first welding thermal cycle, the microstructure of the coarse-grained heat affected zone (CGHAZ) mainly consists of granular bainite and lath bainite, with good impact toughness. After the second thermal cycle with a peak temperature of 650 to 700°C, the CGHAZ microstructure experiences a tempering process and its impact toughness improves due to the precipitation of carbides and reduction of dislocations. With a second peak temperature of 750 to 850°C, part of the CGHAZ microstructure retransforms to austenite and during cooling, martensite-austenite (MA) constituents forms along grain boundaries in the ICCGHAZ microstructure, leading to a sharp drop of toughness. When the second thermal cycle peak temperature is up to 900°C, the CGHAZ microstructure is fully austenitized, and the final microstructure mainly consists of ferrite and granular bainite. After a PWHT process in the temperature range of 560 to 640°C, the impact toughness of ICCGHAZ is markedly improved, primarily due to the decomposition of MA constituents.