Effect of C- and W-content on Microstructure and Toughness of Weld Metal for Low Alloy Cr-Mo Steel

The effect of C- and W-content on the microstructure and impact toughness of the weld metals for 2.25Cr1Mo steel weld via tungsten argon arc welding (TIG) technique was investigated by means of OM, SEM, EPMA, EBSD, thermal expansion- and impact-test. The results show that the microstructure of the multi-layer and multi-pass weld metal was heterogeneous, which can be divided into upper weld metal and intermediate weld metal. The upper weld metal can be divided into melting zone (MZ), coarse grain heat affected zone (CGHAZ), fine grain heat affected zone (FGHAZ), inter-critical heat affected zone (ICHAZ), inter-critically reheat coarse grain heat affected zone (ICCGHAZ) and sub-critical heat affected zone (SCHAZ). Intermediate weld metal composed of necklace-type microstructure which distribute along the prior austenite grain boundary and equiaxed crystal structure. The equiaxed crystal structure was tempered bainite with good toughness. Necklace-type microstructure composed of a large number of M-A constituents, which could easily cause stress concentration and promote crack initiation, whereas, deteriorate the toughness of weld metal. Increasing C content could promote the formation of lath bainite in upper weld metal and necklace-type microstructure in intermediate weld metal, which deteriorated the toughness of weld metal; However, increasing W content could promote the formation of lath bainite in upper weld metal and inhibit the formation of necklace-type microstructure in intermediate weld metal, which is beneficial to improving the toughness of weld metal. © 2021, Editorial Office of Chinese Journal of Materials Research. All right reserved.