Abstract: Repair welding was performed on 4Cr5MoSiV die steel by using TIG welding, followed by post weld heat treatment of quenching and quenching and tempering. The microstructure, hardness, and tensile properties of the joints in three states (welded, quenched and quenched and tempered) were observed and measured by means of scanning electron microscope, microhardness tester, and tensile testing machine. The results indicate that the weld seam microstructure in as-welded state is mainly composed of coarse martensite, austenite, and precipitated carbides. After quenching the weld seam microstructure is mainly martensite, while its microstructure in the quenched and tempered state is mainly tempered martensite and a small amount of precipitated carbides. The average hardness of the weld zone in the as-welded specimens is about 648.7 HV0.1. The hardness of the weld zone in the quenched specimens increases, with an average value of about 670.1 HV0.1, and the hardness is uniform. The hardness value of the weld zone in the quenched and tempered specimens decreases to about 481.7 HV0.1, which is close to that of the base metal zone, and the hardness is also uniform. After post weld heat treatment, both the tensile strength and elongation of the joints are significantly improved. Among them, the tensile strength and elongation of the quenched and tempered joint reach 1270 MPa and 11.3%, which are 29% and 30% higher than those of the as-welded joint, respectively.

Key words: 4Cr5MoSiV die steel, repair welding, post weld heat treatment, microstructure, mechanical properties