Research On Microstructure And Properties Of Welding Repaired New Cold Die Steel Joints

The repairs of Mold are always the interesting technologies in many mold manufacturing enterprises. Among the various repairing technologies, Shielded manual arc welding is widely used in the processing of repairs because of its flexibility and universality of application. However, in some conditions, some of the mold cannot be used even after welding repair due to using some inadaptable welding processes. In order to solve the problems, which must be encountered in the production, this paper completely simulates the actual operating environments, optimizes welding process parameters, and hopes to provide some useful data for the actual production. Tests employ S-700 B.B wear resistant alkaline electrodes, which were usually widely used in some mold manufacturing factory, with a diameter of 3.2mm. The welds were conducted under the conditions of the constant voltage, the welding speed and the 70 A, 90 A and 110 A current intensity respectively. The welding samples were used for the observation of microstructures, and analysis of mechanical behaviors including hardness, tensile strength and wear resistance test.The results show that the microstructure of weld zone consists of martensite and retained austenite after welding repair. Its grains are mostly columnar dendrites. The fusion zone is composed of martensite and retained austenite. The crystalline grain is composed of columnar grains and semi-molten coarse grain. The heat affected zone consists of complete quenching zone, incomplete quenching zone and tempering zone. The complete quenching zone is composed of martensite. The grain is coarse and its hardness is high. The incomplete quenching zone consists of martensite and ferrite. The tempering zone is the softening region of the weld joints, its hardness is even lower than that of the base metal. The tempering zone consists of tempered sorbite, fine martensite and alloy carbide.The hardness test shows that the effect of welding current on the hardness of the welded joint is small, and the gap is 1-2HRC. The hardness of the weld surface is higher than that of the base material, which can satisfy the requirement of the application. The strength test proves that the tempering zone and the fusion zone are the weak zones of the welded joints. The tensile strength of the tempering zone is higher than the fusion zone, and the fusion zone is the main fracture zone. With the increase of welding current, the tensile strength increases gradually. Because of the current increasing, the uniformity of the structure and the tensile strength increase. In the wear resistance test, take a pin on disc wear. With the increase of load, the wear rate and friction coefficient increased gradually. At 110 A current, the wear rate is lower than that at 70 A and 90 A current. In order to get the uniformity of microstructure, higher hardness and lower wear resistance, using the welding current 110 A, so we can get the welded joint with good mechanical properties.