| With the domestic's economy being high rapidly developing, locomotive transport has been necessary and gradually come into people's daily lives. At present, requirement of locomotive keep increasing greatly in domestic market and production keeps increasing also. Locomotive manufacturing industries have good prospects, but many quality problems of parts are still existed. We should especially pay more attention to the quality of key parts of locomotive.42CrMo have good strength and toughness, hardenability and temper brittleness is not obvious. If it is hardened and tempered, it will have a high limit of fatigue, anti-shocks and low temperature impact toughness. It is used in large and important section of machine parts widely. For example, it is used for crankshaft and coupler of high power internal combustion engine, main shaft and impeller for steam turbine generator, and so on. Or it is used for heat-resistant steel below 500℃as high temperature components.Firstly, the weldability of 42CrMo alloy was analyzed and it is poor in the paper. We chose H08Mn2SiA as welding wire and compared the quality of welding under different welding current without preheating. Post weld heat treatment was applied to overcome the brittleness and softening caused by weld in heat affected zone. The result of magnetic particle testing after welding was analyzed. It indicated that difference microstructure in heat affected zone resulted in magnetic trace because difference microstructures have difference magnetic permeability and not because it have crack in the weld. The metallographic microstructure was studied and lots of martensite and cementite was found in heat affected zone, but the microstructure of base metal is sorbite. The microstructure of weld is pearlite and ferrite. Martensite has low magnetic permeability and it cause magnetic field lines spillover. After heat treatment, metallographic structure is homogenizing, the phenomenon of magnetic marks become weak.
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