Repair Welding Of New Type Casting High Cr Hot-Working-Die Steel

National 863 project – "casting hot-working-die steel by Near Net Shape" adopting the new type casting high Cr hot-working-die steel substitutes the new manufacturing technique of hot-working-dies (procedure: casting rough castings by Near Net Shape -heat treatment- finish machining-heat treatment-polishing) for the traditional manufacturing technique of hot working dies (procedure: alloyed steel billet-forging-heat treatment-coarse machining-heat treatment-finish machining). The new type casting high Cr hot-working-die steel has many advantages such as longer service life, simpler working procedure, higher efficiency of raw material usage, shorter producing period, easier rebirth of disused dies, lower cost and fewer resource dissipation. This technique is the newest representation in the field of die manufacturing and will bring about a great advance in the development of dies, automobiles and machine manufacturing industries.The research literatures on repair welding of casting hot-working-die steel were hardly found due to seldom using casting hot-working-dies at present. But the development of repair welding materials and technique for the new type casting high Cr hot-working-die steel is an imperative task to the industrialization of the new type casting high Cr hot-working-die steel. In this thesis the repair welding of the new type casting high Cr hot-working-die steel was performed with commercial H13 welding wires and homogeneous welding materials cut from the base metal. H13 welding joints, homogeneous welding joints and the base metals from the new type casting high Cr hot-working-die steel were studied through the chemical composition analysis, hardness tests, microstructure observing by optical microscope (OM) and scanning electron microscopy (SEM), hot stability tests and thermal fatigue tests.The experimental results showed that H13 welding joints gave microstructure of tempered bainite and hardness of HRC46.7 by contrast to the microstructures of low-carbon-martensite + lower bainite + retained austenite + a few carbides and hardness of HRC39～41 in the base metal. Not only these but also the compositions were different between the H13 welding joints and the base metal. Homogeneous welding joints were consistent with the new type casting high Cr hot-working-die steel on chemical composition, microstructures and hardness.The hot stability of the H13 welding joints was worse than that of the base metal. The hardness of the H13 deposited metal after heated at 620℃ for 16 hours was lower than the hardness of the new type casting high Cr hot-working-die steel. While homogeneous welding joints were as good as the base metal on the hot stability. The hardness of homogeneous welding specimens after heated at 620℃ from 4 hours to 16 hours was a little higher than that of the new type casting high Cr hot-working-die steel. It can be concluded that the homogeneous welding joints were better than H13 welding joints on the hot stability.The thermal fatigue resistance was compared among H13 welding joints, homogeneous welding joints and the base metal at the same heat treatment by self-restraint thermal fatigue experiments. The morphology and the length of thermal fatigue cracks were observed and analyzed and the typical samples were chosen to do SEM analysis. The resistance to nucleation of thermal fatigue cracks for homogeneous welding joints was as good as that of the new type casting high Cr hot-working-die steel and better than that of the H13 welding joints. The resistance to propagation of thermal fatigue cracks for the homogeneous welding joints was better than that of the H13 welding joints but a little worse than that of the new type casting high Cr hot-working-die steel.The thermal fatigue samples were dilacerated by bending along the direction of their thermal fatigue crack propagating to observe the morphology of the fracture surfaces by SEM. Generally speaking, the fatigue cracks near the cut of specimens were found on all cross section. But in the front of the crack propaga...