Study Of High-temperature Wear Performance Of Cast Hot-forging Die Steels

Cast hot-forging dies have been used practically at abroad, because of their good properties and long lives. However, the application of domestic cast die steels is confined owing to their poor elevated temperature wear performance and short lives. In this study, the effects of compositions, processes, microstructures, performances, ambient temperatures, loads and sliding velocities on wear resistance were studied. Moreover, by using SEM, X-ray and EDS, the images, composition and structure of worn surfaces and worn debris were investigated. The elevated temperature wear mechanism of cast hot-forging die steels was discussed through analyzing the morphologies and delaminating patterns of oxide films on worn surfaces, in order to offer some theory grounds for the improvement of their high-temperature wear performance. And this study has important theoretical significance and practical application value.The results show that wear rate of cast hot-forging die steels is not sensitive to the change of hardness. The wide-range thermal strength would meet the need of higher wear resistance. And the high-temperature wear resistance strongly depends on variation of microstructures. It is found that the high-temperature wear resistance of tempered troostite is superior to tempered martensite and bainite complex microstructure and tempered sorbite. The catenulate Cr₂₃C₆ precipitates along grain boundaries of austenite or lath boundaries of martensite in the cast hot-forging die steel with 4.0wt.%Mo, tempered at 600℃, and coarse (RE) ₂O₂S appears in the cast hot-forging die steel with excess addition of RE. Both lower the toughness of the cast hot-forging die steels and deteriorate their high-temperature wear performance. It can be concluded that thermal strength and toughness both are needed for high wear resistance. During high-temperature wear, oxide films and oxide granules are found to form on the worn surfaces. The oxides are composed of Fe₄O₃ and Fe₂O₃. Wear debris of cast die steels is the mixture of Fe, Fe₄O₃ and Fe₂O₃. High-temperature wear mechanism of cast hot-forging die steels is oxidation wear and fatigue desquamation wear. When the second-phase is fine, the oxide film would flake off from the inside of oxide or the interface of oxide film and matrix, which is classified as ordinary oxidation wear, and accords with the theories of Quinn and Wilson etc. In this case, the wear rate has nothing to do with the structure and toughness of matrix. However, when the second-phase is coarse, crack would initiate from the interface of second-phase and matrix and propagate in the matrix, so that the matrix and oxide film would spall together. In this case structure and toughness of matrix would have a strong effect on wear rate.