G20Cr2Ni4A wear-resistant steel plate has good hardenability and toughness, and is one of the main materials for high-strength carburizing steel. After carburizing, a large amount of carbon and alloying elements are dissolved into the matrix, causing the Ms point to drop significantly. After carburizing and air cooling, there is a large amount of residual austenite in the carburized layer. Reheating and quenching cannot reduce the residual austenite content, resulting in the surface layer. The hardness is low and cannot meet the design requirements. In addition, during use, the retained austenite is heated or transformed under strain, causing dimensional changes and stress redistribution, which may also cause grinding cracks during grinding and finishing. At present, the main ways to reduce the amount of retained austenite are high-temperature tempering and cold treatment. Researchers use the hardness gradient of the carburized layer, the depth of the carburized layer, the surface hardness, the microstructure of the carburized layer, etc. Research on the impact of cold treatment and cryogenic treatment on the quality of the carburized layer.
The material selected for this test is G20Cr2Ni4A wear-resistant steel plate, which complies with the standard GB/T3203-1982 "Technical Conditions for Carburized Bearing Steel". The bar stock was machined into several samples with dimensions of 20 mm × 10 mm × 10 mm, and the oxide scale on the surface of the samples was cleaned with sandpaper. Before entering the furnace, put it into an ultrasonic cleaning machine for cleaning for 30 minutes, dry it with a hot air blower and use it as a carburizing sample for this test.
In order to compare the effects of cold treatment and cryogenic treatment on the microstructure and properties of the carburized layer, the samples were treated with different processes using the processes shown in Table 2. Among them, all samples were carburized, high-temperature tempered and cold treated, deep-tempered Tempering after cold treatment is done in the same furnace. The morphology, Rockwell hardness, hardness gradient and depth of the carbides in the carburized layer were tested using a scanning electron microscope, Rockwell hardness tester, and microhardness tester on the samples treated with different processes.
The results show:
(1) After treatment at -75℃×2h and -196℃×2h, the number and dispersion degree of carbide particles on the surface of the sample have been greatly improved.
(2) Cold treatment and cryogenic treatment can increase the hardness of carburized samples. The hardness of the samples after cold treatment at -75℃×2h is on average 1.8HRC higher than that of the non-cooling samples. The hardness of the samples after cryogenic treatment at -196℃×2h is on average 3.0HRC higher than that of the non-cooling samples.
(3) After treatment at -75℃×2h and -196℃×2h, within 1.45mm from the sample surface, the microhardness is higher than that of the sample without cold treatment. The effects of cold treatment and cryogenic treatment on the depth of the carburized layer smaller.
