The Effects Of Cryogenic Treatment On Hardening Behavior And Abrasion Resistance Of High Chromium White Cast Irons Subjected To Destabilization Treatment

As-cast high chromium cast irons (HCCIs) consist of austenite, martensite and eutectic type M7C3 carbide that has high hardness, doesn't join into net and is isolated distribution in the matrix which has high strength and good toughness. The HCCIs have excellent wear resistance because of their particular microstructure and have been widely applied in the mineral industry, metallurgy, manufacture of building materials, power plant and chemical industry and so on. The as-cast HCCIs have not well abrasion resistance and before being put in use they must be properly heat-treated to get better wear resistance, commonly. The heat treatments used commonly are destabilization treatment and sub-critical treatment. As techniques of cryogenic treatment are developing, people begin to use them to improve the wear resistance of the HCCIs.In this paper, we investigate hardening behavior, hardening mechanism and abrasion resistance of HCCIs subjected to cryogenic treatment by means of X-ray diffraction, SEM, TEM, magnetic method, hardness test, and wear test.The studied results show that in heating followed by cryogenic treatment or air cool, with increasing the heating temperature (from 900 °C to 1150°C), the hardness of high chromium cast irons increases until it reaches the maximum, and then begins to fall. But the hardness following the cryogenic treatment is always higher than that following the air cooling. Because in the cryogenic treatment, not only the more retained austemte transforms into the martensite but also more tiny secondary carbides precipitate from the matrix. The secondary carbides play a greater role than the martensite for increasing the bulk hardness. But there is still small retained austenite in microstructures when the hardness reaches the maximum. In destabilization treatment two kinds of secondary carbides (Fe, CrfeCe precipitate from the matrix, one is cubic which has orientation relationship with the austenite, and the other is grainy which has no orientation relationship with the matrix. With holding time prolonging, some fine carbides (Fe, Cr)23C6 have transformed into as- rod carbide M7C3.Results of the wear test indicate that the abrasion resistance of the alloys has relation to their hardness. With increasing hardness, their abrasion resistance can be improved. When the abrasion resistance reaches the maximum there is still small retained austenite in the microstructures, too.