Understanding of the Microstructure Evolution and Wear Resistance of Cr12MoV Die Steel during Deep Cryogenic Treatment

The present study investigated the influence of deep cryogenic treatment (DCT) on the microstructure evolution and wear resistance of Cr12MoV die steel. Four types of heat treatments (quenching + tempering (QT), quenching + DCT (QC), quenching + DCT + tempering (QCT) and quenching + tempering + DCT (QTC)) were utilized to obtain different microstructure and mechanical properties. The hardness, impact toughness, weight loss, frictional coefficient, volume fraction of retained austenite (RA), secondary nanometric carbides (SNC) and worn surface morphology were investigated in detail to discuss the relation between microstructure evolution and wear mechanism changes induced by DCT. The results show that the precipitation of SNC has less effect on hardness than the decrease of carbon content in martensite that caused by DCT and tempering. The principal reason for the high hardness and low toughness without tempering is the high carbon content in martensite. The RA volume fraction has greater effect on the hardness and toughness in condition of tempering. Four types of wear cracks (matrix cracks, boundary cracks, extension cracks and carbide cracks) were identified for the analysis of worn surface morphology. When the matrix hardness is high and RA volume fraction is relatively low, the wear cracks were mainly carbide cracks and extension cracks, accompanied by a few of boundary cracks. When the matrix hardness is low and the RA volume fraction is relatively high, the wear cracks were mainly matrix cracks, accompanied by some boundary cracks and extension cracks. DCT affects the wear mechanism of Cr12MoV die steel by facilitating the transformation of RA and the precipitation of SNC.