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In this paper, we studied the effect on microstructure and properties during low tempering and high tempering temperature of HNC53 and KD11max Cr8 cold working die steel by means of hardness tester, universal tensile testing machine, pendulum impact tester, metallographic microscope, SEM and EDS. The results showed that the microstructure of the two materials were spheroidized pearlite and large eutectic carbide. After quenching then tempering at high temperature 520°C and low temperature 200°C, the microstructure was martensite, eutectic carbide with large particles and small secondary precipitated carbide which was M7C3 type carbide. When KD11max was tempered at high temperature of 520°C, more nano-level secondary carbonization precipitated, and secondary hardening phenomenon was more obvious. The increase of Si and decrease of C could promote secondary hardening and improve the impact toughness of materials. KD11max impact toughness was better than HNC53.

The vacuum low pressure carburizing and high pressure gas quenching processes of 20CrMo, 20CrMnTi and 20Cr2Ni4 with acetylene as carburizing medium were investigated. The results show that the carburizing and diffusion time significantly affected the carburizing performance. Carburized with the process of C2H2 flow rate 10L/min, N2 flow rate 10L/min, carburizing pressure 3kPa, carburizing time 42min, diffusion time 140min and gas quenching pressure 1.5MPa, 20CrMo, 20CrMnTi and 20Cr2Ni4 can obtain the surface carbon content of 0.74%-0.78% and carburizing depth of 0.81mm-0.83mm. Meanwhile, the microstructure showed first grade carbide, and there was no internal oxidation laye on the surface. The carburizing constant and the diffusion time to carburizing time ratio were modified to be more suitable for the carburizing process.

The fracture of a drilling tail made of 18CrNi3MoA steel in the exchanging water hole was analyzed in terms of inclusions, prior austenite grain size, carburized layers, and fatigue fracture morphology by means of optical microscopy, quantitative metallography, and scanning electron microscopy (SEM). Fatigue crack initiation and propagation on the drilling tail were also studied. The results showed that the fracture on the drilling tail was not induced by inclusions and the distribution inhomogeneity of prior austenite grain size. Instead, because the outside surface of the exchanging water hole was not continuously geometric, there was a great deal of stress concentration in those areas under continuous twisting, axial impact, and corrosion of mineral water. Thus three crack sources emerged in these areas. Initial cracks centered on these sources spread from the outside surface to the inside surface, and eventually the drilling tail ruptured. Furthermore, the fractograph of the region around the crack sources exhibited a typical ductile dimple fracture pattern, and cracks initiated on the outside surface of the carburized layers of the exchanging water hole. Three crack sources could be the sources of fatigue crack propagation. Based on the analysis of fatigue crack propagation, it was further demonstrated that fatigue damage originated from the outside surface of the exchanging water hole.