Effects of deep cryogenic treatment on microstructure, impact toughness and wear resistance of an AISI D6 tool steel

Several studies have shown that cryogenic treatments improve many properties of tool steels. Basically the cryogenic treatment reduces the amount of retained austenite and promotes ultrathin carbides precipitation. In this study, samples of AISI D6 were submitted to several heat treatment cycles, aiming to analyze the influence of cryogenic treatment on microstructure, impact toughness and wear resistance. These cycles were quenching and tempering, quenching and tempering followed by deep cryogenic treatment (2 and 24 h) and, finally, quenching and tempering followed by deep cryogenic treatment plus double tempering. By the comparison with other studies and by the application of techniques such as Electron Microscopy, Image Analysis and X-ray Diffraction, it was observed that the hardening step (austenitization time and temperature) plays a fundamental role in the retained austenite level and in the cryogenic treatment results. The thermal cycles act both on the martensite formed directly from the quenching (primary martensite) and on the martensite formed from the retained austenite. It was concluded that cryogenic treatment followed by double tempering produced a reduction in the amount of retained austenite and consequently leads to secondary carbides precipitation. This fact has a marked role on the mechanical properties, keeping the wear values unchanged for the studied conditions.