The effect of subzero treatment on microstructure, fracture toughness, and wear resistance of Vanadis 6 tool steel

Vanadis 6 ledeburitic steel manufactured by powder metallurgy of rapidly solidified particles was conventionally quenched and subzero-treated at temperatures of −90 and −196 °C for 4 and 10 h, respectively, followed by standard double-tempering to a secondary hardening peak at 530 °C for 2 h. Besides quantification of standard microstructural features by using examination by scanning electron microscopy and X-ray diffraction analysis, the fracture toughness, hardness and nanohardness, flexural strength, fracture toughness, and wear resistance have been characterized. The obtained results indicate the following: (i) The microstructure of the material consists of tempered martensite and eutectic, secondary, and small globular carbides. Retained austenite is completely eliminated by application of the given heat-treatment schedules. (ii) The amount of small globular carbides is significantly increased by subzero treatments; the lower the temperature (or the longer the duration) of subzero treatment, the higher the content of these carbides. (iii) The hardness of the material is influenced slightly negatively by subzero treatments followed by tempering in the normal secondary hardening temperature range. (iv) There is no negative impact of the given treatment schedules on either toughness or fracture toughness of the steel. (v) When a hardened ball bearing is used as a counterpart, wear resistance is improved by subzero treatments despite the slightly lower hardness of the examined tool steel.