High temperature friction and wear properties of two hot work die steels

Owing to work at high temperature and high loadings, hot work die steels wear easily, and are especially susceptible to high temperature oxidative wear. Under severe oxidative wear conditions, the wear rate is high, which may lead to premature wear failure of the dies. Therefore, severe oxidative wear should be limited or avoided during the service life of hot work die steels. For service materials, wear resistance is affected by temperature, load, time on the oxide type, plastic deformation, and debris morphology of the surface and sub-surface. Pioneering researchers tended to focus on the influences of temperature, load, and time on wear resistance, and little is known about the wear mechanism of different materials. In this work, the wear mechanism and resistance differences between two hot work die steels, HTCS-130 and DAC55, were studied at temperatures of 100-700℃, using a high temperature friction and wear tester. Surface phase composition, worn surface and cross-section morphology were analyzed by white-light interferometer, scanning electron microscope (SEM), and X-ray diffraction (XRD). The results show that the wear rates of the two steels both increase at first and then decrease at temperatures of 100-700℃. The wear mechanisms of both steels appeared as adhesive wear at 100℃ and adhesive-oxidative wear at 300℃. Then, the wear mechanism changed into oxidative wear at 500℃ and an oxide layer comprising FeO, Fe₂O₃, and Fe₃O₄ was observed on the worn surface. Meanwhile, the subsurface started to soften slightly and a plastically deformed layer appeared. Subsequently, severe oxidative wear occurred at 700℃ and the number of oxides had sharply increased. The materials were severely softened owing to the recovery of the martensite matrix. Meanwhile, a continuous oxide layer formed on the worn surface. Due to the excellent thermal stability of HTCS-130 steel, the high hardness and narrow softened zone of matrix could better support the oxide layer. Therefore, HTCS-130 steel shows better wear resistance than DAC55 steel at 700℃.