Effect of Cryogenic treatment on VIKING cold working tool steel and development of wear mechanism maps

Abstract

Tool steels are classified according to their alloying constituents and will exhibit their real properties upon proper thermal treatment. The process and sequence of heat treatment differ depending upon the percentage of carbon and other metallic elements. VIKING a special purpose tool steel coming under AISI A8 cold working steel classification is widely used for heavy duty blanking and forming operations because of its high wear resistance and equally good toughness. Air or vacuum hardening is recommended as the conventional heat treatment process for steels under this category. These steels are easily machinable in pretreated conditions and will get converted to hardened structure upon appropriate treatment. In this study, we are incorporating a shallow cryogenic cycle within the conventional treatment process before tempering. The thermal treatments at sub zero temperatures up to -90°C was conducted with liquid nitrogen as cryogenic medium and the changes in its microstructure and corresponding improvements in the tribological properties are analyzed. The microstructure is studied using the micrographs taken using optical microscopy which shows more pronounced martenstic conversion and formation of fine carbides. The treated material specimens are then subjected to wear tests on DUCOM TRIBOMETER for different combinations of speed and load under normal temperature without any lubrication. The wear rate and coefficient of friction obtained from these set of experiments are used to develop wear mechanism maps with the help of FUZZY C MEANS clustering. The wear mechanisms maps generated with this computational technique is then compared with the SEM photographs taken and the improvement in properties due to this additional cryogenic treatment is validated. XRD analysis was also performed to identify the phases present before and after cryogenic treatment.