A review of surface integrity in machining of hardened steels

This paper presents a background of surface integrity in the machining of hardened steels for rolling dies, moulds, bearings, shafts, gears, etc. Hardened steels, which are normally harder than 45 HR_C, are considered difficult to cut owing to their combinations of mechanical and physical properties that promote low chip–tool contact area and high forces, and in turn, result in high contact stresses and elevated temperatures. In particular, the high temperatures generated on a newly machined workpiece surface can promote poor surface integrity, such as white layers and microcracks, warping, and consequently reduced fatigue strength. Additionally, tool wear is often significant, which further exacerbates the problem of high cutting temperatures, leading to frequent rejection of hard machined parts during quality control and thus increased manufacturing costs. In this work, the background of surface integrity during machining of hardened steels is introduced, and the main challenges and their solutions from the literature are discussed based on a diverse set of case studies. Based on the state of the art, a series of conclusions and best practices are presented, in addition to an outlook on future directions in the field of machining-induced surface integrity of hardened steels.