Toward understanding of metallurgical behaviours in dry machining of hardened steel: phase transformation and surface oxidation

Dry machining of hard-to-cut materials has been recognized as one of the most prospective manufacturing techniques, while involves complicated thermal and mechanical loadings inducing physical behaviours. Therefore, the metal-physical process associated with metallurgical behaviours may initiate due to highly coupled thermo-mechanical loads, a deep understanding of these encountered phenomena in dry machining is required. In this work, the effects of two factors (cutting speed and feed rate) on prominent material behaviours including chip characteristics, phase transformation and surface oxidation in the dry milling of hardened AISI H13 steel were investigated. A reliable finite element model with implemented user defined subroutine for phase transformation was introduced to predict serrated chip, cutting temperature, cutting forces, and phase compositions simultaneously. Dry milling experiments were carried out for validation. A good consistence concerning chip characteristics and cutting forces was achieved. In the end, X-ray photoelectron spectroscopy (XPS) analysis was utilized to establish the correlation between chip colour and cutting temperature. The obtained results show that the metallurgical behaviours appeared in dry milling is attributed to the excessive rise in cutting temperature and the maximum temperature at the tool-chip interface can be evaluated semi-quantitatively. This research lays a solid foundation for an in-depth understanding of the metal-physical phenomena occurred at cutting zones.