The effect of cryogenic cooling on white layer formation in hard machining

Microstructural phase transformations, commonly named as the white layer on hard turned components, have in recent times become an interesting research topic in machining. Three main theories have been proposed to justify the mechanisms of white layer formation: (i) rapid heating and quenching; (ii) severe plastic deformation; (iii) surface reaction with the environment. Furthermore, coolant application also affects the surface microstructural alterations resulting from machining operations, which have a significant influence on product performance and life. The present work aims at understanding the effects of cryogenic coolant application on machined surface alterations during orthogonal machining of hardened AISI 52100 bearing steel. Experiments were performed under dry and cryogenic cooling conditions using cubic boron nitride (CBN) tool inserts with varying initial hardness and tool shape. Several experimental techniques were used in order to analyze the machined surface. In particular, optical and scanning electron microscopes (SEM) were used for characterizing the surface topography, whereas the microstructural phase composition analysis and chemical characterization have been performed using X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS) techniques. The experimental results prove that the microstructural phase changes are partially reduced or can be totally avoided under certain cryogenic cooling conditions. Therefore, cryogenic cooling has the potential to be used for achieving enhanced surface integrity, thus contributing to improved product life and functional performance.