Experimental analysis of mechanisms related to white layer formation during hard turning of AISI 52100 bearing steel

Microstructural phase transformations, commonly named white layer on hard turned components, are becoming one of the most interesting research subjects for the scientific community. Three main theories have been proposed to justify the mechanisms of white layer formation: rapid heating and quenching, which results in sudden microstructural phase transformation; severe plastic deformation, which produces a homogenous structure and/or a very fine grain size microstructure; and surface reaction with the environment. The present work aims to understand which of the above mentioned mechanisms is the main cause of the white layer formation when AISI 52100 hardened steel is machined by cubic boron nitride inserts. For this reason, an experimental campaign was carried out, and several experimental techniques were used in order to analyse the machined surface. In particular, optical and scanning electron microscope were utilised for surface topography characterisation, while microstructural phase composition and chemical characterisation have been performed by means of X-ray diffraction and energy dispersive spectroscopy techniques. The experimental results prove that the white layer is the result of microstructural alteration, i.e. the generation of a martensitic structure.