Formation of white and dark layers in hard cutting: influence of tool wear

Hard turning technique is considered an attractive alternative to traditional finish grinding operations because of the high flexibility, the capability to achieve comparable workpiece quality, the ability to achieve higher metal removal rates and to operate without the use of coolants. Nevertheless, the surface integrity effects of hard turning need to be better understood due to their influence on the life of machined components. In particular, the formation of a usually undesirable white layer at the surface, and dark layer below it, especially in presence of tool wear, need further investigation. This work deals with the experimental investigation of the effects of tool wear and cutting regime parameters (cutting speed and feed rate) in white and dark layers formation when hardened AISI 52100 bearing steel is machined. In particular, an experimental campaign has been conducted by orthogonal turning tests on disks made of AISI 52100 hardened steel using PCBN inserts. Experimental results are documented including quantification of tool wear and microstructure investigation of the machined surfaces. The results show that cutting regime parameters and, especially, tool wear affect white and dark layers formation.