White layer formation mechanism in dry turning hardened steel

Abstract

The studying white layer produced in dry-hard turning process has important theoretical significance and practical value. The experiments of orthogonal turning AISI 52100 and AISI 4340 steels were employed by using PCBN inserts, a series of experimental methods were employed to study the microstructure, phase component, element segregation and hardness of machined surface. Based on experimental results, the white layer formation mechanism and the effects of cutting parameters and material properties on the white layer thickness were studied. The results show that there is ultra-fine microstructure in the white layer, the carbon segregation occurs in the machined surface and the retained austenite content is larger than that in the substrate, which indicates that the white layer is the product of phase transformation. The grains in the white layer are refined and the hardness of white layer is larger than that of the substrate, which illustrates that the plastic deformation promotes the formation of white layer. In conclusion, white layer is formed under the interaction of phase transformation and plastic deformation. The thickness and retained austenite content of white layer increase at first then decrease with cutting speed. The white layer thickness increases with feed rate, carbon content and hardness of material.

Keywords

• dry-hard turning
• steel
• white layer
• formation mechanism
• cutting parameters
• white layer thickness