Understanding the development of mechanically and thermally induced white layers in AISI 52100 steel during hard turning: Process-microstructure-property relationship

Journal article, 2025

Hard turning offers a cost-effective alternative to traditional grinding, yet the tool wear progression limits the broader industrial adoption. During hard turning, the surface microstructure of AISI 52100 steel transforms into a nanocrystalline structure known as white layer, accompanied by significant surface residual stresses. With optimal cutting conditions, surfaces develop nanocrystalline microstructures with high compressive stresses, known as mechanically induced white layers (M-WLs). In contrast, improper cutting conditions generate thermally induced white layers (T-WLs), associated with tensile stresses. This study investigates the effect of feed rate, cutting speed, and tool wear on the different white layers formed and their influence on the surface integrity. Microstructural analysis reveals that the M-WL formed by dynamic recovery mechanism exhibited fragmented nanocrystalline grains with ∼26 % higher hardness than the bulk material. The presence of elongated lamellar grains with ∼7 % higher hardness in the material drag zone beneath the M-WL suggests the occurrence of a grain subdivision process that initiates M-WL formation. This grain subdivision mechanism generated lamellar grains composed of geometrically necessary boundaries (GNBs) and incidental dislocation boundaries (IDBs), reflecting progressive strain accommodation during severe plastic deformation. In contrast, T-WL is generated by continuous dynamic recrystallization mechanism and features nanograins with ∼27 % higher hardness and an underlying over-tempered dark layer with ∼16 % lower hardness than the bulk material. The M-WL exhibits surface roughness of ∼5 times lower and better surface compressive stress than the T-WL. This research demonstrates a promising hard turning strategy for producing advantageous M-WL with nanocrystalline grains and improved surface integrity.