The Formation of White Layers and its Impact on Surface Integrity of Hard-Turned Tempered Martensitic Bearing Steels

Licentiate thesis, 2024

With the rapid advancement of manufacturing methods, machined components are expected to exhibit improved surface integrity and enhanced functional performance. To achieve better surface integrity and dimensional precision, traditionally hardened steels (> 45 HRC) often undergo grinding and polishing operations. With the recent advancement in the machining process and the progress of new cutting tool materials, high precision, and better surface integrity can be obtained by hard turning process with shorter resetting time using PCBN inserts. However, depending on the cutting conditions, the interaction between the workpiece and cutting tool induces gradients of thermo-mechanical loads on the machined surface resulting in white layer formation.

To understand the white layer formation and its characteristics, the first part of this thesis investigates the effects of cutting parameters and tool geometry on the formation of white layers and their impact on the surface integrity of hard-turned AISI 52100 bearing steel. The results revealed distinctive microstructural morphology in the white layers generated by dominant mechanical and thermal loads, i.e. mechanically induced white layer (M-WL) and thermally induced white layer (T-WL). The M-WL exhibited an elongated, fragmented microstructure with a material drag zone underneath it. M-WL showed ~26% increased hardness, while the material drag showed ~6% increased hardness compared to the bulk material. Additionally, better surface roughness and higher surface compressive residual stresses were observed. In contrast to M-WL, the T-WL exhibited a cellular structure with a dark layer adjacent to it. Under thermal dominant conditions, there was an increase in surface roughness and reduced surface compressive stresses with fresh inserts compared to M-WL, and even observed surface tensile stresses with worn inserts. The T-WL exhibited ~28% higher hardness, while the dark layer beneath resulted in 17% lower hardness than the bulk material.

The second part of the thesis investigates the role of varying retained austenite (RA) content in the white layer formation. The results showed that, regardless of the RA content, M-WLs associated with surface compressive residual stresses were observed at low cutting speed using fresh cutting inserts. With a worn tool, samples with higher RA content resulted in the formation of T-WL which was accompanied by surface tensile residual stresses. A similar observation was made when machining at high cutting speed using a fresh cutting insert. Regardless of RA content, machining at high cutting speed with a worn cutting tool, led to T-WL at the surface, which was accompanied by surface tensile residual stresses.

This work shows the effect of cutting conditions and different retained austenite content on the formation of M-WL and T-WL in AISI 52100 bearing steel. The results highlight the improved surface integrity capability of the M-WL compared to T-WL.