Multi-Scale Characterization of White Layer Evolution in Martensitic Steels after Hard Turning
Doctoral thesis, 2026
The advancement in machining processes demands that components exhibit superior surface integrity and functional performance. Hard turning is an efficient machining process that requires precise process control, especially with hardened steels, to achieve the desired surface integrity. However, the intense thermo-mechanical interactions between the workpiece and the cutting tool often lead to the formation of a white layer (WL) on the hard-turned surface, which typically extends from a few hundred nanometers to few micrometers below the surface. WLs are microstructural alterations characterized by nanocrystalline (NC) grains, appearing featureless and white in the light optical microscopy. Based on established literature, often WLs are detrimental because they consist of brittle, untempered martensite with tensile residual stresses, leading to premature component failure. These typically refers to thermally induced WLs (T-WLs), which form through continuous dynamic recrystallization (CDRX) and reverse martensitic transformation and are often accompanied by the formation of a softer dark layer. However, if the process parameters are carefully controlled, a fundamentally different type of WL is formed below the austenitization temperature, known as mechanically induced white layer (M-WL), which exhibits beneficial compressive residual stresses without the presence of a dark layer. These properties make M-WLs a promising process-induced NC surface for demanding engineering applications. Despite its potential, a detailed understanding of the influence of process parameters, tool geometry, initial microstructure on the formation of M-WL with improved surface integrity is currently lacking. In particular, the underlying mechanisms that control microstructure development in M-WL remain unclear.
This thesis investigates the formation and properties of M-WLs and compares them with T-WLs in AISI 52100 and Hybrid 60 steels after hard turning using a multi-scale characterization approach. The results show that the microstructure development of the M-WL in AISI 52100 steel is primarily initiated by grain subdivision process that lead to lamellar grain formation from the initial lath martensite. This is followed by a mechanically assisted triple junction motion, a dynamic recovery mechanism that leads to the formation of NC grains. Furthermore, compared to T-WL, the M-WL exhibited higher compressive residual stresses, lower surface roughness, and improved nanohardness. A similar M-WL mechanism was observed in Hybrid 60 steel, but in this case it was associated with the dissolution of nanoprecipitates. Nevertheless, this led to an increase in nanohardness due to enhanced grain boundary, dislocation, and solid solution strengthening. Hence, the inherent ability of hard turning to generate severe plastic strain below the phase transformation temperature enables the formation of a tailored microstructure. Achieving this within a single, cost-efficient manufacturing step offers a significant advantage for the production of high-performance surfaces.
This thesis investigates the formation and properties of M-WLs and compares them with T-WLs in AISI 52100 and Hybrid 60 steels after hard turning using a multi-scale characterization approach. The results show that the microstructure development of the M-WL in AISI 52100 steel is primarily initiated by grain subdivision process that lead to lamellar grain formation from the initial lath martensite. This is followed by a mechanically assisted triple junction motion, a dynamic recovery mechanism that leads to the formation of NC grains. Furthermore, compared to T-WL, the M-WL exhibited higher compressive residual stresses, lower surface roughness, and improved nanohardness. A similar M-WL mechanism was observed in Hybrid 60 steel, but in this case it was associated with the dissolution of nanoprecipitates. Nevertheless, this led to an increase in nanohardness due to enhanced grain boundary, dislocation, and solid solution strengthening. Hence, the inherent ability of hard turning to generate severe plastic strain below the phase transformation temperature enables the formation of a tailored microstructure. Achieving this within a single, cost-efficient manufacturing step offers a significant advantage for the production of high-performance surfaces.
hard turning
AISI 52100
microscopy
white layer
nanocrystalline grains
Hybrid 60
martensite
steel
Virtual Development Laboratory (VDL)
Opponent: Prof. Dr.-Ing. habil. Ulrich Krupp, RWTH Aachen University, Germany
Author
Sahith Kokkirala
Chalmers, Industrial and Materials Science, Materials and manufacture
Effect of cuting parameters on the generated surface integrity of hard-turned martensitic AISI 52100 bearing steel
Procedia CIRP,;Vol. 115(2022)p. 154-159
Paper in proceeding
The role of retained austenite on the formation of the nanostructured hard-turned induced white layer in AISI 52100 bearing steel
Procedia CIRP,;Vol. 123(2024)p. 292-297
Paper in proceeding
Understanding the development of mechanically and thermally induced white layers in AISI 52100 steel during hard turning: Process-microstructure-property relationship
Journal of Materials Research and Technology,;Vol. 38(2025)p. 1185-1197
Journal article
S. Kokkirala, S. B. Hosseini, U. Klement. Revealing grain subdivision initiated nanocrystalline white layer evolution in AISI 52100 steel via hard turning using transmission Kikuchi diffraction pattern matching
S. Kokkirala, S. Jakob, M. Thuvander, S. B. Hosseini, U. Klement. Nanocrystalline microstructure evolution and behavior of nanoprecipitates in dual-hardening Hybrid 60 steel under high strain rate deformation induced by hard turning
Process-induced nanostructured steel surfaces by controlled hard turning
The service life of high-performance steel components, such as those used in car engines and wind turbines, is strongly affected by surface quality. Traditionally, these parts are finished by grinding, a precise but energy-intensive process. Hard turning has lately emerged as a faster and more energy-efficient alternative. During hard turning, high temperatures and mechanical forces act on the material surface. These conditions can alter the near-surface region, forming a modified layer known as a white layer. For a long time, white layers have been considered harmful because they are often brittle and prone to cracking during service. However, under carefully controlled hard turning conditions, a fundamentally different and beneficial white layer can be produced. Instead of being generated by excessive heat, this layer forms primarily through intense plastic deformation. As a result, it contains compressive residual stresses, a smoother surface, and higher hardness. These characteristics enhance resistance to wear and fatigue.
This thesis investigates how this beneficial white layer forms. Advanced electron microscopy reveals that it consists of nanocrystalline grains created through progressive grain refinement. This refinement occurs via grain subdivision and triple junction motion, rather than heat-driven phase transformations that typically lead to brittle surfaces. The findings obtained for conventional AISI 52100 steel are successfully transferred to
Hybrid 60, a new steel grade that contains nanoprecipitates. This demonstrates that the formation of a beneficial white layer can be deliberately controlled. Achieving this in a single machining step offers new opportunities to produce steel components with possibility of longer service life, and lower energy consumption.
The service life of high-performance steel components, such as those used in car engines and wind turbines, is strongly affected by surface quality. Traditionally, these parts are finished by grinding, a precise but energy-intensive process. Hard turning has lately emerged as a faster and more energy-efficient alternative. During hard turning, high temperatures and mechanical forces act on the material surface. These conditions can alter the near-surface region, forming a modified layer known as a white layer. For a long time, white layers have been considered harmful because they are often brittle and prone to cracking during service. However, under carefully controlled hard turning conditions, a fundamentally different and beneficial white layer can be produced. Instead of being generated by excessive heat, this layer forms primarily through intense plastic deformation. As a result, it contains compressive residual stresses, a smoother surface, and higher hardness. These characteristics enhance resistance to wear and fatigue.
This thesis investigates how this beneficial white layer forms. Advanced electron microscopy reveals that it consists of nanocrystalline grains created through progressive grain refinement. This refinement occurs via grain subdivision and triple junction motion, rather than heat-driven phase transformations that typically lead to brittle surfaces. The findings obtained for conventional AISI 52100 steel are successfully transferred to
Hybrid 60, a new steel grade that contains nanoprecipitates. This demonstrates that the formation of a beneficial white layer can be deliberately controlled. Achieving this in a single machining step offers new opportunities to produce steel components with possibility of longer service life, and lower energy consumption.
Ultrahigh-strength surfaces on novel corrosion resistant hybrid steel
Swedish Research Council (VR) (2023-04410), 2024-01-01 -- 2027-12-31.
Flexible and sustainable production of high performance rolling bearings
VINNOVA (2021-01274), 2021-06-16 -- 2024-03-31.
Subject Categories (SSIF 2025)
Metallurgy and Metallic Materials
Manufacturing, Surface and Joining Technology
Driving Forces
Sustainable development
Areas of Advance
Production
Materials Science
Infrastructure
Chalmers Materials Analysis Laboratory
DOI
10.63959/chalmers.dt/5797
ISBN
978-91-8103-340-3
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 5797
Publisher
Chalmers
Virtual Development Laboratory (VDL)
Opponent: Prof. Dr.-Ing. habil. Ulrich Krupp, RWTH Aachen University, Germany
Related datasets
DOI: https://doi.org/10.63959/chalmers.dt/5797