White Layer Formation during Hard Turning of Through-Hardened Martensitic and Bainitic AISI 52100 Steel

Increased demands for flexible manufacturing lines, shorter cycle times and reduced impact on the environment drives the need for optimization of today’s manufacturing. For machining of hardened steel parts (>45 HRC), grinding has been the obvious choice for many decades. However, hard turning has often been considered as a replacement due to e.g. shorter processing times and that post surface treatments would not be required for an optimized process. Despite the advantages of hard turning, the process has not yet been widely accepted in industry as a final machining operation. One of the main reasons is the microstructural alteration, referred to as white layer (WL), which might occur at the surface during turning. Due to the limited knowledge regarding the impact from the WLs on the fatigue life, generally post surface treatments are used to remove the WLs. By performing hard turning tests using a set of different machining parameters, surfaces with and without WLs were generated on martensitic and bainitic AISI 52100 steel samples. The machining conditions resulting in WL formation were studied with help of in-process force and temperature measurements combined with advanced characterization techniques for microstructure evaluation such as X-ray diffraction, scanning and transmission electron microscopy and atom probe tomography. It was shown that depending on the cutting speed and tool condition, different types of WLs can be formed, i.e. predominantly mechanically or thermally induced WLs (M-WLs or T-WLs). The M-WLs were formed below the austenitic phase transformation temperature, ATMc1, where the microstructural evolution from a bainitic or martensitic microstructure to a nano- and sub-micrometer sized structure comprising of rectangular shaped sub-grains was controlled by dynamic recovery. The T WLs were formed above ATMc1 temperature and the microstructural transition from the initially bainitic or martensitic microstructure to the substantially refined microstructure was initiated through dynamic recovery and continued by dynamic recrystallization. The microstructure of the T-WLs was characterized by an elongated sub-grain structure, which co-existed with equi-axed grains. The DICTRA simulations showed that the contact times and temperatures involved in hard turning were insufficient to affect secondary carbides and was confirmed by atom probe tomography measurements. The TEM analyses revealed plastic deformation and segmentation of secondary carbides independent of the type of WLs formed. The nano-sized transition carbides were found in the M-WLs by use of atom probe tomography, while it was concluded that they had dissolved in the T-WLs. A descriptive phenomenological model is proposed to describe the formation of different types of WLs induced during hard turning. The model considers the effects of cutting speed and tool wear, i.e. the thermo-mechanical loads by incorporating the effects of high strains and strain-rates, high heating rates, temperatures, and hydrostatic pressures. Moreover, a new transformation temperature, ATMc1, has been defined to consider the transition zone for the thermo-mechanical austenite formation during machining.