Residual Stress, Nanohardness, and Microstructure Changes in Whirlwind Milling of GCr15 Steel

Compared to grinding which has low efficiency, serious pollution, and hard turning together with serious tool wear, whirlwind milling (WM) is a more competitive machining process in manufacturing. However, as a technology under development, the complicated mechanism in WM processes has not yet been fully understood. In this paper, surface integrity of a workpiece processed by a computerized numerical control (CNC) WM machine was investigated using a variety of advanced instruments, e.g., Electron Backscatter Diffraction (EBSD), nanoindentation, X-ray diffraction stress analysis, etc. The results show that surface hardening and deep residual compressive stress could be effectively achieved in the WM process. Based on the EBSD analysis and infrared temperature camera, the residual compressive stress was introduced by volume expansion when the phase transformation from austenite to martensite occurred. According to the Hall–Petch (H-P) relationship, it could be concluded that the higher hardness processed by WM is mainly caused by the finer grains, i.e., the average size was 167 × 102 nm2 in the WM zone, which was less than 194 × 102 nm2 in the non-WM zone. These findings were helpful for developing and optimizing the WM process.