Microstructure and mechanical properties of superficial surface and subsurface layers in the cutting of hardened steel under cryogenic cooling.

Hardened steel parts extensively utilized in manufacturing fields, including automobiles and molds, are difficult to cut and process. Cryogenic cooling is an effective method for reducing tool wear and improving surface quality. However, knowledge about the microstructure and mechanical properties of the hardened steel cutting under a cryogenic cooling condition is severely limited. Herein, low-speed and high-speed cutting experiments with different wear tools were designed to investigate the characteristics and mechanisms of the microstructure, residual stress distribution, and hardness distribution of the superficial surface layer (SSL, including the white and transition layers) and subsurface layer (SL) of hardened steel cutting via cryogenic cooling. Results showed that microstructure changes caused by plastic deformation occurred in both the white and transition layers, whereas no changes occurred in the area below the transition layer. No phase transformation and recrystallization occurred in the white layer, the transition layer, and the area below the transition layer. The intense dislocation slip and strong plastic deformation are the keys to the formation of the white and transition layers. The hardness distribution curve included the surface hardening peak, SL softening valley, and SL hardening peak. The combined effect of fine grain strengthening and dislocation strengthening is the primary mechanism for forming the surface hardening peak. The peak residual tensile stress induces a SL softening valley in the hardness curve, whereas the peak residual compressive stress induces a SL hardening peak in the hardness curve. [ABSTRACT FROM AUTHOR]