High-speed milling of hardened steel under minimal quantity lubrication with liquid nitrogen

Minimal quantity lubrication (MQL) can effectively reduce tool wear and improve surface quality in the high-speed milling of hardened steel, but MQL still suffers from poor cooling effect and oil-film failure at high cutting temperature. Compared with MQL, the cryogenic liquid nitrogen (LN2) or LN2 + MQL method with a highly strong cooling ability can further improve the processing conditions of the high-speed milling of hardened steel. However, the knowledge about the characteristics and mechanisms of the high-speed milling of hardened steel under the cryogenic LN2 and LN2 + MQL cooling is scarce. Thus, high-speed milling of hardened steel under cryogenic LN2 and LN2 + MQL cooling was studied in this paper. The machining characteristics were analyzed, and the machining mechanisms of cooling lubrication, tool wear, and surface deterioration layer formation were discussed. The experimental results show that the tool life of LN2 + MQL is remarkably longer than that of MQL, and high surface quality can be maintained at a long cutting distance. In the high-speed milling of hardened steel under the LN2 and LN2 + MQL modes, adhesion has little effect on tool wear, abrasion and chipping/flaking are the main tool wear mechanisms, and chipping/flaking is the main mechanism of tool breakage. MQL reduces the cutting interface friction using oil-film boundary lubrication, LN2 quickly removes the cutting interface heat using a strong cooling and heat transferability, and LN2 + MQL combines the advantages of both cooling and lubrication. Compared with MQL, the surface hardness of the plastic deformation layer of the LN2 and LN2 + MQL modes is considerably increased, whereas the thickness is smaller, and the strong low-temperature cooling is the main reason for this difference. The increase in tool wear will promote the cutting temperature and material slip deformation, increase the depth of high stress and strain zone and eventually increase the thickness of the plastic deformation surface layer in the LN2 and LN2 + MQL modes. In the two modes of LN2 and LN2 + MQL, there are no oxidation, nitridation, secondary quench hardening, and phase transformation in the surface layer. Severe dislocation strengthening caused by severe plastic deformation of the surface layer is the main hardening mechanism of the workpiece surface layer.