Cryogenic minimum quantity lubrication for improving the surface integrity of AISI 347

Abstract Metastable austenitic steels exhibit a unique hardening mechanism based on a strain-induced phase transformation from ductile austenite to hard martensite, which can be triggered by high plastic deformation at low temperatures. In previous studies, this transformation was successfully promoted during turning by applying large amounts of CO₂. However, the associated high consumption of coolant limits the economic feasibility of this approach. This study introduces a hybrid cooling and lubrication strategy combining CO₂ with minimum quantity lubrication (cMQL). Both media are supplied in very small quantities (0.1–0.2 kg/min CO₂ and 2.60 g/h oil). Two oils specifically developed for use at cryogenic conditions were investigated and compared to pure CO₂ cooling and dry cutting. The objective was to achieve deformation-induced martensite formation while significantly reducing CO₂ consumption. The results demonstrate that the hybrid cMQL approach enables the generation of smooth surfaces together with moderate martensitic transformation. The findings highlight the potential of cryogenic MQL as a resource-efficient alternative to conventional cryogenic cooling for improving surface integrity in the machining of metastable austenitic steels.