The influence of cryogenic supercritical carbon dioxide cooling on tool wear during machining high thermal conductivity steel

In the recent years, there have been increasingly tremendous demands for lightweight automotive parts, in the quest for better energy efficiency. These parts are produced using Ultra High Strength Steel (UHSS), for reasons of high strength and rigidity. However, for the parts blank to have increased strength and hardness, the forming process has to be carried out under elevated temperatures, achieved through preheating and quenching. In addition, to ensure an effective quenching, High Thermal Conductivity Steel (HTCS), having a high thermal conductivity and a high wear resistance, is used as the forming die, possibly resulting in impaired machinability. Therefore, an effective coolant-lubrication technique is required to ensure improved productivity. A cryogenic cooling technique, such as, the use of supercritical carbon dioxide (SCCO2) was reviewed in the machining processes. SCCO2 was selected over the more commonly used liquid nitrogen (LN2), as the cryogenic substance, due to its low gas expansion value, thus ensuring lower environmental hazards to the workers. In short, the non-toxic SCCO2 promotes a healthier, safer and more sustainable working environment. In this study, a milling process of HTCS was performed, where the effectiveness of SCCO2 coolant was compared with that of Near Dry Machining and Dry Machining techniques, in terms of cutting temperature, cutting force, tool wear and wear mechanism. Two methods of SCCO2 cooling conditions were analyzed, with and without lubricant. Both of the cooling conditions were applied under three different input chamber pressures. In comparison with Dry Machining, SCCO2 with lubricant was found to have significantly improved the cutting force and cutting temperature, up to 60% and 55% respectively. In comparison with Dry Machining and Near Dry Machining, meanwhile, the cutting tool life increased to 150% and 87%, respectively. More importantly, it was observed that, adhesion, attrition and abrasion were the dominant wear mechanisms, when HTCS was milled under various coolant-lubricant conditions. The overall results revealed that, cryogenic cooling, using carbon dioxide gas under supercritical state, was the best option for a better control and improvement of tool wear. The prolonged tool life will ensure a highly sustainable production, with less tool wastage and more efficient machining process. From the environmental point of view, the use of SCCO2 as a cryogenic substance is advantageous, as it fulfills the increasing demands for a cleaner manufacturing of HTCS.