Modification of surface morphology during cryogenic turning of metastable austenitic steel AISI 347 at different parameter combinations with constant CO2 consumption per cut

Abstract

For numerous cryogenically machined materials a hardness increase in the workpiece surface layer has already been reported due to strain hardening mechanisms and compressive residual stress. When machining metastable austenitic steels, these hardening mechanisms are overlaid by a deformation induced phase transformation from austenite to martensite. This phase transformation is favored by low thermal and high mechanical loads in the workpiece surface layer; thus the martensite content can be modified by varying the thermo-mechanical load.In this study, the cryogenic turning process is designed to maintain low workpiece temperatures during machining and to maximize the passive forces and consequently the martensite content. The passive forces are adjusted by using indexable inserts with strongly varying chamfer angles and therefore, also varying effective rake angles. The feed rate and thus the cutting cross-section are varied as well to further adjust the passive forces. As higher feed rates lead to shorter cutting times, the cooling time is also shortened, which increases the thermal load and worsens the surface morphology. Therefore, for each variation, the coolant supply was controlled to maintain a constant quantity of CO2 during each cut. Hence, low workpiece temperatures can be achieved even with high feed rates. By controlling the thermo-mechanical load, significantly more martensite can be generated while maintaining a constant CO2 consumption per cut.