Comparison of surface integrity, tool wear and chip morphology in CO2 cryogenic and dry milling of 304 stainless steel

Abstract

In this study, dry and CO2 cryogenic milling conditions on 304 stainless steel is investigated. The cutting speeds of (60, 100, 140 m min−1), feed rates of (0.1, 0.13, 0.16 mm tooth−1) and axial depth of cuts of (0.5, 1, 1.5 mm) are considered in the L9 Taguchi array to design both dry and cryogenic slot milling tests. In cryogenic milling, defects in surface topography and tool wear (flank wear, built up edge and tool chipping) reduced significantly compared to dry machining conditions. Cryogenic milling improved the surface roughness from 4% to 71% compared to the corresponding dry conditions. In dry milling, more serrated chips and more discontinuities are seen in the lateral sides of the chips than cryogenic. In cryogenic compared to dry milling, the surface hardness of the workpiece decreased from at least 8% to a maximum of 12%. Also, the hardness of dry milling surface, increased from 43% to 102% compared to the bulk material. The maximum depth of hardening in cryogenic milling is 23% lower than that of dry condition. Increasing cutting parameters or dry machining will intensify the plastic deformation and compressing the microstructural grains adjacent to the machining surface, as in the deformed surface zone, the grain boundaries are indistinguishable. The maximum reduction in the thickness of the deformed layer was about 16% in the cryogenic milling compared to the corresponding dry condition. Considering XRD diagrams for dry and cryogenic milling compared to non-machined condition, no significant metallurgical phase transformation has occurred in the workpiece material.