Investigation of surface roughness, microhardness and white layer thickness in hard milling of AISI 4340 using minimum quantity lubrication

In this paper, the relationship between cutting parameters and machined surface integrity in the hard milling of 4340 alloy steel using minimum quantity lubricant is investigated. Milling parameters including; cutting speed, feed rate, axial and radial depth of cut were considered in three levels. Then, the effects of these parameters on roughness, topography, microhardness, white layer thickness and surface chemical composition were studied through Response Surface Methodology. The analysis of variance showed that quadratic polynomial models are suitable to estimate the surface roughness and microhardness, while a linear model evaluate the variations of white layer thickness, as well. In addition, the contribution of parameters affecting surface roughness was as follows; feed rate 49.2%, cutting speed 23.1% and radial depth of cut 14.1%. It was found that increasing cutting speed has a significant influence in the reduction of surface defects. Furthermore, all the cutting parameters increased microhardness and white layer thickness. Feed rate with 73.1% and cutting speed with 14.4% had the most influence on microhardness, comparatively. White layer thickness also varied between 7.6 μm and 16.1 μm under different cutting conditions. The cutting speed with 81.3% and feed rate with 9.4% had the most effects on the white layer thickness. EDS analysis of milled surface revealed that in cutting speed of 150 m/min, O found in addition to the base material elements and C weight percent increased.