Effects of the rate and direction on the machinability of low-carbon steels

In this study effects of rate and direction of rolling on the machinability of a low carbon St 44 steel have been investigated. For these purposes steels were subjected to cold and hot rolling at various rates. Cutting forces have been measured by means of a dynamometer, on a shaping machine at three different cutting speeds. The samples have been machined longitudinally, perpendicularly and diagonally with regard to rolling direction by keeping tool material and depth of cut constant. The effect of anisotropic behaviour resulting from the texture and mechanical fibre structure associated with rolling and rolling direction on cutting forces and surface roughness were examined. As a result, it was found that cutting forces of the cold rolled samples decreased with decrease in both the pearlite band thickness and pearlite band and width in 3 cutting directions and raised with increases of cutting speeds. Cutting forces of hot rolled samples increased with the rate of rolling in the longitudinal and perpendicular directions, but there was no considerable change in diagonal direction at the same cutting conditions. The 3 cutting forces in the 11 % hot rolled sample are higher than that of initial material, but lower in the transverse direction at the same machining conditions. With respect to cutting forces of cold rolled samples optimum rate of cold rolling was determined to be 11 %-23 % range in transverse rolling direction. Jaoul-Crussard analysis is employed to determine work hardening behaviour. Cutting forces of cold rolled samplesIV decreased with increasing work hardening rate but there is no linear correlation between cutting forces and rate of work hardening of hot rolled samples. Roughness of surface of cold rolled samples was relatively low compared to that of hot rolled samples. Key words : Cold rolled steel, hot rolled steel, machinability.