Control of machinability in medium-carbon steels

Abstract

A wide variation in machinability can be encountered throughout the range of compositions that are designated ‘medium-carbon steels’. However, this variation can be related primarily to carbon content, and the scatter of results can be broken down into a series of smaller overlapping bands according to the individual specifications in BS 970: Part 1: 1972. By giving greater consideration to specifications, the steel user can select the grade of steel that will provide the best machining performance at the required level of mechanical strength. The improvements in tool life, chip form, and surface finish that can be achieved by the addition of sulphur and lead are described, together with the modifications to the cutting process that are produced by these free-machining additives. By restricting the sulphur range, a higher and more consistent level of machinability can be achieved in both plain-carbon and resulphurized grades. Studies have been made of the effect of silicate and alumina inclusions on the machining characteristics of these steels. Although silicates are detrimental in low-carbon free-cutting steels, the higher temperatures generated in cutting medium-carbon grades soften these inclusions, and there is little effect on machinability. However, alumina inclusions produce a significant reduction in machinability, particularly in resulphurized grades. When grain-refined compositions are required, grain refiners other than aluminium can be considered which have little effect on machinability.