Influence of microstructure on machinability of wrought steels

Abstract

The various ways in which machinability can be evaluated, both in terms of individual or combined machining parameters, or by theoretical energy or force expressions, are reviewed. This background is then used to discuss ways in which the steel microstructure affects the basic requirements of low strength and minimum ductility for good machinability, together with other factors of significance, but excluding the influence of specific additions for free machining. In low-carbon steels, the influence of heat treatment, cold work, and solid-solution strengthening of the ferrite is discussed; spheroidizing heat treatment becomes significant at medium to high carbon levels and may be combined with cold work. In alloy steels, isothermal heat treatment to allow the separation of proeutectoid ferrite during cooling from rolling is a possible alternative to normal annealing. Particular attention is paid to the influence of microstructure on the machinability of highcarbon, chromium bearing steel. The morphology of carbide distribution in both the pearlitic and spheroidized conditions has a marked effect on machinability, as does the introduction of cold work and stress-relief heat treatment.