The effect of electrochemical machining on the surface integrity and mechanical properties of cast and wrought steels

Abstract

The effect of electrochemical machining on the surface integrity of cast and wrought steels has been investigated. Electrochemical machining was carried out using a sodium nitrate electrolyte. It is shown that the resulting surface structure and surface finish are strongly dependent on the current density used during machining. For similar machining conditions the surface finish of the cast steels was inferior to that of the wrought steels. Whilst the tensile strength and hardness of electrochemically machined surfaces is little affected, the fatigue strength is surface sensitive and is, therefore, reduced. The reduction in fatigue strength is related to the current density and is greater in the higher strength alloy steels. In spite of their inferior surface finish the reduction is less in the cast steels owing to their lower notch sensitivity. When machining occurs at an acceptable current density, the reduction in fatigue strength is of the same order as that obtained when mechanically polished specimens are given a stress relief heat treatment. It is clear, therefore, that the reduction in fatigue strength which accompanies electrochemical machining is due mainly to the removal of any pre-existing compressive stresses. The use of a lower current density results in a further deterioration in the surface structure but only in a modest reduction in the fatigue strength. However, light shot peening after electrochemical machining results in a substantial increase in the fatigue strength owing to the reintroduction of surface compressive stresses. After shot peening the fatigue strength is higher than that of the steel prior to electrochemical machining. The results indicate that the surface damage resulting from electrochemical machining is less than that obtained after electrodischarge machining.