An Investigation of Machining-Induced Residual Stresses and Microstructure of Induction-Hardened AISI 4340 Steel

Excessive induction hardening treatment may result in deep-hardened layers, combined with tensile or low compressive residual stresses. This can be detrimental to the performance of mechanical parts. However, a judicious selection of the finishing process that possibly follows the surface treatment may overcome this inconvenience. In this paper, hard machining tests were performed to investigate the residual stresses and microstructure alteration induced by the machining of induction heat-treated AISI 4340 steel (58-60 HRC). The authors demonstrate the capacity of the machining process to enhance the surface integrity of induction heat-treated parts. It is shown how cutting conditions can affect the residual stress distribution and surface microstructure. On the one hand, when the cutting speed increases, the residual stresses tend to become tensile at the surface; and on the other hand, more compressive stresses are induced when the feed rate is increased. A microstructural analysis shows the formation of a thin white layer less than 2 mu m and severe plastic deformations beneath the machined surface.