Surface hardening of AISI 4340 steel by machining: a preliminary investigation

Abstract An attempt has been made to investigate feasibility of applying machining, a material removal process, to surface harden steel parts. The idea is to utilize transient thermomechanical loading induced during machining to purposely generate a hardened layer while simultaneously removing excess materials. This approach, if capable, will be a cost-effective alternative to conventional surface hardening, giving possible benefits such as low production cost, short cycle time, and waste reduction, etc. Feasibility has been examined from thermal and mechanical perspectives. A preliminary experiment using cutting tools with large wear land (1.2 mm) to turn annealed AISI 4340 steel shows distinctly 30 μm deep phase-transformed structures. The machined surface has about 7 vol.% austenite, an evidence of phase transformation. The hardness of the phase-transformed structure increases to a maximum of about 49 HRC versus 28 HRC of the bulk. Residual stresses, measured by X-ray diffraction, were compressive down over 25 μm below the surface. These results suggest the possibility of using machining to surface harden steel parts. However, the challenge is to extensively increase wear land size to achieve hardened depth comparable to that in conventional surface hardening. In addition, surface properties and performance remain keys to justify the usage of machining hardening processes.