Energy Efficient Surface Hardening of 4140 Steel by Friction Stir Processing for Tribological Applications

One of the most energy intensive industrial and manufacturing processes is the heat treatment of ferrous materials for hardening to improve their friction and wear performance. Examples of surface hardening heat treatments include case carburizing, nitriding, boriding, etc. This paper presents the development of a new energy efficient method to harden steel material as a possible replacement for the current energy intensive heat treatment processes. The new process, friction stir processing (FSP) involves severe plastic deformation of the near surface material by a non-consumable rotating and translating tool. FSP has been demonstrated to locally modify material properties such as ductility, fatigue, fracture toughness, etc. Other potential properties that could be positively affected by FSP are wear and friction properties. In a preliminary study, when applied to 4140 alloy steel, this new FSP process produced adequate hardening of the near surface layer compare to conventional furnace heat hardening of the same alloy. The friction and wear performance of the 4140 hardened by FSP was also observed to be superior than heat hardened surface under dry and lubricated sliding contact.