Finite element analysis on the formation and distribution of residual stresses during quenching of low carbon bainitic-martensitic large gears

Applications requiring high strength and wear resistance such as in large gears are expected to benefit from using low carbon bainitic-martensitic (LCBM) steels. Unlike conventional gear steels (SAE 4140 and AISI 5130), post surface hardening treatments namely induction and flame hardening used to increase surface hardness and promote the formation of compressive residual stresses in the teeth are not necessary using LCBM steels. A thermo-mechanical FE model was developed to model the evolution of microstructure, the volumetric changes associated with the kinetics of martensitic phase transformation and the formation and distribution of residual stresses during quenching of LCBM large gears. Two cases, air-cooled and a two stage quenching process of the LCBM large gear were evaluated to determine the residual stress distribution formed in the root of the tooth which is prone to bending fatigue. The results showed that the novel two stage quenching process is most suited for LCBM gears with large root fillet radius since compressive residual stresses achieved in the fatigue prone areas of the gear were found to exceed those of conventionally induction hardened treated SAE 4140 gears. The effects of root fillet radius on the resultant bending stresses incorporating residual stresses in the root of the tooth were analysed for large LCBM gears.