Fatigue life improvement on 20MnCr5 steel through surface modification for auto transmission application

Abstract

This research represents a unique approach in improving the fatigue life of 20MnCr5 (89–91 HR15N hardness) shafts which were surface treated through gas carburizing process in a sealed quench furnace followed by double tempering. Crack initiation on 20MNCr5 transmission shafts always occurs at the end of spline location (Location X), propagates longitudinally and finally ruptures at stem location. This was confirmed through optical microscope and SEM (make:Jeol) images. The fatigue experiments were carried out at room temperature in 11,500Nm torque test machine (Model – MTS-663-144-01). The torque test was carried-out by applying a fully reversed cyclic load with the frequency of 5Hz for the torsional load of ±3000Nm. Effect of double tempering, surface roughness, carbon case depth, Microstructure such as retained austenite and non-martensitic transformation products (NMTP) have been investigated in this research. The outcome of the research shows that increase in case depth (CD) (out of increased gas carburising time) resulting in increased inter-granular oxidation (IGO) and NMTP in its microstructure. The presence of retained austenite on the surface of the shaft has not yielded any remarkable improvement in fatigue life of the shaft. Presence of tempered martensite with lesser percentage of retained austenite on the surface improved the fatigue life drastically from 12,000 cycles to greater than 35,000 cycles at high torque load of ±3000Nm.