Influence of heat treatment on fatigue and fracture toughness behavior of AISI 4140 steel

The present study is concerned with the effect of tempering temperature and time on the microstructure and its correlation with the mechanical properties, endurance strength and fracture toughness of AISI 4140 steel. In this study, the endurance strength of AISI 4140 steel in as received and heat treated conditions (different tempering temperatures and time)were determined using Rotary Bending Fatigue testing machine at a constant stress ratio of R= -1. The fatigue strength was found to increase with decrease in tempering temperature. The fractured surfaces of the failed samples were examined under SEM and the micro mechanisms of fracture are used to explain the variations in the fatigue strength. An attempt has been made to evaluate the fracture toughness i.e. K_IC and J_IC on the CT specimen for as received material and heat treated samples respectively. Fracture toughness values significantly improve after heat treatment as compared to the as received material. This is attributed to change in microstructure from pearlite-ferrite to tempered martensite after heat treatment.