Microstructure and properties of thermomechanically treated and bake hardened AISI 4340 steel

Abstract

Abstract In the present work, response of AISI 4340 steel towards high temperature thermomechanical treatment (HTMT) and bake hardening (BH) studied. During HTMT, steel was austenitized to 1100°C and plastically deformed up to 30% with the help of rolling and immediately quenched in water, followed by tempering at 310±5°C for 2 hours. Optical microstructure, toughness and tensile strength, hardness were evaluated and compared with conventionally hardened and tempered (CHT) steel samples. Observations of HTMT process clarified no alterations in hardness, but a significant improvement in UTS in comparison to CHT samples. Charpy V-Notch (CVN) impact toughness drastically decreased in HTMT samples. Impact toughness of HTMT samples was 13 Joules, but 199 Joules for CHT samples. In HTMT samples, effect of deformation source on process investigated and found that rolling is an effective in improving properties than hammer forging. In addition, significant isotropic properties developed in the case of rolling, compared to hammer forging. Effect of amount of deformation studied; observed 30% deformation was the optimum deformation to achieve the best properties. During bake hardening (BH) flat tensile AISI 4340 steel specimens were pre-strained to 2% and baked at 170°C for 20 minutes a headed with uniaxial tensile testing in which there was no improvement in yield strength after bake hardening observed. This might be because of the low amount of pre-straining or may be the lack of availability of interstitial atoms near to the dislocations generated during pre-straining. Bake hardened specimens were tested at different strain rates; it was observed that yield strength decreases with increasing strain rate, while UTS remains unaffected. FESEM images showed no significant structural changes.