Effect of preheat treatment structure on mechanical characterization of AISI 4340 ferrite bainite dual phase steel

Present study attempts to establish comparison of ductility, hardness and impact strength of AISI 4340 steel between conventional treatments and special treatments (dual phase) where conventional treatments serve as preheat treatments. Dual phase treatment consists of partial austenitising and austempering at upper and lower bainitic temperatures (300 and 400°C respectively). Salt bath of NaNO₂ and NaNO₃ mixture is used for isothermal holding in the bainitic temperature ranges. Dual phase steel contains different amount of high strength bainite phases within ferrite matrix depending upon the austenitisation temperature. On partially austenitising by heating in the intercritical temperature ranges, salt bath quenching followed by isothermal holding at 300°C gives lower bainite while upper bainite is forming by isothermal holding at 400°C. The mechanical characterization is executed as per ASTM standards and the results are analysed. It is observed that the mechanical property of normalized bainite is better than that of annealed bainite. Ductility of normalized is less compared to that of other conditions. Impact resistance of both bainitic phases obtained by different initial phases is higher as compared to bainite free structure. Normalized shows excellent hardness while ductility is maximum in annealed compared to bainitic phases.