Effect of heat input on impact toughness in transition temperature region of weld CGHAZ of a HY 85 steel

Physical weld simulated coarse grain heat affected zone (CGHAZ) specimens of HY 85 steel with heat inputs 15, 22, 50 kJ/cm were prepared by thermo mechanical simulator. Impact toughness was determined at test temperatures from room temperature to −196 °C. Ductile to brittle transition temperature (DBTT) were determined by two methods (i) average impact toughness value of upper and lower shelf energy and (ii) fracture appearance transition temperature (FATT) from the SEM fractographs. Impact toughness and hardness values have been found to decrease with increase in heat input. This decrease is attributed to slower cooling rate, increase in prior austenite grain size and increase in width of bainitic ferrite lath. DBTT was determined for base metal and weld CGHAZ specimens based on upper and lower shelf energy criterion. FATT method based on SEM fractographs is applicable only for base metal but not for weld CGHAZ specimens because they were fully brittle in character. Best impact toughness and hardness for heat input 22 kJ/cm observed are 66 J and 322 VHN respectively.