Mechanical properties of the intercritically annealed 0.15wt%C - 0.32 wt% Mn steel and quenched in SAE engine oil at room temperature were investigated. The steel samples prepared for quenching and those prepared for quenching and then tempering were intercritically heat treated at 750, 760, 770, 780 and 790EC for 1hr in a laboratory muffle furnace and quenched to room temperature in SAE 40 engine oil. The steel samples prepared for quenching and then tempering were further subjected to a low temperature tempering at 150EC for 1 hr and air cooled to room temperature. The results revealed that strength and hardness values increased (from 512.29N/mm2 at 750EC to 674.62N/mm2 at 790ECfor strength and from 197BHN at 750ECto 241BHN at 790EC for hardness) with intercritical annealing temperatures but ductility and notch impact toughness decreased (from 12.18% at 750EC to 7.42% at 790EC for ductility and from 9.08 J/cm2 at 750EC to 5.55 J/cm2 at 790EC for notch impact toughness) with intercritical annealing temperatures. Tempered steel has yield strength values which increased with intercritical annealing temperatures from 180 N/mm2 at 750EC to 258 N/mm2 at 790EC. From the values it was observed that tempering gave rise to a decrease in tensile strength (from 674.62 N/mm2 at 790EC to 637.65 N/mm2 at 790EC ) and hardness (from 241 BHN at 790EC to 190 BHN at 790EC ) and an increase in ductility ( from 7.97% at 750EC to 12.18% at 750EC ) and notch impact toughness (from 9.08 J/mm2 at 750EC to 76.76 J/mm2 at 750EC ) of the as quenched steel. Tempered steels from the values presented better compromise between strength, hardness, ductility and notch impact toughness for automobile and other structural applications.
Metallurgical and Materials Engineering Department, University of Nigeria, Nsukka.