Mechanical properties of SG-iron subjected to variable and isothermal austempering temperatures heat treatment

Mechanical properties of spheroidal graphite (SG)-iron subjected to variable and isothermal austempering temperatures heat treatment have been examined. Variable austempering temperature heat treatment is carried out by austenitizing at 1183 K then quenching into a salt bath held at 593 and 723 K, respectively. After quenching, the former is steadily heated to 723 K while the latter is allowed to cool progressively to 593 K. The tensile properties, impact toughness and hardness are determined and correlated with the microstructure. It has been found that, specimens quenched at 593 K and heated steadily to 723 K have higher 0.2% yield stress, ultimate tensile strength and hardness, but much less ductility than those quenched at 723 K and cooled progressively to 593 K. The elongation of specimens austempered under variable austempering temperature is significantly improved. It reached about three times of that of specimens austempered isothermally at 593 or 723 K.