Austempering of spheroidal graphite cast iron

We study gray spherulitic cast iron, its microstructure, hardness ultimate strength in tension, elongation, and impact toughness as functions of the duration of isothermal austenitizing in the bainite region at 350 and 400 degrees C after austenitizing at 900 degrees C. As the temperature of quenching increases from 350 to 400 degrees C, the microstructure of the analyzed cast iron changes from lower to higher bainite and the amount of retained austenite increases (its maximum is attained after 1 h). At the same time, the ultimate strength in tension and hardness decrease, whereas the elongation and fracture toughness increase.