STEPPED HEAT-TREATMENT FOR AUSTEMPERING OF HIGH MANGANESE ALLOYED DUCTILE IRON

A stepped heat treatment is proposed for overcoming the difficulty of obtaining ductility in an austempered alloyed ductile iron. The method is illustrated for an il on containing 0.67%Mn, 0.25%Mo, and 0.25%Cu, using an austenitising temperature of 920 degrees C, a first step aus tempering temperature of 400 degrees C for 120 min, and a second step austempering temperature of 285 degrees C. The change in the microstru cture and phase characteristics with time during the second austemperi ng step ale described. Related changes in the mechanical properties co mpared with a single austempering treatment at 400 degrees C are an in crease in the ultimate tensile strength from 770 To 970 MN m(-2), an i ncrease in elongation from 2.5 to 7.5%, and an increase in the unnotch ed Charpy impact energy from 40 to 150 J.