Use of austenitising temperature in control of austempering of an Mn-Mo-Cu alloyed ductile iron

Abstract

Measurements of ultimate tensile strength, 0.2% proof strength, elongation, unnotched Charpy impact energy, and austempering kinetics are presented as a function of austempering time over the range 1–4320 min and for an austempering temperature of 375°C after austenitising at 950, 920, 870, 840, and 800°C for a ductile iron of composition Fe-3.39C-2.56Si-0.37Mn-0.25 Mo-0.29Cu-0.04Mg. These measurements are analysed to relate microstructure and mechanical properties, and to define processing windows for the different austenitising temperatures. It is shown that decreasing the austenitising temperature accelerates the stage I reaction and can be used to open a processing window that is closed at a higher austenitising temperature. The introduction of ferrite into the austempered structure, through control of the austenitising temperature, can be used to influence the mechanical properties of the austempered iron. Decreasing the austenitising temperature reduces the iron hardenability.