Effect of Various Heat Treatments Particularly Austempering on Mechanical and Physical Properties of a High Mn Ductile Iron

Abstract

The stable and metastable phase diagrams, microsegregation of carbon and alloying additions and the driving force for single and double austempering are reported for a ductile iron (DI) of composition: 3.5% C, 2.64% Si, 0.25% Cu, 0.25% Mo, 0.67% Mn, 0.007% P, 0.013% S, 0.04% Mg The variation with austempering time of the retained austenite volume fraction (VRA), Unreacted Austenite Volume fraction (UAV), austenite carbon content (CJ, UTS, elongation, and unnotched charpy impact energy is reported for single austempering at 400°C, 285°C, and a double austempering treatment (400°C, 120 min., then austempering at 285°C) after austenitizing at 920°C for 120 min. Finally mechanical and physical properties including strength, ductility, toughness, wear resistance, hardness, thermal conductivity, and electrical properties for the following heat treatments are compared:

ADI (upper bainitic structure) 870°C, 120 min.; 375°C, 120 min.ADI (lower bainitic structure) 870°C, 120 min., 285°C, 1 dayADI (Double austempered) 870°C, 120 min., 375°C, 120 min., 285°C, 1 dayAir cooled (mainly martensitic/ some widmanstatten ferrite): 870°C, 120 min. furnace cooled (50.7% proeutectoid ferrite/49% pearlite): 870°C, 120 min. Step-cooled (15% proeutectoid ferrite/ 85% pearlite): 870°C, 120 min., 650°C 30 min., air cooled As cast structure (39.5% proeutectoid ferrite/ 60.5% pearlite)The comparison shows that double austempering can be used with the high Mn DI to improve elongation and impact energy obtained by a single austempering. Control over the austenitizing and austempering temperatures and times in the double treatment can be used to change the relative improvements in elongation and impact energy. The results show that furnace cooled irons have the best elongation and physical properties.