Influence of austenitizing temperature on fracture toughness of a low manganese austempered ductile iron (ADI) with ferritic as cast structure

• Published in: Materials science & engineering. A, Structural materials : properties, microstructure and processing Structural materials : properties, microstructure and processing, 1999-08, Vol.268 (1), p.15-31
• Main Authors: Putatunda, Susil K, Gadicherla, Pavan K
• Format: Article
• Language: English
• Subjects: Applied sciences; Austenitizing temperature; Carbon content; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Fracture toughness; Low manganese; Materials science; Metals. Metallurgy; Microstructure; Other heat and thermomechanical treatments; Physics; Tensile properties; Treatment of materials and its effects on microstructure and properties; Unalloyed
• ISSN: 0921-5093; 1873-4936
• DOI: 10.1016/S0921-5093(99)00120-3

An investigation was carried out to examine the influence of austenitizing temperature on the resultant microstructure and mechanical properties of an unalloyed and low manganese ADI and with an as cast (solidified) ferritic structure. The investigation also examined the influence of austenitizing temperature on the fracture toughness of this material. Compact tension and round cylindrical tensile specimens were prepared from a nodular cast iron without any alloying elements (e.g. nickel, molybdenum or copper) and with very low manganese content and with an as cast (solidified) ferritic structure. These were then austenitized at several temperatures ranging from 871°C (1600°F) to 982°C (1800°F) and then austempered at a constant austempering temperature of 302°C (575°F) for a fixed time period of 2 h. Microstructure was characterized through optical microscopy and X-ray diffraction. Tensile properties and plane strain fracture toughness of all these materials were determined and correlated with the microstructure. Fracture surfaces were examined under scanning electron microscope to determine the fracture mode. The results of this investigation indicate that the austenitizing temperature above 982°C (1800°F) has a detrimental effect on the fracture toughness of this material. Both volume fraction of austenite and its carbon content increased with austenitizing temperature. The strain hardening exponent of this material was found to increase with increase in the austenitic carbon content i.e. ( X γ C γ ) 1/2 where X γ is the volume fraction of austenite and C γ is the carbon content of austenite. A Hall–Petch type relationship was found to exist between yield strength and mean free path of dislocation motion, d in ferrite. A model for fracture toughness of ADI has been developed. Present test results indicate good agreement with the model.