Dependence of fracture toughness of austempered ductile iron on austempering temperature

• Published in: Metallurgical and materials transactions. A, Physical metallurgy and materials science Physical metallurgy and materials science, 1998-12, Vol.29 (12), p.3005-3016
• Main Authors: PRASAD RAO, P, PUTATUNDA, S. K
• Format: Article
• Language: English
• Subjects: Applied sciences; Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Fatigue, brittleness, fracture, and cracks; Mechanical and acoustical properties of condensed matter; Mechanical properties of solids; Metals. Metallurgy; Physics
• ISSN: 1073-5623; 1543-1940
• DOI: 10.1007/s11661-998-0208-9

Ductile cast iron samples were austenitized at 927 deg C and subsequently austempered for 30 min, 1 h, and 2 h at 260 deg C, 288 deg C, 316 deg C, 343 deg C, 371 deg C, and 399 deg C. These were subjected to a plane strain fracture toughness test. Fracture toughness was found to initially increase with austempering temperature, reach a maximum, and then decrease with further rise in temperature. The results of the fracture toughness study and fractographic examination were correlated with microstructural features such as bainite morphology, the volume fraction of retained austenite, and its carbon content. It was found that fracture toughness was maximized when the microstructure consisted of lower bainite with approx30 vol.% retained austenite containing > 1.8 wt.% carbon. A theoretical model was developed, which could explain the observed variation in fracture toughness with austempering temperature in terms of microstructural features such as the width of the ferrite blades and retained austenite content. A plot of K sub IC exp 2 against sigma sub y (X sub gamma C sub gamma ) exp 1/2 resulted in a straight line, as predicted by the model.