Faculty Publications
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Item Formation of strain-induced martensite in austempered ductile iron(2008) Daber, S.; Prasad Rao, P.P.The present work has been taken up to study the influence of microstructure on the formation of martensite in austempered ductile iron. Ductile iron containing 1.5 wt.% nickel and 0.3 wt.% molybdenum was subjected to two types of austempering treatments. In the first, called as conventional austempering, the samples were austempered for 2 h at 300, 350 or 400 °C. In the second treatment, called as stepped austempering, the samples were initially austempered at 300 °C for 10, 20, 30, 45 or 60 min. These were subsequently austempered for 2 h at 400 °C. Tensile tests revealed considerable variation in the strain-hardening behaviour of the samples with different heat treatments. In the case of samples subjected to conventional austempering, it was found that strain-hardening exponent increased with increasing austempering temperature. In the case of samples subjected to stepped austempering, increased strain hardening was observed in samples subjected to short periods of first step austempering. Study of the microstructures revealed that increased strain hardening was associated with the formation of strain-induced martensite. There was a greater propensity for the formation of strain-induced martensite in the samples containing more of blocky austenite. Retained austenite in the form of fine films between sheaths of ferrite was relatively more stable. Studies revealed that the morphology, size and carbon content of the retained austenite were important parameters controlling their tendency to transform to martensite. © 2007 Springer Science+Business Media, LLC.Item Study of wear behaviour of ductile iron subjected to two step austempering(2010) Kumari, R.; Prasad Rao, P.P.An investigation was carried out to examine the influence of two step austempering on microstructural parameters and the wear behaviour of austempered ductile iron. Ductile iron was austenitised at 900 °C for 30 min, and then austempered successively at two different temperatures. It was first austempered at 300 °C for different durations from 2 min to 30 min and subsequently austempered at 400°C for 2 h, after which it was quenched to room temperature. Resulting microstructures were characterised through optical microscopy and X-ray diffraction. Mechanical properties were studied through hardness measurement and tensile testing. Wear studies were carried out using a pin-on-disc machine. Wear rate was found to decrease with increasing time at the first step temperature of 300 °C. At short austempering times at 300 °C, the amount of austenite was instrumental in improving the wear resistance through formation of deformation induced martensite. Wear rate was found to depend on yield strength, austenite content and its carbon content. © Carl Hanser Verlag GmbH & Co. KG.Item Improvement in fracture toughness of austempered ductile iron by two-step austempering process(2010) Ravishankar, K.S.; Prasad Rao, P.P.; Udupa, K.R.Ductile cast iron samples were austenitised at 900°C and subjected to two types of austempering called as conventional austempering and two-step austempering. Five different temperatures, 280, 300, 320, 350, 380 and 400°C, with an austempering time of 2 h, were chosen for conventional austempering. For two-step austempering process, the first step temperatures were 280, 300 and 320°C. The samples were austempered at each of these temperatures for different times, i.e. 10, 20, 30, 45 and 60 min, and then upquenched to higher temperature of 400°C for 2 h. Fracture toughness and tensile studies were carried out under all these austempering conditions. During conventional austempering, the fracture toughness initially increased with increasing austempering temperature, reached a peak value of 63 MPa m 1/2 and dropped with further increase in temperature. During the two-step austempering, fracture toughness was found to increase with increasing first step time. The curve shifted to higher values of fracture toughness as the first step temperature was decreased and the maximum value of 78 MPa m 1/2 was obtained. The results of the fracture toughness study and the fractographic examination were correlated with microstructural features such as bainitic morphology, the volume fraction of retained austenite, and its carbon content. Ferrite lath size and stability of the retained austenite were found to influence the fracture toughness. © 2010 W. S. Maney & Son Ltd.