JOURNAL OF MATERIALS SCIENCE 29 (1994) 4933-4940 Effect of microstructure on the fatigue strength of an austempered ductile iron P. SHANMUGAM, P. PRASAD RAO, K. RAJENDRA UDUPA, N. VENKATARAMAN Department of Metallurgical Engineering, Karnataka Regional Engineering College, Surathkal, Karnataka 574157, India Rotating bending fatigue tests were carried out on austempered ductile iron containing 1.5 wt% nickel and 0.3 wt% molybdenum. The ductile iron was austenitized at 900 or 1050~ and then austempered at 280 or 400~ for different lengths of time to obtain different microstructures. The fatigue strength was correlated with the amount of retained austenite and its carbon content, which were both determined by X-ray diffraction technique. While the tensile strength decreased with increasing retained austenite content, the fatigue strength was found to increase. Carbide precipitation was found to be detrimental to fatigue strength, Lower austenitizing temperature resulted in better fatigue strength. 1. Introduction In recent years, austempered ductile irons (ADI) have been used for many critical components in auto- mobiles [1-3], such as crank shafts, steering knuckles and hypoid rear axle gears. It is expected that the application of these irons will increase in the future, not only in the automobile industry, but also in many other fields. The interest in these irons is due to the fact that they offer an exceptional combination of high strength, ductility and toughness. This unusual combi- nation of properties comes about because of their unique microstructure, which consists of ferrite and austenite, rather than ferrite and carbide, as in austem- pered steels [4]. The presence of appreciable amounts of austenite should lead to better wear resistance and fatigue strength in these, due to the high work-hardening nature of the austenite. While considerable work has been done in understanding the 'microstructural characteristics of a number of ADIs and their effect on tensile properties [5], impact toughness [6] and frac- ture toughness [7], rather limited information is avail- able on their effect on fatigue properties. One of the earliest investigations is that of Johansson [8]. He carried out rotating bending fatigue tests on notched and unnotched specimens which showed that fatigue strengths of ADI were far superior to those of pearlitic and "quenched and tempered" grades. However, the report lacks information on heat treatment and micro- structural details, which would be helpful in under- standing the correlation between microstructure and fatigue strength. Limited reports are available on other investigations of fatigue characteristics, and these have been summarized by Harding [5]. The present investigation was undertaken to evalu- ate systematically the microstructure and fatigue strength of austempered ductile iron with a view to establishing the correlation between these two. 0022-2461 9 1994 Chapman & Hall 2. Experimental procedure The alloy for the present study was produced at M/s Shanthala Ductile Iron Foundry, Shimoga. The foun- dry practice followed is given elsewhere [9]. The complete chemical analysis is presented in Table 1. Samples for X-ray diffraction, tensile and fatigue tests were machined from the cast slabs. The dimen- sions of the fatigue and tensile samples are given in Fig. 1. X-ray diffraction studies were carried out on samples of dimensions 30 mm x 25 mmx 3 mm. Austempering heat treatment consisted of three variables: austenitizing temperature, austempering temperature and austempering time. Through appro- priate selection of these variables, different micro- structures and mechanical properties were obtained. Heat-treatment combinations employed in the present investigation are illustrated in Fig. 2. The fourth para- meter, of austenitizing time, was kept constant at 60 min for all heat treatments. All the samples were plated with copper prior to heat treatment to minim- ize surface oxidation. Austenitization was carried out in a flowing argon atmosphere while austempering treatment was carried out in salt bath of a mixture of 45% sodium nitrate and 55% potassium nitrate by weight. Temperature was controlled to better than 4- 2 degrees of the set point. Tensile and fatigue samples were subjected to buf- fing after heat treatment, to give them a high degree of surface finish. Four grades of abrasive powders were used: 80, 120, 240 and finished with 500 grit. Fatigue samples were subjected to longitudinal polishing. The samples for X-ray diffraction were initially subjected to a mechanical polish and then to electropolishing to remove a surface layer of at least 50 lam. X-ray diffraction studies were carried out on a Jeol- JDX-8P diffractometer using a copper target and nickel filter, at a scan speed of 0.25~ min-1 over an angular range of 40~ ~ 20. Hardness studies were 4933