Study of wear behaviour of ductile iron subjected to two step austempering

Abstract

Study of wear behaviour of ductile iron subjected to two step austempering 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 8C for 30 min, and then austempered successively at two different temperatures. It was first austempered at 300 8C for different durations from 2 min to 30 min and subsequently austempered at 400 8C 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 8C. At short austempering times at 300 8C, 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.Keywords: Ductile iron; Austempering; Wear; Austenite IntroductionAustempered ductile iron (ADI) is an interesting engineering material because of its unique microstructure and excellent combination of high strength and ductility [1 -3]. While the microstructure of austempered steel consists of ferrite and carbide called as bainite, that of austempered ductile iron consists of ferrite and high carbon stabilised austenite. Austenite is present because of the presence of large amount of silicon in ductile iron which prevents the formation of carbide. This microstructure is generally called ausferrite to distinguish it from the bainite of steel. The ausferrite microstructure can be varied widely by varying the austempering temperature [4 -6]. When austempered at low temperatures in the range of 280 -320 8C, it exhibits very fine laths of ferrite interspersed with very thin layers of austenite. When austempered at higher temperatures, such as 350 -400 8C, the microstructure consists of coarse ferrite laths within bulky austenite. These microstructures are referred to as lower ausferrite and upper ausferrite respectively. While the former shows high strength and low ductility, the latter exhibits lower strength and higher ductility. Wear properties of ADI have been studied by several investigators [7 -10]. The ADIs austempered at low temperatures such as 300 8C show better wear resistance than those austempered at higher temperatures such as 400 8C because of the much higher hardness of the former. However, ADIs generally show better wear resistance than hardened and tempered steels of comparable hardness. This is due to the presence of substantial amounts of austenite. The austenite imparts improved wear resistance to ADI because of its better strain hardening ability.Recently Daber and Rao [11] as well as Daber, Ravishankar and Rao [12] have shown that interesting microstructures can be obtained through a two step ...