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2. ENGINEERING AND TECHNOLOGY
This preliminary study demonstrates that surface engineered austempered ductile iron is a valid alternative gear material to carburised steel. The impact energy of ductile cast iron samples in various conditions, including as – cast, austempered at 240oC and austempered and shot peened using both 0.7mm and 1.4mm steel balls, was measured. As expected the as-cast material exhibited nominal impact energy. This has been attributed mainly to the pearlitic matrix structure.Austempering generally increases the impact energy. The extent of the improvement is however dependent on the austempering temperature and the resulting microstructure. It was shown that the structure following austempering at 240oC consists of fine acicular ferrite needles with high aspect ratio and relatively small quantities of retained austenite. These two features account for the comparatively low impact energy and high hardness values. Increasing the austempering temperature to 375oC leads to structures containing coarser ferrite needled and up to 40% retained austenite. Various investigators have shown that the high impact energy of irons austempered in this temperature range is dependent on the percentage retained austenite content(1, 2) and stability of the latter(3).
It can be seen that a reduction in size of shot used for peening has the effect of moving the maximum subsurface compressive stress closer to the surface. This increases the surface stress, but results in a shallower compressive surface layer. High surface compressive stresses make it more difficult to initiate surface cracks and thus properties such as fatigue and impact energy are bound to improve(4). In the case of high surface compressive stresses, the sub-surface becomes critical and thus the advantage of having a deeper affected zone. This may suggest that in order to obtain maximum benefit from peening duplex size shot should be used.
Samples austempered at 240oC and shot peened have higher impact energy values than as cast samples. This may suggest that the fatigue behaviour of gears austempered at the higher temperatures may be superior to those treated at lower temperatures whilst the tribological characteristics may be inferior. This is however not necessarily the case and needs to be investigated further.
Grech, M., Kennedy, D., Schauperl, Z.: An Investigation on the Suitability of Surface Engineered Austempered Ductile Iron as a Gear Material. In Cost 532 Conference, Triboscience and Tribotechnology, Superior Friction and Wear Control in Engines and Transmission (Holmberg, K. ed), FEUP/ISEP, pp.89-97. Porto, Portugal, 12-14 October, 2005.