Austempered ductile iron (ADI) is produced by an iso-thermal heat treatment process. In recent years, it becomes a good candidate material to replace steel castings and forgings in diverse applications such as automotive, manufacturing and agricultural industries. Excellent mechanical properties of ADI can be achieved due to its unique ausferritic structure consisting of acicular ferrite and carbon-enriched austenite. Austempering temperature, holding time, chemical composition and cooling rate significantly influence the mechanical performance of ADI.
In many mechanical systems such as gears, cam and followers, and connecting rods, contacting components subjected to cyclic load or pressure are frequently used. Rolling contact fatigue damage is often the main failure mechanism for these components. Several methods have been proposed to reduce the possibility of fatigue failure such as improving the component cleanliness level and advanced surface treatments. However, appropriate material selection is required to extend the service life for rolling elements. The use of ADI is a possible alternative to conventional materials used in these applications.
The study of contact fatigue resistance of ADI material has attracted much attention in the last two decades. Dommarco et al. found that ADI had excellent resistance to crack propagation but weak resistance to crack nucleation by using a ball-rod rolling contact fatigue tester. Brunetti et al. studied the effects of sample surface preparation on endurance life of ADI. They found that ground ADI samples had a shorter lifetime than that of polished samples. As compared with polishing processes, tiny cracks were generated after a grinding process, which acted as stress raisers and reduced the fatigue resistance.
The previous research studies generally focused on the contact fatigue resistance of ADI produced by only one austempering temperature and holding time. However, different combinations of austempering temperature and holding time can produce differences in microstructure which significantly affect the mechanical properties of ADI. In this research, the rolling contact fatigue resistance of ADI produced by the same austempering temperature but different holding times was evaluated by using a twin-disk rolling configuration rig. The results were compared with traditional quenched and tempered (Q&T) ductile iron with the same chemical composition. The morphology of the worn surfaces were observed by optical microscopy and scanning electron microscopy (SEM) and the percentage of retained austenite was detected by X-ray diffraction (XRD) to understand the potential failure mechanisms.
The hardness of ADI samples decreased as the amount of acicular structure (ausferrite) increased and became more coarse when increasing the holding time in the furnace at the same austempering temperature.
The selection of holding time had significant effects on fatigue life. Less holding time resulted in ADI which had higher hardness and greater fatigue life than conventional Q&T ductile iron. Longer holding time could shorten the fatigue life of the ADI due to the decrease of hardness.
Metallurgical analysis indicated that both ADI and Q&T ductile iron samples had the same fatigue failure mechanisms. Cracks initiated from the graphite nodules in the subsurface due to stress concentration or the exposed defects caused by the removal of nodules on the surface. Cracks propagated towards either the surface or adjacent nodules. A pit or spall was formed once the linkage of cracks reached a critical size.
ADI samples had better rolling contact fatigue resistance than that of Q&T ductile iron samples for similar macro hardness. This result could be attributed to an increase of micro hardness on and near the surface because of the strain induced transformation of retained austenite with low carbon content into martensite during the tests. After being analyzed by XRD, the change of percentage of retained austenite on the wear track and off the wear track ranged from 4.4% to 26.15%.