The improvement of wear behavior of austempered ductile irons

m THE IMPROVEMENT OF WEAR BEHAVIOUR OF AUSTEMPERED DUCTILE IRONS (PhD Thesis) Melik ÇETİN GAZI UNIVERSITY INSTITUTE OF SCD2NCE AND TECHNOLOGY March 2005 ABSTRACT An investigation was carried out to examine the influence of austenizing temperature and time together with austempering temperature and time on the resultant microstructure, hardness, wear behaviour, friction force and pin temperature of as-cast ferritic and as-cast pearlitic structure of austempered ductile irons (ADI). For this purpose, as-cast ferritic and as-cast pearlitic ductile cast irons were austenitized at 850°C and 900°C for 30, 60 and 90 min and subsequently austempered for 60 and 120 minutes at 250°C, 320°C, 370°C and 400°C. Microstructural characterizations were carried out by optical microscopy examinations. Hardness of the ADIs were determined at room temperature. The wear tests were carried out using a pin-on-disc type apparatus at dry sliding conditions. The wear specimens were rubbed against the hardened AISI 5190 steel disc (62 HRC) at a linear sliding speed of İm s"1, under the loads of 20N, 40N and 60N and sliding distance of 3600m. All tests were performed in ambient air at room temperature. The worn surface of the specimens were examined by scanning electron microscope (SEM). The hardness of the investigated ADI specimens increased with increasing austenitising temperature and time and decreasing austempering temperature. The results indicated that the austempering time above 60 min at 250°C afterIV austenitising at 850°C for 30 min had a detrimental effect on the hardness of both as-cast ferritic and perlitic ductile iron specimens. Wear loss of the specimens austenitised at 850°C decreased with increasing austempering temperature and increasing austenitising time for all the applied loads. Wear loss of the specimens austenitised at 900°C increased with increasing austempering temperature and austenitising time for all the applied loads. The results obtained showed that austempering at 250°C and 320°C after austenitising at 850°C and 900°C for 30,60 and 90 minute improved the wear resistances of the as-cast ferritic and as-cast pearlitic ductile irons more than the both groups austempered at 320°C and 400°C. The improvement in wear performance was explained by the lubricity inherent in the graphite nodules, the increase in initial hardness caused by the ausferritic structure, and the work hardening of the surface as high carbon austenite is transformed to martensite by plastic deformation. It is also found that the ADIs austempered at higher temperatures had higher friction coefficient than those austempered at low temperatures. With increasing surface temperatures of the ADIs during the wear tests, the friction forces decreased. Scanning electron microscopy (SEM) examinations revealed that the dominant wear mechanisms in dry sliding of the ADIs were plastic yield, delemination and oxidation. All these mechanisms were found to be effective in wear losses. The formed oxides were hardened during the tests and acted like abrasives. For the as cast DIs, surface adhesion and plastic deformation were basically found to be responsible from the wear losses. Science code : 626.17.01 Key words : Austempered ductile iron (ADI), Heat treatment, Microstructure, Hardness, Adhesive wear, Friction force, Pin temperature Page number : 246 Adviser : Assist. Prof.Dr. Ferhat GÜL