Tribological and mechanical characteristics of surface modified austempered ductile iron

Abstract

Compared to other ferrous materials, austempered ductile iron (ADI) has marked economic advantages such as low melting temperature, low shrinkage, excellent castability, good machinability and high damping capacity. This makes it a potential candidate material for automotive components such as gears. Gears in service are continuously being subjected to cyclic loads and are therefore required to have a tough and ductile bulk structure, together with superior tribological properties at the surface. In this study, ADI alloyed with 1.6 wt% copper and 1.6 wt% nickel has been investigated as a material for gears. The high toughness and ductility required in the core of gears was obtained by austempering in the temperature range of 360-400°C. A high content of stable austenite in austempered ductile iron is known to yield an optimum combination of mechanical properties. In this study, ductile iron specimens were austempered using different heat treatment parameters with the purpose of obtaining microstructures with contrasting austenite volume fractions and austenite carbon contents. The stability of the austenite was assessed using X-ray diffraction patterns and also by subjecting the austempered samples to cryogenic temperatures. The austempering parameters yielding optimal results were then used throughout the rest of the study. In addition to optimising the bulk properties, shot peening and laser hardening were studied as potential surface treatments for ADI gears. Different peening intensities and different shot materials were used to determine the peening parameters which yield the desired properties at the surface. The optimum parameters (S330 shots and 0.38 mmA intensity) were then applied to shot peen ADI specimens, which were then used to evaluate the resistance to bending fatigue, dry and starved lubricated sliding wear, and rolling contact fatigue. The results showed that while shot peening was beneficial in increasing the bending fatigue life, it did not have a positive influence on all the tribological characteristics. Therefore, laser surface hardening was studied as a second surface engineering technique. In particular, spot matrix hardening was investigated, in which ADI surfaces were irradiated with an array of laser spots. Studies were carried out using single spots and different beam powers, beam durations and beam diameters, with the purpose of determining the parameters which yield a surface with maximum hardness, hardened depth, no cracking and minimal distortion. Following optimisation of the process parameters on single spots (beam duration of 300 ms, beam diameter of 1.75 mm and beam power of 600-660 W), analysis was carried out using different spot patterns. The tribological behaviour of such treated surfaces was studied by carrying out starved lubricated sliding wear tests and rolling contact fatigue tests. Results showed that laser hardening is a suitable surface treatment to improve the tribological characteristics of ADI. Therefore, for the first time, the results obtained on test specimens indicate that shot peening could be applied to the root of Cu-Ni ADI gears in order to increase the bending fatigue resistance. Subsequently, the face and flank of gear teeth can be spot laser hardened with the intent of improving the tribological characteristics of ADI gears.