DEVELOPMENT OF AUSTEMPERED DUCTILE IRON FOR AUTOMOBILE AND MACHINE PARTS

Abstract

An investigation has been carried out on the effect of silicon level on austempered ductile iron in order to determine the suitability of the material in the development of structural materials that can be used in the production of automobile and machine parts that undergo constant cyclic loading. This is in response to pressures on engineering and automobile industries to produce components that provide improved properties and save energy cost or reductions in overall manufacturing cost and also to further increase the opportunity for wider commercial exploitation of cast irons. The study involved initial production of three ductile iron alloys containing different levels of silicon. The effect of austenitising temperature (850,900 and 950°C for 1hour) and rapid quenching and holding in a salt bath maintained at austempering temperature (280, 300 or 350°C for different time intervals 15,30,60,90,or 120) subsequently investigated. The silicon level sensitivity on the fatigue strength, impact toughness and the hardness properties of the three alloys containing increasing silicon levels (2.0%, 2.6%, 3.0%) were characterized using optical micrography and the results were used to correlate the phase relationship with the mechanical properties. Computer modeling was used to measure the volume fraction of retained austenite. The austempering applied to the ductile cast iron alloys produced different bainitic structures, as a function of heat treatment conditions that led to an attractive combination of mechanical properties. These render the low silicon, medium silicon and the high silicon austempered ductile iron useful in the production of automobile and machine parts as an economical substitute for high strength steels. While the bainitic transformation in steels gave a mixture of ferrite and carbides, the presence of silicon in the ductile iron led to significant austenite volume fractions being retained in the microstructure. The best overall results were obtained from a high silicon alloy austenitised at 950°C and austempered at 350°C while optimum impact energy of 70 and 115 Joules respectively was obtained from the medium silicon alloys and high silicon alloys austenitise at 950°C and austempered at 350°C. Both alloys have the same optimum values for fatigue limits, 450Nmm-2 at these austempering conditions. The results show that alloys of different silicon levels can be used to produce machine parts by altering the austempering parameters. The model developed to predict the fatigue limit from the impact toughness value and the opening and closure of the processing window can be used to optimize production processes in foundries.