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I.

INTRODUCTION

MACHINEparts are getting smaller and thinner every year to make machines light, and materials are expected to be stronger and tougher. For this purpose, austempered ductile iron (ADI) has been used in recent years for automotive parts such as gears or crankshafts because of its good molding properties and excellent strength and toughness,tl,21 The mechanical properties are significantly enhanced during the heat treatment.[3~] However, the Young's modulus of ADI is approximately 20 pct lower than that of steel; thus, ADI tends to bend when made into thin parts. The reason is probably that ductile iron, or spheroidal graphite cast iron, contains graphite of over 10 vol pct whose Young's modulus is quite low, approximately 10 GPa. Accordingly, decreasing the graphite content can improve the Young's modulus of austempered castings. The conventional cast steel containing no graphite has a higher Young's modulus, but it has poorer castability and machinability than cast iron. Therefore, we have devoted our attention to spheroidal graphite cast steel containing a little graphite by volume. It was developed in about 1960 t7,8,91but has not been used as widely as spheroidal graphite cast iron. We have austempered this cast steel in order to develop a new material with superior mechanical properties to ADI, and named it AGS (austempered spheroidal graphite cast steel). A standard spheroidal graphite cast steeltS.g]contains not only less carbon but less silicon than a standard ductile cast iron; the TOSHIO TAKAHASHI and SHUJI TADA, Senior Researchers, and TOSHIHIKO ABE, Chief of Materials System Section, are with the Materials Engineering Division, Tohoku National Industrial Research Institute, Sendal 983, Japan. This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled "Atomistic Mechanisms of Nucleation and Growth in Solids," organized in honor ofH.I. Aaronson's 70th Anniversary and given October 3-5, 1994, in Rosemont, Illinois. METALLURGICALAND MATERIALSTRANSACTIONS A

former contains 1.0 to 1.5 pct C and 1.5 to 2.3 pct Si and the latter 3.5 pct C and 2.5 pct Si. Consequently, we have investigated the influence of the amount of silicon and carbon on bainite transformation in AGS. We have also investigated the effects of the transformation on the mechanical properties. The matrix of ADI generally consists of fine bainitic ferrite and retained austenite. It is known that retained austenite is enriched with carbon during the austempering treatment and that the volume fraction of the carbon-enriched austenite affects the mechanical properties of ADI.tl.S] X-ray diffraction analysis has been used to measure the volume fraction and the carbon concentration of retained austenite,t1~ The concentration can be calculated from the lattice parameter. However, few reports have shown interest in the variation of the carbon concentration, or the lattice parameter, of bainitic ferrite and its effects on mechanical properties. One reason is that the variation of carbon content i