EFFECT OF BAINITE TRANSFORMATION AND RETAINED AUSTENITE ON MECHANICAL-PROPERTIES OF AUSTEMPERED SPHEROIDAL GRAPHITE CAST STEEL

Austempered ductile iron (ADI) has excellent mechanical properties, bu t its Young's modulus is low. Austempered spheroidal graphite cast ste el (ACS) has been developed in order to obtain a new material with sup erior mechanical properties to ADI. Its carbon content (approximately 1.0 pct) is almost one-third that of a standard ADI; thus, the volume of graphite is also less. Young's modulus of AGS is 195 to 200 GPa and is comparable to that of steel. Austempered spheroidal graphite cast steel has an approximately 200 MPa higher tensile strength than ADI an d twice the Charpy absorbed energy of ADI. The impact properties and t he elongation are enhanced with increasing volume fraction of carbon-e nriched retained austenite. At the austempering temperature of 650 K, the volume fraction of austenite is approximately 40 pct for 120 minut es in the 2.4 pct Si alloy, although it decreases rapidly in the 1.4 p ct Si alloy. The X-ray diffraction analysis shows that appropriate qua ntity of silicon retards the decomposition of the carbon-enriched reta ined austenite. For austempering at 570 K, the amount of the carbon-en riched austenite decreases and the ferrite is supersaturated with carb on, resulting in high tensile strength but low toughness.