한국주조공학회

Influence of Si contents on the mechanical properties and microstructure of austempered ductile iron was investigated. Four different Si contents between 2.0 and 2.9% were used. Austenitizing was performed at 890℃for 2 hrs and austempering temperatures were both 340 and 380℃ for 0.5, 1, and 2 hrs. Nodule content was more than 300/mm^2 and nodularity was more than 90%. Microstructure was revealed using nital and retained austenite was measured by x-ray diffractometer. Tensile test, no-notch Charpy impact test and wear test were performed. Tensile strength was improved as Si content increased and both elongation and impact toughness had peak at 2.6%Si. The specimen austempered at 380℃ showed lower tensile strength than that of 340℃, but showed higher elongation. However, austempering temperature of 380℃ was desirable because that of 340℃ was close to lower bainite transformation. As austempering time increased, tensile strength and elongation were improved and optimum condition was obtained for 2 hrs heat treatment.