Effect of quenching and tempering on the microstructure, tensile properties, and wear behavior of spheroidal graphite cast iron

The effects of austenitizing and tempering temperatures on the microstructure, tensile and wear properties of spheroidal graphite cast iron subjected to quenching and tempering (QT) treatment were systematically investigated. Specimens were austenitized at temperatures between 900 and 1000 °C, oil-quenched, and subsequently tempered at 200 °C and 600 °C. Hardness, tensile properties, friction behavior, retained austenite fraction, carbon concentration in austenite, and microstructural evolution were quantitatively evaluated. The hardness of the as-quenched specimens decreased from 58.6 to 53.5 HRC with increasing austenitizing temperature, which was associated with an increased fraction of retained austenite and enhanced carbon enrichment in the austenite. A similar decreasing trend in hardness was observed after tempering at 200 °C. In contrast, after tempering at 600 °C, the hardness remained nearly constant in the range of 35.4–35.9 HRC, regardless of the austenitizing temperature. As-quenched and 200 °C-tempered specimens fractured prematurely; 600 °C-tempered specimens exhibited yield strengths of 857–873 MPa, with ultimate strengths (1016–1089 MPa) and elongations (4.5–5.6%) increasing with austenitizing temperature. During tempering, graphite precipitation occurred instead of cementite formation, and secondary graphite was observed only after tempering at 600 °C. Although the size and volume fraction of secondary graphite increased with higher austenitizing temperatures, its influence on hardness was negligible. These results indicate that the hardness behavior of QT-treated spheroidal graphite cast iron, particularly under high-temperature tempering conditions, is predominantly controlled by retained austenite stability and matrix softening rather than by graphite precipitation. Coarser, more abundant secondary graphite reduced the friction coefficient, with QT1006 reaching the minimum (∼0.55).