Fe-2.4C-1.6Si-0.1Mn-32Ni-5Co低熱膨張球状黒鉛鋳鉄の熱膨張係数及びマルテンサイト変態挙動に及ぼす均質化熱処理の影響

<p>  In this study, we investigated the effects of homogenizing heat treatment on the coefficient of thermal expansion and martensitic transformation behavior of low thermal expansive ductile cast iron (Fe-2.4C-1.6Si-0.1Mn-32Ni-5Co) with eutectic composition. By increasing the holding time at a homogenizing heat treatment temperature of 1100℃, the segregation degree (δ) (which is defined as the ratio of the difference in maximum and minimum concentrations (ΔC) in heat-treated samples to the ΔC in as-cast samples) of several alloying elements decreased to 0.2-0.4 at a holding time of 4 hours, and became 0 at a holding time of 100 hours, achieving complete homogenization. With the decrease in δ, the coefficient of thermal expansion also decreased from 4 × 10^-6/℃ to 3 × 10^-6/℃. However, when the heat treatment time exceeded 4 hours, despite the decrease in δ, the coefficient of thermal expansion remained constant at 3 × 10^-6/℃. We conducted investigations on the possible causes, and found that in as-cast samples, martensitic transformation could be observed in the micro final solidification region. However, after homogenizing heat treatment at 1100℃ for more than 4 hours, martensitic transformation hardly occurred even in liquid nitrogen (-196℃). On the other hand, the sample homogenized at 1100℃ for 1 hour showed an increased martensite area fraction compared to the as-cast sample. This may be because the redistribution of alloy elements during the homogenizing heat treatment process increases the region where the Ms point is higher than the cooling temperature.</p>