HIGH-TEMPERATURE STRENGTH OF CHROMIUM WHI TE CAST-IRON CONTAINING SPHEROIDAL GRAPHITE

The structure of chromium white cast iron containing spheroidal graphi te (SG-alloy) consists of three phases of M7C3 carbide, ferrous matrix and spheroidal graphite. The SG-alloy is produced by addition of Ce m isch metal to a flaky graphite type of white cast iron (FG-alloy). A c ompression test was conducted at high temperatures for the SG-alloy. A shape of the stress-strain curves is classified into two types, that is, one type shows a shape with large work hardening up to a maximum s tress level, which appears at lower temperatures below Tm/2 (700 K, Tm is the absolute melting temperature of the SG-alloy), and another one a shape with gradual work softening followed by the maximum stress, w hich appears at high temperature about Tm/2. The strain rate dependenc e of the maximum stress becomes remarkable at high temperatures above 873 K, and the values of strain rate sensitivity (m-value) is in Me ra nge from 0.11 to 0.14. The apparent activation energy (Q) for deformat ion is estimated to be from 300 to 400 kJ/mol in a higher temperature range above Tm/2, which is close to the activation energy for creep de formation of iron and steel. This suggests that the high temperature d eformation process at high temperatures is controlled by the deformati on of the matrix phase, while the strength of the SG-alloy is supporte d by the hard carbide phase. Spherodized graphite on the SG-alloy is e ffective to increase the strength below Tm/2, but it has no effect on the strength above Tm/2. This suggests that the high temperature defor mation dose not depend on the morphology of graphite. The fracture sur face after bending tests appears brittle below 673 K, and ductile abov e 873 K.