Abstract
The structure of chromium white cast iron containing spheroidal graphite (SG-alloy) consists of three phases of M7C3 carbide, ferrous matrix and spheroidal graphite. The SG-alloy is produced by addition of Ce misch metal to a flaky graphite type of white cast iron (FG-alloy). A compression 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 stress 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, which appears at high temperature about Tm/2. The strain rate dependence of the maximum stress becomes remarkable at high temperatures above 873 K, and the values of strain rate sensitivity (m-value) is in the range from 0.11 to 0.14. The apparent activation energy (Q) for deformation 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 deformation of iron and steel. This suggests that the high temperature deformation process at high temperatures is controlled by the deformation of the matrix phase, while the strength of the SG-alloy is supported by the hard carbide phase. Spherodized graphite on the SG-alloy is effective to increase the strength below Tm/2, but it has no effect on the strength above Tm/2. This suggests that the high temperature deformation dose not depend on the morphology of graphite. The fracture surface after bending tests appears brittle below 673 K, and ductile above 873 K.
