Abstract
Effects of grain and grain boundary strengths on the creep crack propagation at 700°C were investigated using an austenitic heat resisting steel, JIS SUH38. It is found that all creep cracks propagate through grain boundaries due to intergranular cracks and cavities formed by creep deformation, and that the empirical formula for creep crack propagation rate, d(2l)/dt=V^^·/θ (V^^·: notch opening displacement rate, θ: crack opening rate), which was previously proposed by the authors, is valid even for specimens having various levels of grain-and grain boundary strengths. From the results obtained, it is concluded that in order to increase the resistance to creep crack propagation the strength of grains should be increased in harmony with the strength of grain boundaries and that it is rather harmful to increase the resistance to creep crack propagation to strengthen excessively the grains alone, especially when the grain boundaries are weaker.
