Abstract
Effects of grain size on creep-fatigue properties of an austenitic stainless steel under multiaxial stress states were investigated. Creep-fatigue tests under uniaxial and torsional loading conditions were conducted on 18Cr-9Ni-3Cu-Nb-N steels which were solution treated at three different temperatures. The average grain sizes of the tested materials were 18, 29 and 76μm depending on the temperature of solution treatment.It was clarified that the creep-fatigue life under the torsional loading increased with decrease of the grain size as well as under the uniaxial loading. Observation results of surface crack growth behavior indicated that the initiation life of cracks of 100∼200μm length was longer in the fine grained steel than that in the coarse grained steel. The grain size had larger influence on the fatigue life under CP (slow-fast) type strain waveforms than that under PP (fast-fast) type strain waveforms in both uniaxial and torsional loading conditions. These experimental results were closely related to the tensile and creep ductility of fine and coarse grained steels. It was also found that the creep-fatigue life evaluation method which was based on the strain range partitioning concept and modified with multiaxiality factor, MF, was effective for all tested steels independent of grain size and loading condition.
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