The effect of rapid thermal cycling on the creep properties of alpha iron

Abstract

The effect of rapid, low amplitude thermal cycling on the creep properties of alpha iron was investigated. It was found that creep rates under thermal cycling conditions are lower than the creep rates calculated from steady-state isothermal creep experiments. This relative decrease of the creep rate was dependent on the dwell time and the applied stress. The maximum decrease in creep rate occurred when the time at the lower temperature was very short. In single cycle creep experiments it was found that an abrupt temperature decrease was followed by a delay period of zero creep rate. When the temperature was again increased, a period of inverse transient creep was observed. It is concluded that the inverse transient creep is responsible for the relatively lower creep rates observed under thermal cycling conditions. Investigations of the dislocation substructure with the electron microscope did not show any significant changes attributable to thermal cycling. The lowering of the creep rate is tentatively explained on the basis of dislocation pinning by point defects during the cooling part of the cycle which inhibits subsequent dislocation motion during the rest of the cycle and hence decreases the overall creep rate.