Effects of Temperature and Stress on Transient Creep Behavior of Alpha Iron

The transient creep characteristics of polycrystalline alpha iron were investigated mainly at temperatures ranging from 500 to 650°C using the constant stress creep equipment with a servo divider and the rapid heating method developed newly for determining the activation energy for transient creep. The results obtained are summarized as follows: (1) The transient creep strain exhibits a normal time dependence and inverted transient creep has not been observed. The stress dependence of the initial creep rate, Vi, which is the rate just after applying a load is larger than that of the steady-state creep rate, Vs, and the value of Vi⁄Vs, increases as the stress increases within the range up to about twice the 0.01% offset stress. (2) Of some proposed equations for creep time relation, an equation based on the assumption given by Akulov satisfies the experimental results fairly well. The calculated values of the creep rate approximate the experimental ones at the creep strain more than 1%. (3) The activation energy for transient creep depends on the creep strain. The dependence can be divided into three regions at 700°C as reported previously. However, the first region is not observed at 800°C, and the values of the activation energy obtained at the creep strain of nearly zero are smaller than that of self-diffusion on the contrary to the results at 700°C.