Effects of Temperature and Stress on Transient Creep Behavior of Fe-19Cr Alloy

Abstract

The transient creep characteristics of a polycrystalline Fe-19wt%Cr alloy were investigated at temperatures ranging from 600 to 800°C under stresses up to 5 kg/mm2, using the constant stress creep equipment with a servo divider and the rapid heating method, developed newly for determining the activation energy in transient creep. The results obtained are summarized as follows: (1) The shape of creep curve changes with the variation of the testing condition. In some cases, the incubation phenomenon is observed in the range of creep strain below 10−3. The incubation phenomenon disappears by applying a slight prestrain (6×10−3) and is not observed at higher temperatures above 750°C or at lower stresses below 1.0 kg/mm2. (2) The temperature dependences of the initial creep rate, Vi, which is the rate just after applying a load, and the maximum creep rate, Vm, which is the tangent at the inflexion point of S-type creep curve, are higher than those of the steady-state creep rate, Vs, at temperatures below the Curie temperature (about 660°C), but at temperatures above the Curie temperature they are conversely lower. The stress dependences of Vi and Vm are the same above and below the Curie temperature, and higher than that of Vs. (3) The activation energy for transient creep measured by the new method (heating rate=500°C/sec) shows a higher value in comparison with the value for steady-state creep, and exhibits a rapid decrease as the creep strain increases within the strain range where the incubation phenomenon appears. Thus, it is thought that the incubation phenomenon is controlled by some complex process which is different from that for steady-state creep.