Creep of a low carbon steel at low stresses and intermediate temperatures

Abstract

The creep of a low carbon steel in the form of helical coils was investigated in the temperature range 698–923 K (0.39 to 0.51 T m), for grain size 6–23 μm and stresses up to 7 MPa. Unlike previous studies on pure iron which showed substantial enhancements in creep rate compared with theoretical predictions, good agreement was obtained for recrystallised material between observed strain rate per unit stress and predictions of diffusional creep theory for the temperature range 698–823 K over which grain growth did not occur. The associated activation energy of 169 ± 12 kJ/mol confirms grain boundary diffusion creep as the dominant creep mechanism under these conditions. At 873 and 923 K, stress independent concurrent grain growth appears to result in enhancement of the creep rate. Transient enhancement at 773 K is inversely proportional to a test duration parameter P 0 = 2D g t d 3 , where w is grain boundary width, D g is grain boundary diffusivity, t is test duration and d is grain size. Creep exhibited a threshold stress with a strong temperature dependence exemplified by an activation energy of 68 ± 14 kJ/mol. This is substantially larger than in previous work on pure iron possibly as a result of interactions between grain boundary dislocations and impurities. An increased enhancement of strain rate per unit stress was observed at 698 K with increase of extent of cold work prior to testing, persisting even after extended creep times. This indicates that increased dislocation density resulting from cold work had a greater effect on creep rate than any creep strengthening arising from the elongated grain shape that also results from prior cold work.