The influence of grain size, stress and temperature on the steady state creep of a 25 Cr-20 Ni austenitic stainless steel without precipitates

Abstract

This work was performed in order to study the steady state creep behaviour of a modified 25 Cr-20 Ni stainless steel which has no precipitates. The test temperature range was 1171 to 1211 K, the stress range 4.9 to 19.6 MPa, and the grain size was 40 to 600μm. The steady state creep rate,$\mathop {\dot \in }\nolimits_S$, decreases with increase in grain size, especially at the lowest stress;$\mathop {\dot \in }\nolimits_S$ is proportional to 1/d ² at 4.9 MPa, whered is a mean grain diameter. The variation of$\mathop {\dot \in }\nolimits_S$ with grain size is smaller in the middle and coarse-grained specimens than in the fine-grained specimens, the stress exponent,n, gradually decreases from ~ 4 to ~ 1.5 with reducing stress, but in the middle- and coarse-grained specimens, a discontinuous point appears on a ∥ log$\mathop {\dot \in }\nolimits_S$ versus logσ plot. The activation energy for the steady state creep of the coarse-grained specimens tends to be larger than that of the fine-grained specimens, and the tendency is remarkable in the higher stress level. It is indicated that the creep mechanisms in the fine-grained specimens are essentially different from those in the coarse-grained specimens, and that the creep at the lowest stresses and smallest grain size is similar to that predicted by a vacancy creep model involving grain-boundary diffusion.