Correlation between Creep and Relaxation Behaviour in a Cr Martensitic Steel

There is a growing number of papers in literature concerning the possibility, for high temperature component design purpose, to generate creep rate data using stress relaxation tests and viceversa. According to these papers, relatively short stress relaxation tests could allow to estimate the creep strain rates spanning in several decades. Similarly, quite simple and inexpensive creep tests could predict stress relaxation performances. In order to check if stress relaxation tests can be a quick experimental way to obtain long term creep data, the primary creep and stress relaxation behaviour of a martensitic steel, has been investigated at 350°C with tests lasting up to 15.000h. Different kind of stress relaxation tests have been performed, in particular: - multiple stress relaxation tests where the specimen, after some stress relaxation, is reloded at the initial load and stress relaxed again, - stress relaxation tests performed on specimens crept up to the steady state. The experimental results show that anelastic-reversible-kinematic hardening processes control the initial stages of creep and relaxation, while creep-irreversible-isotropic hardening mechanisms control the creep and relaxation behaviour at longer times. The results indicate that the experimental curves obtained in multiple stress relaxation tests can give important information on the steady state creep behaviour of the alloy. Finally, the experimental results have been modelled using coupled differential equations of the Kachanov form, that are consistent with physical deformation mechanisms of the studied alloy. The parameters of the proposed constitutive equation are functions of microscopic parameters like the density of mobile dislocations and their curvature between pinning points, but they are also related to easily measurable characteristics of the experimental creep curves, so that they can be determined using macroscopic creep data only. The use of differential formalism of the equations allows to describe the creep behaviour also at variable stress and temperature, and to cope also with more complicated load/temperature histories as, for example, variable loading creep, constant strain rate and stress relaxation tests without appealing to arbitrary strain or time hardening rules. In order to further validate the proposed equations, creep tests with one step change of load and/or temperature have been performed. The proposed differential equations have been successful both in interpolating the constant load/temperature tests and in predicting the stress relaxation behaviour and the effect of step changes in load on the creep behaviour of the alloy.