Abstract
In order to clarify the effect of retightening on stress relaxation behavior for high temperature bolts used in a steam turbine, reloading stress relaxation tests have been carried out at 500°C and 550°C for 1Cr-0.5Mo-0.25V and 12Cr-1Mo-1W-0.25V bolting steels, respectively. The tests were made by repeating loadings up to thirty times at a specific value of initial stress for specific reloading time intervals. The initial stress at each loading was 30.3kgf/mm2 for 1Cr-0.5Mo-0.25V steel and 27.9kgf/mm2 for 12Cr-1Mo-1W-0.25V steel. The specific reloading time intervals were 24, 72, 240, and 720 hours.
The reloading stress relaxation data obtained for both steels showed an increase in relaxation strength with the number of loadings, although they showed no increase in relaxation strength over a certain number of loadings. From the discussions on the dependence of residual stress, creep strain and testing time on relaxation plastic strain rate and the dependence of total strain on the relation between relaxation plastic strain rate and residual stress, it was shown that the strain hardening law, the time hardening law, and the constitutive equations for steady state creep could not be applied for the present data.
This paper focused particularly on examination of relaxed stress versus accumulated plastic strain relations obtained from the data after specific reloading time at each number of loadings. Numerical analysis was made for these relations using an inductive procedure. An empirical equation to describe the reloading stress relaxation data at specific reloading time intervals for both steels was proposed as follows:
σr=σ0-atibNc
where σr is the residual stress, σ0 is the initial stress, ti is the specific reloading time interval, N is the number of loading, and a, b, c are constants.
