抄録
Reloading stress relaxation tests have been carried out to clarify the effect of retightening on stress relaxation behaviors of high temperature bolts used in a steam turbine. The tests were made for 1 Cr-0.5 Mo-0.25V and 12 Cr-1 Mo-1 W-0.25V bolting steels at 500°C and 550°C, respectively. Repeating loadings were undertaken up to twenty-five times when stress was relaxed to a specific value of residual stress. The initial stresses (σ0) at each reloading for 1 Cr-0.5 Mo-0.25V and 12 Cr-1 Mo-1 W-0.25V steels were 30.3kgf/mm2 and 27.9kgf/mm2, respectively, which correspond to the values of stress for the total strain of 0.20 percent. The specific residual stresses were 0.8 σ0, 0.7 σ0, 0.6 σ0, 0.5 σ0, and 0.4 σ0kgf/mm2.
The reloading stress relaxation data obtained for both steels showed that the relaxation strength increased with increasing number of loadings, but it became constant or somewhat decreased above a certain number of loadings.
Discussions were made for the dependence of residual stress, creep strain and testing time on the relaxation plastic strain rate, and the dependence of total strain on the relaxation plastic strain rate versus residual stress relations. It was found that the strain hardening law and the time hardening law for transient creep, and the creep constitutive equations for steady state creep could not be applied to the present data.
The present paper was forcused on examinations of the accumulated plastic strain versus number of loadings relations and the testing time versus accumulated plastic strain relations obtained when stress was relaxed to each specific residual stress. Numerical analysis was made for these relations using an inductive procedure. An empirical equation to describe the reloading stress relaxation data after relaxing stress to the specific residual stresses for both steels was proposed as follows:
log ti=a0+a1log(σ0-σri)+a2{log(σ0-σri)}2+b log N
where ti is the testing time, σri is the specific residual stress, (σ0-σri) is the specific relaxed stress, N is the number of loadings, and a0, a1, a2, b are constants.
