Temperature Dependence of Reloading Stress Relaxation at Specific Relaxed Stress for High Temperature Bolting Steels

Abstract

In order to clarify the effect of retightening on stress relaxation behavior of high temperature bolts used in steam turbines, a series of studies on the retightening (reloading) stress relaxation were conducted previously^1)∼4).
Examinations were made, in the present paper, for the effect of test temperature on reloading stress relaxation behavior when stress was relaxed to a specific value of residual stress for 1Cr-0.5 Mo-0.25V, 12Cr-1Mo-1W-0.25V and 19Cr-9Ni-1.4Mo-1.4W-Nb bolting steels.
The relation between the testing time (t_j) and the number of loading (N) at each test temperature (T; 4∼5 levels) was analyzed numerically for the case when the initial stress (σ₀) was relaxed to a specific value of residual stress (σ_rj=0.6σ₀), and an empirical equation was obtained to represent these data at various test temperature conditions. This equation was combined with our previously proposed equation²⁾³⁾ which concerns the data with various specific relaxed stress ratios {σ_Rj=(σ₀-σ_rj)/σ₀; 5∼9 levels} at a constant test temperature.
In order to describe the retightening stress relaxation behavior of high temperature bolts at various temperatures and specific relaxed stress ratios, the following constitutive equation is proposed:
t_j=KN^aσ_Rj^{b₀+b₁logσ_Rj}exp(c₀+c₁logN/T)
where K, a, b₀, b₁, c₀, and c₁ are constants.