Determination of Activation Energy and Prediction of Long-Term Strength of Creep Rupture for Alloy Inconel 740/740H: A Method Based on a New Tensile Creep Rupture Model

Abstract

The activation energy of creep rupture is determined based on a new tensile creep rupture model and then applied to rationalize the creep rupture data measured over a wide range of stresses and temperatures, which enabled the predictions of 100,000 hours and 200,000 hours creep rupture strengths to be made at different temperatures in the range of 650 °C to 875 °C for alloy Inconel 740/740H. The reliability of such long-term predictions is also analyzed.

Appendix

Equation [3] can be simply rearranged into following form:

$$ \left[ {\left( {r{-}{ 1}} \right)/r} \right] \, = \, {-}k_{{1}} \left[ {t_{{\text{r}}} {\text{exp}}\left( {{-}Q_{{\text{c}}}^ */\left( {RT} \right)} \right)} \right]^{u} $$

(A1)

where r = σ/σ_TS, k₁ = exp(–A*/n*) and u = 1/n*. Now, when r → 1, i.e., when σ → σ_TS, the following relationship becomes valid

$$ \left[ {\left( {r{-}{ 1}} \right)/r} \right] \, \approx {\text{ ln}}r $$

(A2)

Thus, lnr = lnσ/σ_TS ≈ –k₁[t_rexp(-Q_c*/(RT))]^u, or σ/σ_TS ≈ exp{–k₁[t_rexp(-Q_c*/(RT))]^u}, which is the Wilshire model. Therefore, when σ → σ_TS, the new creep rupture model, i.e., Eq. [3], approximately equals the Wilshire model.

Fig. 1

Plot of lnt_r versus lnσ based on the conventional creep rupture model

Fig. 2

Plot of tensile strength σ_TS versus temperature

Fig. 3

Plot of lnt_r measured at three almost the same constant stress ratio σ/σ_TS values versus 1/T. (The numeric figure near each datum point is the value of stress ratio σ/σ_TS under which that datum point is measured.)

Fig. 4

Plot of lnt_r^-1exp[Q_c*/(RT)] versus ln[σ/(σ_TS – σ)] for all the available experimental data

Fig. 5

Comparison between the predictions (solid curves) based on the new creep rupture model and experimental data (data points)