91Ó°ÊÓ

(a) Estimate the activation energy for creep (i.e., \(Q_{c}\) in Equation \(\left.8.25\right)\) for the S-590 alloy having the steady-state creep behavior shown in Figure 8.32. Use data taken at a stress level of 300 MPa (43,500 psi) and temperatures of \(650^{\circ} \mathrm{C}\) and \(730^{\circ} \mathrm{C}\). Assume that the stress exponent \(n\) is independent of temperature. (b) Estimate \(\dot{\epsilon}_{s}\) at \(600^{\circ} \mathrm{C}(873 \mathrm{~K})\) and \(300 \mathrm{MPa}\).

Steady-state creep data taken for an iron at a stress level of \(140 \mathrm{MPa}(20,000 \mathrm{psi})\) are given here: $$ \begin{array}{cc} \hline \dot{\boldsymbol{\epsilon}}_{s}\left(\boldsymbol{h}^{-\mathbf{1}}\right) & \boldsymbol{T}(\boldsymbol{K}) \\ \hline 6.6 \times 10^{-4} & 1090 \\ \hline 8.8 \times 10^{-2} & 1200 \\ \hline \end{array} $$ If it is known that the value of the stress exponent \(n\) for this alloy is \(8.5\), compute the steady-state creep rate at \(1300 \mathrm{~K}\) and a stress level of \(83 \mathrm{MPa}\) \((12,000 \mathrm{psi}) .\)

(a) Using Figure 8.31, compute the rupture lifetime for an \(S-590\) alloy that is exposed to a tensile stress of \(400 \mathrm{MPa}\) at \(815^{\circ} \mathrm{C}\). (b) Compare this value to the one determined from the Larson-Miller plot of Figure \(8.33\), which is for this same S-590 alloy.

Cite three metallurgical/processing techniques that are employed to enhance the creep resistance of metal alloys.