Strain dependent rate equation to predict elevated temperature flow behavior of modified 9Cr-1Mo (P91) steel

True stress–strain data from isothermal hot compression tests on modified 9Cr-1Mo steel over a wide range of temperature (1173–1373 K) and strain rate (0.001–100 s−1) were employed for constitutive analysis following the sine-hyperbolic Arrhenius equation. The correction for shear modulus and diffusivity in the constitutive equation showed a clear deviation from power-law at higher stresses and this was accounted for by considering the contribution from pipe diffusion. The stress dependence was found to obey rate equation of the form ε ˙ / D L = constant [sinh(αLσ/G)]nh, where DL is lattice diffusivity, G is shear modulus and, αL and nh are constants. After incorporating the influence of strain on material constants, the developed constitutive equation could predict flow stress in the strain rate range 0.1–100 s−1 at all temperatures with very good correlation and generalization. Though deviation in prediction was observed at lower strain rates 0.001 and 0.01 s−1, a higher correlation coefficient (R = 0.99) and a lower average absolute relative error (7.3%) for the entire investigated hot working domain revealed that the prediction of flow stress was satisfactory.