Resisting stress for constitutive analysis of hot deformation in modified 9Cr-1Mo (P91) steel

Flow stress data from isothermal hot compression tests on modified 9Cr-1Mo steel over a wide range of strain rate (0.001-100 s−1) and temperature (1173-1373 K) were found to follow the universal Dorn power-law equation. Distinct stress regimes were observed with stress exponent values of ∼5 and ∼10 for low and high stress regimes, respectively. The flow behavior is rationalized by invoking resisting stress σR for dislocation motion and the modified stress exponent n0 was close to 5 for the entire stress regime. At low stresses, σR/G=K(σ/G) and approaches a constant threshold stress (σR/G=σH/G) in the high stress regime. This has been attributed to the transition in the mechanism from dislocation climb by-pass over particles at low stresses to Orowan bowing at high stresses. The stress dependence is found to obey rate equation of the form (ε̇kT/DLGb)=A0[(σ−σR)/G]n0 and the constitutive parameters A0, n0, K and σH/G evaluated at different strains were employed for predicting flow stress. The successful prediction of flow stress is reflected by a higher correlation coefficient (R=0.99) and a lower average absolute relative error (6.62%) for the entire investigated hot working domain.