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 ( ε ̇ k T / D L G b ) = A 0 [ ( σ − σ R ) / G ] n 0 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.