Hot Deformation of Heat-Resistant Steels
The full recrystallisation temperature and the carbon-free bainite phase transformation temperature are determined by the slope-change points in the curve of mean flow stress versus the inverse of temperature. Constitutive equations including the stress exponent and an activation energy term are applied to analyse the hot deformation behaviour of nitride-strengthened 9Cr-Nb-V martensitic heat-resistant steels. There exist two different linear relationships between critical stress and critical strain due to the augmentation of auxiliary softening effect of the dynamic strain-induced transformation (DSIT). The stress–strain curves up to the peak are divided into four regions, in sequence, representing four processes, namely hardening, dynamic recovery, DSIT and dynamic recrystallisation. The microstructures under different deformation conditions are analysed. The lower carbon content in steel would increase the fraction of precipitates by increasing the volume of DSIT ferrite during deformation. In the calculation of processing maps, with the change of Zener-Hollomon value, there are three domains of different levels of workability, namely excellent workability region with equiaxed-grain microstructure, good workability region with ‘stripe’ microstructure and poor workability region with martensitic-ferritic blend microstructure.
KeywordsFlow Stress Deformation Temperature Stack Fault Energy Deformation Condition Critical Strain
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