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The isothermal hot compression tests of 43CrNi steel was carried out at temperatures 800°C to 1 050°C at an interval of 50°C and at constant strain rates of 0.01, 0.1, 1.0 and 10 s − 1 for total true strain of 0.7. The values of experimental stress were corrected for adiabatic heating which was used for flow stress modeling, and further its predictability was verified with experimental results. The true stress-true strain curves exhibit peak stresses followed by softening due to occurrence of DRX. The activation energy was calculated based on sinh type equation and found to be increasing with increase in strain. The obtained stress exponent 5.4, suggests that the mechanism of hot deformation is dislocation glide controlled by dislocation climb. Strain rate sensitivity maps and processing maps were developed using dynamic materials model (DMM), modified DMM with different instability criteria. The instability parameter, κ j , is directly related to the dissipative function which is associated to microstructural changes, J, whereas the instability parameters, ξ and κ, are associated with the strain rate sensitivity parameter, m. Therefore, the instability parameter, κ j , is more mathematically precise as it eliminates m from its calculations. The microstructures of deformed specimens were studied and correlated with the domains of processing maps to validate the workability region.KEY WORDS: medium carbon low alloy steel; processing maps; hot deformation; constitutive analysis; thermo mechanical simulator.