Hot deformation behaviour of 40CrNi steel and evaluation of different processing map construction methods

40CrNi steel is a low alloy steel with medium hardenability and has been widely used in the manufacturing of crankshafts, wind turbine forgings, etc. As forging is usually involved in the manufacturing of these components, optimization of the hot deformation process of 40CrNi steel is quite critical. Towards this, the prediction of flow stress and the determination of optimal processing parameter windows are of great importance. In this research, isothermal uniaxial hot compression tests were carried out for 40CrNi steel with true strain rates of 0.01, 0.1, 1, 10 and 30 s−1 and temperatures of 850–1200 °C. Both the strain compensated hyperbolic sine constitutive equation and the modified Johnson-Cook model were adopted to predict the flow stress curves, with average absolute relative errors of 0.034 and 0.061, respectively. Processing maps of the tested steel were constructed to determine the optimal deformation parameter windows, and the characteristics of the prior austenite microstructure generally correspond very well to the processing map established. Furthermore, different processing map construction methods were compared and it was found that power-dissipation efficiency maps strongly rely on the σ- ε˙ relationship. Assuming a certain type of constitutive equations leads to monotonic efficiency value patterns. Calculation methods based on experimental results are helpful to reflect the meaningful information carried by the experimental σ- ε˙ relationships.