The paper deals with the possibilities of the determination of the hot deformation activation energy and the flow stress course at hot forming conditions. By means of the Gleeble 3800 plastometer, the set of uniaxial compression tests has been carried out for the AISI 4140 steel. The tests were carried out within the temperature range of 850 - 1 250 °C and the strain rate range of 0.1 - 50 s-1 when the strain reached up to 1.0. From the obtained stress - strain curves the values of peak stress were determined and subsequently used to determine the value of the activation energy at hot forming according to the classical equation of the hyperbolic-sine type. Enumeration of its coefficients allow at the same time to predict the maximum flow-stress values of the investigated steel in dependence on the temperature and strain rate. The value of the activation energy has been used to assembly the mathematical models which allow to describe the flow stress course and the kinetics of the dynamic recrystallization of this steel at hot-forming conditions. By this way obtained results have been compared with the previously examined steel of the grade of AISI 4130.Keywords: stress - strain curves, hot deformation activation energy, AISI 4140 steel, uniaxial compression test
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