PSI - Issue 43

Maroš Eckert et al. / Procedia Structural Integrity 43 (2023) 318–323 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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3.2. Microstructure of Deformed Samples After dilatation experiments, metallographic samples were prepared from deformed steels and prepared for observation of the microstructure using an optical microscope. The sample surfaces were etched with 3 % Nital solution to reveal the individual structures. This solution reacts significantly with the martensitic structure, with very little ferritic structure and carbides. This creates a contrast on the surface, based on which it is possible to determine the composition, shape and size of these structures. Fig. 3 shows the microstructure of 100MnCrW4 steel after deformation. There was a significant change in structure and enlargement of the grains using DRX at a higher transformation rate. This is due to the adiabatic nature of the process, which causes a local increase in temperature, which acts as a driving force for dynamic recrystallization. In Fig. 3c), 3d), only the dependence of the grain size change on the transformation rate of 100MnCrW4 steel can be observed. The grains again underwent complete recrystallization with precipitation of carbide phases at the boundaries of the new grains. In this case, the grain size was significantly affected by the forming temperature, its increase accelerates not only the dynamic recrystallization, but also the grain growth rate itself, which also enhances the softening due to the recrystallization effect. This is because the nucleation of dynamic recrystallization is controlled by thermal activation (Lia et al. 2020). Increasing temperature also increases the difference in free energy between the new and original phases, leading to increased growth rates. In addition, it increases the driving force of the grain cores, and the deformed grains are gradually replaced by dynamically recrystallized grains. a) b) Fig. 3. Microstructure of 100MnCrW4 steel after deformation at: a) temperature 800 °C and strain rate 0.001 s -1 ; b) temperature of 800 °C and strain rate of 10 s -1 , c) temperature of 1200 °C and strain rate of 0.001 s -1 ; d ) a temperature of 1200 °C and strain rate of 10 s 1 3.3. Constitutive Modelling The Arrhenius equation, which is a phenomenological in fact approach, was used to predict constitutive equation, which gives the flow stress and strain at different temperatures and expresses the Z parameter, known as the Zener Hollomon parameter. The Z parameter represents as the temperature compensated strain rate, which has been widely used to characterize the behaviour of materials in hot working (Shang et al. 2014). The Z parameter is formulated as: = ̇exp ( ) (1) where ̇ is the strain rate, T is the temperature in the unit of Kelvin, R is the gas constant (R=8.314 J·mol -1 ·K -1 ) and Q is the activation energy. The detailed procedure for obtaining individual parameters is based on the usual procedure and is defined e.g., in Krbata et al. (2019). The resulting equation for calculating the stress with respect to the strain rate is then: = f 1 1 ( ) { ( ̇ exp ( f 3 ( ) ) exp[f 4 ( )] ) f 1 2 ( ) + [ ( ̇ exp ( f 3 ( ) ) exp[f 4 ( )] ) f 2 2 ( ) + 1 ] 1 2 } (2) b) c)