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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2.2. Experimental Procedure The relationships between the actual stress and strain of the experimental materials at elevated temperatures and different strain rates were created using a DIL 805A / D dilatometer (TA Instruments). The device is designed for physical modeling of heat treatment processes (so-called hardening mode) and hot deformation of metallic materials (so-called deformation mode). Its greatest use is in the industry dealing with heat treatment and forming of metals and their alloys, as well as in the field of research and development of new materials and technologies. When measuring by deformation mode, the experimental sample of material is in the form of a bar with a diameter of 5 mm and a length of 10 mm. This sample is inserted horizontally into the working chamber between two ceramic pistons. A thermocouple is attached to the sample surface using a spot weld. The experiment itself consists of an initial heating by means of an induction coil to the desired temperature controlled by a heating rate of 0.5 °C ∙ s -1 . After reaching the desired temperature, the endurance at the given temperature follows to ensure that the whole sample is overheated for 30 minutes. The vacuum in the chamber is maintained at 50-100 mPa throughout the test. The length change of the sample during the experiment is measured by a movable measuring rod connected to the LVDT sensor. The computer and the data acquisition system record the change in length depending on the force. From the known sample geometry, the results are evaluated in the form of stress curves depending on the actual deformation. Deformation temperatures of 800 °C, 900 °C, 1000 °C, 1100 °C and 1200 °C were used in the experiments. The temperatures were chosen with respect to the recrystallization temperature of the investigated materials. The strain rates were chosen based on standard forming rates in the industry and with regard to the capabilities of the measuring device, namely 0.001 s -1 , 0.01 s -1 , 0.1 s -1 , 1 s -1 , 10 s -1 . The maximum value of the true strain was 0.8. At the end of the measurement, each sample was cooled with helium gas to ambient temperature at a rate of 100 ° C ∙ s -1 . 3. Results and Discussion 3.1. Flow Curves The stress curves as a function of the true strain in the dilatometric deformation test of the selected tool steel at deformation rates from 0.001 s -1 to 10 s -1 and temperatures from 800 °C to 1200 °C are shown in F ig. 2. The values and course of stresses are highly dependent on the temperature and the rate of transformation. It can be seen from the observation curves that an increasing conversion rate or a decreasing temperature causes an increase in the voltage level. In other words, it prevents softening due to dynamic recrystallization and dynamic recovery and causes the material to show strain hardening. This is due to the fact that higher conversion rates and lower temperatures provide shorter energy storage times and lower grain boundary mobility, leading to nucleation and growth of dynamically recrystallized grains and removal of dislocations.

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d) e) Fig. 2. True stress-strain curves of high-strength steel X153CrMoV12 under different strain rates, a) strain rate 0.001 s -1 , b) strain rate 0.01 s -1 , c) strain rate 0.1 s -1 , d) strain rate 1 s -1 , e) strain rate 10 s -1 .