PSI - Issue 43

Michal Krbaťa et al. / Procedia Structural Integrity 43 (2023) 270 – 275 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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the authors (Jandová et al. 2015, Black et al. 2002), secondary carbides Fe 2 MoC also occur in the structure. These carbides have the shape of fine needle particles with a thickness of several nanometres, which are not recognizable in our work. Since the investigated steel contains more than 5 % chromium, it can be highly assumed that secondary carbides of the M 7 C 3 and M 23 C 6 type will also be present in the materials, as published by the author (Michaud et al. 2007). The density of M 23 C 6 carbides is higher than that of M 7 C 3 carbides, because only about 1.2 % Cr is needed to begin their formation, in contrast to M 7 C 3 carbide, where the initial limit of their formation is 2.7 % Cr. It should also be noted that carbides of the M 23 C 6 type are formed only up to a 5 % Cr content. Of course, other alloying elements as well as carbon content also play an important role in their occurrence. Therefore, these limits may be slightly reduced or increased. However, the content of these carbides is lower compared to the number of other types of carbides in a given structure. In the third cooling curve, 1 °C/s, the matrix also forme d only martensite. The boundaries of the primary austenitic grains are also visible in the given figure, these were also observed in the other samples (Fig. 3b). Next, the cooling rate of 0.1 °C/s was evaluated. In contrast to the previous curves, this mea surement started a bainitic transformation at 398 °C (Fig. 3c). We can see that the shape of the derivative curve is very similar to the previous one. We might assume that this dilatation curve represents a continuation of the martensitic transformation. However, this is not the case because the increase in the transformation temperature B s by 46 °C compared to the previous measurement at a cooling rate of 0.5 °C/s and a temperature of M s 352 °C is too large. The onset of the martensitic transformation coul d not be recorded, but we can assume that this transformation proceeds smoothly after the bainitic transformation at a temperature of approximately 350 °C and produces one dilatation deviation. At a cooling rate of 0.1 °C/s, two types of microstructure hav e already been recorded in the material (Fig. 3d). The first martensitic was prevalent. The second structure was bainite, which was about 20 %. Primary tungsten carbides of the MC type were very difficult to observe. The results from the last lowest coolin g rate of 0.01 °C/s are visible in Fig. 3e. Only one type of phase transformation occurred in this measurement. At 752 °C the pearlitic transformation of P s took place and at 743 °C the end of this transformation P f was recorded. A pearlitic matrix occurred in the last metallographic sample, which had the slowest cooling rate of 0.01 °C/s. The pearlitic matrix was composed of ferrite and globular carbides of the M 3 C type. These carbides are formed based on Fe and Cr . As up to 5 % Cr is present in the investigated steel, it is assumed that most of these carbides have been enriched in Cr (Guillermet 1991). Primary carbides of the M 6 C and MC types also occurred in the structure (Fig. 3f). The bainitic structure no longer appeared in the sample. 3.2. CCT diagram for tool steel X37CrMoV5-1 The resulting CCT diagram of X37CrMoV5-1 tool steel (Fig. 4) is compared with the CCT diagram, which is calculated using JMatPro software. Solid red lines indicate A c1 and A c3 temperatures, which indicate the transformation of the initial state microstructure to austenite when the samples are heated. As we can see, the temperatures of A c1 and A c3 were 861 °C and 929 °C, respectively. Temperatures A c1 and A c3 with the JMatPro software again had a lower value, namely 825 °C and 890 °C, respectively. The black horizontal line shows the martensitic transformation start curve M s in the resulting CCT diagram. Comparing this curve with the calculated M s temperature, we observe that the experimental measured curve is approximately 50 °C higher. The experimental curve of the beginning of the martensitic transformation of M s generally maintains its constant temperature until at a cooling rate of 0.1 °C/s it begins to decrease slightly a nd approaches the calculated temperature M s according to the software. According to experimental measurements, the critical cooling rate when only a martensitic matrix is formed in the structure is between a cooling rate of 0.5 and 0.1 °C/s. The area of the beginning of the formation of the bainitic transformation B s is shown in purple in the CCT diagram. According to the evaluation of the dilatation curves, it started at a cooling rate of approximately 0.3 °C/s. Metallographic analysis supports this cl aim. When comparing the bainitic areas, we see that their shape is again very similar, only the beginning of this transformation in the experimental measurement is shifted slightly to the right. However, this shift is not important because the onset of the bainitic transformation is in both cases between cooling rates of 0.5 and 0.1 °C/s. Above this bainitic transformation, there is another area, namely the area of pearlitic transformation. According to the dilatation analysis, this transformation occurred only in the last two cooling curves. We can observe that the experimentally measured perlite area is smaller compared to the area calculated by software or the beginning of this pearlitic transformation P s is shifted slightly to the left according to the calculations and thus the phase change should occur even at a cooling rate