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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of 0.1 °C/s. However, this was not experimentally confirmed in our case. The authors (Pastor et al. 2015, Wang et al. 2019) point out in their works that a pearlitic transformation does not occur in a given steel, but a ferritic transformation. The experimental measurements performed by us do not confirm this statement, but it should be noted that steels containing Cr up to 1.5 % do not form typical chromium-based carbides Cr 7 C 3 or Cr 23 C 6 . Chromium in these steels only enriches Fe 3 C carbides and replaces some iron atoms. At higher Cr contents (above 3.5%) than in this case, Cr 7 C 3 carbides are preferably formed and Fe 3 C is no longer present, because C has a higher affinity for Cr than for Fe (Liu et al. 2016).

F ig. 4 a) CCT diagram X37CrMoV5-1; b) HV5 hardness values for different steel cooling rates X37CrMoV5-1 The resulting structure is formed by ferrite grains and areas of fine carbides. It can be stated that this is not a typical perlite, because it is defined as an interstitial solid solution of Fe α + Fe 3 C and this carbide is not present. The ferrite grains contain fine globular dispersed secondary Cr-based carbides, which replace M 3 -based M 3 C-type carbides. Overall, it can be stated that the experimental diagram is very similar in shape, approximately 90 % with the calculated CCT diagram. 3.3. Hardness of curves in steel X37CrMoV5-1 The Fig. 5 shows the hardness measurements for each cooling rate used. It is clear that the hardness decreases again as the cooling rate decreases. In the first four measurements, only a martensitic structure enriched with various fine primary and secondary carbides occurred. The highest hardness value was up to 665 HV5. Then the slow hardness decreased to 582 HV5. The reason for the reduction in hardness, although the resulting structure in these four measurements was the same, is the refinement of the martensitic structure. At a cooling rate of 0.1 °C, the bainitic component was also confirmed in the sample structure and accounted for approximately 20 % of the material content. This result was determined by metallographic analysis and was not visibly recorded on the dilatation curve. At the penultimate cooling rate of 0.05 °C/s, the resulting structu re was highly heterogeneous. In addition to the bainitic component, it was already formed by a pearlitic structure. The one we know from the metallographic analysis was made up of a ferritic component containing globular carbides based on M 3 C. Globular perlite shows less hardness compared to conventional lamellar perlite and is also more deformable (Allain et al. 2019, Nutal et al. 2010). At a given rate, the matrix was formed by bainite with a low content of globular pearlitic structure. Therefore, the resulting hardness of 503 HV5 does not show such a significant decrease compared to the previous sample at a cooling rate of 0.1 °C/s. At the slowest cooling rate, a significant decrease in hardness of more than 100 HV to 395 HV5 was recorded. This decrease is caused by the so-called homogeneous pearlitic structure and the extinction or termination of the bainitic phase transformation, respectively. It can also be noted that the measurement uncertainty tends to increase with decreasing cooling rate as the microstructure changes from single-phase martensitic to multiphase with more pronounced heterogeneity. It should also be noted that the increase in hardness from the lowest cooling rate to the highest cooling rate is only 67 % compared to the previous experimental tool steel. Also in this case, it can also be