PSI - Issue 43

Róbert Cíger et al. / Procedia Structural Integrity 43 (2023) 312–317 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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3. Result and discussion The measured values indicate that M398 steel has higher hardness values compared to M390 steel due to the cooling procedure used. The highest hardness was achieved on M398 steel, which was cryogenically hardened. The hardness value in this case was 864 HV10 at a tempering temperature of 500 °C (Table 3). M390 steel, which has been cryogenically hardened, shows comparable results with M398 steel, which has not undergone the process. As a result, we can state that the steel M398 quenched without deep freezing can fully replace the used steel M390, which was quenched with deep freezing, in terms of hardness. M398 steel, when tempered at higher temperatures, there is a percentage reduction in residual austenite, which is converted to ferrite and cementite. However, it is clear from the Fig. 1 that the incorporation of the cryogenic hardening process increases the resulting hardness values, and thus it is more advantageous to use this process. However, it should be noted that from an economic point of view, the production price of this M398 steel is rising. The measured average values of notch toughness are shown in Fig. 2.

Table 3. Hardness measurement of M390 and M398 steel after heat treatment

Tempering temperature [ °C ]

Hardness [HV 10]

200

300

400

500

600

Quenched

760.7 814.3 647,3 851,7

788.3 823.3 628,3 773,7

746.0 805.3 635,7 809,3

800.0 864.0

520.3 531.0 521,3

M398

Quenched with subzero cooling

Quenched

717

M390

Quenched with subzero cooling

827,3

495

450 500 550 600 650 700 750 800 850 900 950 1000

M398 Quenched M390 Quenched

M398 Deep freezing M390 Deep freezing

Hardness [HV10]

200

300

400

500

600

Tempering temperature [ ° C]

Fig. 1. Dependence of the hardness of M390 and M398 tool steel on the tempering temperature

Fig. 2 is a graph showing average values of notch toughness as a function of sample tempering temperature. This dependence mostly indicates that samples that were tempered in the temperature range of 200 ÷ 400 °C achieved comparable results of notch toughness. From other results it can be noticed that significantly different values were obtained by the sample, which was with the material M390, which did not undergo the process of cryogenic hardening. Here we observe that the highest value of notch toughness which was 8 J.cm -2 and was obtained on a sample which was tempered at a temperature of 300 °C (Table 4). As the tempering temperatures subsequently increased, this value gradually decreased. The results also point to the fact that samples that were cryogenically treated showed lower values of notch toughness compared to samples without cryogenic hardening. This statement is true for both types of experimental materials.