PSI - Issue 43

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

316

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Table 4. Notch toughness measurement of M390 and M398 steel heat treatment

Tempering temperature [ °C ]

Notch toughness [J.cm -2 ]

200

300

400

500

600

Quenched

3

3.1 2.5

2.7 2.1 7.1 2.9

4.2 2.8

2.5 2.3 3.2 4.1

M398

Quenched with subzero cooling

2.5

Quenched

6 3

8 3

6

M390

Quenched with subzero cooling

3.1

10 12

M398 Quenched M390 Quenched

M398 Deep freezing M390 Deep freezing

0 2 4 6 8 Impact energy [J.cm -2 ]

200

300

400

500

600

Tempering temperature [ ° C]

Fig. 2. Dependence of the impact energy of M390 and M398 tool steel on the tempering temperature

Fig. 3 shows an example of fracture surface of M390 material without cryogenic cooling that has been tempered to 300 °C. In Fig. 3a is a macroscopic image of the fracture showing the entire fracture area. Throughout its cross section, the sample showed an intercrystalline fatigue of the brittle fracture, during which the crack moved along the grain boundaries. In Fig. 3a, 3b, and 3c we can observe the propagation of this fracture across the boundaries. Due to the same crystallographic orientation of the adjacent grains, the strengths of the atomic bonds at the grain boundaries are reduced compared to the bonds inside the grain, which are caused due to the presence of another phase or grouping of precipitates at the grain boundaries. Since the steel is produced by powder metallurgy, it has a very fine microstructure with a high content of fine carbides and is therefore more susceptible to intercrystalline brittle fracture. All other experimental samples showed the same type of brittle fracture. 4. Conclusion One of the ways to make better use of new metallic materials is to gain a deeper understanding of their properties and behavior in difficult stress conditions. The aim of the study is to constantly improve them and to be able to provide the designer with quantitative data on the behavior of materials under stress conditions at such a level as to avoid, as far as possible, equipment failures caused by material failure. The results of the notch toughness test and subsequent inspection of the specimens and fracture surfaces demonstrated the susceptibility of M390 and M398 steels to brittle intercrystalline fracture in the presence of martenitic microstructure. The presence of a martensitic microstructure is necessary to achieve optimal operating properties of screw injection molding machines. From the measured data of hardness and notch toughness, we observed a significant increase in hardness and a decrease in notch toughness of M390 steel using hardened freezing versus conventional hardening. However, M398 steel using conventional hardening achieves a hardness comparable to M390 steel after hardening with freezing in the