PSI - Issue 43

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

314

3

of the light spectrum emitted from the plasma arc burning between the electrode and the sample in an inert gas environment. The result of the measurement is a quantitative evaluation of the mass proportion of chemical elements in the sample. The basic mechanical properties are listed and compared in Table 2.

Table 1. Chemical composition of the investigated M390 and M398 steels (wt. %) Element C Si Mn Cr

Mo

V

W

BOHLER M390

1.90 1.98 2.70 2.65

0.70 0.79 0.50 0.55

0.30 0.38 0.50 0.51

20.00 20.37 20.00 20.09

1.00 0.85 1.00 1.00

4.00 4.02 7.20 7.10

0.60 0.53 0.70 0.43

Spectral analysis M390

BOHLER M398

Spectral analysis M398

Table 2. Basic mechanical properties of the investigated M390 steel

Impa ct KV/ Ku [J]

Proof strength Rp 0.2 (MPa)

Modulus of Elasticity [10 3 N/mm 2 ]

Tensile strength [MPa]

Elongat ion [%]

Thermal conductivity [W/m°K]

Specific heat [J/kg°K]

M3 90 M3 98

33

227

34

898

172

16.5

480

31

231

35

1078.5

183

15.2

490

Experimental materials were delivered as rods with a diameter of 50 mm. The materials were delivered in a soft annealed state with a maximum hardness of 280 HB for M390 steel and 330HB for M398 steel. For experimental work, samples from both materials were heat treated. The samples were heated to 1150 °C and then quenched into an oil bath. Ten samples were prepared for each steel, of which 5 samples were hardened and then tempered to 200, 300, 400, 500 and 600 °C. The remaining 5 samples were quenched followed by cryogenic cooling to - 78 °C using dry ice, followed by tempering that was similar to the previous samples. Two conductors (platinum and tungsten) were welded to the samples. Depending on the temperature, small electricity is generated at the ends of the conductors for accurate temperature monitoring during cryogenic hardening. In terms of heat treatment, the manufacturer states that the hardening temperature of the given experimental steels affects the final hardness of the material, while the highest hardness is achieved when hardening to a negative temperature, which reaches - 70 °C. According to the manufacturer, freezing to lower temperatures no longer has a significant effect on hardness and would also be economically demanding. Like M398 steel, M390 steel retains high tempering strength at temperatures up to 500 °C. Subsequently, at temperatures of 600 °C, a significant decrease in hardness occurs. The area of consumption of the final component must be considered to select the correct tempering temperature, as M390 and M398 powder steels can be heat treated for increased corrosion resistance or abrasion resistance. By cooling to negative temperatures, a reduction in the amount of residual austenite is achieved, however, this reduction can also be achieved by tempering. Here, the manufacturer states that at a tempering temperature of 540 °C, the proportion of residual austenit e decreases significantly to less than 1%. Tempering temperatures in the range T = 200 ÷ 300 °C are suitable for both st eels when the material is designed for high corrosion resistance. Wear resistance for material that has not been turbid to sub-zero temperatures is in the range of tempering temperatures 540 ÷ 560 °C. For materials that have been frozen, this area shifts between 510 ÷ 530 °C. Hardness was measured using a Vickers hardness testing machine (Instron Wolpert 930) equipped with diamond indenter under the applied load of 98,1 N and dwell time of 10 s. A PSWO 30 Charpy hammer was used for experimental "V" notch toughness tests. The energy of the pendulum hammer was 300 J with an impact speed of 5.6 m/s. The tests were performed on samples with the above designation at room temperature 22 °C.