PSI - Issue 43

Miroslav Polášek et al. / Procedia Structural Integrity 43 (2023) 306–311 Author name / Structural Integrity Procedia 00 (2022) 000 – 000

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parameter is the longest possible service life while adhering to the specified quality parameters, such as non-increase of dispersion above the specified manufacturer limit, some manufacturers or users also set other parameters of bullet stabilization (side shots). These requirements require the material to be more resistant to adhesive, abrasive, and oxidative wear (Chen et al. 2022). This is further emphasized when using new types of monolithic bullets based on CuZn39Pb3 brass. In this case a bullet is harder than the lead or shell projectiles used so far. Other factors influencing the increase of wear of the main small-caliber weapons are the maximum pressure and temperature of the burned gun powder, the maximum warming of the barrel (depending on the firing time), the reaction of combustion products to the borehole (depending on the powder used), firing rate (firing mode), increasing the speed of small caliber bullets (Seguard et al. 2018). When we design a barrel where we want to use only a lead bullet, we also define that the muzzle velocity will not exceed 400 m/s. At higher speeds, there is a high probability of tearing the projectile during the flight in the barrel, even if we used lead with additives such as antimony to increase the hardness of the resulting projectile ( Lawrence et al. 2016). With this type of projectile, a barrel steel without expensive elements of chemical composition will suffice. We can make do with ingredients such as carbon and manganese. Typical examples of these steels are steels C45E (mesh standard EN ISO) / 12050/1.1191 or E335 (mesh standard EN ISO) /11600/St60-2/1.0060. If we design a projectile with a muzzle velocity higher than 400 m/s. We need higher quality steel with more additives and also with expensive additives such as Vanadium, Chromium, Molibden or Nickel. Typical representatives of these steels are 42MnV7, 42CrMo4, 30CrMoV9, 34CrNiMo6. To extend the life of these steels at higher loads, the technology of internal chrome plating of the bore barrel was used. Chromium plating of the surface mainly prolonged the life of the barrel, but it also had disadvantages (Kim et al. 2021). When chromium plating was applied to the inner surface the chromium layer was not regular. Currently, this technology is at a standstill due to its poor environmental impact. If we wanted steel from an even higher possible load, we chose 30CrNiMo8 steel, which has a higher chromium and nickel content compared to 34CrNiMo6 steel. We made the selection based on the requirements for the main steel, which are high toughness, the highest yield strength, homogeneity (no Kraft cracks, phosphorus, and sulphur content together up to 0.04-0.06%), the highest strength even at higher temperatures. 30CrNiMo8 steel has high strength while maintaining suitable impact strength even at - 40 °C. In this work we will measure dry sliding friction on selected experimental steels 42CrMo4 and 30CrNiMo8, compare their wear in contact with the G40 bearing ball which was also used by the author Gehlen et al. 2020 and Budinski et al. 2021, at different surface roughnesses of steel. In our case, the entire friction measurement was performed. The main parameter in UMT TriboLab, was a load of 2.5 N and various surface roughness parameters of the measured steels. Fig. 1 shows a typical coefficient of friction curve, which can be divided into two main parts. The first part is called the transition part consisting of the lead-in part with a mostly short linear part. At the end of this section, it is usually the peak of the coefficient of friction. The second part of the curve is the steady state period, when the coefficient of friction curve stabilizes. Depending on the selected parameter and the types of contact pairs, the second part of the curve can have either a slightly decreasing or ascending direction.

Fig. 1. Typical friction coefficient curve