The experimental materials used in this contribution were two special tool steels manufactured by Böhler, namely M390 and M398. Steels are produced using powder metallurgy with the HIP (Hot Isostatic Pressing) method [11 (link)]. The description of the production process is described in the publication by Ciger et al. [12 (link)]. Due to the production method and chemical composition, the steel provides extremely high resistance to mechanical wear as well as corrosion resistance. The prerequisite for the use of steel is the production of screws for injection molding machines. The main concept for increasing the macro-hardness is the high content of MC and M7C3 carbides, which can be observed in the microstructure itself in Figure 1 . The basic microstructure of test tool steels was investigated by scanning electron microscopy (SEM, Tescan Vega 3, Tescan Orsay Holding, Brno, Czech Republic). The samples in this case were heat-treated (Q + T) to the highest hardness.
These steels have their uses in the plastics industry, specifically to produce injection molding screws. The helix performs a rotational movement which moves the solid granulates from the hopper into the space in front of the screw. It is gradually heated and melted to the injection temperature depending on the type of plastic polymer in the temperature range of 160–250 °C. The injection of plastic takes place by means of a straight movement of the screw to the front position. The back flow cylinder prevents the backflow of the molten plastic granulate. The investigated material M398 is a newly developed material, the task of which will be to replace material M390. Other properties that predetermine M398 steel to produce screws include, high dimensional stability during heat treatment, good resistance to chemical corrosion, and the possibility of polishing to a mirror finish.
Table 1 shows and compares the values of the prescribed chemical composition from Böhler and the chemical composition measured by the authors using a Spectrolab Jr. chemical analyzer. The basic mechanical properties are listed and compared in Table 2 . The manufacturer did not provide the exact chemical range of the individual elements.
In the processes of all realized experiments, dry sliding friction on powdered tool steels M390 vs. M398 and comparison of wear in contact with ceramic bearing ball Al2O3 at a constant measuring temperature of 200 °C were investigated. The whole measurement process took place on an instrument, the UMT TriboLab (Bruker Austria GmbH, Wien, Austria), where the main changing parameter was the different tempered temperatures of the samples (200 °C, 400 °C, 600 °C). Half of each material was also cryogenically turbid to reduce the residual austenite. The marking of the samples consisted of the marking of the experimental material and the subsequent tempering temperature. The samples are marked with the type of steel, and the subsequent number determines the tempering temperature of the samples. The letters DF (deep freezing) indicate samples that have undergone a cryogenic hardening process. Samples that did not have DF at the end were quenched to 50 °C and then tempered to the prescribed temperatures written on their labels. Wear results and roughness measurements were measured by light optical microscopy (LOM) and atomic force microscopy (AFM, Oxford Instruments, Abingdon, UK), respectively. Using the AFM microscope, 3D topographies of the ceramic balls, as well as the formed grooves, were also obtained, and their final texture surface was determined after individual experiments. All the obtained results for the coefficient of friction and wear are discussed in the next part of the article.
These steels have their uses in the plastics industry, specifically to produce injection molding screws. The helix performs a rotational movement which moves the solid granulates from the hopper into the space in front of the screw. It is gradually heated and melted to the injection temperature depending on the type of plastic polymer in the temperature range of 160–250 °C. The injection of plastic takes place by means of a straight movement of the screw to the front position. The back flow cylinder prevents the backflow of the molten plastic granulate. The investigated material M398 is a newly developed material, the task of which will be to replace material M390. Other properties that predetermine M398 steel to produce screws include, high dimensional stability during heat treatment, good resistance to chemical corrosion, and the possibility of polishing to a mirror finish.
In the processes of all realized experiments, dry sliding friction on powdered tool steels M390 vs. M398 and comparison of wear in contact with ceramic bearing ball Al2O3 at a constant measuring temperature of 200 °C were investigated. The whole measurement process took place on an instrument, the UMT TriboLab (Bruker Austria GmbH, Wien, Austria), where the main changing parameter was the different tempered temperatures of the samples (200 °C, 400 °C, 600 °C). Half of each material was also cryogenically turbid to reduce the residual austenite. The marking of the samples consisted of the marking of the experimental material and the subsequent tempering temperature. The samples are marked with the type of steel, and the subsequent number determines the tempering temperature of the samples. The letters DF (deep freezing) indicate samples that have undergone a cryogenic hardening process. Samples that did not have DF at the end were quenched to 50 °C and then tempered to the prescribed temperatures written on their labels. Wear results and roughness measurements were measured by light optical microscopy (LOM) and atomic force microscopy (AFM, Oxford Instruments, Abingdon, UK), respectively. Using the AFM microscope, 3D topographies of the ceramic balls, as well as the formed grooves, were also obtained, and their final texture surface was determined after individual experiments. All the obtained results for the coefficient of friction and wear are discussed in the next part of the article.
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