2. Materials and Methods
Water atomized HSS M3/2 grade powder and Höganäs NC 100.24 iron powder, both finer than 150 µm (more than 85% of the particles are less than 75 µm), were used in the experiments. HSS powder was used in the annealed condition. Its chemical composition is shown in Table 1
, and its morphology in Figure 1
In HSSs, the microstructure of the powder particles is very important because due to the low temperatures used during the powder metallurgy production process, it is easy to maintain it in the finished product. Its microstructure consists of fine carbides (MC and M6
C types) embedded in a ferritic/bainitic matrix. The typical particle microhardness is 284 ± 17 HV0.065.
The microstructures of the used powders are shown in Figure 2
The powder mixtures of 50 wt% Fe with 50% wt% HSS were prepared by mixing for 30 min in a Turbula T2F (WAB, Muttenz, Switzerland) shaker-mixer and then sintering in an HP D 25/3 (FCT Systeme, Rauenstein, Germany) spark plasma sintering furnace. The on:off ratio of pulsed DC was set at 125:5 (in ms). The powder was sintered using a set of graphite tools under a vacuum of 5 × 10−2 mbar at sintering temperatures of 900, 950, and 1000 °C and the compaction pressure of 50 MPa. The heating rate was 100 °C/min, and the holding time was 2.5 min. For this purpose, tools made of graphite grade 2333 (Mersen, Gennevilliers, France) were used. The loading chamber in the set of graphite tools was filled with the powder mixture. For technological reasons, Papyex N998 graphite foil (Mersen, Gennevilliers, France) was placed between the powder mixture and the die and the punches. The set of tools prepared in this way was placed in the sintering chamber of the HP D 25/3 furnace in order to carry out the sintering process. Samples with dimensions of Ø40 mm × 10 mm were produced.
The spark plasma sintered samples were subsequently tested for density by the Archimedes method, Brinell hardness (tungsten carbide ball with a diameter of 2.5 mm and 1640 N load), flexural strength, the tribological properties and subjected to microstructural examinations by means of a SU-70 (Hitachi, Tokyo, Japan) scanning electron microscopy. The phase identification of the materials was carried out using a TUR-M62 (Carl Zeiss, Jena, Germany) X-ray diffractometer with CuKα
radiation (λ = 1.5406 Å). Three samples were sintered under the same conditions, then the properties were determined for each of them, and next, the standard deviation was calculated. In the three-point flexural test, a rectangular sample is stressed, and the corners are subjected to maximum stresses and strains. Failure will occur when the deformation or elongation exceeds the material limits. The transverse rupture strength (TRS) test was carried out using the three-point flexural method on a ZIM mechanical press. The wear tests were carried out with a T-05 (ITeE, Radom, Poland) block-on-ring tester (Figure 3
During the test, a rectangular tribological sample was mounted in a holder equipped with a hemispherical insert, ensuring proper contact between the tested sample and a steel ring rotating at a permanent speed. The friction surface (F) of the sample was perpendicular to the load (L) direction. A double lever system was used to push the sample towards the ring with a loading accuracy of ± 1.5%. The wear test conditions were as follows:
dimensions of the test sample: 20 mm × 4 mm× 4 mm,
rotating ring: heat-treated steel 100Cr6, 55 HRC, Ø49.5 mm × 8 mm,
rotational speed: 136 rpm,
velocity: 0.35 m/s,
load: 200 N,
sliding distance: 250 and 500 m.
The friction coefficient was defined as the ratio of the average friction force (expressed in Newton’s) to the load, expressed in Newton’s. The sample surfaces after the tribological tests were examined under a LEXT OLS 4100 (Olympus, Tokyo, Japan) confocal microscope.
Conceptualization, methodology and writing—review and editing, M.M., B.L.-M., and D.G.; investigation, B.L.-M. and M.M.; writing—original draft preparation and visualization, M.M. All authors have read and agreed to the published version of the manuscript.
This research was funded by the Polish State Committee for Scientific Research, grant number 18.104.22.1683.
Conflicts of Interest
The authors declare no conflict of interest.
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SEM morphologies of powders: (a) M3/2 high speed steel, (b) NC 100.24 iron; SEM.
Microstructure of powders: (a) M3/2 high speed steel, (b) NC 100.24 iron; SEM.
Diagram of the test using T-05 tribometer.
Relative densities of spark plasma sintered M3/2–50% Fe materials.
Brinell hardness of spark plasma sintered M3/2–50% Fe materials.
Flexural strength of spark plasma sintered M3/2–50% Fe materials.
The microstructure of M3/2–50% Fe material spark plasma sintered at 900 °C; SEM.
The microstructure of M3/2–50% Fe material spark plasma sintered at 950 °C; SEM.
The microstructure of M3/2–50% Fe material sintered at 1000 °C; SEM.
The microstructure of M3/2–50% Fe material spark plasma sintered at 1000 °C: 1–MC carbides, 2–M6C carbides, 3–HSS matrix; SEM.
XRD diffraction pattern of spark plasma sintered M3/2-50% Fe materials.
Linear analysis of element distribution on iron—high-speed steel boundary.
Wear rate of spark plasma sintered M3/2-50% Fe materials.
Coefficient of friction of spark plasma sintered M3/2-50% Fe materials.
The surface of M3/2-50% Fe materials spark plasma sintered at 900 °C after examining wear resistance: (a) 2D intensity mode, (b) 3D intensity mode.
The surface of M3/2-50% Fe materials spark plasma sintered at 1000 °C after examining wear resistance: (a) 2D intensity mode, (b) 3D intensity mode.
Alloying elements and their proportion by weight in M3/2 grade steel.
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