## 2 Materials and Methods  
In order to determine the properties of the material that reaches recipients, it was decided to conduct the research on the material produced on an industrial scale. Carbonsteel used for machine elements operating at variable loads was analyzed. As the materials used for the elements of different machines must have different properties, it was decided to create different microstructures in the tested steel. For this purpose, it was decided to carry out hardening with tempering at different temperatures. This treatment ensured a hard microstructure of tempered martensite, an elastic microstructure of medium-tempered martensite and a plastic microstructure of highly tempered martensite. The structural steel for the tests was heated in an industrial electric arc furnace with a capacity of 140 tons. The furnace input was pig iron with about 25% steel scrap. The tests were carried out in seven independent heats. The metal from the furnace was poured into a ladle with a capacity of 7 tons. The steel was desulfurized and then refined with argon. Argon was introduced into the ladle of liquid steel through a porous brick. The time of the refining process was established experimentally and was about 9 min. After the completion of the metal crystallization in the molds, it was rolled into billets with a diameter of 100 mm \(\times\) 100 mm using conventional methods. Samples were taken from the billets in order to determine: the chemical composition, metallographic studies, the relative volume of non-metallic inclusions and the phase composition of oxide inclusions. The contents of chemical elements in the steel were estimated using LECO analyzers, the AFL FICA quantometer and using conventional analytical methods. The relative volumes of non-metallic inclusions with dimensions of 2 \(\upmu\)m and above were determined with a Quantinet videomicroscope at a magnification of 400\(\times\). The contents of non-metallic inclusions were determined separately for diameters of 2 \(\upmu\)m and above, 5 \(\upmu\)m and above, and 10 \(\upmu\)m and above. The volume of all non-metallic inclusions was determined by chemical extraction methods. The relative volume of non-metallic inclusions with dimensions below 2 \(\upmu\)m was determined analytically by determining the difference between the volume of all non-metallic inclusions and the volume of inclusions with a diameter of 2 \(\upmu\)m or greater. The determination of the relative volume of inclusions was possible on the assumption that the quotient of the number of particles per area divided by the area was equal to the quotient of the number of particles in the volume divided by the volume. The volume of non-metallic inclusions in the Me-S system was lower than the error value in determining their share. For this reason, Me-S inclusions were excluded from further studies. Oxygen-based inclusions were thoroughly analyzed. The real average chemical composition of the analyzed steel from seven heats is presented in Table 1.  
Bars with a diameter of 10 mm were taken from the billets. The symmetry axes of the bars were parallel to the direction of the plastic processing of the material. The bars were austenitized for 30 min at 880 \({}^{\circ}\)C. Cooling was carried out in water. Then, the bars were tempered for 2 h at the following temperatures: 200 \({}^{\circ}\)C, 300 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 500 \({}^{\circ}\)C and 600 \({}^{\circ}\)C and cooled in air. The average mechanical properties of the tested steel from seven heats, depending on the tempering temperature, are presented in Table 2.  
The steel fatigue strength test was carried out on a VEB Werkstoffprufmaschinen rotary testing machine enabling rotary bending at 6000 rpm. A cylindrical sample with a constant cross-section of 10 mm in diameter was mounted in the rotating holders of the two-point loading testing machine. The tests were carried out in accordance with PN-74/H-04327 standards [49]. The samples were cut from 100 mm \(\times\) 100 mm billets for each heat.