## 3 Results
### Mechanical Properties
#### 3.3.2 Strength and Hardness  
The general strength decrease during tempering can be attributed to the diffusion of carbon atoms, decomposition of retained austenite, and dislocation recovery effects [24] in the tempering temperatures used in the current study. As stated earlier, the precipitation of cementite was observed in the FESEM images, and the microstructures of the 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C samples differed greatly from the quenched and low-temperature tempered samples. Now, the effect on the mechanical properties can be distinguished in Figure 6. Ultimate tensile strength (UTS) began to decrease already with the 150 \({}^{\circ}\)C tempering temperature. The measured difference was very small (WQ: 2233 \(\pm\) 9 MPa vs. 150 \({}^{\circ}\)C: 2168 \(\pm\) 28 MPa) and almost within the limits of measuring error. The decrease of UTS was more prominent for the 200 \({}^{\circ}\)C tempered variant. The temperature increase of 50 \({}^{\circ}\)C from the 150 to 200 \({}^{\circ}\)C tempering temperature had a significant effect on the tensile strength, when comparing to the clearly less dramatic difference in tensile strength between the WQ and 150 \({}^{\circ}\)C samples. Further, the tempering at 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C caused the ultimate tensile strength to decrease even more. The hardness for the water quenched steel was on the expected level reaching 653 \(\pm\) 16 HV10. The low tempering at 150 \({}^{\circ}\)C also resulted in very high hardness exceeding the water quenched sample. However, the values were very close to each other, almost within the limits of error, similar to the tensile strength values. Hardness is generally considered to be related to the tensile strength, and therefore it can be stated that both the tensile strength and hardness were both almost on par with the WQ and 150 \({}^{\circ}\)C samples. Nevertheless, the lowest tempering temperature did not significantly reduce the strength nor the hardness. Tempering at 200 \({}^{\circ}\)C produced a slightly more distinct decrease of hardness as it also did for the tensile strength. Yet, the hardness was above 600 HV10 for the 200 \({}^{\circ}\)C tempered sample. Higher tempering temperatures caused a more severe drop of hardness to levels below 500 HV10. The ratio of tensile strength to hardness varied within 3.2 to 3.4 (R\({}_{\rm m}\)/HV10) and 2.3 to 3.2 for yield strength and hardness (R\({}_{\rm p0.2}\)/HV10). The latter showed an increase with increasing tempering temperature, following the increasing yield-to-tensile strength ratio. Figure 6: Mechanical properties of the tested steels: (**a**) engineering tensile test curves, (**b**) material hardness and plastic component of the uniform elongation (UE), (**c**) yield strength (0.2% offset, R\({}_{\rm p0.2}\)) and tensile strength (R\({}_{\rm m}\)), and (**d**) Charpy-V impact test energies at +20 and \(-\)40 \({}^{\circ}\)C temperatures. Standard deviations are included in (**b–d**). Yield strength of the tested steels did not directly follow the same behavior as the tensile strength. Low-temperature tempering increased the yield strength to \(1606\pm 2\) MPa (150 \({}^{\circ}\)C) and to \(1616\pm 13\) MPa (200 \({}^{\circ}\)C), as the water quenched variant had yield strength of \(1528\pm 13\) MPa. The yield-to-tensile strength ratio increased steadily from 0.66 to 0.68 (HR and WQ) to 0.92 for the 400 \({}^{\circ}\)C tempered variant. Uniform elongation decreased linearly for the quenched and tempered samples. A plateau for the elongation was reached at 400 \({}^{\circ}\)C as the martensite strength decrease was already substantial. [PERSON] et al. [5; 6] have discussed the yielding phenomena in martensitic steels, noting that uniform elongation may increase with increasing strength level, i.e., tempered samples have decreased uniform elongation as was also seen in the current study. They argued that the relaxation of internal stresses was the main cause for earlier instability that subsequently led to the decrease of uniform elongation.