## 3 Results
### Mechanical Properties
#### 3.3.3 Impact Toughness  
For the studied steels, the tempering up to 200 \({}^{\circ}\)C improved the impact toughness, but eventually the higher temperatures in the temper martensite embrittlement (TME) range (300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C) resulted in even or slightly decreasing Charpy-V impact energies when tested at \(-\)40 \({}^{\circ}\)C (Figure 6d). Interestingly, the hot-rolled sample also fitted this curve quite well, though other mechanical properties did not seem to have any direct linear relationship with the quenched and tempered variants. Only the yield-to-tensile strength ratio was very close for the HR and WQ steels. The room temperature testing (+20 \({}^{\circ}\)C) showed that the 400 \({}^{\circ}\)C tempered sample had better impact toughness over the 300 \({}^{\circ}\)C sample. The main reason for the higher room temperature impact toughness energy was presumably a shift of the transition temperature region towards lower temperatures. Nevertheless, impact energies are all relatively low when tested at subzero or room temperatures. The improved Charpy-V result for the 400 \({}^{\circ}\)C might have been due to the tempering temperature surpassing the temper embrittlement temperature range. Temper embrittlement in martensitic steels is an extensively studied phenomenon [12; 13; 14; 31; 34], and a reduction of impact toughness is often found in steels tempered at these temperatures. It is related to the formation of cementie in the temperature range of approximately 300 \({}^{\circ}\)C to 400 \({}^{\circ}\)C, depending  
Figure 7: Engineering stress–strain curves for the water quenched (WQ) and 400 \({}^{\circ}\)C tempered samples. on the alloying content. Interlath retained austenite decomposes and creates brittle cementie films, which form pathways for cracks to propagate. Concurrently, impurities may also segregate to the grain boundaries inflicting brittle behavior by providing weak interfaces for crack propagation. The severity of the tempered martensite embrittlement is affected by chemical composition. For example, silicon shifts cementie formation to higher temperatures [31]. In the current tests, it was found that samples tempered in the TME region showed a slight decrease of Charpy-V impact energies in the subzero temperature testing (\(-40\)\({}^{\circ}\)C) when compared to the low-temperature tempered samples. The microstructure of these steels (\(300\)\({}^{\circ}\)C and \(400\)\({}^{\circ}\)C) already revealed the formation of cementie (Figure 3e,f), which causes the temper embrittlement. In addition, the XRD results showed that retained austenite was present in the quenched and low-temperature tempered samples but was diminished with the higher tempering temperatures. FESEM images taken from near the notch tip (a-b) and the middle of the fracture surface area (d-f) of chosen samples are presented in Figure 9. Surprisingly, even the fracture surface of the WQ sample consisted of a significant amount of small and scattered dimples with most of the surface area consisting of separate cleavage facets and locally sheared features between the narrow multi-void-coalesced strips (Figure 9a). As the tempering temperature increases, the fraction of the dimpled area increases steadily from WQ to the samples tempered at 400 \({}^{\circ}\)C. The respective impact energies for the given samples in Figure 9 were \(18\pm 5\) J/cm\({}^{2}\) (WQ), \(31\pm 2\) J/cm\({}^{2}\) (200 \({}^{\circ}\)C), and \(43\pm 2\) J/cm\({}^{2}\) (400 \({}^{\circ}\)C), i.e., the absorbed energy almost doubled from the WQ sample to 400 \({}^{\circ}\)C tempered sample. This is credited to the increased dimple fraction, and the increase of shear lips, of which the latter consumes about half the energy of ductile fracture [36]. These observations are also in line with the ones made by [PERSON] et al. [35] in case of fracture toughness specimens of tempered AISI 4340.