## 3 Results
### XRD Results  
Table 2 presents the microstrains, crystallite sizes, dislocation densities, and retained austenite contents of tested materials based on the XRD measurements. Tempering increased the crystallite size and reduced the amount of internal strains, which can be assumed to be attributed from the reduced amount of dislocations after tempering. Calculated dislocation densities correlated well with the condition of the steels giving the highest value of \(8.31\times 10^{15}\) m\({}^{-2}\) in the case of the untempered, martensitic water quenched steel, and decreasing with higher tempering temperature as seen also from Figure 10. The strengthening effect of dislocation density on martensitic steel can be calculated using Equation (4) presented by [PERSON] et al. [37].  
\[\sigma_{\text{disl.}}=K_{D}\rho^{1/2} \tag{4}\]  
where \(K_{D}\) is 0.38 \(\upmu\)b for bcc metals, \(\mu\) is the shear modulus (used here 82 GPa for Fe), and \(b\) is the Burgers vector, which is 0.2485 nm in the present case. Based on the calculations using Equation (1), the strengthening effect of measured dislocation densities starting from WQ steel moving towards the higher tempering temperatures were 668 MPa (WQ), 620 MPa (150 \({}^{\circ}\)C), 522 MPa (200 \({}^{\circ}\)C), 444 MPa  
Figure 9: FESEM images of fracture surfaces taken from the upper part (**a**) water quenched, (**b**) tempered at 200 \({}^{\circ}\)C, (**c**) tempered at 400 \({}^{\circ}\)C and the middle part (**d**) water quenched, (**e**) tempered at 200 \({}^{\circ}\)C, (**f**) tempered at 400 \({}^{\circ}\)C of selected impact toughness samples tested at +20 \({}^{\circ}\)C.  
(300 \({}^{\circ}\)C), and 373 MPa (400 \({}^{\circ}\)C). Therefore, part of the softening, especially at tempering temperatures of 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C, can be attributed to clearly lower dislocation density.  
The decrease of dislocation density with increasing tempering temperature followed the results found in the literature [22; 25; 38; 39; 40]. [PERSON] et al. [38] reported dislocation density for both direct quenched (DQ) and reheated and quenched (RAQ) around \(4.0\times 10^{15}\) m\({}^{-2}\) for martensite containing 0.1% C, which explains some of the difference with the current results. Using the same Williamson-Hall method as used here, in the case of 0.3% C martensite, [PERSON] et al. [39] found dislocation densities in the as-quenched state in the range 8-10 \(\times\) 10\({}^{15}\) m\({}^{-2}\), depending on PAGS, while [PERSON] et al. [40] found a dislocation density of \(2.0\times 10^{16}\) m\({}^{-2}\).  
Furthermore, retained austenite contents were determined from the XRD data showing \(\sim\)3% austenite content in the case of WQ, 150 \({}^{\circ}\)C and 200 \({}^{\circ}\)C steels. Tempering at 300 \({}^{\circ}\)C and 400 \({}^{\circ}\)C decomposed the retained austenite, as seen from retained austenite values in Table 2. The previously discussed transformation of retained austenite during tempering was therefore confirmed by the XRD measurements. This also affects the mechanical properties of the steel along with precipitation and carbon redistribution during tempering.