*3.3. Results of Fractographic Analysis*

Figures 24–28 show representative images of fracture surfaces of Hardox Extreme welded joints in conditions both after welding and after heat treatment operations. Fractographic analyses were made at the temperatures of impact testing, i.e., +20 ◦C and −40 ◦C. In each of the analyzed cases, fractures after impact tests do not show a significant share of plastic side zones (Figure 24), which proves relatively small energy expenditure during their creation.

This statement especially refers to fractures of the joint after welding, which were subjected to impact testing at the negative temperature. In addition, macroscopic analysis showed that fractures of the specimens after welding are characterized by highly diversified surface topography, resulting from the presence of coarse-grained structure on the fusion line, partially under the notch (frames A1 and A3 in Figure 24), in the central zone (frames B1 and B3 in Figure 24) and in major part of the

final fracture zone (frames C1 and C2 in Figure 24). In turn, fracture surfaces of the heat treated joint can be found to be uniform and rough on their entire area, showing a characteristic run according to the crystallization direction after welding (Figure 24). In order to reveal a detailed structure of individual zones, all fracture surfaces were subjected to further examinations by using scanning electron microscopy.

Transcrystalline fractures of Hardox Extreme welded joints in the condition after welding, subjected to examination at both ambient and reduced temperatures, are fractures of mixed nature, with irregularities on the separation surface (steps) and with a clearly visible "river" system (Figures 25 and 26).

**Figure 25.** SEM images of fracture surfaces of Hardox Extreme welded joints after welding, shown in Figure 24, after impact testing at +20 ◦C. (**a**) Area marked with the frame A1; (**b**) area marked with the frame B1; (**c**) area marked with the frame C1. R—"river" system; SL—slides; C—microcracks with fine steps. Scanning microscopy, unetched.

**Figure 26.** SEM images of fracture surfaces of Hardox Extreme welded joints after welding, shown in Figure 24, after impact testing at −40 ◦C. (**a**) Area marked with the frame A3; (**b**) area marked with the frame B3; (**c**) area marked with the frame C3. R—"river" system; SL—slides; C—microcracks with fine steps. Scanning microscopy, unetched.

In both cases, structures with micro-voids of different sizes can be also distinguished on fracture surfaces, where no inclusions of phases derived from alloying microadditives are observed. A characteristic feature of fracture surfaces of the specimens after welding is presence of numerous transverse microcracks with agglomerations of fine steps. Such a state is observed mostly in the zone under the notch (Figures 25a and 26a) and locally in the final fracture zone (Figure 25c). Moreover, almost in all areas of the fractures not subjected to heat treatment, slides are visible, which is characteristic for cleavage fractures.

Qualitative differences in fracture structures are clearly demonstrated on the specimens subjected to heat treatment after welding. On the surfaces, areas of micro-voids are observed, separated by plastic areas with band-like arrangement of "scaly" steps (Figures 27b,c and 28c).

**Figure 27.** SEM images of fracture surfaces of Hardox Extreme welded joints after heat treatment, shown in Figure 24, after impact testing at +20 ◦C. (**a**) Area marked with the frame A2; (**b**) area marked with the frame B2; (**c**) area marked with the frame C2. R—"river" system; C—microcracks with fine steps; S—"scaly" steps. Scanning microscopy, unetched.

**Figure 28.** SEM images of fracture surfaces of Hardox Extreme welded joints after heat treatment, shown in Figure 24, after impact testing at −40 ◦C. (**a**) Area marked with the frame A4; (**b**) area marked with the frame B4; (**c**) area marked with the frame C4. R—"river" system; C—microcracks with fine steps; S—"scaly" steps. Scanning microscopy, unetched.

The described topography of a fracture surface is created by slips and decohesion, which results in appearance of microcracks in the planes {100} [29] and creation, after their separating walls are merged, of scales overlapping in a characteristic way. Parabolic contours of micro-voids indicate that the fracture is initiated by plastic deformation—slip—provoked by tangent forces in the process of fracture creation. It is worth mentioning that the cracking itself proceeds along the specific crystallographic planes.

In addition, it can be stated that identification of these planes is practically impossible because of the characteristic "river" relief occurring on fracture surfaces of the heat-treated specimens. This is caused by the fact that the meandering "river" system creates on a large area a system of micro-void coalescence characteristic for a plastic fracture. Mixed nature of the fractures contributes to the creation, during cracking, of steps increasing the amount of absorbed energy and thus decreasing the brittleness threshold.
