*3.2. Results of Microscopic Examinations and Hardness Measurements*

Figures 5–23 show macroscopic images, hardness measurements and an overview of characteristic microstructures in the entire area of the welded joint. It should be stated on the grounds of the performed examinations that the welding process provoked in the considered steel diverse structural changes that resulted in wide zones with decreased hardness, generally designated as heat-affected zones (Figures 5 and 7). Width of this zone was mainly decided by the used technology, parameters and conditions of welding, the value of delivered heat input, chemical composition of steel, and also by the structure of the base material before welding (Figure 9). High influence of the latter parameter on properties of the welded joint can be explained by tempering the processes of post-martensitic structures occurring mainly in the heat-affected zone. In the case of Hardox Extreme steel, the entire zone affected by the temperature exceeding 125 ◦C is characterized by wide areas of reduced hardness (Figure 7). Experiences of the author indicate that, in many cases, the width of this zone exceeds 60–100 mm. In turn, application of high-energy welding methods to a steel with chemical composition close to that of Hardox Extreme very often results in cold cracking and reduced brittleness threshold determined in impact tests.

Conclusions similar to these above cannot be formulated in relation to the specimens after heat treatment. In the macroscopic image of the joint in this condition (Figure 6) no heat-affected zone is observed, which is additionally confirmed by hardness measurements (Figures 7 and 8). It should be also stressed that, from the viewpoint of real chemical composition of Hardox Extreme (*CEV* = 0.90; Table 3 and Figure 1), application of generally accepted weldability criteria to low-alloyed steels can give rise to serious doubts. In this connection, the author believes that, with regard to very high mechanical properties of the base material, operations of post-weld heat treatment should be obligatorily considered at welding processes of the steel Hardox Extreme. Such an approach makes it possible to restore microstructure and mechanical properties of the entire area of a welded joint.

Analysis of structural changes of the examined welded joints showed, in the zone of base material after welding (BM in Figure 5), microstructure characteristic for toughening processes, i.e., tempered sorbite (Figure 9b). This structure shows the tempering processes occurring in the welded material, resulting in significant loss of hardness in comparison to the condition before welding (Figure 7). The carried-out thermal operations made it possible to obtain, in the analogous zone, structures very well corresponding with the Hardox base material in an as-delivered condition. After heat treatment, the base material zone (BM in Figure 6) showed microstructure of fine-lath hardening martensite, almost identical to that of Hardox Extreme in as-delivered condition (Figure 9a,c). The above finding is confirmed by hardness measurements taken in the considered zones. After welding, level of hardness in the BM zone reached 320–350 HV (Figure 7). Heat treatment resulted in an increase of hardness to over 650 HV (Figure 7), which only slightly declines from the value declared by the manufacturer of Hardox Extreme in an as-delivered condition, i.e., ca. 700 HV (min. 650 HBW, see Table 1).

**Figure 5.** Macroscopic image of cross-section of a Hardox Extreme welded joint after welding. WM—weld metal zone, FZ—fusion zone, BM—base material zone, CGH(AZ)—coarse-grained heat-affected zone (overheating), FGH(AZ)—fine-grained heat-affected zone (normalization and recrystallisation), ICH(AZ)—intercritical heat-affected zone (incomplete normalization). Stereoscopic microscopy, etched with 3% HNO3 and Adler's etchant.

**Figure 6.** Macroscopic image of cross-section of a Hardox Extreme welded joint after heat treatment. WM—weld metal zone, FZ—fusion zone, BM—base material zone, HAZ—heat-affected zone. Stereoscopic microscopy, etched with 3% HNO3.

**Figure 7.** Hardness distribution of a Hardox Extreme welded joint along the line A shown in Figure 2. AW—condition after welding, HT—condition after heat treatment.

**Figure 8.** Hardness distribution of a Hardox Extreme welded joint along the line B shown in Figure 2. AW—condition after welding, HT—condition after heat treatment.

Hardness level in the weld metal zone (WM1, WM2 and WM3 in Figure 6) after heat treatment was ca. 550 HV, slightly lower than that of the base material (Figure 7). A drop of hardness by ca. 100 HV can be explained by slightly lower percentage of carbon in the weld axis, which decides saturation degree of ferrite and thus hardening capacity of the material. However, it is worth mentioninging that the obtained hardness on the entire thickness of the heat-treated welded joint was nearly linear at the level of 550 HV (Figure 8). This feature clearly distinguishes this material condition from that after welding, where hardness along the weld thickness changes within 220 to 310 HV, which also determines individual usable properties of the joint depending on sequence of applying the weld layers. Structural examinations indicate that the above conclusions concerning hardness should be related to microstructure changes in representative areas of the joint. In the zones designated WM1, WM2 and WM3 in Figure 5, microstructure after welding was typical for diverse temperatures and cooling rates. Generally, the microstructure of the weld metal is of a dendritic nature and composes of bands of martensite and small quantity of bainite on the background of non-equilibrium ferrite grains with features of a Widmanstätten structure in the WM1 zone (Figure 10).

**Figure 9.** Microstructure of base material Hardox Extreme: (**a**) condition before welding—structure of tempered martensite; (**b**) condition after welding—BM zone in Figure 5, characteristic structure of tempered sorbite; (**c**) condition after heat treatment—BM zone in Figure 6, structure of hardening martensite. Light microscopy, etched with 2% HNO3.

**Figure 10.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked WM1 in Figure 5; (**b**) an enlarged image marked with frame in Figure 10a. Morphologically diverse structure composed of bands of martensite (M) with colonies of hardening troostite (T) on the background of non-equilibrium grains of ferrite. Light microscopy, etched with 2% HNO3.

Microstructure in the zone WM2 is composed of lath hardening martensite on the background of tempered sorbite, small quantity of bainite and colonies of troostite (Figure 11). Microstructure in the transition zone between the weld layers 1 and 2 (WM3 in Figure 5) is composed mostly of hardening sorbite with band-like martensite, hardening bainite and colonies of troostite (Figure 12). It is also worth to indicate the significant change of structure in the zone WM3, which can determine general impact strength of the entire welded joint. Thus, from the viewpoint of fracture mechanics, the microstructure of the transition zone between the weld layers 1 and 2 (Figure 2) should be taken into account during the selection of technology and welding parameters for this type of metallic materials.

**Figure 11.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked WM2 in Figure 5; (**b**) an enlarged image marked with frame in Figure 11a. Strongly diversified structure with dendritic nature, composed of bands of martensite (M) and bainite (B) and few colonies of troostite (T) on the grounds of hardening sorbite. Light microscopy, etched with 2% HNO3.

**Figure 12.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked WM3 in Figure 5; (**b**) an enlarged image marked with frame in Figure 12a. Clearly visible, morphologically diversified fusion lines 1 and 2 composed of bands of martensite (M) and bainite (B) and colonies of troostite (T) on the background of hardening sorbite. Light microscopy, etched with 2% HNO3.

After heat treatment, in all zones of the additional material (WM1, WM2 and WM3 in Figure 6), very similar microstructures (corresponding with that of as-delivered base material) were obtained, composed of fine-lath hardening martensite (Figures 13–15). With regard to the applied technological operations and their parameters, a remainder of band-like structure after welding can be observed in the recorded microstructures. Nevertheless, in the context of industrial applications, its complete removal would not be technologically and economically justified. However, it is worth mentioning that the applied heat treatment practically eliminated the observed structure changes in the zone marked WM3 in Figure 15.

**Figure 13.** Microstructure of Hardox Extreme welded joint after heat treatment: (**a**) in the area marked WM1 in Figure 6; (**b**) an enlarged image marked with frame in Figure 13a. Structure of fine-lath hardening martensite with clear banding features resulting from former dendritic structure. Light microscopy, etched with 2% HNO3.

**Figure 14.** Microstructure of Hardox Extreme welded joint after heat treatment: (**a**) in the area marked WM2 in Figure 6; (**b**) an enlarged image marked with frame in Figure 14a. Structure of fine-lath hardening martensite. Light microscopy, etched with 2% HNO3.

**Figure 15.** Microstructure of Hardox Extreme welded joint after heat treatment: (**a**) in the area marked WM3 in Figure 6; (**b**) an enlarged image marked with frame in Figure 15a. Structure of fine-lath hardening martensite with weak banding features resulting from former dendritic structure. Faintly outlined fusion line between weld layers 1 and 2 (FL) is marked with arrows. Light microscopy, etched with 2% HNO3.

Observations of microstructures in the fusion zones indicate a very wide range of their kinds and morphologies. It can be generally found that the zones marked FZ1 and FZ2 in Figure 5 are characterized by a very clearly outlined fusion line (FL in Figures 16 and 17) composed of coarse-lath hardening martensite, areas of upper bainite and non-equilibrium grains of ferrite, and also hardening sorbite and troostite (Figures 16 and 17). In addition, the recorded microstructures are characterized by strongly heterogeneous morphology, even within the same type of structure. It should be stressed that creation of a complete characteristic of structures in the considered Hardox Extreme welded joint requires examinations with the use of transmission electron microscopy (TEM) that is currently being carried-out.

**Figure 16.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked FZ1 in Figure 5; (**b**) an enlarged image marked with frame in Figure 16a. Very clearly outlined fusion line (FL) with strongly diversified microstructure. Visible islands of hardening martensite (M) and bainite (B), troostite areas (T) and acicular ferrite (F). Light microscopy, etched with 2% HNO3.

**Figure 17.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked FZ2 in Figure 5; (**b**) an enlarged image marked with frame in Figure 17a. Very clearly outlined fusion line (FL) with strongly diversified microstructure. Visible islands of hardening martensite (M) and bainite (B), small number of troostite colonies (T) and acicular ferrite (F). Light microscopy, etched with 2% HNO3.

The performed heat treatment operations led to homogeneous structure in the fusion zones (FZ1 and FZ2 in Figure 6), with respect to both type and fineness. In both considered fusion zones, structures of fine-lath hardening martensite were observed on the very weakly outlined fusion line (Figures 18 and 19). The above findings are also confirmed by hardness measurements (Figure 7) that do not show clear differences in comparison to the joint after welding.

**Figure 18.** Microstructure of Hardox Extreme welded joint after heat treatment: (**a**) in the area marked FZ1 in Figure 6; (**b**) an enlarged image marked with frame in Figure 18a. Structure of fine-lath hardening martensite. The arrows (FL) indicate the very weakly outlined fusion line. Light microscopy, etched with 2% HNO3.

**Figure 19.** Microstructure of Hardox Extreme welded joint after heat treatment: (**a**) in the area marked FZ2 in Figure 6; (**b**) an enlarged image marked with frame in Figure 19a. Structure of fine-lath hardening martensite. The arrows (FL) indicate the very weakly outlined fusion line. Light microscopy, etched with 2% HNO3.

Analysis of the other characteristic areas of heat-affected zone in the condition after welding, i.e., CGHAZ, FGHAZ and ICHAZ (marked in Figure 5), indicates much diversified structures in each case, resulting in significantly varying hardness levels. In the coarse-grained heat-affected zone (CGHAZ in Figure 20), fine- and coarse-lath hardening martensite is observed, with bainite separated on grain boundaries of former austenite, and also small quantity of hardening sorbite. In the fine-grained heat-affected zone (FGHAZ in Figure 21), microstructure includes mostly fine-lath hardening martensite with colonies of troostite and small number of bainitic areas. The intercritical heat-affected zone (ICHAZ in Figure 22) is composed of very fine, band-like arranged martensite with tempered sorbite. The morphology of this zone is clearly different than that of the base material, composed of fine-lath martensite (left side in Figure 22a,b).

**Figure 20.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked CGHAZ in Figure 5; (**b**) an enlarged image marked with frame in Figure 20a. Lath-like and locally acicular martensitic structure with bainite (B) on grain boundaries of former austenite. Light microscopy, etched with 2% HNO3.

**Figure 21.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked FGHAZ in Figure 5; (**b**) an enlarged image marked with frame in Figure 21a. Structure of fine-lath hardening martensite with inclusions of bainite (B) and troostite (T). Light microscopy, etched with 2% HNO3.

**Figure 22.** Microstructure of Hardox Extreme welded joint after welding: (**a**) in the area marked ICHAZ in Figure 5; (**b**) an enlarged image marked with frame in Figure 22a. Structure of fine-lath band-like arranged hardening martensite (M) with hardening sorbite (S). Light microscopy, etched with 2% HNO3.

The performed heat treatment operations of the joint resulted in uniform microstructure in the entire heat-affected zone. The microstructure of this zone (HAZ in Figure 6) is composed of fine-lath hardening martensite with maintained banding features resulting from the process of thermo-mechanical rolling of Hardox sheet metal (Figure 23). As such, the hardness course in this zone is nearly linear at the average level of 650 HV (Figure 7). It can be stated on this ground that heat treatment of the joint brought the structure of heat-affected zone to the structure of the base material in the as-delivered condition.

**Figure 23.** Microstructure of Hardox Extreme welded joint after heat treatment: (**a**) in the area marked HAZ in Figure 6; (**b**) an enlarged image marked with frame in Figure 23a. Structure of fine-lath hardening martensite. Light microscopy, etched with 2% HNO3.
