**1. Introduction**

During recent years, metallic materials classified by their manufacturers as low-alloyed abrasive-wear resistant martensitic steels, have been more and more widely applied. Regardless of their declared abrasion resistance, the common features of all these materials are very high mechanical parameters, which are maintained even for very thick steel sheet material. This feature is obtained with strictly selected chemical compositions depending on sheet thickness (in particular, the microaddition of 0.002% to 0.005% of boron), a reduced content of harmful admixtures of phosphorus and sulfur, and also by thermo-mechanical treatment. Results from generally available advertising information [1] and the author's own experience suggest that the currently available commercial steels of the considered material group reach a tensile strength exceeding 2000 MPa, with maintained satisfactory plasticity and impact strength. It is also worth stressing that these indices are obtained for carbon content up to 0.50 wt%, which is of crucial importance from the viewpoint of welding techniques. Tables 1 and 2 show properties of selected abrasive-wear resistant steels, which are declared by their manufacturers, and chemical compositions of these steels. In Table 2, in addition to the most commonly used *CEV* carbon equivalent (determining the metallurgical weldability of steel according to the International Institute of Welding), the carbon equivalent *CET* is also given. This indicator, according to the SS-EN 1011-2 standard, determines the preheating temperature to avoid hydrogen cracking of welded joints of fine-grained, non-alloyed, and low-alloy steels.

Information from manufacturers [1–5], literature data [6–9] and our own results [10–15] concerning the steels Hardox 400 and Hardox 500 confirm their good weldability and relatively high mechanical properties of welded joints. However, in each of the considered cases, thermal processes occurring during welding caused adverse structural changes in heat-affected zones of the welded materials, resulting in a significant reduction of their abrasive-wear resistance. Results of some research works [16]

suggest that changes of structure and hardness distribution occurring in welded joints of low-alloyed martensitic steels, adverse from the viewpoint of abrasive-wear resistance, can be eliminated by the application of additional heat-treatment operations. With regard to this, the author believes that it is worth complementing knowledge about execution and optimization of properties of welded joints of Hardox Extreme steels. In the previously published examination results of Hardox 600 [17], the authors indicated that it was possible to obtain static tensile strength of welded joints over 1500 MPa by proper selection of welding conditions and parameters, followed by suitable heat treatment.


**Table 1.** Declared mechanical properties of selected abrasive-wear resistant steels [1–5].

*R*p0.2—yield strength, *R*m—ultimate tensile strength, *A*5—percentage elongation after fracture for proportional specimens with the original gauge length *L*<sup>0</sup> equal to 5 times diameter, *KCV*−40—Charpy V-notch toughness at −40 ◦C, *HBW*—Brinell hardness, NA—no available data.

**Table 2.** Chemical compositions and declared carbon equivalents of selected abrasive-wear resistant steels [1–5].


*CEV*T—typical carbon equivalent according to International Institute of Welding, *CET*T—typical carbon equivalent according to SS-EN 1011-2, #—sheet thickness for the given chemical properties, NA—no available data.

Development of the subject matter concerning welding of high-strength, abrasive-wear resistant martensitic steels seems to also be well-grounded in the context of numerous negative opinions of users of these steels, undertaking technological activities similar to these discussed here. These opinions predominantly result from big discrepancies between the data published in material data sheets and the results of our own examinations. The carbon equivalent value given by manufacturers is most often a typical value that is significantly different from that calculated on the grounds of real chemical composition of the given grade and sheet thickness. For this reason, activities undertaken by the users on the grounds of inaccurate data often do not provide positive results. This is why, in most cases, the use of the considered grades are given up in favor to their equivalents with much lower mechanical properties, but which are characterized by better carbon equivalents.

The performed examinations concerning the chemical and structural properties of low-alloyed abrasive-wear resistant steels [16,18–26] make it possible to formulate a general statement with regard to the good weldability of steels Hardox 400 and 450, as well as the satisfactory weldability of Hardox 500. This standpoint is also confirmed by the location of chemical compositions of these steels in the diagram *C-CEV* (Figure 1) close to the zones of low (I) or conditions-dependent (II) susceptibility to cracking. However, in the case of Hardox Extreme, this statement does not seem to be well-grounded (Table 3 and Figure 1) because of big discrepancies between the manufacturer's data (designation "P" in Figure 1) and own results (designation "O" in Figure 1). The discrepancy also concerns Hardox 600.


**Table 3.** Real chemical compositions and carbon equivalent values calculated on their grounds for sheet metal 8–15 mm thick of selected low-alloyed steels [18,24].

*CEV* [%] = C + Mn/6 + (Cr + Mo + V)/5 + (Cu + Ni)/15; *CET* [%] = C + (Mn + Mo)/10 + (Cr + Cu)/20 + Ni/40, *CEV*—carbon equivalent according to International Institute of Welding, *CET*—carbon equivalent according to SS-EN 1011-2, #—sheet thickness for the given chemical properties.

**Figure 1.** Susceptibility to cracking in function of carbon content and *CEV* of selected abrasive-wear resistant steels. H—Hardox, BR—Brinar, P—manufacturer's data, O—own results. Development based on data from Table 3 and [27].

The above-described discrepancies decidedly provide evidence against these steels with regard to their metallurgical weldability, shifting *CEV* values from the zone (II) to the zone (III) of high susceptibility to cracking in any welding conditions. According to users and manufacturers of abrasive-wear resistant steels, the most often indicated problems of weldability of Hardox Extreme (and also Hardox 600) are related to the susceptibility of the made welds to the brittle cracking (including also delayed cracking) and very wide zones with lower hardness in comparison to the base material. In this connection, the purpose of the presented research work is the identification of macroand microscopic structures of Hardox Extreme welded joints in the as-welded condition (after welding) and determination of the area of structural changes within the entire welded joint, as well as provoking, through heat treatment, structural changes in order to eliminate or minimize the previously existing adverse structures. It is worth mentioninging that similar examinations of Hardox 600 were also carried out. However, with regard to the very comprehensive set of results, they will be elaborated on separately. It should be also mentioned that the author is currently conducting examinations of Hardox welded joints with regard to their abrasive wear in real conditions of soil abrasive mass.
