**4. Summary**

It can be said on the grounds of the carried-out examinations that, in spite of limited weldability declared in the manufacturer's information materials, elements of the martensitic abrasive-wear resistant steel Hardox Extreme can be joined by welding techniques. It was shown in this elaboration that, through a proper selection of technology and welding parameters, it is possible to obtain an imperfection-free welded joint that is also characterized by very favorable strength indices. The above-mentioned assumptions were realized by proper selection of technology and welding parameters, as well as by use of additional heat treatment after welding, composed of normalization followed by volumetric quenching in oil and low tempering (stress-relieving). In the considered case, the applied thermal operations made it possible to restore, in the entire area of the welded joint, a structure and—to a large degree—a hardness level similar to those of the base material. The presented statement was positively verified in course of the carried-out strength and impact testing. The problems of execution and heat treatment of welded joints made of Hardox Extreme sheet metal can be characterized in the following ways:


It is worth noting that the brittleness threshold of constructional materials is accepted as impact strength of 35 J/cm<sup>2</sup> [30], which results from maintaining at least 50% share of ductile fracture. Therefore, the obtained impact strength value after heat treatment and results of fractographic analysis make it possible to conclude that there was an occurrence of a favorable "shift" of plastic properties of the welded joint beyond the accepted brittleness threshold.

Irrespective of the conclusions formulated above, it should be also mentioned that the examination results of the steel Hardox Extreme, presented in this paper, constitute a fragment of the cycle of research works concerning low-alloyed martensitic, abrasive-wear resistant steels with an addition of boron, realized by the author for several years. Thus, it can be stated that problems of weldability, announced before, are not limited to the steel Hardox Extreme only, but concern almost the entire considered group of materials. The materials already considered by the author include the steels: Hardox 400, Hardox 500, Hardox 600, HTK700H, HTK900H, AR400, XAR 600, Creusabro 4800, Creusabro 8000, TBL Plus, B27, Brinar 400, Brinar 500, and others. In almost all of the considered cases, problems with welding of these steels were observed, although the manufacturers declared their weldability. Therefore, attempts to weld high-strength abrasive-wear resistant steels with subsequent post-weld heat treatment seem to be very well grounded. In addition, it is also worth considering the application of advanced methods of hybrid welding and dedicated induction heating stations for welding and heat treatment of the above-mentioned materials. Optimum usage of this type infrastructure would make it possible to apply welding techniques for joining high-strength abrasive-wear resistant martensitic steels, while maintaining their very profitable mechanical properties and usable features.

**Funding:** This research received no external funding.

**Acknowledgments:** The author would like to thank Józef Ptak from Stal-Hurt S.C. company for providing sheets of Hardox steels and for Zbigniew Konat and Eugeniusz Szymanowicz from the Wroclaw Shipyard for assistance in the implementation of welded joints.

**Conflicts of Interest:** The author declares no conflict of interest.
