## 1 Introduction
Mechanical characteristics of the HAZ of the HSLA steel weld were dominant to both, resulting in mechanical characteristics and mechanical behavior of the welds during exploitation [13; 14; 15]. Mechanical characteristics, in correlation with evaluation of microstructures in specific zones of welds, as well as analysis of the microhardness distribution, are the basis for design of welded structuresmade of HSLA steels. The full advantages provided by the related material usage can be achieved only by this design perspective [14; 15]. Influence of the HAZ characteristics on overall characteristics of the welds of low-carbon steels is analyzed in [16]. That zone is characterized by altered microstructures and high levels of residual stresses. The authors concluded that rough grains of microstructure appearedwithintensive heat input in the HAZ, whichthen caused lower values of hardness. Authors in [17] studied the influence of the welding technology, byexecuting a \"single V\" butt multiple-pass welded joint, on impact toughness of welded joints of high-strength low-alloyed steels. It should be noted that a much more detailed literature review on problems related to welding of the HSLA steels is presented in that article. Based on that review, the authors concluded that the contemporary referent literature sources discuss the impact only of certain parameters on the microstructure and in certain zones of the weld and that it is necessary to establish the correlation between the microstructure parameters and mechanical characteristics of the welded joints. Authors of [18] were considering using different cutting techniques for HSLA steels. They presented results of the impact of the cutting technology on changes of hardness in the cutting zone of Hardox 400 and 450 steels. Influence of the welded joints' quality on safety and reliability in operation was studied in [19]. The poorly executed welded joint of steels can be causes of fracture and even crashes of plants. Authors observed various defects, like pores, blowholes, slags, lack of fusion, non-plastic non-metallic particles, cracks and shrink holes in the welded joint area, which resulted in fracture of rails and pipes made of HSLA steels. A trend in the automotive industry for reducing the mass of cars and semi-trailers was considered in [20], where an important factor is guaranteed safety of use and safe operation life of such vehicles. The authors presented results of fatigue tests in the range of low and high cycle fatigue of wear resistant quenched and tempered martensitic Hardox steel 400 and 450 steels and discussed the influence of the grain size on the shape of fatigue curves and compared those results to fatigue tests of S355J2 structural steel results. In [21] are presented selected experimental results of investigation of the fatigue resistance of structural steels. The authors performed experiments on nine structural steels including high strength steels, DOMEX 700 MC, HARDOX 400, HARDOX 450 and 100 Cr6 (SSAB, Oxelosund, Sweden) (UTS from 446 MPa to 2462 MPa) at high-frequency cyclic loading (f = 20 kHz, T = 20 \(\pm\) 5 \({}^{\circ}\)C, R = -1) in the range of the loading cycle numbers from \(2\times 10^{6}\) to \(2\times 10^{9}\). The continuous decrease of fatigue strength in terms of the increasing number of loading cycles was observed. Since the cooling rate is variable and decreases with the temperature decrease, the cooling time from 800 to 500 \({}^{\circ}\)C (ts\({}_{85}\)) is usually taken as the parameter that best characterizes the cooling conditions of the HAZ, with respect to the lowest stability of austenite of the majority of steels. The cooling time, in that temperature range, can be calculated according to different expressions, as well as determined experimentally. Knowing that cooling time and relevant material properties enables prediction of characteristics of the welded joint and the heat affected zone [22; 23; 24].