# MAG Welding Tests of Modern High Strength Steels with Minimum Yield Strength of 700 MPa
## 1 Introduction  
The recent environmental regulations require structures with lighter weight and improved performance. Steel is one of the most important structural materials worldwide, and therefore the developments in structural steel have a global industrial impact. When producing durable structures from the steel, weldability is an important material property because most constructions require welding in some point of the production chain. Recently, the weldability of a high strength steel (HSS) has been improved based on the ability to achieve high yield strength also with low carbon content. In addition, enlightened usage of micro-alloying elements helps improving properties, including the impact toughness, of the welded components [1]. Still, the biggest challenge in welding of HSS isto maintain the good balance of strength and toughness after welding because the fusion welding induces high thermal gradients and severe microstructural changes in the weld zone. From the beginning of 1990s, the weldability of structural steels with tensile strength of more than 780 MPa has been researched widely [2]. One challenge is the excessive softening of the HAZ. [PERSON] et al. [3] researched the microstructures and mechanical properties of weld-repaired HSLA steel. The hardness of the base material (BM) ranged between 224 and 264 HV, whereas the hardness of the softest zone of the heat affected zone (HAZ) was approximately 150 HV [3], emphasizing a significant mismatch when comparing BM hardness to the minimum HAZ hardness. Depending on the composition and the initial microstructure of the material, different mechanisms can cause the softening. For example, [PERSON] et al. [4] studied the softening mechanisms during laser welding in dual-phase steels using Gleeble simulation. The peak temperatures reached in the softest HAZ were between Ac1 and Ac3 (730-870 \({}^{\circ}\)C in this case). Some softening occurred also in lower temperatures caused by the tempering of the martensite as the maximum temperature exceeded 250 \({}^{\circ}\)C [4]. In multi-pass welding complex, an HAZ configuration occurs based on overlapping regions [5]. Hence, intercritically reheated and tempered regions form and these regions may have completely different mechanical properties than unaltered HAZ regions. The softening can be reduced by inhibiting the excessive grain growth in the HAZ. It has been observed that grain coarsening and excessive softening in the heat-affected zone can be inhibited by the usage of micro-alloying elements like V, Nb or Ti and hence the weldability of HSS can be improved [1]. Though the effect of these elements in preventing grain growth depends significantly on the local temperature history as, e.g., the TiN precipitates are stable even at temperatures approaching 1300 \({}^{\circ}\)C, whereas the amount of Nb as NbC precipitates undergoes dissolution above 1000 \({}^{\circ}\)C [6; 7]. Nano-sized carbides in ferrite matrix formed by micro-alloying elements help avoiding excessive softening in the HAZ [8]. Welding methods, in which the heat input is restricted, may result in improved weld properties as reported by [PERSON] et al. [9]. [PERSON] et al. reported about fiber laser welding (10 kW) of direct quenched ultra-high strength steel (yield strength of 960 MPa). High quality welds were obtained with good impact toughness values up to \(-40\)\({}^{\circ}\)C [9]. The weld geometry and mechanical properties of the weld can also be improved by small modifications in the welding process, like utilizing activating fluxes during arc welding with solid wire. For example, [PERSON] [10] evaluated the effect of three different activating flux aided MIG/MAG (metal inert/active gas) on the weld joint characteristics for low carbon steel AISI 1020. The usage of the fluxes was beneficial for the mechanical properties of the weld and also angular distortion was reduced [10]. In addition to the softening, the loss of toughness properties in HAZ is characteristic for the welds of high strength steels.