# MAG Welding Tests of Modern High Strength Steels with Minimum Yield Strength of 700 MPa
## 2 Materials and Methods  
The base materials used in this study comprised three different modern HSS produced by SSAB. The chemical composition of the S700 MC-1 and the S690 QL will be disclosed, but the S700 MC-2 is a recent tentative development from the S700 MC-1, and thus its chemical composition is confidential, although its carbon equivalent will be disclosed. Anyway, including this new steel in the research results emphasizes how small adjustments in the micro-alloying elements, which results in some important differences in mechanical performance. The S700 MC-1 and S700 MC-2 steels were produced by TMCP meeting the requirements set in the standard EN 10149-2 [22]. The chemical composition of the S700 MC-1 is [max. wt.%] 0.12 C; 0.25 Si; 2.1 Mn; 0.015 (min.) Al; 0.01 S; 0.02 P; 0.09 Nb; 0.15 Ti; 0.2 V (note: sum of Nb + Ti + V \(\leq\) 0.22 wt.%). The carbon equivalent CEV (also named CE\({}_{\rm{IIW}}\)[23]) of the S700 MC-1 is CEV = 0.37. The S700 MC-2 is known to be similar as in the S700 MC-1, with small adjustments in contents of micro-alloying elements, namely with higher Cr, resulting in a CEV = 0.44. A third of the test materials was the S690 QL with chemical composition [max. wt.%] 0.2 C; 0.6 Si; 1.6 Mn; 0.8 Cr; 0.01 S; 0.02 P; 2.0 Ni; 0.3 Cu; 0.7 Mo; and 0.005 B. The S690 QL was produced by quenching and tempering (Q&T) method fulfilling the requirements in the standard EN 10025-6:2004 + A1 [24]. The CEV of the S690 QL is CEV = 0.42. The plate thickness of all base materials was 8 mm. Figure 1 depicts the microstructure of the base materials. Figure 1: Optical (above) and SEM (below) micrographs of microstructures of the three base materials. The microstructures of TMCP (MC) steels are fairly similar between each other, consisting of fine ferritic grains with small amount of second phase constituents at the grain boundaries. Deformation from the rolling can be seen as elongated grains. Base material of Q&T steel consists mainly of tempered martensite. The base materials were welded using a Kemppi Promig 530 automatized system with matching 1 mm in diameter Bohler X 70-IG wire (EN ISO 16834-A: G 69 5 M Mn3 Ni1 CrMo). The shielding gas was MISON 8 (Ar + 8% CO\({}_{2}\) + 0.03% NO) with a flow of 19 L/min. The groove was V-shaped with 50\({}^{\circ}\) angle as shown in Figure 2. Two weld passes were applied in flat welding position, with no backing bar. The weld bead length was 1000 mm deposited parallel to the final rolling direction, joining plates 200 mm wide, before welding. The stand-off distance was about 15 mm in all welds. The root pass was always made with the same heat input: 0.7 kJ/mm (I = 253 A DC+; V = 25.8 V; v = 560 mm/min). A second pass was made with four different values of heat inputs, namely: 0.7 kJ/mm (I = 268 A DC+; V = 27.8 V; v = 638 mm/min); 1.0 kJ/mm (I = 270 A DC+; V = 28.6 V; v = 463 mm/min); 1.2 kJ/mm (I = 269 A DC+; V = 29.0 V; v = 390 mm/min); and 1.4 kJ/mm (I = 271 A DC+; V = 29.0 V; v = 336 mm/min). The heat input is calculated according to EN ISO 1011-1 considering a thermal efficiency of 0.8 (MAG welding process). These four different heat input values resulted in four different cooling rates, expressed as t\({}_{8/5}\) [s] cooling times and estimated analytically based on three-dimensional heat conduction according to guidelines established in [25]. The four different t\({}_{8/5}\) values obtained were: 5 s; 10 s; 15 s; and 20 s, respectively. Figure 2 presents the joint design and welding sequence schematically. Tensile tests were primarily implemented using an MTS 1200 kN testing machine. The preparation of the specimens followed the standard SFS-EN ISO 4136 [26]. Gauge length L\({}_{0}\) of 70 mm was used. As the welding direction was parallel to the final rolling direction, tensile tests of the welds were carried out is transversal direction in relation to the rolling direction. Testing temperature was the ambient room temperature.