# MAG Welding Tests of Modern High Strength Steels with Minimum Yield Strength of 700 MPa
## 4 Conclusions  
From the presented research work, the following conclusions can be drawn:  
* The welding tests showed that the properties of both S700 MC-1 and S700 MC-2 were the least deteriorated by welding. The Q&T S690 QL was the most prone for detrimental effects induced by welding. The most notable differences occurred in the impact toughness tests with the notch locations at FL + 1 mm and at FL + 3 mm. For example, at FL + 1 mm at \(-40\)\({}^{\circ}\)C, the S700 MC-2 has absorbed energies of 43 to 73 J. The values decrease almost linearly with the increasing heat input, whereas S690 QL has the energy of 62 J with the lowest heat input (t\({}_{8/5}\) = 5 s) and with three other higher heat inputs (t\({}_{8/5}\) = 10 s, 15 s and 20 s) energies of 22 to 24 J. These differences in toughness are eventually related with the fact that, even at relative low heat input welds, the S690 QL presented carbide concentration combined with coarse bainitic structures at FL + 1 mm, and both TMCP steels showed lower tendency to form martensitic/bainitic phases at the vicinity of the FL, although grain coarsening and formation of second phases and precipitates contribute to reduce their toughness; this only happens at the highest heat input level tested.
* In addition, differences between the Q&T and TMCP steels were observed in tensile tests: the welding decreased the elongation of all materials, but the elongation of S690 QL was more affected by increasing heat input than the elongation of TMCP steels.  
Figure 20: Carbide concentrations in detail from location D of the HAZ of S690 QL produced with the high heat input weld (t\({}_{8/5}\) = 20 s).  
* A decreasing trend in the yield strength exists along the weld line, from the start towards the end of the weld joint. This effect is more pronounced at the start and at the end of the joint.
* The combined results from the microstructural analysis, tensile tests with DIC measurements, and hardness profiles revealed that the most problematic domain in terms of strength/hardness is the recrystallized zone. When the higher heat input is used, the soft domain widens and becomes continuous through the plate thickness. In that case, the weakening effect is significant, as shown by the DIC measurements.
* According to the hardness profiles, the softening at the root side is more severe than the softening at the face side along increasing heat input. This corresponds also to the strength values at the face and at the root side as shown by bending tests. Bending tests indicated that, when the tensile stress was applied to the root side, more explicit sensitivity to increasing heat input was observed. This is explained by the accumulative thermal cycle effect of both passes on the microstructure at the root side. On the face side, it is mainly the second pass that causes the softening.  
[PERSON] carried out the experimental work. All authors provided critical feedback and helped shape the research, analysis and wrote manuscript. [PERSON] and [PERSON] supervised the project.  
This study was funded by Tekes/FIMECC and was a part of the project BSA SP2 P3. The experiments were conducted at both the research center of SSAB in Rahae, Finland and Aalto University in Espoo, Finland.  
No conflict of interest.