# MAG Welding Tests of Modern High Strength Steels with Minimum Yield Strength of 700 MPa
## 3 Results and Discussion
### Microstructural Analysis  
SEM-images were taken from the different zones at the HAZ as presented in Figure 13. For each material, specimens with the lowest (t\({}_{8/5}\) = 5 s) and the highest (t\({}_{8/5}\) = 20 s) heat inputs were analyzed. The location A, B and C represent the CGHAZ at the root side, at the center, and at the face side of the plate and they are located at about 0.5 mm from the fusion line. Face/root side location is defined at about 1.5 mm from the top/bottom surface. Location D represents a recrystallized zone at the center of the plate, which is located at about 1.5 mm from the fusion line in low heat input weld (t\({}_{8/5}\) = 5 s) and in high heat input weld (t\({}_{8/5}\) = 20 s) about 2.5 mm from the fusion line. The images of all three materials of these locations for the heat inputs t\({}_{8/5}\) = 5 s and t\({}_{8/5}\) = 20 s are presented in Figures 14-19. Figure 14: SEM images from low heat input (t\({}_{8/5}\) = 5 s) weld HAZ for steel S700 MC-1. (**A–D**) are locations established in Figure 13.  
Figure 13: Locations of the SEM images in the cross section of weld specimens: (**a**) low heat input, t\({}_{8/5}\) = 5 s; (**b**) high heat input, t\({}_{8/5}\) = 20 s.  
Figure 16: SEM images from low heat input weld (\(\rm{t_{8/5}}\) = 5 s) HAZ for steel S700 MC-2. (**A**–**D**) are locations established in Figure 13.  
Figure 15: SEM images from high heat input (\(\rm{t_{8/5}}\) = 20 s) weld HAZ for steel S700 MC-1. (**A**–**D**) are locations established in Figure 13.  
Figure 17: SEM images from high heat input weld (t\({}_{8/5}\) = 20 s) HAZ for steel S700 MC-2. (**A**–**D**) are locations established in Figure 13.  
Figure 14:0 SEM images from low heat input weld (t\({}_{8/5}\) = 5 s) HAZ for steel S690 QL. (**A**–**D**) are locations established in Figure 14:1  
According to Figures 14-19, all three materials present a somewhat different microstructural behaviour. A brief comparison and overview about the essential observations from each material based on the previous SEM images are presented below:  
* S700 MC-1 has the lowest tendency to form martensitic/bainitic phases. Only in location C, with low heat input (high cooling rate), structure consisting of laths can be observed. In location A (and less but also in B), there is a reheated effect of the second weld pass over the first weld pass. In S700 MC-1 with high heat input, significant grain coarsening can be observed, also at the location D, deteriorating the mechanical properties. Titanium nitrides in sizes of a few micrometers are commonly found in S700 MC-1 because of the higher titanium content than in the other materials. Although not experimentally confirmed, in Figure 15 (position A), an arrow was positioned to identify possible titanium nitride (TiN). The dissolution temperature of titanium nitrides is high and therefore they can be found also in the vicinity of the weld. Because of a sharp-edged form, mechanical properties like fatigue behavior or impact toughness can be impaired by TiN:s as reported by [PERSON] et al. [33]. * S700 MC-2 have showed significantly higher tendency to form hard phases at the HAZ compared to S700 MC-1. Also with high heat input at locations B and C, bainitic structure can be seen. In addition, the tendency to form islands of a second phase is considerably higher than in S700 MC-1 as emphasized by the arrows in (position D) of Figures 16 and 17. With higher heat input, the size of these islands increases clearly, which can be the reason for clear decrease in measured impact toughness values among increasing heat input, noticed at the FL + 1 mm. * The influence of higher carbon content of S690 QL can be seen in the images. The material tends to form martensite when cooling rate is high. In addition, coarse and brittle bainitic structures can be observed at the coarse grained zones of high heat input weld. Figure 20 depicts with high magnification a detail of the location D where plenty of carbides can be seen.