# High-Speed Welding of Stainless Steel with Additional Compensatory Gas Jet Blow Molten Pool
## 4 Results and Discussion
### Analysis of Microstructure  
The microstructure of a high speed butt weld is closely related to the mechanical properties of the weld. Segments with and without compensatory jets impinging on the molten pools were selected, cut as samples, and ground. The bright sides of the samples were corroded by concentrated hydrochloric acid (HCl) and concentrated nitric acid (HNO\({}_{3}\)) at a volume ratio of 3:1. The butt weld of sample 1# and 2# was selected form Figures 5 and 6 for microstructure analysis as shown in Figure 7.  
In Figure 7, the molten depth varies significantly in the cross-sections and at the bottom of the fusion zone. With otherwise equivalent parameters, the difference is caused by the presence or absence of the gas jet impinging on the molten pool. The non-blown weld has only approximately 2/3 penetration, while the penetration of the blown weld is improved greatly. The penetration of sample #1 is better than #2 due to more heat input. The fusion line in the weld toe zone is steep in the weld made with no gas jet, and the edge line of the weld above the parent material has a large angle with the parent material. However, in the weld toe zone of the weld made with the gas jet,  
Figure 6: Weld bead shape of sample 2#: (**a**) Front view; (**b**) back view. the welding fusion line is flat, the angle between the weld edge line and the parent material is smaller, and the fusion line is wider compared with the weld toe zone of the weld made with no gas jet. The top of the fusion zone with no gas jet shows thick dendrites with a fan-shaped distribution; a zone of concentrated impurities near the surface is obvious. The microstructure of the fusion zone of sample 1# and 2# is similar. There are almost no defects such as pores and cracks in the fusion zone of both sample 1# and 2#. The top of the fusion zone of the weld made with a gas jet shows a broad area of fine equiaxed grains in the near-surface area. In the lower part of this area, dispersed columnar crystals appear with grains smaller than those seen in top of the fusion zone of the weld made with no gas jet. The middle of the fusion zone without compensation and middle of the fusion zone with compensation show the structures in the middles of the welds. Without compensation, thick dendrites are intermixed with short columnar crystals, while with compensation, relatively fine columnar grains form a staggered distribution. At the bottom of the fusion zone of the weld made with and without compensation show significant differences. The compensatory weld is well fused with the parent material, and crystallization is staggered in the middle. The weld without the compensatory gas jet shows fan-shaped crystallization that grows to the top. These results show that the compensatory argon gas jet can blow on and stir the molten pool, with heat-transfer effects on the molten pool, parent material, and welding zone. This helps the weld to dissipate heat quickly, thus forming fine and uniform grains. However, as the weld without the compensatory argon jet is cooled naturally without external force, thick and directional dendrites grow along a fixed direction. The protective gas jet impacts the weld metal of the unsolidified molten pool in a vertical direction, so that the hump bead metal periodically accumulated in the original natural form is spread around the periphery under the influence of the jet impact force before solidification. Macroscopically, the jet impact force makes the hump and trough disappear, the melting width significantly increase, the depth of fusion slightly increase, and the excessive weld metal reduce. Microscopically, since the jet stirs the molten pool, the conditions for the formation of coarse dendrites during the cooling process are interrupted, a large number of equiaxed crystals appear, and the crystals are thereby refined. Crystallization in the center of the weld is omnidirectional; the top crystalline impurity layer is thinned, as is the grain, reducing the possibility of cracks on the surface of the weld.