# High-Speed Welding of Stainless Steel with Additional Compensatory Gas Jet Blow Molten Pool
## 4 Results and Discussion
### Rockwell Hardness Test and Analysis  
Hardness testing samples were fabricated with reference to the national standard \"Welding Joint Hardness Test Method\" (GBT2654-2008), with polished weld surfaces. As shown in Figure 8, in the test a spherical indenter was adopted with a 150-kgf (1.47-kN) test force applied for 10 s. The center of the weld was set as the center of the test point and one-half of the molten pool depth was set as the test line of the parallel parent material. A test point was selected every 2 mm (seven total). Test points were located across the weld center, fusion zone, heat-affected area, and parent material. The test was conducted in accordance with the predetermined procedure, including the test point, indentation forming, indentation observation, and measurement. The hardness data is shown in Table 2 and the weld hardness curve comparison chart is shown in Figure 9.  
Complex processes such as the melting of the welding wire and parent material, heat cycling in the molten pool, and phase changes in the solidification process during welding, yield different organizational structures and characteristics among the welding metal, fusion zone, heat-affected area, and parent material. Among these differences, the effect of heat cycling causes grain coarsening in the heat-affected zone, thus rendering a heat-affected zone with relatively weak performance. In particular, because austenitic stainless steel has a low thermal conductivity and large linear thermal expansion coefficient, large residual stresses are easily formed in the heat-affected zone as the weld cools.  
In the central part, comprising the fusion zone and the heat-affected area, the hardness values of welds #1 and #2 with compensatory gas jets are all higher than those without the compensatory gas jet, the difference is greatest in the central part. This indicates that the compensatory jet impinging on the molten pool has a significant impact on hardness. It is possible that during solidification, blow by the jet stream causes violent movement in the molten pool that refines the grains.  
The hardness of the central part, fusion zone and heat-affected area on welds #1 and #2 with compensatory jets is higher than the hardness of without compensatory jets, and the difference of hardness in the central area is the biggest, which clearly shows the effect on hardness that compensatory jet flow shocking the molten pool has. This is because the molten pool is shocked by jet flow during  
Figure 8: Weld hardness test point.  
Figure 9: Comparison of hardness curves of welds formed with and without compensatory gas jets.  
solidification to generate dramatic movement and refine the grain. Hardness in the weld joint area is higher than that in other areas, and the maximum hardness is found near the fusion line at two sides of the weld joints (31-36 HRC). The weld made without the compensatory jet firstly forms a crystal nucleus in the fusion zone at the time of crystallization for the molten pool and grows towards the center of the molten pool in the shape of columnar crystal. During this process, the segregation function makes the central part of the weld joints gather more impurities, which makes hardness values in the central area of the weld joint lower than those at both sides (30 HRC). The location with the lowest hardness is near the fusion line of weld joints and parent metal.  
The results show two main findings: firstly, that airflow impacts the molten pool to produce a faster cooling rate; and secondly, the gas jet stirs the molten pool to refine the grain in each region. The finer the metal grain, the more grain boundaries there are, and since the grain boundary strength is greater than the intragranular strength, the finer the metal grain is, the greater the strength will be. Moreover, a large number of grain boundaries can also prevent the diffusion of cracks, so the finer the metal grain is, the better the toughness will be.