# Numerical Simulation Study on Welding Process of Upper Frame of Hydropower Unit
## 3 Simulation Analysis of T-Joint Welding Based on TEPM
### Welding Process Impact Analysis and Optimization
#### 3.3.2 Analysis of Influence of Interlayer Cooling Time  
Based on the analysis that the optimal welding speed for the first two welds is 5.5 mm/s, the influence of interlayer cooling time on the overall welding result is analyzed. Simulation calculations were carried out for five cases of interlayer cooling time: 0 s, 30 s, 60 s, 90 s, and 120 s, respectively.  
When the interlayer cooling time is determined, the temperature between the layers before the final weld reaches its maximum value, as shown in Figure 15. According to the calculation results, the maximum temperature before the last weld with five different cooling times is 1090.6 \({}^{\circ}\)C, 286.9 \({}^{\circ}\)C, 220.2 \({}^{\circ}\)C, 187.5 \({}^{\circ}\)C, and 167.8 \({}^{\circ}\)C, respectively. In the welding manufacturing standard for the upper frame of the hydropower unit, it is stipulated that the interlayer temperature of the weld seam cannot exceed 200 \({}^{\circ}\)C during welding. This requirement can only be met when the interlayer cooling time is greater than 90 s.  
Figure 14: \(Z\)-direction deformation.  
Figure 13: The calculation results of the Z-displacement. (**a**) v = 5 mm/s; (**b**) v = 5.5 mm/s; (**c**) v = 6.67 mm/s; and (**d**) v = 8 mm/s.  
A longer interlayer cooling time is conducive to the uniform distribution of the weld temperature field after welding, and the final residual stress distribution cloud diagram calculated for different interlayer cooling times is shown in Figure 16.  
Different interlayer cooling times did not change the distribution trend of residual stress. The maximum equivalent residual stress obtained by simulation was 736.19 MPa, 701.67 MPa, 687.78 MPa, 684.30 MPa, and 688.85 MPa, respectively. The final residual stress of the specimen was higher when the interlayer cooling time was short. This may be because with shorter cooling times, the interlayer temperature of the weld is higher, and the material remains in the high-temperature region for a longer period, which leads to a more thorough transformation process from austenite to bainite and martensite. When the interlayer cooling time is 90 s, the maximum residual stress is reduced by 51.89 MPa compared to 0 s.  
Figure 17 shows the Z-direction deformation of the specimen's flange bottom perpendicular to the welding direction centerline under different interlayer cooling times. With the increase in interlayer cooling time, the maximum Z-direction deformation slightly increases, but the variation is not significant.  
The effects of different interlayer cooling times on welding temperature field, residual stress, and deformation are analyzed comprehensively. When the interlayer cooling time is 90 s, the welding process requirements can be met, and the residual stress and deformation after welding are small. Compared with the 120 s interlayer cooling time, the 90 s interlayer cooling time is more conducive to improving the manufacturing efficiency of welds in actual welding operations.  
Figure 15: Temperature field cloud diagram. (**a**) Cooling time is 0 s; (**b**) cooling time is 30 s; (**c**) cooling time is 60 s; (**d**) cooling time is 90 s; and (**e**) cooling time is 0 s.