Zhuojin Pipe Fitting Co., LtdDesign and Application of Tees in Transformer Connecting Pipe Welding
Literature Overview
This paper by Ping Yumin from Baoding Tianwei Baobian Electric Co., Ltd., published in Transformer (2013, Vol. 50, No. 7, pp. 21-23), addresses the design and application of tee fittings in the connecting pipe welding of electrical transformers. Transformer connecting pipes are critical components that carry insulating oil between different parts of the transformer, and the integrity of the tee fittings is essential for preventing oil leakage and ensuring the reliable operation of the transformer. The paper describes the method of drawing tees directly on the main bus pipe and highlights the improvements in weld quality and oil leakage prevention achieved through this approach.
Core Technical Approach
The traditional approach to creating tee fittings in transformer connecting pipes involves welding a separate tee fitting to the main pipe. This method introduces additional weld joints and potential leakage points, which can compromise the integrity of the oil-containing system. The approach described in the paper involves drawing (or forming) the tee directly on the main bus pipe, eliminating the need for a separate tee fitting and reducing the number of weld joints.
Drawing Process for Tees
The drawing process involves the following steps:
| Step | Description | Key Parameters |
|---|---|---|
| 1. Marking | Mark the tee location and dimensions on the main pipe | Accuracy of ±1 mm |
| 2. Pre-heating | Pre-heat the drawing area to 300-400°C | Uniform temperature distribution |
| 3. Drawing | Use a forming tool to draw the tee shape from the main pipe | Controlled deformation rate |
| 4. Stress relief | Post-drawing stress relief at 400-500°C | Reduce residual stresses |
| 5. Welding | Weld the branch pipe to the drawn tee opening | Low-hydrogen welding procedure |
The pre-heating step is critical to prevent cracking during the drawing process. The pre-heat temperature of 300-400°C is sufficient to reduce the yield strength of the steel, allowing it to be deformed without excessive strain hardening or cracking. The uniform temperature distribution is essential to ensure consistent deformation and to avoid localized strain concentrations.
The drawing step involves the use of a forming tool to gradually deform the main pipe into the tee shape. The deformation rate must be controlled to avoid excessive strain hardening and to prevent cracking. The forming tool must be designed to distribute the deformation evenly across the drawing area, avoiding localized thinning or cracking.
The stress relief step is critical to reduce the residual stresses introduced during the drawing process. The stress relief temperature of 400-500°C is sufficient to reduce the residual stresses without causing significant microstructural changes or grain growth. The stress relief time should be proportional to the wall thickness, typically 1 hour per 25 mm of thickness.
The welding step involves welding the branch pipe to the drawn tee opening. The welding procedure must be optimized to minimize the heat input and to prevent cracking. Low-hydrogen welding consumables must be used, and the welding sequence must be balanced to minimize distortion. The welding heat input should be limited to prevent excessive grain growth and to maintain the mechanical properties of the base metal.
Weld Quality and Oil Leakage Prevention
The paper emphasizes that the drawing approach improves the weld quality and effectively controls oil leakage. The key factors contributing to this improvement are:
- Reduced number of weld joints: By drawing the tee directly on the main pipe, the number of weld joints is reduced, which reduces the potential for leakage.
- Improved weld geometry: The drawn tee provides a more favorable weld geometry, with a smoother transition between the main pipe and the branch pipe, which reduces stress concentrations and improves weld integrity.
- Controlled residual stresses: The stress relief step reduces the residual stresses introduced during the drawing process, which reduces the risk of cracking and leakage.
- Optimized welding procedure: The low-hydrogen welding procedure and balanced welding sequence minimize the risk of cracking and distortion, which improves the overall weld quality.
The improvement in weld quality is particularly important for transformer connecting pipes, which are subjected to cyclic thermal and mechanical loading during transformer operation. The reduced number of weld joints and the improved weld geometry reduce the risk of fatigue cracking and leakage, which is critical for the long-term reliability of the transformer.
Engineering Practice and Lessons Learned
The paper describes the results of multiple trials to optimize the drawing process and welding procedure. The key lessons learned from these trials are summarized below:
| Trial Parameter | Optimized Value | Rationale |
|---|---|---|
| Pre-heat temperature | 300-400°C | Reduce yield strength for controlled deformation |
| Drawing deformation rate | Controlled, gradual | Avoid excessive strain hardening |
| Stress relief temperature | 400-500°C | Reduce residual stresses without microstructural changes |
| Welding heat input | Limited to <20 kJ/mm | Prevent excessive grain growth |
| Welding sequence | Balanced, symmetric | Minimize distortion |
| Welding consumables | Low-hydrogen E7018 or equivalent | Prevent hydrogen-induced cracking |
The optimization of these parameters is critical for achieving the desired weld quality and oil leakage prevention. The pre-heat temperature must be carefully controlled to ensure that the steel is sufficiently soft for deformation but not so hot that it causes microstructural changes or oxidation. The drawing deformation rate must be gradual to avoid excessive strain hardening and cracking. The stress relief temperature must be sufficient to reduce residual stresses but not so high that it causes grain growth or phase transformations.
The welding heat input must be limited to prevent excessive grain growth in the heat-affected zone (HAZ). Excessive grain growth reduces the toughness of the HAZ and increases the risk of cracking. The welding sequence must be balanced and symmetric to minimize distortion, which can compromise the alignment of the branch pipe and the main pipe. The welding consumables must be low-hydrogen to prevent hydrogen-induced cracking, which is a common failure mode in transformer connecting pipe welds.
Key Questions and Reflections
The study raises several important questions for further consideration. First, the paper does not address the long-term performance of the drawn tees under cyclic thermal and mechanical loading. In transformer service, the connecting pipes are subjected to repeated thermal cycling and mechanical vibration, which can lead to fatigue cracking. The long-term fatigue life of the drawn tees should be evaluated through fatigue testing or accelerated life testing. Second, the paper does not discuss the effect of the drawing process on the microstructure and mechanical properties of the base metal. The drawing process introduces plastic deformation, which can alter the microstructure and mechanical properties of the steel. The effect of this deformation on the long-term performance of the tee should be evaluated through metallographic examination and mechanical testing. Third, the paper does not address the effect of the welding procedure on the microstructure and mechanical properties of the HAZ. The welding heat input and cooling rate can significantly affect the microstructure and mechanical properties of the HAZ, which can affect the long-term performance of the weld.
The approach of drawing tees directly on the main pipe is a significant improvement over the traditional method of welding separate tee fittings. However, the long-term performance of the drawn tees under cyclic loading and the effect of the drawing and welding processes on the microstructure and mechanical properties of the base metal and HAZ require further investigation. These aspects are critical for ensuring the long-term reliability of transformer connecting pipes.
Study Insights and Implications
This study provides a practical and effective approach for creating tee fittings in transformer connecting pipes. The drawing approach reduces the number of weld joints, improves the weld geometry, and effectively controls oil leakage. The optimization of the pre-heat temperature, drawing deformation rate, stress relief temperature, welding heat input, welding sequence, and welding consumables is critical for achieving the desired weld quality and long-term reliability.
For engineering practice, the study emphasizes the importance of optimizing the drawing and welding processes to ensure the long-term reliability of transformer connecting pipes. The approach of drawing tees directly on the main pipe is a significant improvement over the traditional method, but further investigation is needed to evaluate the long-term performance under cyclic loading and the effect of the drawing and welding processes on the microstructure and mechanical properties. The findings of this study should be incorporated into the design and manufacturing guidelines for transformer connecting pipes, and the optimized drawing and welding procedures should be implemented as standard practice to ensure the long-term reliability of these critical components.