Zhuojin Pipe Fitting Co., LtdDeformation Analysis During Welding of Pressure Steel Pipes in Hydropower Projects
Literature Overview
This paper, published in Hot Working Technology (2006, Vol. 35, No. 3), authored by Chen Guoxin and colleagues from Shaoyang University, addresses a critical construction challenge encountered during the installation of pressure steel pipes at the Three Gorges Hydropower Station. The study focuses on the impact of eliminating expansion joints on the welding process and analyzes deformation patterns under two alternative installation schemes. The authors employ both empirical formulas and theoretical calculations to determine transverse and longitudinal shrinkage amounts during welding, ultimately recommending the optimal installation approach.
Core Technical Content
The fundamental problem addressed is how to manage welding-induced deformation in large-diameter pressure steel pipes when expansion joints are removed from the design. Expansion joints traditionally serve to accommodate thermal expansion and contraction, but their removal creates a rigid pipe system where welding deformation must be carefully controlled to avoid excessive residual stresses and geometric distortions.
The study compares two installation schemes:
| Parameter | Scheme A (with expansion joint) | Scheme B (without expansion joint) |
|---|---|---|
| Thermal accommodation | Yes, joint absorbs expansion | No, rigid system |
| Welding shrinkage control | Easier, joint compensates | Difficult, full restraint |
| Transverse shrinkage | Lower magnitude | Higher magnitude |
| Longitudinal shrinkage | Distributed | Concentrated at weld seams |
| Construction complexity | Higher (joint assembly) | Lower (continuous pipe) |
| Long-term reliability | Joint may leak over time | No joint failure risk |
Welding Deformation Mechanism Analysis
The deformation during welding of pressure steel pipes can be categorized into three primary types:
- Transverse shrinkage - This occurs perpendicular to the weld seam direction and results from the cooling contraction of the weld metal and heat-affected zone (HAZ). For large-diameter pipes with wall thicknesses typically ranging from 30 mm to 60 mm, transverse shrinkage can accumulate significantly across multiple circumferential welds.
- Longitudinal shrinkage - This occurs parallel to the weld seam and is caused by the restraint of the weld contraction by the surrounding base metal. In a rigid pipe system without expansion joints, longitudinal shrinkage forces are transmitted through the entire pipe length, creating substantial axial stresses.
- Angular distortion - For butt-welded joints, uneven cooling rates across the weld cross-section can produce angular distortion, particularly problematic in thick-walled pressure pipes.
The empirical formulas used in the study are based on the principle that welding shrinkage is proportional to the heat input per unit length and inversely proportional to the effective cross-sectional area of the restraint. The theoretical approach employs thermal stress-strain analysis considering the elastic-plastic behavior of the steel during the welding thermal cycle.
Engineering Practice Implications
For engineers working on large hydropower or water conveyance projects, several key takeaways emerge:
| Deformation Type | Typical Range (mm/m) | Countermeasure |
|---|---|---|
| Transverse shrinkage | 0.3-0.8 | Pre-fit with compensating gap; symmetric welding sequence |
| Longitudinal shrinkage | 0.5-1.5 | Allowance in fit-up; temporary restraining frames |
| Angular distortion | 0.1-0.3 | Multi-pass welding with balanced heat input |
The study's recommendation to remove expansion joints while managing deformation through careful welding sequence planning and fit-up allowance represents a practical engineering trade-off. The elimination of expansion joints reduces long-term maintenance concerns but demands more rigorous deformation control during construction.
Key Technical Insights
The paper demonstrates that welding sequence significantly influences the final deformation state. A symmetric welding approach, starting from the center and proceeding outward in both directions, minimizes cumulative distortion. For circumferential welds on large-diameter pipes, the use of multiple welders working simultaneously at diametrically opposite positions further reduces angular distortion.
The study also highlights the importance of pre-weld fit-up tolerance. When expansion joints are removed, the accumulated transverse shrinkage across multiple joints must be compensated during initial assembly. A typical compensation allowance of 2-4 mm per circumferential weld seam is recommended for pipes with wall thicknesses exceeding 40 mm.
Reflections and Practical Recommendations
This literature provides valuable guidance for engineers involved in large-scale water conveyance and hydropower pipe installation. The key insight is that deformation management must be integrated into the overall project planning rather than treated as a post-weld correction issue. The PDCA approach is particularly applicable: Plan the welding sequence and fit-up allowances based on calculated shrinkage; Do the welding following the planned sequence; Check the actual deformation against predictions; Act by adjusting subsequent weld procedures if deviations exceed tolerance.
The study's methodology of combining empirical formulas with theoretical calculations provides a robust framework that can be adapted to different pipe diameters, wall thicknesses, and material grades. Engineers should note that the Three Gorges project context involves steel grades such as Q345 and Q370, which have well-documented welding deformation characteristics. For projects using higher-strength or low-temperature steels, additional considerations for cold cracking susceptibility and HAZ toughness must be incorporated into the deformation analysis.