Zhuojin Pipe Fitting Co., LtdApplication of Pre-Deformation Method in Tube Sheet Overlay Welding
Literature Overview
The 1996 paper by Xu Zailin and Sheng Likuan from Changzhou Chemical Machinery Factory, published in "Welding Technology," addresses a well-known engineering challenge in pressure vessel fabrication: the control of welding distortion during overlay welding of tube sheets. Tube sheets are large, thick plate components that serve as the interface between the pressure vessel shell and the tube bundle in heat exchangers and reactors. They are typically fabricated from carbon steel or low-alloy steel and require overlay welding of stainless steel or nickel-based alloys on the tube-side surface to provide corrosion resistance. The paper proposes the use of a pre-deformation method to counteract the inherent welding distortion, representing a proactive rather than reactive approach to distortion control.
Distortion Mechanism Analysis
The "pot-bottom" deformation described in the paper is a classic manifestation of welding distortion in large plate components. The mechanism can be understood through the following analysis:
| Distortion Type | Cause | Effect |
|---|---|---|
| Bowing (pot-bottom) | Unequal thermal expansion on upper and lower surfaces | Plate curves inward toward the weld side |
| Angular distortion | Asymmetric heat input relative to neutral axis | Edge rotation |
| Shrinkage | Longitudinal and transverse contraction | Dimensional reduction |
The root cause of pot-bottom deformation in tube sheet overlay welding is the asymmetric thermal gradient created by the overlay weld deposit on one surface only. The heated surface expands more than the cooler back surface, and upon cooling, the contraction of the weld zone pulls the plate into a concave shape.
Quantitative Considerations
The magnitude of distortion depends on several factors:
- Plate thickness: Thicker plates are more resistant to bending but develop higher residual stresses.
- Overlay area: Larger overlay areas produce greater total contraction.
- Heat input: Higher heat input increases the thermal gradient and distortion.
- Welding sequence: Poor sequencing exacerbates distortion.
Pre-Deformation Methodology
The pre-deformation method proposed in the paper involves deliberately introducing a controlled deformation in the opposite direction before welding, such that the welding-induced deformation brings the plate back to its final required geometry. This approach is analogous to the concept of "springback" compensation in sheet metal forming.
Implementation Steps
- Distortion prediction: Based on plate thickness, overlay area, and welding parameters, estimate the expected pot-bottom deformation magnitude.
- Pre-deformation application: Mechanically deform the plate in the convex direction (opposite to expected pot-bottom) using hydraulic pressing, mechanical bending, or thermal pre-heating of the back surface.
- Welding execution: Perform overlay welding with optimized parameters and sequencing.
- Post-weld verification: Measure final geometry and apply corrective measures if necessary.
| Parameter | Typical Range | Notes |
|---|---|---|
| Pre-deformation amount | 1-3 mm for 30-50 mm thick plates | Depends on overlay area and heat input |
| Pre-deformation method | Hydraulic pressing or mechanical bending | Must be uniform across the plate |
| Welding sequence | Symmetric, multi-directional | Minimize net distortion vector |
| Heat input control | Low to moderate | Reduce thermal gradient |
| Welding material | 309L or 310L stainless steel | Match overlay specification |
Welding Sequence Optimization
The paper emphasizes that the welding sequence is critical to distortion control. An optimal sequence for a large tube sheet overlay would involve:
- Starting from the center and progressing outward in a symmetric pattern.
- Using a multi-directional approach where each weld pass is oriented at angles to the previous pass.
- Maintaining a consistent interpass temperature to avoid cumulative thermal effects.
- Avoiding continuous welding in one direction, which creates a strong directional contraction.
Engineering Practice and Reflections
The pre-deformation method is a practical and cost-effective solution to tube sheet distortion, particularly in fabrication environments where post-weld straightening would be expensive or impractical. However, its effectiveness depends on accurate prediction of the welding distortion magnitude, which requires experience or computational modeling.
From my decades of experience in pressure vessel fabrication, I can attest that the pre-deformation approach, while conceptually simple, demands precise execution. The pre-deformation must be uniform and accurately quantified; over-correction results in a convex plate that is equally unacceptable as an under-corrected pot-bottom shape. In modern fabrication, finite element analysis (FEA) has become a standard tool for predicting welding distortion, allowing more accurate pre-deformation calculations than the qualitative analysis described in this 1996 paper. Nevertheless, the fundamental principle remains valid and widely applied.
The paper's approach also highlights the importance of welding material selection. The use of 309L or 310L stainless steel for overlay on carbon steel tube sheets is standard practice, but the dilution characteristics of these materials must be considered. Excessive dilution with the carbon steel base can reduce the corrosion resistance of the overlay, while insufficient fusion can create lack-of-bond defects. The pre-deformation method does not address these metallurgical concerns, which must be managed through proper welding procedure qualification and consumable selection.