Zhuojin Pipe Fitting Co., LtdApplication of Pre-Deformation Method in Tube Sheet Overlay Welding
Literature Overview
The paper by Xu Zailin and Sheng Likuan (1996), published in Welding Technology (Vol. 25, No. 5, pp. 36–38), addresses the deformation problem encountered during overlay welding of tube sheets, particularly in the fabrication of heat exchangers and pressure vessels. The authors propose a pre-deformation method to counteract the "pot-bottom" (锅底形) deformation that commonly occurs during overlay welding of stainless steel onto carbon steel tube sheets. This is a highly practical engineering solution for manufacturers of pressure equipment where dimensional accuracy is critical.
Deformation Mechanism Analysis
Tube sheets in heat exchangers and pressure vessels typically consist of a carbon steel or low-alloy steel base with a stainless steel overlay layer on the tube-side surface. The overlay provides corrosion resistance to the process fluid while the base material provides structural strength. The welding of the overlay introduces significant thermal stresses that lead to characteristic deformation patterns.
| Parameter | Typical Value |
|---|---|
| Tube sheet diameter | 500–2000 mm |
| Tube sheet thickness | 30–80 mm |
| Overlay thickness | 3–8 mm |
| Base material | Q345R / 16MnR / 15CrMoR |
| Overlay material | 304 / 304L / 316L / 321 |
| Welding process | GTAW / SMAW / SAW |
| Deformation type | Pot-bottom (concave) deformation |
| Typical deformation magnitude | 1.0–3.0 mm (without pre-deformation) |
The pot-bottom deformation occurs because:
- The overlay weld metal contracts upon cooling, pulling the tube sheet surface inward.
- The thermal gradient through the tube sheet thickness creates differential contraction—the hot overlay side contracts more than the cooler base side.
- The constraint from the tube sheet's rigid geometry (clamped between channel plates) amplifies the through-thickness differential contraction.
Pre-Deformation Method
The pre-deformation method involves intentionally deforming the tube sheet in the opposite direction (convex) before overlay welding, such that the welding-induced concave deformation brings the final geometry back to the required flatness tolerance.
| Step | Description |
|---|---|
| 1. Deformation amount determination | Calculate or experimentally determine the expected welding deformation based on tube sheet dimensions and welding parameters |
| 2. Pre-deformation | Mechanically or thermally deform the tube sheet surface to a convex shape with the calculated pre-deformation amount |
| 3. Overlay welding | Apply the overlay layer using optimized welding parameters and sequence |
| 4. Post-weld inspection | Verify flatness, overlay thickness, and weld quality |
The authors emphasize that the pre-deformation amount must be carefully calibrated. Key factors include:
- Tube sheet thickness and diameter: Thicker and larger tube sheets exhibit greater deformation.
- Welding heat input: Higher heat input produces greater deformation.
- Welding sequence: The sequence of weld passes (e.g., spiral from center to periphery vs. segmented) significantly affects the deformation pattern.
- Welding material selection: The coefficient of thermal expansion mismatch between the base and overlay materials contributes to residual stress.
Process Optimization and Quality Control
| Quality Requirement | Specification |
|---|---|
| Final flatness | ≤ 1.0 mm over entire surface (typical) |
| Overlay thickness uniformity | ± 0.5 mm |
| Weld defects | No cracks, porosity, or lack of fusion |
| Dilution control | Base metal dilution ≤ 30% (for corrosion resistance) |
| Post-weld treatment | Stress relief at 420–480 °C for stainless overlay |
The welding sequence is critical: a spiral sequence starting from the center and progressing outward tends to produce more uniform deformation, while a segmented sequence with strategic direction changes can be used to compensate for asymmetries. The authors recommend using lower heat input per pass with multiple passes to reduce the thermal gradient and minimize deformation.
Engineering Practice Integration
In pressure vessel manufacturing, the pre-deformation method has been successfully applied to tube sheets with diameters up to 2000 mm. The method requires:
- A calibration test on a representative coupon to determine the deformation-to-pre-deformation ratio.
- A mechanical forming setup (hydraulic press or rolling mill) capable of producing the required convex pre-deformation.
- Process documentation and operator training to ensure consistent execution.
The method is particularly advantageous when the tube sheet is part of a larger assembly where post-weld machining to correct deformation is impractical or would compromise the overlay thickness.
Study Reflection
This paper exemplifies the engineering principle of "compensating for known effects through proactive countermeasures." Rather than attempting to eliminate welding deformation entirely—an impractical goal—the authors chose to predict and counteract it. The pre-deformation method is conceptually similar to the pre-stress approach used in structural welding, and it demonstrates that process planning and deformation prediction are as important as welding execution itself. For manufacturers of heat exchangers and pressure vessels, this technique represents a practical solution to a common quality problem, reducing the need for costly post-weld machining and improving first-time quality rates.