CLADDING TECHNOLOGY SHANXI CO., LTDAnalysis of Deformation After Tube Sheet Weld Overlay and Control Measures
Literature Overview
This 1991 publication by Zhou Dapeng and Xu Hui from the Nanjing Second Chemical Machinery Factory addresses one of the most challenging problems in heat exchanger manufacturing: controlling deformation in tube sheets after weld overlay cladding. Published in the journal Welding Technology, this work tackles a problem that directly affects product quality, assembly fit, and ultimately the reliability of heat exchangers used in chemical processing, petrochemical, and power generation applications.
Tube sheets are critical structural components that must maintain dimensional accuracy to within tight tolerances (typically ±0.5 mm for tube hole positions and ±0.1 mm for surface flatness) after all fabrication operations, including weld overlay cladding. The thermal stresses induced by welding inevitably cause distortion, and managing this distortion is the central engineering challenge addressed in this literature.
Core Technical Content
Deformation Mechanisms in Tube Sheet Weld Overlay
Tube sheet weld overlay introduces several deformation mechanisms that act simultaneously:
- Thermal expansion and contraction: The localized heating of the overlay zone causes differential expansion, creating compressive stresses in the heated region and tensile stresses in the surrounding cooler material
- Phase transformation strains: In low-alloy steel tube sheets, the austenite-to-ferrite transformation during cooling can introduce additional volumetric strain
- Residual stress relaxation: Pre-existing stresses from prior fabrication operations (rolling, drilling, machining) are partially relieved by welding heat, causing shape changes
- Plastic deformation: Localized yielding in the heat-affected zone produces permanent deformation that does not recover upon cooling
Quantitative Deformation Analysis
The authors present a systematic approach to predicting and measuring deformation. Based on the engineering context, typical deformation patterns include:
| Deformation Type | Typical Magnitude | Direction | Primary Cause |
|---|---|---|---|
| Overall warping | 1.5-3.0 mm over 800 mm span | Convex toward weld side | Asymmetric thermal input |
| Local bulging at weld zones | 0.3-0.8 mm | Perpendicular to tube sheet | Localized thermal expansion |
| Tube hole distortion | 0.05-0.15 mm ovality | Aligned with weld direction | Differential thermal strain |
| Edge curling | 0.5-1.5 mm at edges | Inward or outward | Edge constraint effects |
| Thickness change in HAZ | 0.1-0.3 mm reduction | Through thickness | Thermal thinning |
Weld Overlay Process Parameters
The specific welding process used for tube sheet cladding significantly affects deformation magnitude:
| Parameter | Submerged Arc Welding (SAW) | Gas Metal Arc Welding (GMAW) | Electroslag Welding (ESW) |
|---|---|---|---|
| Heat Input (kJ/mm) | 1.5-3.0 | 0.8-1.5 | 2.0-4.0 |
| Welding Speed (mm/min) | 200-400 | 300-600 | 100-200 |
| Number of Layers | 2-3 | 3-5 | 1-2 |
| Distortion Magnitude | Moderate | Low | High |
| Productivity | High | Moderate | Very High |
| Surface Quality | Good (requires dressing) | Excellent | Poor (requires machining) |
Control Measures
Design-Based Controls
The most effective deformation control begins at the design stage:
- Symmetrical cladding layout: Distribute weld passes symmetrically about the tube sheet centerline to balance thermal input
- Reduced overlay thickness: Specify the minimum overlay thickness required for corrosion resistance (typically 3-5 mm for 304/316 stainless steel on carbon steel)
- Intermittent weld pattern: Use segmented welds with gaps rather than continuous welds to reduce total heat input
- Stiffening ring design: Incorporate internal stiffening rings that provide rigidity against warping during welding
Process-Based Controls
| Control Measure | Implementation Method | Effectiveness |
|---|---|---|
| Backing plate | Rigid steel backing plate bolted to tube sheet backside | High - constrains bulging |
| Welding fixture | Custom fixture with clamping points around perimeter | High - controls edge curling |
| Backing rod technique | Copper or steel backing rod against the weld root | Medium - controls root-side deformation |
| Step-back welding | Weld from center outward in alternating directions | Medium - balances thermal input |
| Low heat input | Use lower current, higher speed parameters | High - reduces HAZ and distortion |
| Interpass grinding | Grind each layer flush before welding next layer | Medium - removes deposited material and stress |
| Pre-weld machining | Machine tube sheet to final dimensions before cladding | Low - deformation still occurs |
Post-Weld Correction
When deformation exceeds acceptable limits, post-weld correction is necessary:
- Mechanical correction: Use hydraulic presses or mechanical presses to flatten warped sections, followed by re-machining
- Thermal correction: Apply controlled heating at specific points to induce counter-deformation (requires careful thermal control to avoid material property degradation)
- Final machining: Remove the tube sheet surface to flatness specification after all welding is complete, accepting the material removal as the cost of dimensional accuracy
Quality Verification
The final verification of tube sheet quality after weld overlay and correction involves:
| Test | Specification | Method |
|---|---|---|
| Surface flatness | ≤ 0.1 mm over any 300 mm | Straightedge and feeler gauge |
| Tube hole position accuracy | ±0.1 mm from nominal | Coordinate measuring machine |
| Tube hole roundness | ≤ 0.05 mm ovality | Bore gauge |
| Overlay thickness | ≥ 3.0 mm minimum (for 304 SS on carbon steel) | Ultrasonic thickness gauge |
| Bond strength | ≥ 40 MPa | Peel test or micro-tensile test |
| Hardness of overlay | 150-250 HV (for austenitic SS) | Vickers hardness tester |
| Intergranular corrosion | No intergranular attack after 24h ASTMA262 Practice E | Metallographic examination |
Engineering Practice Integration
The practical significance of this work extends beyond individual tube sheet fabrication to the broader heat exchanger manufacturing process. Tube sheet deformation directly affects:
- Tube insertion fit: Distorted tube holes can prevent proper tube insertion or create stress concentrations at the tube-to-tube-sheet joint
- Channel cover sealing: Warped tube sheets do not seal properly against channel covers, leading to leakage
- Hydrostatic test results: Residual stresses from deformation can cause pressure-induced distortion during hydrostatic testing
- Long-term fatigue life: Residual tensile stresses in the tube sheet reduce fatigue life under cyclic thermal loading
The authors emphasize that the most effective strategy combines multiple control measures rather than relying on a single technique. In practice, the optimal approach typically involves:
- Design the cladding pattern for maximum symmetry
- Use a rigid backing plate during welding
- Employ low-heat-input welding parameters
- Grind between layers to remove deposited material
- Machine to final dimensions after all welding is complete
- Verify flatness and hole accuracy before assembly
Key Questions and Reflections
Several questions arise from studying this literature that merit further investigation:
- How does the number of tube holes and their distribution affect the deformation pattern? A tube sheet with a high hole density may behave differently from one with fewer, larger holes.
- What is the effect of base material thickness on deformation? Thinner tube sheets are more susceptible to warping but may also have less material to accommodate plastic strain.
- How do modern welding monitoring systems (such as real-time thermal imaging and strain gauges) improve deformation prediction compared to the analytical methods used in 1991?
Summary
This 1991 publication provides a systematic treatment of tube sheet deformation during weld overlay cladding, combining analytical understanding of deformation mechanisms with practical control measures. The work establishes that deformation control is a multi-faceted challenge requiring attention at the design, process, and post-weld stages. The key insight is that no single control measure is sufficient; rather, a combination of symmetrical design, process optimization, mechanical constraint, and final machining is required to achieve the dimensional accuracy demanded by heat exchanger assembly. This holistic approach to deformation control remains the foundation of modern tube sheet cladding practice, and the principles articulated in this literature continue to guide engineering decisions in heat exchanger manufacturing worldwide.