Zhuojin Pipe Fitting Co., LtdWelding Process Research for Tube Sheet Overlay
Literature Overview
This paper, published in Henan Chemical Industry (2010, Vol. 27, No. 9, pp. 51–53), presents a focused study on the welding process for tube sheet overlay, conducted by Wang Hengxiang of Luoyang Longhui Petrochemical Engineering Co., Ltd. The paper addresses the practical challenges of overlay welding on tube sheets, including weldability analysis, dilution management, microstructural evolution, and the formation of low-plasticity bands. The author emphasizes that careful analysis of welding conditions and operating parameters, combined with reasonable process measures, can produce high-quality weld joints.
Weldability Analysis
Material Compatibility
Tube sheet overlay welding typically involves the joining of dissimilar metals—carbon steel or low-alloy steel base materials with austenitic stainless steel overlay layers. The weldability of this combination is influenced by several factors:
| Factor | Effect on Weldability |
|---|---|
| Carbon content mismatch | Carbon diffusion from base to overlay, forming a hard, brittle zone |
| Thermal expansion difference | Residual stresses, distortion |
| Dilution | Reduced corrosion resistance of overlay |
| Thermal conductivity difference | Uneven heat distribution |
Microstructural Evolution
The microstructure of the overlay weld is complex and includes several distinct regions:
- Base metal: Unaffected or minimally affected by welding.
- Heat-affected zone (HAZ): Transformed microstructure due to thermal cycling, potentially including martensite in high-carbon regions.
- Weld metal (overlay): A mixture of base metal and filler metal, with microstructure depending on dilution level and cooling rate.
- Low-plasticity band (LPB): A thin region at the fusion line where the microstructure is characterized by coarse grains and reduced ductility.
The LPB is a critical concern because it is susceptible to cracking during welding and in service. The formation of the LPB is influenced by:
- Carbon diffusion from the base metal into the weld metal.
- Rapid cooling at the fusion line, promoting martensitic transformation.
- Segregation of alloying elements at grain boundaries.
Process Design and Optimization
Welding Parameters
The following table summarizes typical welding parameters for tube sheet overlay:
| Parameter | GMAW (MIG) | SAW |
|---|---|---|
| Current | 180–260 A | 400–600 A |
| Voltage | 22–28 V | 25–32 V |
| Travel speed | 150–250 mm/min | 200–350 mm/min |
| Wire diameter | 1.2–1.6 mm | 1.6–2.4 mm |
| Shielding gas | Ar + 2–5% CO₂ | Flux-covered |
| Preheat | 50–100 °C | 100–150 °C |
| Interpass temp | ≤ 150 °C | ≤ 200 °C |
Dilution Control
Dilution is the most critical parameter affecting the corrosion resistance of the overlay layer. The dilution ratio can be estimated using the following approach:
- For GMAW: Dilution typically ranges from 20–35% for the first layer.
- For SAW: Dilution typically ranges from 15–25% for the first layer.
- For multi-layer overlay: Subsequent layers have lower dilution (5–15%).
The following measures are recommended to control dilution:
- Use of a high-alloy filler metal (e.g., ER309L, ER309MoL) with excess chromium and nickel content.
- Application of multiple overlay layers, with each subsequent layer having lower dilution.
- Use of a backing layer or transition layer to reduce carbon diffusion.
- Verification of overlay chemistry through spectrometric analysis.
Thermal Stress Management
Thermal stresses during overlay welding can lead to cracking, distortion, and reduced service life. The following measures are recommended:
- Preheating: Controlled preheating at 50–100 °C reduces thermal gradients and minimizes residual stresses.
- Low heat input: Lower current and higher travel speed reduce the heat-affected zone and minimize thermal distortion.
- Symmetrical welding: Welding in a symmetrical pattern distributes heat evenly and reduces warping.
- Post-weld stress relief: Stress relief annealing at 550–620 °C reduces residual stresses.
Defect Analysis and Prevention
Common Defects
| Defect | Location | Cause | Prevention |
|---|---|---|---|
| LPB cracking | Fusion line | Carbon diffusion, rapid cooling | Preheat, low heat input, post-weld heat treatment |
| Overlay cracking | Weld metal | High carbon content, excessive dilution | Use low-carbon filler, control dilution |
| Porosity | Weld metal | Contamination, inadequate shielding | Clean surface, ensure gas flow |
| Lack of fusion | Fusion line | Low travel speed, improper angle | Optimize parameters, maintain torch angle |
| Excessive dilution | Weld metal | High heat input, low travel speed | Reduce current, increase travel speed |
LPB Formation and Mitigation
The low-plasticity band (LPB) is a thin region (typically 0.1–1.0 mm) at the fusion line where the microstructure is characterized by coarse grains, martensite, and reduced ductility. The formation of the LPB is influenced by:
- Carbon diffusion: Carbon from the base metal diffuses into the weld metal, increasing the carbon content and promoting martensitic transformation.
- Rapid cooling: The fusion line cools rapidly, promoting martensitic transformation and grain coarsening.
- Segregation: Alloying elements segregate at grain boundaries, reducing ductility.
Mitigation measures include:
- Use of a low-carbon filler metal (e.g., ER309L) to reduce the carbon content of the weld metal.
- Controlled preheating and interpass temperature to reduce cooling rates.
- Post-weld stress relief annealing to temper martensite and reduce residual stresses.
- Multi-layer overlay to dilute the LPB region.
Engineering Practice Recommendations
Based on the findings of this study, the following recommendations are provided for tube sheet overlay welding:
- Material selection: Use ER309L or ER309MoL filler metal for austenitic stainless steel overlay on carbon steel base materials. The low carbon content reduces the risk of sensitization and improves weldability.
- Process selection: GMAW is preferred for smaller tube sheets or where precise control is required. SAW is suitable for larger tube sheets where high deposition rates are needed.
- Layer design: A minimum of two overlay layers is recommended. The first layer should be designed to have controlled dilution, and the second layer should achieve near-full dilution.
- Quality control: 100% penetrant inspection of the overlay layer is recommended for critical applications. Chemical analysis of the overlay should be performed to verify compliance with corrosion resistance requirements.
- Post-weld treatment: Stress relief annealing at 550–620 °C is recommended to reduce residual stresses and temper martensite in the HAZ and LPB.
Conclusion
This study provides valuable insights into the welding process for tube sheet overlay, emphasizing the importance of weldability analysis, dilution control, microstructural management, and defect prevention. The key technical challenges—dilution, LPB formation, thermal stress, and distortion—are addressed through a combination of process optimization, material selection, and quality assurance. The findings reinforce the principle that careful analysis of welding conditions and operating parameters, combined with reasonable process measures, is essential for producing high-quality tube sheet overlay welds. Engineers should carefully evaluate the specific requirements of each application and tailor the welding procedure accordingly.