Zhuojin Pipe Fitting Co., LtdStrip Electrode Overlay Welding Technology for Large Diameter Tube Sheets
Literature Overview
The paper by Guan Yunsheng, published in China Chemical Equipment (Volume 13, Issue 1, 2011, pages 45–48), describes the application of submerged arc strip electrode overlay welding (SAW strip electrode) on a large-diameter (φ4000 mm) tube sheet fabricated from 20MnMo forged steel. The overlay material is 309LMo, a low-carbon, molybdenum-enhanced austenitic stainless steel. The paper addresses the challenges of achieving high-quality overlay welds on large-diameter components, including the control of weld dilution, ferrite content, and welding distortion. This is a technically demanding application that requires careful process planning and execution.
Core Technical Analysis
Component and Material Background
The tube sheet is a critical component in heat exchangers and reactors, serving as the boundary between the process fluid and the tube bundle. For a φ4000 mm diameter tube sheet, the component is typically a thick-walled forging (wall thickness of 100–200 mm) that requires overlay welding to provide corrosion resistance on the process-side surface.
| Parameter | Specification |
|---|---|
| Base material | 20MnMo forged steel |
| Component diameter | 4000 mm |
| Overlay material | 309LMo (EN 1.4407 / UNS S30908 with Mo addition) |
| Welding process | SAW with strip electrode |
| Key challenges | Dilution control, ferrite control, distortion control |
The selection of 309LMo as the overlay material is based on its excellent resistance to intergranular corrosion (due to low carbon content) and its enhanced resistance to pitting and crevice corrosion (due to molybdenum addition). The Mo addition also improves the weldability of the 309L alloy by reducing the risk of solidification cracking.
Process Design and Parameter Optimization
The strip electrode SAW process is well-suited for large-area overlay welding because it provides a high deposition rate, consistent weld quality, and the ability to achieve uniform overlay thickness over large surfaces. However, the large diameter of the tube sheet introduces several challenges:
- Heat input management: The large mass of the tube sheet acts as a heat sink, which can lead to excessive cooling rates at the weld. This can result in hard, brittle microstructures in the weld metal and the heat-affected zone (HAZ). To counteract this, the process must be designed with adequate preheat and interpass temperature control.
- Dilution control: The dilution rate of the base material into the overlay weld is a critical parameter. Excessive dilution reduces the alloy content of the overlay, compromising its corrosion resistance. The paper reports that the dilution rate was controlled to be below 30% for the first pass and below 15% for subsequent passes by using appropriate wire feed speeds, arc voltages, and travel speeds.
- Ferrite control: The ferrite content in the 309LMo overlay must be controlled to be in the range of 5–15% AF (austenite ferrite) to prevent cracking while maintaining adequate ductility. The paper describes the use of a magnetic ferrite gauge to monitor the ferrite content during production.
- Distortion control: The large diameter and thick section of the tube sheet make it susceptible to warping and distortion during welding. The paper describes the use of a multi-directional welding sequence, symmetric welding from the center outward, and the use of clamping fixtures to minimize distortion.
Transition Layer Design
A critical aspect of the overlay design is the transition layer between the 20MnMo base material and the 309LMo overlay. The transition layer serves to reduce the dilution rate of the base material into the final overlay and to provide a metallurgically compatible interface. The paper describes the use of a two-pass transition layer:
- First pass: A 309L wire electrode to provide initial alloy dilution control.
- Second pass: A 309LMo strip electrode to achieve the final overlay composition.
This two-pass approach ensures that the final overlay layer has a composition that is sufficiently alloyed to provide the required corrosion resistance, while also minimizing the risk of cracking at the interface.
Defect Prevention and Quality Control
The paper identifies several potential defects and their countermeasures:
| Defect Type | Cause | Countermeasure |
|---|---|---|
| Solidification cracking | Excessive sulfur/phosphor in base material | Use of low-sulfur, low-phosphor electrodes; controlled heat input |
| Cracking at weld toe | High residual stress | Post-weld heat treatment (PWHT); controlled welding sequence |
| Excessive dilution | High travel speed, low wire feed rate | Optimized wire feed speed and travel speed; multiple thin passes |
| Distortion | Asymmetric heat input | Symmetric welding sequence; use of clamping fixtures |
| Ferrite content out of range | Inconsistent wire composition | Regular ferrite gauge measurements; adjustment of welding parameters |
Engineering Practice Integration
The successful application of this technology to a φ4000 mm tube sheet demonstrates the scalability of strip electrode SAW overlay welding for large components. The key engineering lessons are:
- The welding sequence must be carefully planned to minimize distortion, with symmetric welding from the center outward and multi-directional coverage.
- The dilution rate must be monitored and controlled through the entire overlay build-up, with regular composition analysis of the final overlay layer.
- Post-weld heat treatment is essential to relieve residual stresses and to temper the HAZ, particularly for thick-section components.
- The use of a transition layer is a cost-effective approach to achieving the required overlay composition without excessive dilution.
Study Insights and Implications
This paper provides a comprehensive case study of large-diameter tube sheet overlay welding that is directly applicable to the design and fabrication of large heat exchangers and reactors. The key insight is that the strip electrode SAW process, when properly designed and executed, can achieve high-quality overlay welds on large-diameter components with consistent composition and controlled distortion. For engineers involved in the fabrication of large pressure vessels, this paper offers a validated approach to overlay welding that balances quality, productivity, and cost. The broader implication is that large-diameter tube sheet overlay welding is a mature technology that can be applied to a wide range of industrial applications, provided that the process is carefully designed and the quality control measures are rigorously implemented.