Strip Electrode Overlay Welding Technology for Large Diameter 20MnMo Tube Sheet

Literature Overview

This paper by Guan Yunsheng, published in China Chemical Equipment (2011, Vol. 13, No. 1, pp. 45-48), addresses the technical challenges of overlay welding a large-diameter (4000 mm) 20MnMo forged tube sheet with 309LMo stainless steel using submerged arc strip electrode welding. The study focuses on controlling dilution of alloy elements, managing ferrite content, and minimizing welding distortion. The application is in the manufacture of heat exchanger tube sheets for petrochemical equipment. The paper represents a significant engineering achievement in scaling up overlay welding technology to large-diameter components.

Technical Challenges and Solutions

Scale-Related Challenges

Overlay welding a 4000 mm diameter tube sheet presents unique challenges that differ significantly from smaller components:

Material Selection and Layer Strategy

The base material 20MnMo is a low-alloy steel containing approximately 0.20% C, 0.9-1.4% Mn, and 0.25-0.35% Mo, providing good strength and toughness at moderate temperatures. The overlay material 309LMo (UNS S30908) is a low-carbon austenitic stainless steel with approximately 22-24% Cr, 12-14% Ni, and 0.75% Mo, selected for its excellent corrosion resistance and compatibility with the base material.

The layer strategy is critical to achieving the desired performance:

1. Transition layer: A single layer of 309L (or 309LMo) applied directly to the base metal to ensure good metallurgical compatibility. This layer will have higher dilution but serves to prevent cracking at the fusion line.
2. Intermediate layers: Additional layers of 309LMo to gradually transition from the high-dilution transition layer to the final composition.
3. Cover layers: Final layers of 309LMo with controlled dilution to achieve the target composition and properties.

The dilution rate is a critical parameter that must be carefully controlled. For the transition layer, dilution may reach 40-60%, while subsequent layers should achieve dilution below 20% to ensure adequate corrosion resistance in the final overlay.

Process Parameter Optimization

The strip electrode submerged arc welding process was selected for its high deposition rate and excellent weld quality. Key process parameters include:

Distortion Control Strategy

Pre-Weld Preparation

Controlling distortion in a 4000 mm diameter tube sheet requires careful preparation:

1. Preheating: The entire tube sheet should be preheated to 200-250°C to reduce thermal gradients and residual stress. This also helps prevent cold cracking in the base metal.
2. Backing plate: A heavy backing plate (typically 20-30 mm thick steel) should be clamped to the back of the tube sheet to provide thermal mass and support. The backing plate should be of similar thermal conductivity to the tube sheet material.
3. Support structure: The tube sheet should be supported on a rigid frame that allows for some thermal expansion while preventing excessive deflection.
4. Welding sequence design: The welding sequence should be designed to minimize residual stress by:

• Starting from the center and working outward in a spiral pattern
• Alternating between opposite sides of the tube sheet
• Using stringer beads with adequate spacing to allow heat dissipation
• Avoiding continuous long welds that create excessive heat buildup

Post-Weld Treatment

After welding, the tube sheet should undergo the following treatment:

1. Stress relief: Post-weld heat treatment at 620-650°C for 2-4 hours to relieve residual stresses. This temperature is below the austenitizing temperature of the overlay to prevent phase transformation.
2. Machining: The overlay surface should be machined to achieve the required flatness and surface finish for tube installation.
3. Inspection: The overlay should be inspected for:

• Ferrite content (target 5-20% for 309LMo to prevent sensitization)
• Dilution (metallographic analysis)
• Surface quality (visual and dye penetrant testing)
• Thickness uniformity (UT measurement)

Engineering Practice Integration

Quality Control Plan

A comprehensive quality control plan for this application should include:

Lessons Learned and Best Practices

The paper highlights several important lessons for engineers working with large-scale overlay welding:

1. Process validation is essential: Before production welding, a detailed procedure qualification should be performed on a coupon of similar thickness and material to establish the optimal parameters and verify the achievable properties.
2. Dilution monitoring is critical: The dilution rate must be monitored throughout the welding process, as it directly affects the composition and properties of the overlay. Any deviation from the target dilution should trigger a parameter adjustment.
3. Thermal management is paramount: The large size of the tube sheet creates significant thermal challenges. Adequate preheating, controlled cooling, and stress relief are all essential to prevent distortion and cracking.
4. Layer thickness control: Each layer should be deposited with a consistent thickness to ensure uniform dilution and properties throughout the overlay. Excessive layer thickness can lead to cracking, while insufficient thickness increases the number of layers and reduces productivity.

Study Insights and Reflections

This case study demonstrates the successful application of strip electrode submerged arc welding to a challenging large-scale overlay welding application. The technical approach—combining a transition layer strategy with careful process parameter control and comprehensive distortion management—represents best practice in the field.

One area that deserves further reflection is the long-term performance of the overlay in service. The 309LMo overlay, while providing excellent corrosion resistance, is susceptible to sensitization if exposed to temperatures in the 450-850°C range for extended periods. In heat exchanger applications, the tube sheet may experience thermal cycling that could promote sensitization at the overlay surface. Engineers should consider this in the design and maintenance planning.

Additionally, the paper raises important questions about the economic optimization of the overlay thickness. Thicker overlays provide greater corrosion resistance but increase cost and distortion. The optimal overlay thickness should be determined through a life-cycle cost analysis that considers the expected corrosion rate, inspection interval, and repair costs.

The successful application of this technology also highlights the importance of skilled welders and rigorous procedural control. Strip electrode SAW, while highly productive, requires careful setup and monitoring to achieve consistent results. The skill of the welder and the quality of the procedure are both critical to the success of the operation.

In conclusion, this literature provides valuable technical guidance for engineers working with large-scale overlay welding applications. The key takeaways are the importance of systematic process development, careful thermal management, and comprehensive quality control. The successful application of strip electrode SAW to a 4000 mm tube sheet demonstrates that even challenging large-scale overlay welding can be achieved with the right technical approach and engineering discipline.