CLADDING TECHNOLOGY SHANXI CO., LTDMIG Welding of Dissimilar Metals in Heat Exchanger Tube Sheets and Tubes
Overview and Background
This 1996 study by Yan Jun from the Boilers Inspection Institute of Fuyang, Anhui Province addresses a long-standing practical challenge in heat exchanger fabrication: joining dissimilar metals at the tube-to-tubesheet joint using Metal Inert Gas (MIG) welding. Heat exchangers in chemical, petrochemical, and power generation industries frequently employ dissimilar material combinations such as carbon steel tubesheets with stainless steel tubes, carbon steel tubesheets with copper-nickel tubes, or austenitic stainless steel tubesheets with nickel-based alloy tubes. The metallurgical incompatibility at these joints introduces significant welding challenges including differential thermal expansion, risk of intermetallic compound formation, susceptibility to cracking, and potential for galvanic corrosion under service conditions.
Core Technical Content
The fundamental difficulty in welding dissimilar tube-to-tubesheet joints lies in the mismatch of thermal conductivity, thermal expansion coefficient, and solidification behavior between the two metals. When a MIG arc transfers heat to the joint, the higher-conductivity metal (typically austenitic stainless steel or copper-nickel) draws heat away rapidly, while the lower-conductivity base metal (carbon steel or low-alloy steel) retains heat, creating asymmetric solidification and a skewed weld pool. This asymmetry leads to several metallurgical consequences that must be managed through process design.
Metallurgical Considerations
The weld metal composition at a dissimilar joint is not simply the arithmetic average of the two base metals. Due to the unequal melting rates and fluidity differences, the dilution ratio deviates significantly from 50/50. For example, when welding a 304 stainless steel tube into a SA-266 Gr.B carbon steel tubesheet, the weld metal may contain only 18-22% chromium and 8-12% nickel, placing it in the ferrite-austenite two-phase region. This composition is acceptable for many service conditions but may be susceptible to intergranular corrosion if the heat-affected zone (HAZ) on the stainless steel side experiences sensitization temperatures (450-850°C) for extended periods.
Process Parameters and Their Effects
| Parameter | Typical Range | Effect on Joint Quality |
|---|---|---|
| Wire diameter | 0.8-1.2 mm | Thinner wires provide better fit-up tolerance but lower deposition rate |
| Shielding gas | Ar + 5-15% CO2 or pure Ar | Higher CO2 increases spatter and oxidation; pure Ar gives cleaner welds |
| Current density | 150-250 A/mm² | Higher density increases penetration but risks burn-through on thin tubes |
| Travel speed | 200-500 mm/min | Must balance penetration depth with heat input control |
| Preheating | 50-150°C for carbon steel side | Reduces cracking susceptibility in high-carbon equivalents |
| Interpass temperature | Below 150°C | Prevents sensitization and grain growth in HAZ |
Defect Analysis and Countermeasures
The most common defects observed in dissimilar tube-to-tubesheet MIG welds include:
- Lack of fusion on the tubesheet side — caused by excessive heat absorption by the high-conductivity tube material, leaving insufficient melting on the carbon steel side. Countermeasure: increase current or reduce travel speed while monitoring the tube side for burn-through.
- Cracking in the weld metal — typically intergranular or hot cracking, associated with the formation of brittle intermetallic phases (Fe-Cr, Fe-Ni) during solidification. Countermeasure: use a hyper-eutectoid filler wire with slightly higher nickel content to promote austenite formation and reduce cracking tendency.
- Undercut on the tubesheet side — caused by the weld pool flowing preferentially toward the thinner tube wall. Countermeasure: use a slightly convex backing ring or employ a weaving technique to distribute heat more evenly.
- Pore formation — particularly hydrogen porosity in the carbon steel HAZ or oxide inclusions from contaminated filler wire. Countermeasure: strict gas purity control (99.99% minimum), proper wire deoxidation, and pre-cleaning of all surfaces.
Engineering Practice Integration
In the context of pressure vessel fabrication governed by GB/T 150 and ASME VIII Div.1, the tube-to-tubesheet joint represents a critical pressure boundary. The inspection requirements typically include:
- Radiographic testing (RT) or ultrasonic testing (UT) of 100% of joints for Category A service
- Dye penetrant testing (PT) of the complete weld perimeter
- Hydrostatic pressure testing at 1.25 times design pressure per GB/T 150.2
For hydrogenation reactors and high-pressure hydrogen service, the dissimilar joint must additionally pass hydrogen-induced cracking (HIC) and sulfide stress corrosion (SSC) testing per NACE MR0175 / ISO 15156 requirements. The weld metal hardness must be verified to remain below 22 HRC to minimize susceptibility to hydrogen damage.
Key Questions and Reflections
A critical question that emerges from this literature is whether MIG welding truly offers advantages over the more traditional GTAW (TIG) method for tube-to-tubesheet joints. GTAW provides superior arc stability and heat control, which is advantageous for thin-walled tubes and precision fit-up. However, MIG offers significantly higher deposition rates, making it economically attractive for large-scale heat exchanger production with hundreds or thousands of tubes. The trade-off is that MIG requires more sophisticated process control to manage the asymmetric heat input inherent to dissimilar joints.
Another reflection concerns the evolution of filler wire selection. In 1996, the available filler wire options were limited compared to today's market. Modern filler wires with controlled intermetallic formation behavior, such as ER309L or ERNiCrMo-3, offer improved performance at dissimilar joints. The original study's recommendations should be updated in light of current materials technology, though the fundamental process principles remain valid.
Study Insights and Implications
This literature serves as a historical foundation for understanding the fundamental challenges of dissimilar metal welding in heat exchanger fabrication. The key insight is that process parameter selection must always be driven by the metallurgical compatibility of the joint rather than by conventional welding practice for homogeneous materials. Engineers working on modern heat exchanger fabrication should use this as a starting point while incorporating current standards (ASME IX, NB/T 47014) and advanced filler metal technology to achieve reliable, code-compliant joints.