Zhuojin Pipe Fitting Co., LtdMIG Welding of Dissimilar Metals in Heat Exchanger Tube Sheets
Literature Overview and Background
The 1996 paper by Yan Jun, published in Welding Technology (Hanshi Jishu), addresses a long-standing practical challenge in chemical equipment manufacturing: the MIG welding of T3 copper heat exchange tubes to 1Cr18Ni9Ti stainless steel tube sheets. This dissimilar metal joint is common in chemical processing equipment where copper provides superior thermal conductivity for heat exchange tubes while stainless steel offers corrosion resistance for the pressure-containing tube sheet and shell. The paper, though concise (a single page), encapsulates decades of field experience accumulated by the Fuyang Boiler Inspection Institute in Anhui Province, China.
This topic is particularly relevant to engineers working on heat exchangers, condensers, and evaporators used in petrochemical, power generation, and marine applications. The metallurgical incompatibility between austenitic stainless steel and copper-based alloys creates unique challenges that are not encountered in homogeneous weldments.
Metallurgical Challenges of Dissimilar Metal Welding
The fundamental difficulty in welding T3 copper to 1Cr18Ni9Ti stainless steel lies in the significant differences in their physical and metallurgical properties:
| Property | T3 Copper | 1Cr18Ni9Ti (321 equivalent) | Implication for Welding |
|---|---|---|---|
| Thermal conductivity | ~390 W/(m·K) | ~16 W/(m·K) | ~24x difference causes extreme heat asymmetry |
| Melting point | 1083°C | 1400-1450°C | Copper melts much earlier |
| Coefficient of thermal expansion | 16.5 × 10⁻⁶/°C | 17.3 × 10⁻⁶/°C | Moderate mismatch |
| Electrical resistivity | 0.0175 μΩ·m | 0.70 μΩ·m | Affects arc stability and heat input |
| Intermetallic formation | — | Cu-Ni, Cu-Fe phases | Brittle intermetallics at fusion line |
The thermal conductivity mismatch is the most critical factor. During MIG welding, the arc heat is absorbed preferentially by the copper tube due to its high thermal conductivity, causing rapid melting of the copper side while the stainless steel side receives comparatively less heat. This leads to an asymmetric weld pool, with excessive copper dilution on the stainless steel side and potential incomplete fusion on the copper side. Furthermore, the molten copper tends to flow away from the weld zone due to its lower melting point and higher fluidity, creating a concave weld profile.
Intermetallic Compound Formation
When copper and austenitic stainless steel melt together, they form intermetallic compounds such as Cu-Ni, Cu-Fe, and Cu-Cr phases. These intermetallics are inherently brittle and have limited solubility at room temperature. Their formation at the fusion boundary creates a zone of reduced ductility and crack susceptibility. The formation of Cu-Ni phases is particularly problematic because they precipitate during cooling in a range that overlaps with the sensitization temperature range of austenitic stainless steels, potentially exacerbating grain boundary carbide precipitation.
Welding Process Parameters and Recommendations
Based on the author's field experience, the following MIG welding parameters and practices are recommended for this dissimilar metal joint:
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Welding process | MIG (GMAW) with spray transfer | Stable arc, moderate heat input |
| Shielding gas | 80% Ar + 20% CO₂ | Adequate protection for both metals |
| Wire feed speed | Moderate (avoid high) | Reduce copper dilution |
| Arc length | Short to medium | Improve arc stability on copper |
| Travel speed | Relatively fast | Minimize heat input to copper |
| Preheat | Minimal or none | Avoid excessive copper melting |
| Post-weld treatment | None (or controlled cooling) | Avoid slow cooling that promotes intermetallics |
The author recommends using a filler wire that is compatible with both base metals. In practice, a nickel-based filler (such as ERNiCrMo-3 or a copper-nickel alloy wire) is often used to act as a buffer, reducing the direct interaction between copper and stainless steel at the fusion boundary. The nickel content in the filler helps to dilute the copper concentration in the weld metal and suppresses the formation of brittle Cu-Fe and Cu-Cr intermetallics.
Practical Welding Sequence
For tube-to-tube-sheet joints, the welding is typically performed from the inside of the heat exchanger (tube side). The sequence involves:
- Tack welding to secure the tube in the tube sheet hole with minimal heat input.
- Full penetration weld from the tube side using a controlled arc oscillation pattern.
- The arc is kept slightly toward the stainless steel side to compensate for the thermal asymmetry.
- Multiple passes may be required for thicker tube sheets, with interpass temperature control to prevent excessive thermal cycling.
Engineering Practice Reflections
From my experience in reviewing heat exchanger weldments, the key lessons from this paper are:
- Thermal management is paramount: The 24-fold difference in thermal conductivity between copper and stainless steel means that conventional welding parameters optimized for either material alone will fail. The process must be deliberately designed to manage the asymmetric heat flow.
- Filler metal selection is a compromise: No single filler wire is ideal for both metals. Nickel-based fillers provide the best metallurgical compatibility but at the cost of increased material expense. Engineers must balance cost against the risk of intermetallic-induced cracking.
- Inspection focus: The critical inspection zone is the fusion boundary on the stainless steel side, where intermetallic compounds are most likely to form. Radiographic testing (RT) and ultrasonic testing (UT) should be supplemented with macrographic examination of the weld cross-section to detect intermetallic formation.
- Service environment matters: If the heat exchanger operates in a chloride-containing environment, the presence of copper-nickel intermetallics in the weld may affect crevice corrosion resistance. This is particularly relevant for heat exchangers used in marine or chemical service.
This paper, while brief, captures the essence of dissimilar metal welding in heat exchanger applications. Its value lies in the practical wisdom it conveys — the kind of knowledge that is often lost in purely academic treatments of welding metallurgy. For engineers designing or inspecting heat exchangers with copper tubes and stainless steel tube sheets, the principles outlined here remain valid and directly applicable.