A bimetal flange is a composite flange manufactured by permanently bonding two dissimilar metals into a single, pressure-tight component. Unlike a standard flange machined from a single material, a bimetal flange exploits the structural strength of one metal (typically carbon steel or stainless steel for the flange body) and the thermal, electrical, or corrosion-resistant properties of a second metal (typically aluminium, copper, or brass) for the pipe-side insert or collar.
The bond between the two metals is achieved through controlled brazing or soldering — metallurgical joining processes that create a hermetically sealed, void-free interface. Because the two metals are joined at the atomic level, there is no exposed junction for electrolyte ingress, eliminating the galvanic corrosion that would occur if dissimilar metals were simply bolted or mechanically assembled together.
Tesco Steel & Engineering manufactures four types of bimetal flanges — Bi-Metallic Brazing Flanges, Bi-Metallic Blanking Brazing Flanges, Bi-Metallic Soldering Flanges, and Solid Brazing Flanges — for refrigeration, HVAC, heat exchanger, marine, and industrial piping applications. All types are available in nominal sizes DN15 to DN300 (½" to 12") and are exported from Mumbai to 96 countries.
⚡ Why Bimetal? — The Galvanic Corrosion Problem Explained
When copper and aluminium are in direct contact in the presence of moisture or refrigerant oil/moisture mix, aluminium corrodes rapidly — copper is 0.34 V nobler on the galvanic series, driving aluminium anodic dissolution. In HVAC and refrigeration systems, this manifests as pinhole leaks in aluminium evaporator headers within 2–5 years. A bimetal brazing flange eliminates this interface entirely: the copper-to-aluminium bond is hermetically sealed by the brazing filler (BAlSi-4), with no exposed metal-to-metal junction available for electrolyte attack.
| Type | Body Metal | Insert Metal | Filler Metal | Temp Range | Max Pressure |
|---|---|---|---|---|---|
| Bi-Metallic Brazing | CS / SS 304 / SS 316L | Al 6061 / Cu C10200 | BAlSi-4 / BAg-7 | −45°C to +150°C | PN25 / Cl.300 |
| Blanking Brazing | CS / SS 304 / SS 316L | Al 6061 (solid) | BAlSi-4 | −45°C to +150°C | PN25 / Cl.300 |
| Bi-Metallic Soldering | Cu C10200 / Brass CW617N | Al 1050A / Al 3003 | SnZnAl / SnAg3.5 | −30°C to +120°C | PN16 |
| Solid Brazing | Cu C10200 / C11000 / Brass | None | BCuP-5 / BCuP-6 / BAg-7 | −196°C to +200°C | PN25 / Cl.300 |
Bar length represents relative maximum pressure rating. PN25 ≈ 25 bar; PN16 ≈ 16 bar at 20°C.
Connecting copper refrigerant lines (compressor, condenser) to aluminium evaporator coils and headers in commercial and industrial refrigeration plants. Compatible with R410A, R32, R134a, R404A, R448A, R449A, and R1234yf refrigerants.
Air-handling unit connection headers, chilled water aluminium distribution manifolds, VRF (Variable Refrigerant Flow) multi-split system interconnections, and fan coil unit connections where copper and aluminium piping systems interface.
Shell-and-tube and plate heat exchangers where aluminium tube bundles connect to steel or copper shell nozzles. Plate heat exchanger copper/aluminium port flanges. Industrial process coolers and condensers with dissimilar metal tube-to-header interfaces.
Copper-nickel to aluminium transitions in seawater cooling circuits aboard vessels. Aluminium superstructure cooling pipe connections to copper-based heat exchange systems. Offshore platform HVAC systems requiring corrosion-resistant dissimilar metal transitions.
Generator cooling circuits, transformer oil coolers, auxiliary cooling water systems, and air-blast heat exchanger connections in power stations where copper conductors and aluminium cooling fins must interface at flanged connections.
Food processing plant refrigeration, pharmaceutical cold storage, chemical plant cooling circuits, cryogenic liquid nitrogen storage systems (solid copper brazing flanges to −196°C), and any process application requiring a pressure-tight, corrosion-resistant transition between copper and aluminium piping.
| Standard | Scope |
|---|---|
| EN 1254-4 | Copper fittings for refrigerating systems — end connections, dimensions, materials |
| EN ISO 14903 | Qualification of brazing and soldering personnel and processes for refrigerating systems and heat pumps |
| EN 1092-1 | Flange face dimensions — PN10, PN16, PN25 for steel/copper/aluminium flanges |
| ASME B16.5 | Flange bolt circle and face dimensions — Class 150, 300 (applied to bimetal flange face dimensions) |
| AWS A5.8 | Filler metal specification — BCuP-5, BCuP-6, BAg-7 brazing alloys |
| DIN EN 1044 | Brazing filler metals — BAlSi-4, BAlSi-5 designation for aluminium brazing |
| EN 573-3 | Wrought aluminium alloys — EN AW-6061, EN AW-1050A, EN AW-3003 designation |
| ASTM B152 | Copper sheet, strip, and foil — C10200 and C11000 copper for solid brazing flanges |
🔧 Field Brazing Guidance for Bimetal Flanges
When connecting bimetal brazing flanges in the field, the pipe-side insert (aluminium or copper collar) is brazed to the pipe using standard HVAC/refrigeration brazing practice. The flange body is then bolted to the mating counter-flange using the appropriate gasket. The brazing filler and flux must be compatible with the insert material — use BAlSi-4 flux (noncorrosive aluminium flux per AWS A5.31 Class FB3-A) for aluminium inserts; BCuP (self-fluxing on copper) for copper inserts. Always purge the pipe interior with dry nitrogen during brazing to prevent internal oxidation. After brazing, pressure-test to 1.5× design pressure with dry nitrogen before charging with refrigerant.
Tesco Steel & Engineering maintains ready stock of all standard bimetal flange types and sizes. Request a price list with current stock availability, lead times, and full documentation package.
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