At a glance: a bimetal flange = two metals engineered into one component — typically a steel flange body with a copper or brass brazing interface — so copper refrigeration tubing can bolt into steel piping without dissimilar-metal welding. The workhorse of HVAC-R, ammonia and CO₂ refrigeration and cryogenic skids. Specify by five things: size, type (brazing/blanking/solid), material pair, rating and refrigerant.
What Are Bimetal Flanges?
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.
Types of Bimetal Flanges
Material Combinations & Technical Specifications
| 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 |
Pressure Rating Comparison by Type
Bi-Metallic Brazing — PN25 (Class 300)
Blanking Brazing — PN25 (Class 300)
Solid Brazing (Copper) — PN25 (Class 300)
Bi-Metallic Soldering — PN16 (Class 150)
Bar length represents relative maximum pressure rating. PN25 ≈ 25 bar; PN16 ≈ 16 bar at 20°C.
Applications of Bimetal Flanges
🌡 Refrigeration & Cold Storage
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.
❄ HVAC & Chiller Systems
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.
🔥 Heat Exchangers
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.
⚓ Marine & Offshore
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.
⚡ Power Generation
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.
🏭 Industrial & Process
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.
How Our Bimetal Flanges Are Manufactured
1
Component manufacture — the steel flange body is forged and machined to the flange standard, while the copper or brass sleeve/insert is machined to the tube size it will braze to; both from certified heats.
2
Joining — the two metals are metallurgically bonded (brazed) under controlled conditions, creating the permanent steel-to-copper transition that defines the product.
3
Machining & facing — the assembled flange is finish-machined: face, bolt holes and the brazing socket to size.
4
Leak & pressure testing — joints are pressure-tested against the rated duty, since a refrigerant transition joint has zero tolerance for leaks.
5
Testing & marking — dimensional inspection and marking of type, size, rating and traceability number.
6
Certification & packing — EN 10204 3.1 MTC per component (3.2 witnessed on request), faces and sockets protected, export-packed.
Applicable Standards
| 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.
How to Specify & Order a Bimetal Flange
A complete bimetal flange specification has five elements — with the refrigerant and tube size doing work no ordinary flange spec needs:
1
Type — brazing flange, blanking (bi-metallic blind) flange, or solid brazing flange.
2
Size — flange pattern and the copper tube OD the brazing socket must receive.
3
Material pair — e.g. carbon steel + copper, stainless + copper, steel + brass — matched to the system metals.
4
Service — refrigerant (NH₃, CO₂, HFCs), design pressure and temperature range, so the rating and braze alloy suit the duty.
Example of a complete line item: “Bimetal Brazing Flange, DN50, steel body + copper socket for 54 mm tube, CO₂ transcritical duty, EN 10204 3.1 — 10 pcs.”
Frequently Asked Questions — Bimetal Flanges
A bimetal flange is a composite flange that permanently bonds two dissimilar metals — typically a steel or copper body with an aluminium or copper insert — through brazing or soldering. It is used wherever two different metal piping systems must be connected without galvanic corrosion at the interface. In refrigeration and HVAC, the most common need is connecting copper refrigerant lines to aluminium evaporator coils — a direct copper-to-aluminium contact would corrode the aluminium rapidly; a bimetal flange provides a hermetically sealed, corrosion-resistant transition.
Brazing uses filler metal above 450°C (BAlSi-4 at ~590°C for aluminium, BCuP at ~750°C for copper-to-copper). The higher process temperature produces a stronger bond — tensile strength ≥ 170 MPa for BAlSi joints — enabling PN25/Class 300 pressure ratings and service from −45°C to +150°C. Soldering uses filler metal below 450°C (SnZnAl at ~220°C), producing a softer joint with lower tensile strength (~50–80 MPa), suitable only for PN10/PN16 service at −30°C to +120°C. Brazing is required for all high-pressure refrigerants (R410A, R32); soldering is sufficient for lower-pressure HVAC air-side applications.
Bimetal brazing flanges are compatible with all HFC, HFO, and HCFC refrigerants: R410A, R32, R134a, R407C, R404A, R448A, R449A, R1234yf, R290 (propane), and others. Both copper and aluminium are chemically inert to these refrigerants and their lubricating oils (POE, PAO, mineral oil). Exception: Ammonia (R717) corrodes copper and copper alloys. Ammonia refrigeration systems must use steel or aluminium flanges throughout — bimetal copper/steel or solid copper brazing flanges are not suitable for ammonia service.
Bi-metallic blanking brazing flanges (also called bimetal blind flanges) have a solid face with no bore — they seal and cap the end of a bi-metallic pipe run. Common uses: (1) Capping a branch during hydrostatic or pneumatic pressure testing; (2) Permanently sealing a decommissioned pipe branch while maintaining system integrity; (3) Closing the end of a bi-metallic header manifold; (4) Providing a future connection point by capping with a blanking flange that can later be replaced by a brazing flange when the branch is needed. They mate directly with standard brazing flanges using the same bolt circle and face standard.
A solid brazing flange is technically a single-metal flange (copper or brass), not a bimetal composite. It is grouped in the bimetal flange family because it is joined to piping by the same brazing process and is designed to work in conjunction with bi-metallic brazing flanges in refrigeration and HVAC systems. A solid brazing flange provides the all-copper termination point — at a compressor, valve, or equipment nozzle — while a bi-metallic brazing flange provides the copper-to-aluminium transition further along the circuit. Together they form a complete bi-metallic piping system.
CO₂ transcritical high-side pressures reach 100–130 bar — far above standard PN16/PN25 bimetal flange ratings. Standard bimetal flanges are not suitable for CO₂ high-side service. The CO₂ low-side (35–40 bar, −40°C) may be feasible with special high-pressure bimetal flanges designed to Class 300 or Class 600 ratings. Contact Tesco Steel's technical team to discuss CO₂-specific requirements. For CO₂ systems, fully stainless steel or copper-nickel flanged connections to Class 600/900 are the more common engineering solution.
Bimetal flange faces follow EN 1092-1 (PN10/PN16/PN25) or ASME B16.5 (Class 150/300) dimensions for the bolt circle and face OD, ensuring compatibility with standard counter-flanges, valves, and equipment. Face types available are Flat Face (FF) and Raised Face (RF). Gasket selection: PTFE full-face sheet (most common — inert to all refrigerants), EPDM sheet for low-pressure HVAC, or spiral-wound gaskets with aluminium inner winding ring for higher-pressure refrigerant service. Do not use graphite-based gaskets with copper or aluminium flanges in refrigeration service.
Tesco Steel supplies bimetal flanges with full documentation: (1) Material Test Certificates (MTCs) per EN 10204 3.1 for both metal components (body and insert); (2) Brazing/soldering process qualification records per EN ISO 14903 or AWS B2.2; (3) Hydrostatic or pneumatic pressure test reports at 1.5× design pressure; (4) Dimensional inspection report per EN 1092-1 or ASME B16.5; (5) Third-party inspection (TPI) by BV, DNV, Lloyds, SGS, or other agencies on request. EN 10204 3.2 (TPI-witnessed) certificates are available for marine, offshore, and nuclear-adjacent applications.
Tesco Steel manufactures bimetal flanges in nominal sizes DN15 to DN300 (½" to 12"), covering the full range of commercial and industrial refrigeration and HVAC applications. Bore sizes are matched to standard copper tube ODs per EN 12735-1 (HVAC/refrigeration copper tube) or ASTM B280 (ACR tubing), as well as aluminium tube ODs per EN 12451. Custom bore sizes for non-standard pipe ODs are available on request with a manufacturing lead time of 4–6 weeks.
Tesco Steel & Engineering exports bimetal flanges from Mumbai, India to 96 countries including the United Arab Emirates, Saudi Arabia, Qatar, Kuwait, Oman, Bahrain, United Kingdom, Germany, Italy, Netherlands, Belgium, Spain, France, Australia, Singapore, Malaysia, Thailand, South Africa, Nigeria, Egypt, and the United States. All export consignments are packed in wooden export crates or heavy-duty plastic-wrapped pallets with moisture-proof film and desiccant sachets. Commercial invoices, packing lists, certificates of origin (CoO), and GSP/FTA documentation are provided for all exports.
Five elements plus commercial terms: (1) type — brazing flange, blanking (bi-metallic blind) or solid brazing flange; (2) size — the flange pattern and the copper tube OD the brazing socket must receive; (3) material pair — e.g. carbon steel + copper, stainless + copper, steel + brass; (4) service — refrigerant (ammonia, CO2, HFCs), design pressure and temperature range, so the rating and braze alloy suit the duty; (5) certification — EN 10204 3.1 or 3.2 and pressure-test records. Add the quantity and destination and we return price, weight and delivery — normally within 24 hours.
Tesco Steel & Engineering is an ISO 9001:2015 certified manufacturer based in Mumbai, India, producing bi-metallic brazing, blanking and solid brazing flanges — steel or stainless bodies metallurgically bonded to copper or brass brazing interfaces, pressure-tested and supplied with EN 10204 3.1/3.2 certification. Flanges serve HVAC-R, ammonia and CO2 refrigeration, and cryogenic systems, and are exported to more than 50 countries.
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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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