Standard folded B-tube
Roll formed from one clad strip; the folded edges form the centre web and are brazed to the opposite wall in the CAB furnace. The lightest option, typically 0.20 – 0.30 mm wall.
Guide · Heat exchanger tube
Aluminium B-Type Tubes for Radiators and Heater Cores
A B-type tube is a flat, thin-wall aluminium tube with an internal web that splits it into two channels, so its cross-section looks like the letter B. It is roll-formed from brazing-clad strip, either folded or high-frequency (HF) welded, and brazed with fins and headers into radiator and heater cores. This guide explains the tube types, clad material structures, sizes, brazing, testing and how to match an existing core design.
- Forming
- Folded or HF welded
- Material
- 4343 / 3003 or 3005 / 7072
- Wall
- About 0.20 – 0.40 mm
- Use
- Radiator and heater cores
What is an aluminium B-type tube?
Modern brazed aluminium radiators are built from rows of flat tubes with corrugated fins between them. Coolant flows inside the tubes; air flows through the fins. The B-type tube is a refinement of the plain flat tube: a central web joins the two broad faces, so the tube resists internal pressure and does not balloon, even with walls of only 0.20 – 0.30 mm.
That web is the reason B-tubes became standard in passenger-car radiators. A plain flat tube needs a thicker wall to withstand the same pressure, because its long, flat faces bend outwards under load. By tying the faces together in the middle, the B-shape halves the unsupported span, so a thinner, lighter strip can be used. Thinner walls mean less metal, a lighter core and slightly better heat transfer, all of which matter to vehicle makers counting every gram.
B-tubes are made in two main ways. In a folded tube, the strip edges are rolled inwards and folded back to form the central web, and all joints are closed later in the brazing furnace. In an HF welded tube, the strip is formed into a flat tube and its seam is welded by high-frequency induction or contact welding on the forming line, with internal ribs, dimples or channels formed into the profile.
Folded, dimpled, ribbed and HF welded B-tubes
Dimpled folded B-tube
Small dimples pressed into the faces add internal support points and create turbulence in the coolant, which improves heat transfer at low flow rates.
Rib-reinforced B-tube
Combines longitudinal ribs with dimples for cores that see higher pressure cycling or need extra stiffness during core assembly.
HF welded B-tube
Strip clad with braze alloy on both sides, formed and seam-welded. Used where a core was originally validated on welded tube; walls of about 0.22 – 0.40 mm.
Multi-channel welded tube
Two-, three- or four-channel internal layouts, sometimes dimpled, for wider tubes in heavy-duty radiators where a single web is not enough.
Double-B and special profiles
OEM-specific cross-sections such as double-B shapes, made to an approved drawing with profile-specific tolerances.
| Comparison | Folded B-tube | HF welded B-tube |
|---|---|---|
| Forming route | Roll forming and folding; web joint closed in the brazing furnace | Roll forming, then seam welding on the line |
| Typical wall | ≈ 0.20 – 0.30 mm | ≈ 0.22 – 0.40 mm |
| Typical clad structure | Braze alloy / core / waterside sacrificial layer | Braze alloy / core / braze alloy |
| Strength of joint before brazing | Mechanical fold only | Fully welded seam |
| Best suited to | New lightweight designs, passenger-car radiators and heaters | Cores already validated on welded tube, wider multi-channel tubes |
| Main validation focus | Fold geometry, braze fillet at the web, burst pressure | Weld integrity, profile accuracy, burst pressure |
Clad material structures: 4343, 3003, 3005 and 7072
B-tube strip is a brazing sheet: a core alloy roll-bonded with thin layers of other alloys, each with a job. The cladding is usually a few to around fifteen per cent of the strip thickness per side, specified as a clad ratio.
| Layer | Alloy | Main alloying element | Function |
|---|---|---|---|
| Braze clad | 4343 (or 4045) | Silicon about 6.8 – 8.2 % (4343) | Melts below the core during brazing and flows into joints with fins, headers and the internal web |
| Core | 3003 | Manganese 1.0 – 1.5 %, small copper | Carries pressure and gives post-braze strength; well proven and economical |
| Core (higher strength) | 3005 or modified 3xxx | Manganese with added magnesium | Higher post-braze strength for thinner walls; Mg must be limited for CAB flux compatibility |
| Waterside liner | 7072 | Zinc about 0.8 – 1.3 % | More electronegative than the core, so it corrodes sacrificially and protects the core from coolant-side pitting |
Folded tubes typically use a three-layer structure such as 4343 / 3003 / 7072 or 4343 / 3005 / 7072: braze alloy on the face that forms the external and web joints, and the zinc liner facing the coolant. HF welded tubes more often use 4343 / 3003 / 4343 or 4343 / 3005 / 4343, with braze alloy on both faces so the inside ribs and outside fins can both be joined.
Because the waterside liner and core alloys are chosen together with the fin alloy (often a zinc-containing fin stock that is itself sacrificial to the tube), the whole core forms a corrosion-protection system. Changing one material without reviewing the others can shift which component corrodes first, so always share the full material list when matching a tube. See our brazing sheet and fin stock pages for the companion materials.
Common B-type tube sizes (W × H × T)
B-tubes are described by outer width, outer height (the tube "thickness" in the core) and wall thickness in millimetres. The sizes below are typical of the market; most projects match an existing OEM profile or drawing rather than choose from a catalogue.
| Tube type | Representative sizes W × H × T (mm) | Typical use |
|---|---|---|
| Folded B-tube | 16 × 1.4 × 0.22 · 20 × 2.0 × 0.20 · 21 × 1.3 × 0.25 · 26 × 1.5 × 0.22 · 27 × 1.6 × 0.25 · 29 × 1.4 × 0.23 · 34 × 1.75 × 0.20 | Passenger-car radiators, heater cores |
| Dimpled folded B-tube | 26 × 1.73 × 0.20 / 0.26 · 28 × 1.4 × 0.24 · 42 × 1.65 × 0.30 / 0.35 | Radiators needing internal turbulence |
| Rib-reinforced B-tube | 26 × 1.2 × 0.22 · 34 × 1.75 × 0.28 | Special core geometries |
| HF welded, two-channel | 16 × 2.0 · 22 × 2.0 · 26 × 2.0 · 32 × 2.0 with walls 0.20 – 0.30; 40 × 2.0 × 0.30 / 0.35; 52 × 1.8 × 0.35 / 0.40 | Validated welded-tube cores |
| HF welded, three-channel | 21 × 1.2 × 0.25 · 27 × 1.4 × 0.25 · 32 × 1.8 × 0.25 · 42 × 1.65–1.8 × 0.28–0.30 · 52 × 1.6–1.8 × 0.28–0.30 | Wider tubes, extra flow separation |
| HF welded, dimpled four-channel | 26 × 1.5 × 0.26 · 27 × 1.4 × 0.25 · 42 × 1.7 × 0.28 · 52 × 1.8 × 0.30 | Heavy-duty and off-road radiators |
For special welded profiles, drawing tolerances in the order of ±0.05 mm on width and height and ±0.01 mm on wall are typical of what OEM drawings call for. Tubes are supplied cut to length, with clean, burr-free ends ready for insertion into header slots.
Brazing considerations for B-type tubes
Most aluminium radiators are joined by controlled atmosphere brazing (CAB): the assembled core is coated with a non-corrosive potassium fluoroaluminate flux and heated in nitrogen to about 590 – 610 °C. The 4343 clad layer (melting range roughly 577 – 613 °C) melts and flows by capillary action, forming fillets at every tube-to-fin, tube-to-header and web joint, while the 3003 core (melting from about 640 °C) stays solid.
- Web joint quality: on folded tubes the internal web is sealed only in the furnace, so flux must reach the fold and the fold must be tight and even along the length. Poor fillets here lower burst pressure.
- Magnesium content: magnesium in the core or fin diffuses to the surface and poisons CAB flux. Higher-strength 3xxx cores therefore keep Mg low, or need more flux.
- Erosion: too much braze clad or too long at peak temperature lets molten silicon dissolve the thin core. Clad ratio and furnace profile must suit the 0.20 – 0.30 mm wall.
- Header fit: tube width, height and corner radius must match header slots so the joint gap is right for capillary flow.
- Cleanliness: forming oils must be evaporative or removable so they do not interfere with fluxing and brazing.
For a new design, share the header drawing, fin material and your CAB conditions so tube material and clad ratio can be checked against them before samples are made.
Inspection and testing of B-type tubes
| Check | What it confirms |
|---|---|
| Vision or optical measurement | Width, height, wall thickness and critical dimensions against the drawing |
| Profile projector / cross-section | Shape of the B-section, fold geometry, web position and internal structure |
| Eddy current testing | Continuity of the tube wall and weld seam, detecting holes, cracks and defects in line |
| Burst and pressure testing | That brazed tube samples withstand the specified internal pressure |
| Metallography | Clad thickness, weld or braze joint quality, and core erosion after a test braze |
| Material certificate | Chemical composition and mechanical properties of the clad strip, to EN 10204 3.1 |
For OEM and new-development projects, the inspection plan, sampling frequency, burst pressure and dimensional report format should be agreed during quotation and written into the drawing or quality agreement. Typical automotive programmes also require a first-article or PPAP-style sample approval before series supply.
Where B-type tubes are used
Passenger-car radiators
The main market. Folded and dimpled B-tubes allow lightweight cores with thin walls and good pressure resistance.
Heater cores
Small cores inside the HVAC unit, using narrow tubes such as 16 – 29 mm wide with walls around 0.20 – 0.25 mm.
Heavy-duty and off-road radiators
Wider multi-channel welded tubes for trucks, tractors and construction machinery, where vibration and pressure cycles are severe.
Aftermarket radiators
Replacement and performance radiators that must match the original core dimensions and tube profile.
Other coolant heat exchangers
Charge-air and auxiliary coolers in some designs, though many use microchannel or flat-oval tubes instead.
Alternatives
For refrigerant systems, extruded microchannel tube is usually preferred; for older cores, flat-oval radiator tube.
How to specify a B-type tube enquiry
- ProfileW × H × T and a cross-section drawing or sample, stating folded or welded design and the channel, rib or dimple layout.
- MaterialClad structure (e.g. 4343 / 3003 / 7072), clad ratios, temper and any OEM material specification.
- Length and endsCut length and tolerance, end squareness and burr limits.
- TestingBurst pressure, eddy current, dimensional report and approval sample requirements.
- LogisticsAnnual volume, packing that prevents tube deformation, and delivery terms such as FOB, CIF or DAP.
Metline supplies heat-exchanger tubes and the related clad strip, fin stock and brazing materials to core makers worldwide, matched to drawings and supplied with mill test certificates.
References and standards
- The Aluminum Association – International Alloy Designations and Chemical Composition Limits (3003, 4343, 7072)
- ASTM B209 – Aluminum and Aluminum-Alloy Sheet and Plate (includes clad products)
- EN 573-3 – Aluminium and aluminium alloys: chemical composition and form of wrought products
- ASM Handbook, Vol. 6 – Welding, Brazing and Soldering: brazing of aluminium alloys
- US Patent 6,615,488 – Method of forming heat exchanger tube (folded B-tube)
Aluminium B-type tube: frequently asked questions
Why is it called a B-type tube?
Because its cross-section has two flat channels separated by a central web, which looks like the letter B. The web ties the broad faces together so the tube resists internal pressure with a very thin wall.
What is the difference between a folded and an HF welded aluminum B-tube?
A folded tube forms its web by folding the strip edges inwards and seals all joints in the brazing furnace; it allows the thinnest walls (about 0.20 – 0.30 mm). An HF welded tube has a welded seam made on the forming line and typically 0.22 – 0.40 mm walls, and is used where cores were validated on welded tube.
What does 4343/3003/7072 mean?
It is the layer structure of the clad strip: 4343 aluminium-silicon braze alloy on one face, a 3003 aluminium-manganese core, and a 7072 aluminium-zinc liner on the coolant side that corrodes sacrificially to protect the core.
Can you match an existing radiator tube without changing my core design?
Yes, that is the usual route. Send the drawing or a sample with material structure and length; the same W × H × T can hide different internal structures, so the cross-section must be matched, not only the outer size.
How are B-type tubes brazed?
Usually by controlled atmosphere brazing at about 590 – 610 °C with non-corrosive fluoride flux. The silicon-rich clad melts and forms joints with fins, headers and the internal web while the core stays solid.
What tests should I ask for?
Dimensional checks against the drawing, cross-section inspection, eddy current testing for wall and seam defects, burst testing of brazed samples, and a material certificate for the clad strip.
Need B-type tubes matched to your core?
Send the tube drawing or sample, material structure, length and volume, and our team will review the profile and reply with a quote.
