What temperature is used for vacuum brazing of stainless steel?

Vacuum brazing of stainless steel with BNi-2 (AWS A5.8) nickel-based filler is performed at 1010–1066°C. For BNi-7 (lower melting point), the brazing temperature is 980–1000°C, which avoids sensitisation of 300-series stainless. Copper-based filler (BCu-1) brazes at 1090–1149°C under vacuum (10⁻⁴ to 10⁻⁵ mbar). Silver-based fillers are used for lower-temperature applications (620–870°C) but require controlled atmosphere or flux rather than high vacuum.

What joint clearance is recommended for brazing?

Optimal brazing joint clearance (at brazing temperature) is typically 0.025–0.125 mm (0.001–0.005 in) for most silver and nickel-based fillers. Clearances in this range allow capillary action to draw filler through the joint. Too tight ( 0.25 mm) causes weak, poorly filled joints. Allow for thermal expansion when setting room-temperature fixture clearances — for stainless steel brazed at 1050°C, add approximately 0.01–0.02 mm per 10 mm joint length.

Can aluminium be vacuum brazed?

Yes — aluminium vacuum brazing uses aluminium-silicon filler alloys (e.g. AWS BAl-Si-4, 11.7% Si) at 590–613°C. A high vacuum (10⁻⁵ to 10⁻⁶ mbar) is required to eliminate the tenacious aluminium oxide layer and allow wetting. The process is widely used for heat exchangers, aerospace structures, and automotive components. Magnesium (from the base alloy or from an Mg getter) acts as an oxygen getter and aids de-oxidation of the surface.

What is the difference between brazing and welding for heat exchanger manufacture?

Brazing joins materials at temperatures below the base metal melting point using a filler metal, relying on capillary action to fill tight joints. It produces lower distortion than welding, can join dissimilar metals, and seals complex geometries (tube-to-header, plate-fin) inaccessible to welding. However, brazed joints have lower strength than parent metal and are limited to temperatures below the filler's solidus. For heat exchangers operating above 400°C under pressure, welding or diffusion bonding is typically preferred.

Brazing Temperature & Filler Metal Selection Guide — Vacuum & Furnace Brazing

Brazing Filler Metal Reference (AWS A5.8)

AWS Class	Alloy System	Brazing Temp (°C)	Process	Base Metals
BCu-1	Copper (99.9%)	1090–1149	Vacuum / H₂	Steel, Ni alloys
BAg-7	Ag-Cu-Zn-Sn	650–760	Atmosphere / torch	Steel, Cu, stainless
BAg-24	Ag-Cu-Zn	700–815	Atmosphere / torch	Steel, Cu alloys
BNi-2	Ni-Cr-Si-B	1010–1066	Vacuum	Stainless, Ni alloys
BNi-7	Ni-Cr-P	980–1000	Vacuum	Stainless, low C steel
BAl-Si-4	Al-Si (11.7% Si)	590–613	Vacuum	Aluminium alloys
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Guide Coverage

AWS A5.8 filler metal properties and selection
Joint clearance by filler type and base metal
Vacuum level requirements by filler and base metal
Controlled atmosphere selection (H₂, N₂, dissociated NH₃)
Heating rate and soak time for complex assemblies
Post-braze heat treatment for age-hardening alloys

Base Materials Covered

Carbon and low-alloy steels
Austenitic and martensitic stainless steels
Nickel and cobalt superalloys
Aluminium alloys (1xxx, 3xxx, 6xxx series)
Copper and copper alloys
Titanium (with appropriate filler and atmosphere)

Selecting the Right Brazing Temperature and Filler Metal

Brazing temperature selection involves balancing three factors: the filler metal liquidus (must be exceeded for flow), the base metal temperature capability (must not cause distortion, softening, or sensitisation), and the process atmosphere capability (vacuum furnaces typically limited to ~1200°C). Choosing a filler metal with the lowest brazing temperature that gives adequate joint properties minimises base metal degradation and furnace energy consumption.

For vacuum furnace brazing — Bloor Engineering's primary process — nickel-based and copper-based fillers dominate. The selection tool matches base metal compatibility, service temperature requirements, and your furnace's maximum temperature and vacuum capability.

Select Your Brazing Filler & Process

Enter base material, joint geometry, and service requirements — get filler metal recommendations and brazing temperature in seconds. Free with a registered account.

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Brazing Temperature & Filler Metal Selection Guide

Guide Coverage

Base Materials Covered

Selecting the Right Brazing Temperature and Filler Metal

Select Your Brazing Filler & Process

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