Brazing-stainless,

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Brazing-stainless steel is considered easy to perform if a basic understanding of the materials is obtained and if the applicable processes are mastered.

In particular it is advantageous for joining dissimilar materials that cannot be welded together.

When properly applied Brazing-stainless permits to build strong and leak tight joints in thin wall items with minimum deformation.

Delicate assemblies can be performed even in inaccessible joint locations. Suitable methods are available for mass production.

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Brazing-stainless Steel Base Metals

It should be remarked that Stainless Steel is not a well defined material. This general name indicates all those iron base materials containing at least 10% Cr. The five main classes are briefly described in our page on Stainless Steel Welding.

When Brazing-stainless Austenitic (non hardenable) attention is required to avoid carbide precipitation (see a.m. reference).

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Ferritic types, non hardenable, (405,430, 446) may be subject to interfacial corrosion. BAg-21 is suitable for brazing type 430. High temperatures promote grain growth and fragilization. Self fixturing required.

Martensitic types, hardenable, low carbon (403, 410 and 416) and high carbon (440A, B or C) must have brazing combined with hardening thermal cycle.

Precipitation hardening types have brazing combined with hardening thermal cycle. Nickel plating or vacuum brazing is needed.

Duplex types need special attention to avoid embrittlement. BNi-7 was used successfully in hydrogen atmosphere.

The chromium content that gives to stainless steels their most important properties, mainly protection from rusting, produces on the surface a chromium oxide film that must be removed before wetting (by the molten filler metal) and Brazing-stainless can occur.

The same applies to titanium oxide that forms in stainless steel titanium stabilized grades (321). These oxides are tenacious and refractory, and must be prevented from forming again while heating, either by a suitable flux or by the presence of adequately protective atmosphere.

Once a brazing process has been developed with successful results, rigid adherence to process control measures must be assured.

Filler Metals

The selection of suitable filler metal for Brazing-stainless is based on the mechanical properties of the joints, on their corrosion resistance, on the service conditions including operating temperature, stresses and environment, on heat treatment requirements, on the Brazing-stainless temperature and on the heating method.

Furthermore joint geometry and thickness, brazing process, production required per unit time, cost, provisions against sensitization of unstabilized alloys and safety considerations must be taken into account.

Many types are available based on different material families. Most of them are included in

ANSI/AWS A5.8M/A5.8:2011
Specification for Filler Metals for Brazing and Braze Welding
Edition: 10th
American Welding Society / 17-Jun-2011 / 62 pages

Additional proprietary filler metals may be found in suppliers' catalogs.
Silver base Brazing-stainless filler metals are among the most used.
Alloys containing Zn or Cd get off noxious fumes that should not be inhaled by operators. The same are not suitable for furnace brazing.

If the Brazing-stainless temperature is very near to that of sensitization (See Stainless Steel Welding), there is the risk to impair corrosion resistance of the stainless steel involved unless the time at temperature is very short. In case of concern either the base metal or the filler would better be changed.

Additional filler metals were developed for use in vacuum service conditions.

The following Table lists some of the important Silver Base Filler Metals.

Table
Silver Base Brazing-stainless Filler Metals

AWS A5.8 Composition % Temperature C/F
Class Ag Cu Zn Cd Ni Other Solidus Liquidus
BAg-1 45 15 16 24 - - 605/1125 620/1145
BAg-1a 50 15.5 16.5 18 - - 625/1160 635/1175
BAg-2 35 26 21 18 - - 605/1125 700/1295
BAg-2a 30 27 23 20 - - 605/1125 710/1310
BAg-3 50 15.5 15.5 16 3 - 630/1170 690/1270
BAg-4 40 30 28 - 2 - 670/1240 780/1435
BAg-5 45 30 25 - - - 675/1250 745/1370
BAg-6 50 34 16 - - - 690/1270 775/1425
BAg-7 56 22 17 - - Sn=5 620/1145/ 650/1205
BAg-8 72 28 - - - - 780/1435 780/1435
BAg-8a 71.7 28 - - - Li=0.3 765/1410 765/1410
BAg-9 65 20 15 - - - 670/1240 720/1325
BAg-10 70 20 10 - - - 690/1275 740/1360
BAg-11 75 22 3 - - - 740/1365 790/1450
BAg-13 54 40 5 - 1 - 720/1325 855/1575
BAg-13a 56 42 - - 2 - 770/1420 895/1640
BAg-18 60 30 - - - Sn=10 1115/600 1325/720
BAg-19 92.5 Rem - - - Li=0.2 760/1400 890/1635
BAg-20 30 38 32 - - - 675/1250 765/1410
BAg-21 63 28.5 - - 2.5 Sn=6 690/1275 800/1475
BAg-22 49 16 23 - 4.5 Mn=7.5 680/1260 700/1290
BAg-23 85 - - - - Mn=15 960/1760 970/1780
BAg-24 50 20 28 - 2 - 660/1220 705/1305
BAg-25 20 40 35 - - Mn=5 740/1360 760/1455
BAg-26 25 38 33 - 2 Mn=2 705/1305 800/1475
BAg-27 25 35 26.5 13.5 - - 605/1125 745/1375
BAg-28 40 30 28 - - Sn=2 650/1200 710/1310
BAg-33 25 30 27.5 17.5 - - 640/1180 715/1320
BAg-34 38 32 28 - - Sn=2 650/1200 720/1330
BAg-35 35 32 33 17.5 - - 685/1265 755/1390
BAg-36 45 27 25 - - Sn=3 640/1185 680/1260
BAg-37 25 41 32 - - Sn=2 690/1270 780/1435

Notes:
  • BAg-1 and
  • BAg-1a: General purpose brazing alloys for thin clearance joints. All metals. Not for corrosion resistance.
  • BAg-2 and
  • BAg-2a: Economic general purpose low temperature brazing for variable gaps. Higher brazing temp. Not to be used for prolonged heating time because of liquation.
  • BAg-3: Recommended for brazing stainless steel and tungsten carbide tips. Improved corrosion resistance. Not for food.
  • BAg-4: Recommended for brazing stainless steel and other metals. Food compatible.
  • BAg-5: Suitable for pipe brazing, oil coolers, lamp assemblies. Cadmium free and
  • BAg-6: General purpose for higher brazing temperature.
  • BAg-7: Cadmium free low temperature brazing. Food compatible.
  • BAg-8: For vacuum furnace brazing of silver, copper or nickel base alloys. Also ceramic to metal.
  • BAg-8a: For furnace brazing stainless in protective atmosphere.
  • BAg-9 and
  • BAg-10: Color matching with silver ware. Used also for iron and nickel base.
  • BAg-13:For furnace brazing stainless in protective atmosphere.
  • BAg-13a: For fluxless brazing stainless in dry hydrogen atmosphere. No zinc fumes.
  • BAg-18:For vacuum applications and for contact with salt water.
  • BAg-19: For fluxless furnace brazing of stainless steels.
  • BAg-20: General purpose brazing filler for use on non ferrous metals.
  • BAg-21: For brazing 400 series stainless steels, corrosion resistant. Cd free, for food handling. Suitable for welding ferritic stainless type 430.
  • BAg-22: For brazing tungsten carbide tip. May be used for stainless and carbon steels.
  • BAg-23: For applications on stainless steel requiring good strength at elevated temperature.
  • BAg-24: Recommended for brazing stainless steel and other metals. Improved corrosion resistance. Food compatible.
  • BAg-26: Low silver, economic filler for steel and stainless steel.
  • BAg-27: Low silver, economic filler for all metals with larger clearance.
  • BAg-28: General purpose brazing filler for steel copper and nickel alloys.
  • BAg-33: For joining various materials with tight clearance.
  • BAg-34: Recommended for low temperature brazing steels, copper and nickel alloys. Suitable for refrigeration and air conditioning equipment.
  • BAg-35: Intermediate temperature filler, used with different materials.
  • BAg-36: Low temperature filler, cadmium free. Limited silver content. For narrow gaps.
  • BAg-37: Low silver, economic filler for steels and non ferrous alloys.

Nickel Base Brazing-stainless filler metals, suitable for higher service temperatures, are also quite frequently used, generally in furnace brazing. These materials are generally supplied in the form of powder or paste (powder with binder). Paste containers are generally kept under refrigeration before use, because the binder has an expiry date beyond which the paste should not be used.

Table
Nickel Base Brazing-stainless Filler Metals

AWS A5.8 Composition % Temperature C/F
Class Cr B Si Fe C Other Solidus Liquidus
BNi-1 14 3 4.5 4.5 0.8 - 975/1790 1035/1900
BNi-1a 14 3 4.5 4.5 0.06 - 975/1790 1075/1970
BNi-2 7 3 4.5 3 0.06 - 970/1780 1000/1830
BNi-3 - 3 4.5 0.5 0.06 - 980/1800 1035/1900
BNi-4 - 2 3.5 1.5 0.06 - 980/1800 1065/1950
BNi-5 19 0.03 10 - 0.10 - 1080/1975 1135/2075
BNi-6 - - - - 0.10 P=11 880/1610 880/1610
BNi-7 14 0.01 0.10 0.2 0.08 (1) 890/1630 890/1630
BNi-8 - - 7 - 0.1 (2) 890/1630 890/1630
BNi-9 15 3.6 - 1.5 0.06 - 1055/1930 1055/1930

Notes:
For all alloys in this Table: Ni=bal., S=0.02, Al=0.05, Ti=0.05, Zr=0.05
P= 0.02 unless otherwise indicated
(1) - Other: P=10, Mn=0.04
(2) - Other: Mn=23, Cu=4.5

Boron containing BNi-1 to 5 tend to corrode thin sheets.
Boron free filler suitable for nuclear reactors (boron absorbs neutrons).
BNi-5 has the highest melting point.

Additional filler metals based on copper, gold and precious metals, and cobalt are also in use for special applications.

Fluxes

Fluxes are necessary for Brazing-stainless steels in air, by torch or by induction. Manufacturers' recommendations as to their temperature of activity should be followed, depending on the filler metal used.

ANSI/AWS A5.31-92
Specification for Fluxes for Brazing and Braze Welding
American Welding Society, 24-Apr-1992, 23 pages

Atmospheres

The protective atmospheres used for Furnace Brazing-stainless steels are, besides vacuum, dry hydrogen and dissociated ammonia. The dew point should be under control and kept low for best results.

Occasionally an inert atmosphere of dry argon can be used if hydrogen is prohibited because of its reactivity to some metals.

Quality

The usual quality requirements are applied, depending on the application. Once the Brazing-stainless Procedure Specification has been approved, process control must be applied in all stages of production from preparation to final delivery.

Safety

Safety should always be a concern, but especially for the danger of inhalation of zinc and cadmium by operators in torch brazing, and for the danger of explosion if hydrogen atmosphere is used. In this case the furnace must have facilities for suitable management of emergencies like sudden loss of power. Training and refreshing of instructions should be repeated at regular intervals.

Resources

Modern Brazing of Stainless Steel
http://www.aws.org/wj/2004/10/030/

Brazing-stainless Steels
http://www.bssa.org.uk/topics.php?article

Brazing-stainless steel using Amorphous Brazing Foil
http://www.metglas.com/downloads/lit/br_st_steel.pdf

Brazing & Soldering Materials
http://www.brazing-and-soldering-materials.com/en/products/2762.html

The European Association for Brazing and Soldering (EABS)
http://www.twi.co.uk/j32k/unprotected/band_1/eabs.html

Fundamentals of Brazing [...]
PWL#035B and Bulletin 77.

An Article on New Silver-Free Brazing Filler Metals was published (4) in Issue 140 of Practical Welding Letter for April 2015.
Click on PWL#140 to see it.

An Article on Filler Metal for superconducting radiofrequency cavities was published (4) in Issue 145 of Practical Welding Letter for September 2015.
Click on PWL#145.

An Article on Aim for Small, Concave Braze Fillets was published (3) in Issue 148 of Practical Welding Letter for December 2015.
Click on PWL#148.

An Article on A new method of brazing stainless steel parts was published (7) in Issue 151 of Practical Welding Letter for March 2016.
Click on PWL#151.

Commercial Sources Publications

Hot-Zone Design Meets Tight Brazing Temperature Requirements
Industrial Heating.

High Temperature Brazing Alloys (16 pages)
Metal Joining.

Frequently Asked Questions
Metal Joining FAQ.

Filler Metal Technical and Safety MSDS Documents
Lucas Milhaupt.

Download your copy of the Brazing Book from
Handy & Harman.

Watch the following Video on

Brazing Brass to Stainless at FABTECH 2011

http://www.youtube.com/watch?v=-pbPddrtiTI

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