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.

Base Metals

The chromium content that gives to stainless steels their most important properties, 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 titanium stabilized grades. 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.

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 use, on the Brazing-stainless temperature and on the heating method.

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

ANSI/AWS A5.8/A5.8M:2004
Specification for Filler Metals for Brazing and Braze Welding
American Welding Society, 01-Jan-2004, 46 pages
Click to Order.

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.

The folllowing 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.
• BAg-2 and
• BAg-2a : Economic general purpose low temperature brazing for variable gaps.
• BAg-3 : Recommended for brazing stainless steel and tungsten carbide tips. 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.
• BAg-7 : Cadmium free low temperature brazing. Food compatible.
• BAg-8 : For 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.
• 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. 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

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
Click to Order.

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 prohibiteded because of its reactivity to some metals.

Quality

Usual quality requirements are applied, depending on the application. Once the Brazing 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

Commercial Sources Publications

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

High Temperature Brazing Alloys
Metal Joining.

Filler Metal Technical and Safety MSDS Documents
Lucas Milhaupt.

Cadmium Free Silver Base Filler Metals
Handy & Harman.

Frequently Asked Questions
Metal Joining FAQ.

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