Brazing Stainless Steel

Brazing Stainless Steel

Brazing Stainless Steel -
Introduction
Brazing of stainless steels is now performed as a routine operation. Stainless steel can be brazed, when welding is impracticable, using a silver-brazing alloy. Brazing can join stainless steel to a variety of dissimilar metals to which it cannot be satisfactorily welded.
The quality of brazed joints can often be improved by judicious selection of brazing thermal process, brazing temperature, filler metal composition and protective atmosphere or flux. In brazing, carbide precipitation can be minimized by making thermal cycle as short as possible.

Filler Materials
Stainless steels may be brazed with the help of following filler material rods:

(A) Silver Aloy Filler Rods
	Ag	Cu	Zn	Cd	Ni	Sn	(% Composition)	Brazing Temperature °C
(i)	45	15	16	24	-	-	...	620 to 760
(ii)	50	15.5	15.5	16	3	-	...	690 to 815
(iii)	60	Rest	-	-	-	10	...	720 to 845
(B) Nickel Alloy Filler Rods
	Cr	Si	B	Fe	Others	Ni	(% Composition)	Brazing Temperature °C
(i)	14	4	3.5	4.5	0.75	Rest	...	1065 to 1205
(ii)	-	4.5	3.1	1.5 Max	-	Rest	...	1010 to 1175
(C) Copper Filler Rods
Cu 99.9% Min								1095 to 1150
Cu 86.5% Min								1095 to 1150

Silver alloy (ii) is probably the most widely used of the silver alloys. Silver brazed joints cannot be used for high temperature service. Recommended maximum service temperature is 205°C.
If corrosion resistance in service is sufficiently critical, either an extra low-carbon type or a columbium-tantalum-stabilized type filler rod should be used. Nickel-base filler rods are preferred where extreme heat and corrosion resistance are required, e.g., in the manufacture of jet and rocket engines, chemical processing equipment, etc. A filler rod with melting point around 650°C.is recommended minimizing the heat-affected-zone in the base metal.
Fluxes
A flux in powder; paste or liquid form is required for torch, induction and resistance brazing. Furnace brazing if carried out in protective atmosphere * needs no flux.

A stainless steel brazing flux contains
Boric acid
Fluorides
A wetting agent, etc
Surface Preparation
Before brazing the surface of the job should be cleaned of grease, oil and oxides, either mechanically or chemically. Mechanical cleaning involves the use of abrasive paper or cloth, filing or machining. Chemical cleaning can be done using a mixture of 15 to 20% HNO3 and 2 to 5% HF at 60 to 82°C.
Lap joint and butt joint are two basic types of joint designs used in brazing. A lap joint is perhaps better because of increased contact area that compensates for the lower strength of the alloy used as the filler material for brazing.

Brazing Methods
Stainless steel can be brazed by using following methods:
Torch brazing
Dip brazing
Induction brazing
Resistance brazing
Furnace brazing
Stainless steel can be brazed to other commonly used metals except
Aluminium
Magnesium and the reactive metals.
When torch brazing the stainless steels
(i) A strongly reducing flame is employed to prevent oxidation of the work-metal surfaces.
(ii) Silver alloy is used as filler material.
Induction brazing of stainless steels may be carried out in an air atmosphere (using a suitable flux) or in a protective atmosphere or in vacuum.

Given the same power input, ferritic and martensitic steels heat faster than austenitic stainless steels which are non-magnetic.
Furnace brazing of stainless steels is carried out either in
(i) A protective atmosphere i.e., dry hydrogen and dissociated ammonia.
(ii) Vacuum.
Furnace atmosphere is selected depending upon the degree of protection that must be given to base metal, the flow characteristics of the filler metal, the brazing temperature and cost.

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