Brazing stainless steels
Most stainless steel types, with the exception of titanium or niobium stabilised grades, can be brazed.
The three main methods in common use for brazing stainless steels are:
brazing under reducing atmosphere
Brazing in air with flux
For brazing stainless steels in air with flux low-temperature silver brazing alloys are generally used. Details of this category of filler materials is to be found in Table AG in the BS/EN 1044:1999. These include 56% Ag: Cu: In: Ni, with a melting range of 600-710oC and 60% Ag: Cu: Sn, with a melting range of 602-718oC.
These are recommended for use where crevice corrosion failure of a brazed joint can be anticipated.
Fillers containing either cadmium and zinc can result in corrosion to the stainless steel due the formation of phases that can result in some preferential corrosion.
The fluxes used are often mixtures of alkali-metal salts, which are solid at room temperature.
These need to be melted before they can begin to dissolve the oxide film on the surface of the stainless steel to be joined. The flux must remain sufficiently fluid, even when heavily laden with dissolved oxides, for it to be ‘flushed out’ of the capillary gap by the advancing front of molten brazing alloy.
To do this the flux must become active, and start dissolving oxides, at a temperature which is at least 50oC below the solidus temperature of the brazing alloy being used and remain active, continuing to dissolve oxides, at a temperature that is at least 50oC higher than the liquidus of the brazing material being used.
The flux must also be capable of wetting, and remaining on, vertical surfaces and the residue should be capable of being removed easily from the work-pieces at the end of the brazing cycle.
No single flux satisfies all these requirements and as result a whole series of proprietary fluxes are available. The various families of flux are detailed in BS/EN1045: 1999.
As a general rule, flux is best applied to the joints as a paste.
Flux paste should be evenly applied to the mating surfaces of the joint and the area immediately adjacent to it with particular care being taken to apply a liberal quantity to any sharp edges on the components in the vicinity of the joint.
Pre-application of flux to an assembly is better than the application of flux to the joint during the heating cycle.
A summary of the points to consider in air brazing with a flux is
the quantity of flux applied to the joint needs to be adequate
the brazing time needs to be as short as possible
the brazing temperature needs to be as low as possible
the heat in-put to the work balanced so that no portion of the joint experiences an excessive temperature.
Brazing under reducing atmospheres
The reducing atmosphere furnace brazing of stainless steel has seen rapid, and ongoing expansion since the mid-1990’s. This was in response to demands from the automotive industry for fuel-rails and systems fabricated in stainless steel.
In this type of application chemical reduction of the surface oxides is relied upon to provide the oxide-free surface need to permit wetting and flow by the molten filler material. It is for this reason that brazing is generally carried out in a continuous conveyor furnace that is lined throughout with heat-resisting alloys so that the atmosphere can be contained.
Careful control of hydrogen, oxygen and water vapour levels is important in furnaces used for reducing atmosphere brazing.
Generally, copper, or copper-base alloys are used as the filler material in reducing atmosphere furnace brazing, and this means that brazing temperatures are typically in excess of 1085oC
In most applications where stainless steels are to be brazed in vacuum the high-temperature brazing filler metals are employed. The range of materials that are widely available being those listed in Table NI in BS/EN 1044:1999.
Vacuum brazing temperatures are usually ‘high’ i.e. in excess of 1000oC. This provides an opportunity to do some heat treatments as part of the brazing operational cycle.
As part of the process the furnace can be ‘back-filled’ with an inert gas in order to assist in ‘flushing out’ any residual air from the capillary paths of the part to be brazed. This gas is removed before the brazing operation begins. Inert gas can be used to speed up cooling after the filler has solidified.
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