Compatibility with base metals is an important factor when considering brazing processes. Some materials are readily brazed in their commercially available state, while others require one or more types of treatment prior to brazing.
Most materials used in the fabrication of assemblies for high-temperature service, such as 18-8 chromium-nickel and the nickel-chromium-cobalt materials, have oxides which are readily reduced by pure dry hydrogen or high-vacuum atmospheres. These materials can be adequately furnace brazed, therefore, without the use of flux and will come out of the furnace bright and clean.
Materials containing more than trace quantities of such elements as aluminum or titanium, if not treated prior to brazing, will form hydrides or oxides not reducible in a pure dry hydrogen atmosphere. With such base metals it is necessary to use a high-vacuum atmosphere in the furnace, or to treat them with a special surface preparation such as nickel plating or acid treatments prior to brazing in a hydrogen atmosphere. Zirconium, selenium and magnesium are other elements which can introduce problems in furnace brazing.
Brazing Stainless Steels
When brazing stainless steels in pure dry hydrogen, an atmosphere in the furnace having a dew point of -40°C (-40°F) or below is required to adequately reduce chromium oxide. Reduction of chromium oxide is necessary to facilitate flow and wetting of the nickel-chromium brazing alloy. The atmosphere requirements of stainless steels cannot be met by a brick-lined furnace, because the refractories or metal oxides will themselves reduce under the brazing conditions required.
Advantages of Furnace Brazing
(1) Furnace brazing can be used to simultaneously assemble as many as a thousand joints, depending on size. This means high production and reduced brazing costs.
(3) Dimensional control and contour stability are much improved in furnace brazing when compared to processes involving localized heating.
(4) Assemblies can be fabricated by furnace brazing that cannot readily be manufactured by other joining methods.
(5) Complex assemblies can be mass produced by furnace brazing without highly skilled personnel.
(7)Furnace brazing, when used to back up spot welds, improves the fatigue characteristics of the joints.
Furnace Atmospheres for Brazing
Specialized equipment is required for brazing base metal alloys for high-temperature service. This equipment generally includes a tightly sealed container or box, such as a retort furnace, in which pure dry hydrogen and other atmospheres can be maintained. A vacuum atmosphere is one of the modes used for certain applications.
While a pure dry hydrogen atmosphere is the most frequently used for brazing stainless steels, other atmospheres such as dissociated ammonia, argon, helium and vacuum can be used. The dissociated ammonia atmospheres are usually avoided when brazing alloys containing boron are used, because boron is readily nitrided and the brazing alloy will neither melt nor flow.
Argon and helium atmospheres are very compatible with nickel-chromium-boron brazing alloys, because the boron content makes the alloy self-fluxing, resulting in excellent wetting and flow properties. Similarly, at high temperatures, reduction or dissociation of metal oxides actually occurs; this is probably due to the increase in dissociated pressure of the metal oxide.
The lap joint is the most frequently used in the sign of brazed joint assemblies. A lap of approximately 4 to 5 times the thinner sheet thickness is generally preferred. Such a joint will seldom fail; rather, any failure in the brazed assembly will occur in the base metal.
Butt joints exhibit higher unit strength than lap joints, but are seldom used in brazed assemblies because of the difficulty in controlling clearance and alignment of adjoining sheet metal thicknesses. Many combination lap-butt joints are successfully brazed, however.
A poor braze is likely to result from a design in which the joint is either totally enclosed or located in a blind hole. Air bubbling through the brazed joint while cooling will cause porosity. In such cases a vent must be left in the chamber to eliminate the pressure and to purge the part if furnace brazing is used.
One important requirement of brazed designs is the need for inspection to assure brazing alloy flow. To provide a means of inspection, holes are incorporated in the back sides of a joint. When one side of the joint is sealed within the assembly, it is a good practice to put the brazing alloy on the inside. This forces the alloy to flow to the outside of the assembly, and permits easy and positive inspection.
When brazing with alloys such as nickel-chromium-boron and other nickel-base brazing alloys, certain characteristics of these brazing materials must be considered in the design of the assembly. The nickel-base brazing alloy is soluble in stainless steel base metals. It will, therefore, readily alloy with the base metal while flowing through the joint. It is this solubility and the resulting alloying action which provides the desired high-temperature strength, but it can cause some difficulties in the brazing operation itself if the assembly is not carefully designed. The primary problem is caused by erosion, which occurs when too much brazing alloy is applied in a single location, or when the brazing alloy flows to the bottom of the joint. The erosion problem is easily solved by providing proper flow paths for the brazing alloy, and by applying the correct quantity of alloy in the correct manner.
The erosion phenomenon is not confined to nickel-base brazing alloys. Aluminum brazing of all kinds, silver brazing of copper, copper brazing of nickel copper, and many others require the same attention to design and the same care in applying the brazing alloy to avoid the erosion problem.
Erosion can generally be avoided by providing a clearance of 0.05 to 0.10 mm (0.002 to 0.004 in.) for a joint of 3.2 mm (1/8 in.) or longer. This clearance allows sufficient time for the alloy to proceed through the joint before it picks up sufficient base metal to bring its melting point above the brazing temperature.
As in all designs, the proof of effectiveness is the service test of the actual part. Service tests of many assemblies brazed with nickel-base alloys indicate excellent performance at operating temperatures as high as 1100″C (2000 OF).