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Brazing-cast-iron:what to do and why.SOLUTIONS with Effective, Practical Advice
Brazing-cast-iron is a simpler joining method than welding. This occurs because cast irons, due to their high carbon content, are not easily weldable. However these are special problems that can be taken care of. The problems stem from the fact that graphite and silicon interfere with wetting of the surface by molten filler metal. Wetting is essential to generate the joint metallurgical bond. Therefore those repelling elements must be thoroughly removed by aggressive cleaning methods. Our previous page on Brazing treated in general of these important joining processes. Other pages, like this one, are devoted to different materials, as detailed further down.
Brazing-cast-iron is easiest to do on Malleable Cast Iron because it has lower carbon and silicon content. Moreover a special Heat Treatment causes carbon to agglomerate in irregular shape nodules of graphite. Next comes Ductile Iron that, due to the addition of minute amounts of magnesium to the molten iron, forms spherical graphite nodules. Proper preparation consists in removing the nodules from the surface. Third is Gray Iron where free carbon is present in the form of graphite flakes that interfere with wetting of the surface. White Cast Iron is rarely brazed if at all. Special Cleaning As hinted at above, cleaning is critical to the success of Brazing-cast-iron. It is suggested to check the efficiency of cleaning by testing all brazing variables. Oils and contaminants are usually removed by degreasing or alkaline cleaning. Abrasive blasting with steel or nickel shot is used to remove graphite, silicon and sand inclusions from the rough surface of ductile or malleable cast iron. Other pickling or chemical treatments can be used if they demonstrate adequate cleaning action. Carbon removal can be performed by searing the surface with an oxidizing flame or by furnace treating in a strongly decarburizing atmosphere between 870 and 900 °C (1600 and 1650 °F) for as long as needed. Gray cast iron castings are most effectively cleaned by immersing them in fused salts at 400 to 480 °C (750 to 900 °F). The cleaning action can be enhanced by superposing a direct current into the bath, and by changing polarity a few times. The so prepared castings must then be rinsed in water and dried to have them ready for Brazing-cast-iron. Filler Metals ANSI/AWS A5.8M/A5.8:2011
Most used for Brazing-cast-iron are silver base brazing alloys, a Table of which is reported in our page on Brazing Stainless Steel. These alloys have low brazing temperatures, suitable for a broad range of applications. In particular filler alloy types BAg-3, BAg-4 or BAg-24 containing Nickel have been used successfully with a black type flux (AWS FB3-C or AMS 3411) (See hereafter H&H publication). Nickel base filler metals also reported in our above page can be used for applications requiring chemical or corrosion resistance. Copper base filler metals exhibit higher brazing temperatures and may allow higher service temperatures. Although they cost less than silver base alloys (per weight units), they may need protective atmosphere furnaces, which tends to increase the associate costs. A Table of Copper Base Brazing Filler Metals can be found in our page on Brazing Copper. Some special copper base alloys, not included in AWS specifications, contain nickel and manganese to improve their elevated temperature properties. If not provided by hand when performing manual brazing, filler metal is usually preplaced at the joint as preforms or paste. Flux or Atmosphere Brazing-cast-iron needs the action of a flux (containing borates, fluorides and possibly boron to improve wetting) or of a protective atmosphere to prevent oxidation during the heating cycle. Although suitable, vacuum is not used because it is more expensive than acceptable alternatives. The usual methods described in our page on Brazing Heating are applied for Brazing-cast-iron. Fixturing To hold parts aligned while welding, the use of fixtures may be necessary except if the assemblies are self fixturing by interlocking provisions. Difference in the Coefficient of Thermal Expansion (CTE) must be taken into account and possibly exploited to the advantage of the process. Heat Treatments The methods of heating for Brazing-cast-iron should not degrade the mechanical properties of the base metal. The selection of filler metal should suit the heat treatment requirements. The best way would be to have all treatments performed before brazing and then to make sure that brazing will be at such low temperature and for such a short time that does not influence the previous heat treatment. Otherwise the brazing temperature should be carefully selected either to be the same as that required for heat treatment, or to be higher so that subsequent heat treatments will not damage the joint. A new website page is available on Casting-Repair. A new Resource page on Weld Repair of Castings is available at the link for Bulletin 105. Commercial Resource Publications Brazing Cast Iron - No Longer a Problem? A Flux for every Brazing Need
Note: Lacking a proper video example where brazing is done to join a capillary gap, the following video is suggested as an illustration of a cast iron braze-welding application. See the Calling this filler metal a Silver Solder is a misnomer as a solder liquidus should be less than 840°F (450°C). The filler metal used here, called SSF-6, is a commercial product that might be useful in appropriate cases. No endorsement or recommendation is intended in this presentation. Details on the filler metal, including composition and properties, are in the page
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Brazing
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