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One of the first things I do Bill, is determine what the casting is. You can look at the grain structure or do a spark test on the part to determine if it is cast iron or cast steel. Cast steel would be welded with a different filler metal and procedure than cast iron. A spark test will also tell you if the cast iron is ductile or not. The longer/brighter the sparks, the softer the material. Hard Durite castings exhibit very short, dull red sparks, for example. All cast irons start out as carbon steel. Carbon is added during the manufacturing process and generally ends up in the range of 3% to 4%. Mild steel is around .03% C for instance. Steel men refer to .03% as 3 point steel. When determining what process and filler metal to use, there are a few key considerations. Keep in mind that the rate of cooling when manufacturing cast iron is a large part of what determines the type of cast iron you will create. Slow cooling allows the graphites to form in layers in ductile iron. When cooled rapidly, the carbon is not allowed to precipitate into free grahites and a hard brittle grain structure results. A piece of malleable iron is transformed into white cast iron, sometimes referred to as "chilled" cast iron, when allowed to cool rapidly from critical temp. Keeping this in mind, think about the mass of the part. Is it practical to heat the entire part? If not, you will only create a large localized area of hot metal by brazing. The result will be quenching of the HAZ ( heat affected zone) by the cold mass. This will change the grain structure and most likely lead to stress cracking. On parts too large to pre-heat and maintain proper temps, the only other option is to cold weld the part. This is done with an arc welding process. An arc welding process allows you to employ techniques to limit heat input to the base material. Techniques to relieve residual stress in the casting are used in conjunction. There is not enough room here to explore all of the known techniques or advanatges of the different processes and filler metals. There really is no shotgun approach. We handle each case on an individual basis when making recommendations or writing procedures. We currently have over 7 different types of filler materials ( and 4 different processes) for cast iron repair and joining cast iron to itself or to unlike materials.
HTH,
Steve
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