Why is welding cast iron so difficult ????

In its molten state, weld metal is at its largest
(by volume)state. When weld metal cools it shrinks. This shrinkage "pulls" the casting
causing warpage and distortion. Cast iron is not ductile. If these stresses are high enough, and the restraint is strong enough, cracking will occur. Preheat/postheat is required - so that both the weld metal and the base metal will shrink fairly evenly and possibly avoid stress cracking. Welding iron castings is a both an art and a science. The welder has to know what is going on where, and why.

Ken has correctly stated that cast iron consists of one of two crystalline structures.

That grain structure goes through what is called Phase Transformation when subjected to elevated temps. Something known as a Carbon Diagram is used to observe these transformations.

Cast iron consists of pure iron with the addition of C in quantities greater than 2.1%. Anything less than 2.1% is steel. Steel manufacturers use alloying constituents to maintain certain compounds (that occur naturally at elevated temps) at room temp in order to obtain the desired charactistics for end use of the casting. If you are ever in need of a good sleep aid, try studying the grain structures such as pearlite, cementite, and the phases of austenite, ferrite, and martensite.

In general terms, the heat created by welding and brazing will duplicate one or more of the various transformation stages. You are remanufacturing metal every time you weld or braze. The rate of cooling probably has the greatest effect. There are other considerations too. The filler metal will mix with the base metal causing a re-alloying effect.

If someone is not sure of all the various considerations it is good to know a supplier who is knowledgeable enough to shed some light on them. These considerations consist of but are not limited to: type of cast iron, thickness, shape, mass, end use, previous environment, finishing requirements such as drilling, tapping, machining, joining to dissimilar metals, etc.

One cannot simply use one process, one filler metal, one procedure in every case. The same can be said for hardfacing and soldering. Otherwise as they say, "Your results may vary".
Steve

There is really no short answer to your question other than cast iron has a different grain structure than other metals.

Cast iron contains a high content of silicon and carbon. When the silicon content is high and the metal is allowed too cool slowly, gray cast iron is the result. When heated the carbon seperates in the form of graphite and this seperation of carbon from the iron that makes grey cast iron brittle.
They then add nickle, copper and chromium to give gray cast iron higher corrosion resistances and greater strength.

T_Bone