I think it’s fascinating and Thank You! Can you tell me what it means when a seller of cast cannons describes it as “grey iron”..?? Is that somehow stronger….or not…?Without getting too far into Metalurgy 101, suffice it to say that cast iron's most identifying trait is it has WAY too much carbon to form anything strong - like steel for instance which is between 0.2 and 0.95 percent carbon. Yes, the whole vast worldwide steel industry lives between zero and one percent carbon. (look up "iron-carbide diagram"). Dead-soft steel has around 0.2 % C and the highest-zoot tool steels have around 0.95% C.
Cast irons typically contain 4 to 6 percent carbon. The excess C forms tiny islands in the grain structure, greatly reducing tensile strength. Its cheap and easy to make in odd shapes, so its used for lots of things. Just don't pull on it too hard. Most cast iron is 3 or 4 times weaker in tension than in compression - like concrete. Welding it causes widespread thermal micro-cracks all around the area that can't be eliminated without re-melting the whole thing. The part is always compromised after that.
There are lower-carbon alloys like ductile or malleable iron which are stronger but also more expen$ive to make. Sorry for the lecture, I used to teach this stuff to engineering students, and I fully recognize "a little knowledge is a dangerous thing". There are a lot of people sticking cast iron back together thinking they are "welding" it - but in almost all cases it's just a bigger bag of invisible internal cracks with way less than original strength. Dick B.