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Recently, I was observing someone cut through a cast iron pipe and a cast iron radiator and even using a very powerful reciprocating saw, the cut was very slow. I have also previously had discussions with contractors who have also said that cutting through cast iron is about 5 times slower than cutting through steel.

Does anyone know why this is the case and exactly how much more difficult it is to cut through cast iron than steel? I had assumed that the lubricating effect of the carbon in the iron might be the cause but it would be really useful to know the actual reason.

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Cast iron is strong but brittle. If you hit it hard enough (like with a sledgehammer) it will just shatter. If you are pulling it out it is often easier just to break it up that way. –  auujay Jul 15 '11 at 16:08
    
Well it is weird.. cast steel is stronger.. but cast iron has more carbon.. maybe its just a safety reason.. maybe cutting cast iron to quick could cause it to shatter and ricochet into your body and potentially cut an artery and bleed to death.. i suppose. –  ppumkin Jul 15 '11 at 16:19
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@ppumkin: It is not that people intentionally cut it slowly (at least with a recipricating saw) you just can't get through the material. If you are actually cutting cast iron pipe I would highly recommend getting pipe cutter designed for the job. A recipricating saw can do almost anything, just not this. –  auujay Jul 15 '11 at 16:25
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2 Answers

Cast iron in it pure state is called grey cast or pig iron and in bygon times was used to tip cutting tool because it is one of the hardest metals they had .Normaly cast Iron is very easy to cut and drill as the carbon acts like wax ,but if the iron as been chilled when made or quenched in water it reverts back to gray cast iron.ps this also happens when casting thin pipes like drain pipes.

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A quick read gives the necessary information. Interestingly, cast iron is not truly pure iron. In fact, a simple steel may be closer to pure iron than than is cast iron. The difference is that steel has a carbon content of less than 2.1% by weight. More carbon makes the mix harder, more brittle, so less ductile. Cast iron contains MORE carbon than that rough upper limit of 2.1% for steel.

What happens? That carbon prevents dislocations from forming when you try to deform (bend) the cast iron. The cast iron simply won't bend. Instead, it breaks, it shatters under stress. This is why when one is removing cast iron, the best solution is often to shatter it into smaller pieces.

Ok, so what does this mean in terms of true cutting with a sharp blade, instead of abrasive cutting with a grinding wheel? If you attempt to cut steel with a tough, sharp edge that is harder than the steel, the edge actually peels a thin slice of steel away. The steel deforms as it is being cut. Thus a saw blade or a drill bit truly cuts off chips of steel. But if you look at each chip, you will see they are curved pieces of metal.

A grinding wheel is composed of an aggregate of pieces of a very hard material, typically very hard materials like Aluminum Oxide, Silicon Carbide, Diamond, and Cubic Boron Nitride. Each tiny piece of this hard material will also cut into the softer cast iron. But it cuts only a very tiny swath, resulting in much heat production and very fine particles flying off.

You should logically conclude that cutting the comparatively ductile steel is far faster, and far more efficient in terms of the work involved, since each cut produces a relatively large chip.

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That all sounds to me like a fairly good and interesting reason why. –  Ian Turner Jul 16 '11 at 12:23
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protected by Steven Jan 24 at 18:33

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