Difference between cast iron and high carbon steel?

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I don't understand the difference between cast iron and high carbon steel. They both are an iron-carbon alloy and they both have a high amount of carbon within the iron metal. Both will oxidize at the same rate without a coating to protect the metal. Stainless steel will oxidize, it just takes way longer for it to happen. And I have seen both black rust and red rust forming on stainless steel and I know the difference between the 2 is the ratio of iron to oxygen.

But anyway, back to cast iron and high carbon steel. Both are very strongly magnetic compared to martenistic stainless steel(which is the most magnetic stainless steel there is)

The only real difference I have seen is that cast iron is more brittle and is always seasoned to prevent iron oxide from forming, whereas high carbon steel isn't nearly as brittle and isn't seasoned.

But why is there that difference when the amount of carbon is high in both, magnetism is strong in both unless it is treated to be demagnetized, and both will oxidize at the same rate if not seasoned? I have heard that with steel, they blow oxygen into the furnace but for cast iron they just melt the iron without blowing oxygen into it. But how does some extra oxygen make any difference? Does it oxidize the carbon? Does it oxidize the iron? What difference does that oxygen make that makes cast iron more brittle than high carbon steel when the ratio of iron to carbon is the same or at least similar?
 

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  • #2
Borek
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AFAIR blowing the oxygen is done to remove easy to oxidize impurities (including carbon) completely. Later you can easily add exactly the amount of carbon needed to get the requested composition.
 
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CWatters
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AFAIR blowing the oxygen is done to remove easy to oxidize impurities (including carbon) completely. Later you can easily add exactly the amount of carbon needed to get the requested composition.
I think that's the key to the Bessemer process. Prior to his invention nobody could work out how to control the amount of carbon in the steel because the ore starts with a variable amount. So the trick was to remove all the carbon then chuck in a known quantity.

Google suggests cast iron is a mix of iron and silicon and more than 2% carbon. Whereas high carbon steel usually has less than 2% carbon.

But I'm no expert.
 
  • #4
Baluncore
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Cast Iron is an old technology, a low cost material, with 2.06% to 6.67% carbon. The carbon is introduced from the fuel during smelting. The excess carbon takes the form of graphite flakes that weaken the material and make it brittle. Fundamentally, Cast iron goes through a eutectic reaction during cooling, while carbon steel does not.

The Bessemer process is used to lower the carbon content of cast iron to make a steel.

Carbon Steel is a high technology material with less than 2.06% carbon and a carefully controlled microstructure. The carbon is held as cementite, Fe3C. Carbon steel is used to make springs and knives because it is both flexible and hard.

See diagram and notes; https://en.wikipedia.org/wiki/Steel#/media/File:Steel_pd.svg
Which is from; https://en.wikipedia.org/wiki/Steel#Material_properties
 

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