So we have pure iron, and we add carbon and we call it steel. Ok cool, so then we also have cast iron, which is where you take iron and add more carbon. Yeah okay. And then we get pig iron, which is where you take iron and add even more carbon to it. So with steels we get high carbon steel and low carbon steel. Low carbon steel has more carbon than iron but less than iron. High carbon steel has less carbon than iron but much more than iron.
Huh? Cast iron has additives? I thought it was pure iron and was just brittle because it was cast and not forged
This is exactly the shit I'm talking about! You would be totally forgiven for thinking that and if you Google "what's the difference between Iron and Steel" you'll probably get some non-helpful answer like that, actually hang on:
Pure elemental iron has almost zero commercial uses, really only certain chemical processes that need it as a catalyst or raw material and even a lot of those use alloys to improve performance (see: iron catalyst in the Haber process)
Cast iron and pig iron are both extraordinarily high carbon iron alloys, which is why they're black. They're called iron because it's what you get if you smelt and forge iron ore at high temperatures with a charcoal or coke furnace, the carbon just naturally gets in there, although of course nowadays you carefully control cast iron carbon content to ensure it meets demands.
Wrought iron is almost pure iron, using a lower temperature process that literally hammers the impurities out of hot iron. Nowadays you get mild steel, also known as low carbon steel, which is used for anything low strength made of steel that needs to bend and flex. They're made in different ways but have similarly low carbon contents and thus similar ductilities.
The major innovation of steel is the ability to reduce and carefully control the carbon content in iron alloys during smelting and forging rather than haphazardly dumping tons of it in during the forging process. Cast iron is hard and brittle because iron alloys get really stupidly hard once you put more than like 1.5% carbon in there, and that's basically always going to happen in a hot charcoal fuel burning furnace. Steel metallurgy is a trip and a thing I'm not really qualified to talk about, I should read some metallurgy textbooks sometime.
Low carbon steel is like 0.005% or less carbon, high carbon steel is about 2%, cast iron is pretty much anything higher than that. There's overlaps between what's called iron and what's called steel if you're only looking at carbon content and there's some very technical distinctions made between them by metallurgists. Steel also tends to encompass a wide array of additional alloys like silicon steel, tungsten steels, etc.
As somebody who had worked with too many weird stainless steel alloys, you tend to forget about carbon content when you’re just trying to find something that doesn’t have so much damn nickel in it
The iron carbon phase diagram is very normal and totally clears this up.
It... kinda makes sense but the irony is not lost on me that really Cast Iron is Steel with Carbon added.
L + δ-ferrite + carbon ratio
