08-30-2013, 07:31 PM
Incidentally this thread and all the talk about chilled or white iron, has made me think about my Metallurgy lessons from twenty years ago and I thought it might be interesting to those here to know a little more about why cast iron acts as it does, when really it's just Iron and carbon pretty much like steel is.
1. Cast iron has a lot of carbon compared to steel so much that the carbon clumps together into flakes of graphite when cooled slowly(precipitation) but even so, this graphite is in a matrix of steel so why is it so easy to machine? well, due to the other impurities mainly sulphur the "steel" matrix is a form of free machining sulphurised steel similar to F12L07 (or free machining leaded steel).
2, Chilled iron is when the the iron goes from it's fully liquified state where the carbon is bonded to the iron to a solid state very quickly, the "Iron Carbides" that are present in liquid form normally can't exist as solids but when rapidly cooled, there isn't enough time for the carbides to break down, so we end up with solid Iron carbide, now think about how hard silicon carbide can be and you'll realise why chilled iron is so tough, many people say that white iron has the carbon chilled out of it, or that there is no carbon present, but that isn't true the carbon is there but so well bonded that it can't be seen. You may have heard of "Pearlite" grains that are present in hardened steels well these are layered crystals of ferrite and carbides that also give great strength but with more ductility from the ferrite(pure iron) layers, incidentally they are called pearlite as a result of how they look under a microscope, they look kind of pearl like, very pretty.
3. more commonly known is why grey iron doesn't need lubricant, where many other metals do, this is as a result of the graphite flakes falling out of the surface as the material is machined and so lubricating the cutting or wearing action.
4, spheroidal graphite iron, or malleable iron is where additives (usually magnesium) are added to cast iron and these help the graphite to form little balls or spherites, these balls form less stress concentration points than flakes and also the process eats up some of the sulphur which reduces the free machining characteristics of the steel matrix, these two factors combined makes the SG iron more malleable, less brittle and causes the little curly shavings seen only on SG irons.
Please keep in mind this is straight out of my memory, so go easy on me if there are any errors but hopefully it might help explain why certain things happen when casting iron.
Best regards
Rick
1. Cast iron has a lot of carbon compared to steel so much that the carbon clumps together into flakes of graphite when cooled slowly(precipitation) but even so, this graphite is in a matrix of steel so why is it so easy to machine? well, due to the other impurities mainly sulphur the "steel" matrix is a form of free machining sulphurised steel similar to F12L07 (or free machining leaded steel).
2, Chilled iron is when the the iron goes from it's fully liquified state where the carbon is bonded to the iron to a solid state very quickly, the "Iron Carbides" that are present in liquid form normally can't exist as solids but when rapidly cooled, there isn't enough time for the carbides to break down, so we end up with solid Iron carbide, now think about how hard silicon carbide can be and you'll realise why chilled iron is so tough, many people say that white iron has the carbon chilled out of it, or that there is no carbon present, but that isn't true the carbon is there but so well bonded that it can't be seen. You may have heard of "Pearlite" grains that are present in hardened steels well these are layered crystals of ferrite and carbides that also give great strength but with more ductility from the ferrite(pure iron) layers, incidentally they are called pearlite as a result of how they look under a microscope, they look kind of pearl like, very pretty.
3. more commonly known is why grey iron doesn't need lubricant, where many other metals do, this is as a result of the graphite flakes falling out of the surface as the material is machined and so lubricating the cutting or wearing action.
4, spheroidal graphite iron, or malleable iron is where additives (usually magnesium) are added to cast iron and these help the graphite to form little balls or spherites, these balls form less stress concentration points than flakes and also the process eats up some of the sulphur which reduces the free machining characteristics of the steel matrix, these two factors combined makes the SG iron more malleable, less brittle and causes the little curly shavings seen only on SG irons.
Please keep in mind this is straight out of my memory, so go easy on me if there are any errors but hopefully it might help explain why certain things happen when casting iron.
Best regards
Rick
Whatever it is, do it today, Tomorrow may not be an option and regret outlasts fatigue.