Cast Iron?

Was amused by the suggestion that cast iron is only cast iron once it's been cast.

Does it not start off as pig iron albeit cast into ingots these days from the blast furnace?. So the foundry raw material for making iron castings will be an iron casting which is cast iron before it is melted. If you melt cast iron then it must me molten cast iron I think.

regards Martin

;O)

Carried pig iron to china once in the 80s and it's pretty crummy stuff, as you say, it's ingots

The problem I believe is in the mix of materials, it's like soup, once something is in you can't get it out so for good quality cast iron(or anything) you need a controlled quality mix from the very start

China produced millions of tons of junk pig iron in the cultural revolution, each town had to produce so many tons for the state or they got into trouble. The state realised after chopping down lots of forests etc that large dedicated foundries produce far better quality metal at far less cost, giant steelworks were the only real way for large quantities

Edited By Ady1 on 18/03/2022 14:41:26

A chemical engineer would roll his eyes at that! Chemistry is all about extracting and purifying elements and molecules from mixtures. In principle, almost anything can be extracted or produced by chemical and physical manipulations. Cheaper if the input is clean and pure, but nothing is impossible.

200 years ago the soup analogy was reasonable because no-one understood exactly what impurities were in ores, or what good and bad effects they had on metals, or how to remove them. Nowadays it's easy to get a chemical analysis, and there are many well-understood reactions and physical techniques that can be applied to remove unwanted content.

Cast Irons are usually made from pig-iron for cheapness, but can easily be made from mild-steel if need be. Cast Iron from alloy steel is possible but unlikely because there's no profit in it - there are better ways to recycle expensive alloy steels because Cast Iron is a cheap product.

Dave

Right chaps, lets dispell some myths here;-} When coke, limestone and iron ore are placed in a blast furnace and melted together, the limestone acts as a flux, and combines with the impurities in the ore, and the molten pure iron runs down through the coke to the bottom of the furnace, picking up a large amount of carbon on its way. When the furnace is tapped for iron, the iron runs out into radial channels with each channel having a series of oblong bowls with rounded ends arranged down either side. These are called pigs because they look like many small pigs feeding from a sow! They were sized at what was reckoned to be what 1 man could lift. This is what is known as "pig iron" The metal is pure iron with a high carbon content that makes it strong, but brittle, and is commonly known as cast iron. If you melt steel, especially mild steel, which has a low carbon content, and then cast it in a mould, the result will be cast steel, because it has a much lower carbon content.

Cast iron is used, very often straight from the furnace, to make steel. This involves removing most of the carbon, and then adding back various other elements and compounds that produce the specific alloy that is being made Varying the amount of carbon produces different kinds of steel, from mild steel with a carbon content of 0.16% to 0.29% to tool steel with a carbon content of 0.5% to 1.5% The only difference between cast iron and steel is the carbon content.

Edited By Phil Whitley on 18/03/2022 18:19:43

Well said Phil ! Though your last sentence is a sweeping generalisation that should and likely will be challenged ! Noel.

Keep it simple Noel!

OK, my go! The only difference between cast iron and carbon steel(s) is the carbon content.

Bill

The big local one I can think of is Mr Beaverbrook cut all the wrought and cast Iron railings in Edinburgh down for the war effort

This metal sat in a huge pile at the Carstairs Railway Junction until someone carted it off in 1946

You would think that someone could have chucked it into a big melt for making bomb casings or suchlike war consumables but no-one went near it

Edited By Ady1 on 18/03/2022 18:54:58

Correct Bill! Perhaps I should have said "The BASIC difference! I seem to remember a pasage from Tubal Cains book on hardening and tempering that a cube of cast iron with 24" (?) sides contains as much carbon as a sack of coke!

Phil

You are correct Phil, there is a great difference between the properties of cast iron and cast steel.

Interestingly enough, I had some cast steel cylinders made for a full size Norwegian railway locomotive. I went to the works and was very surprised at the high tech systems. Nothing remotely like a cast iron foundry!

This must have been 25 years ago, I still have not worked out why the cylinders were cast steel. Every loco I have worked on appears to have had cast iron cylinders! One of life's little mysteries, unless anyone on the forum has an explanation.

Andrew

If you type into google “phase equilibrium diagrams” it will show you how carbon content changes the steel into cast iron .And if you learn to read it it’s good for heat treating temps

Phase equilibrium diagram for steel

.

I realise that your rhetorical question was probably posted ‘for sport’ … but I’m intrigued :

the ‘suggestion’ by whom? and in what context?

MichaelG.

.

This makes reasonable sense of it: **LINK**

https://civilblog.org/2017/05/29/pig-iron-cast-iron-wrought-iron-whats-difference/

It was a thowaway byline in MEW letters by David Hall as a response to the statement "when molten cast iron is remarkably fluid" and goes on to comment that surely cast iron is not cast iron until after it has been cast. In the same spirit of humorous mischief I extended the time frame to ponder that had been cast after the smelting process so had some claim on the cast moniker.

On a more sensible note maybe the 'molten cast iron' can be considered as being so when considered as a mixture rather than as a structure as it is no longer crystaline.

regards Martin

Thanks, Martin

I must confess, I have been rather busy with domestic matters and haven’t even opened MEW yet.

MichaelG.

That's broadly right, but look closely and there are many different Cast Irons just as there are many different Carbon Steels. Better to see Cast Iron as a large family of related alloys rather than as a single metal. Machinery's Handbook lists 17 classes of Grey Cast Iron, which is a different group from White Cast Iron, Chilled Cast Iron, Alloy Cast Iron, Malleable Iron and Nodular Cast Iron. I guess at least 30 different cast-irons are available. They're all alloys of Iron containing between 1.7 and 4.7% Carbon, plus small quantities or none of Phosphorous, Silicon, Manganese and Sulphur. Alloy Cast Iron might contain Nickel, Chromium, Molybdenum or Copper as well.

Depending on how it's made Cast-iron varies from being outrageously nasty to an expensive high-end engineering material. The best and worst cast-irons are as different as chalk and cheese.

Ballast and sash weights were often crudely made by melting random scrap in a dirty coke furnace with almost no TLC: doesn't matter if sash weights contain inclusions and are chilled glass-hard. More care is needed making street furniture and ornaments, but not much. Backyard operations, scaled up. At the other extreme, high-end cast-iron is carefully made in a clean high-tech electric-arc furnace, centrifugally cast, and is good enough for aero-space.

The cast-iron used to make machine tools varies considerably: pre-WW2 machines often feature blow-holes, slag inclusions and other imperfections but it rarely matters unless something cracks. Later machines are frequently made of better Cast-iron, one of the Meehanites, because these have superior vibration absorbing qualities and are stronger. Not sure about hobby machines but I'd expect the cast iron to vary because they're made down to a price.

Dave

If you melt mild steel and cast it, the result will be cast steel, it will not become cast iron unless you add a large quantity of carbon to the melt!

Phil

The easy answer is I don't know without doing some research! It could be the fact that the piston rings are cast iron, and cast iron on steel is a good bearing surface Look at the number of lathes with steel spindles and cast iron bearing caps and linings. I would think that the steel would also stand higher pressures than the cast if the engine was worked really hard, but I am no expert on steam by any stretch of the imagination!!

Phil

Just a guess, but I think it's do with heat.

Cast Iron absorbs more heat than steel, and emits it too. Furthermore, because cast steel is much stronger than cast iron in both compression and expansion, the cylinder can be made much lighter.

When steam arrives in a metal cylinder, a large quantity of heat is wasted bringing the cylinder up to temperature, and keeping it there. Energy that should be turned into work by the piston is lost. A heavy thick-walled cast-iron cylinder wastes more heat than a thin lightweight cast steel cylinder, making the engine more costly to run than it need be. Locomotive cylinders being stuck on the side and air-cooled doesn't help.

Another possibility is less maintenance because cast-steel wears much less quickly than cast iron, reducing rebores and shifting wear to easily replaced consumable piston rings. Maintenance is a financial disaster because every minute off the track is lost revenue in addition to the cost of maintenance itself.

On a model engine, or full-size loco doing low mileage, I doubt cast steel has any advantage, but it would benefit a working engine averaging 80,000 miles per year for a few decades.

Dave

I am afraid that link contains many innacuracies, Coal is totally unsuitable for blast furnace fuel, and was tried, but the resulting iron was useless, so the changeover from charcoal to coke (Abraham Darby, 1709) meant that very good quality cast iron could be made in large quantities in a blast furnace. A cupola furnace is a smaller furnace and is used to remelt pig iron and scrap cast iron for use in a foundry moulding shop where many sand moulds have been made up to cast different items. It was charged with coke fuel and limestone to remove impurities, but the idea that PIG iron is an inferior quality is wrong, the quality depended on how well the blast furnace men operated their furnace, and the higher the qualty of iron they produced, the higher the price! wrought iron CAN be welded but

" Wrought Iron bends better than mild steel and is very corrosion resistant, hardly rusting over long periods. It is tough, malleable, ductile and easily welded, but the slag inclusions make it extremely difficult to obtain a porosity-free and crack-free weld. "

Phil