A question for those who know about casting techniques....
Was watching this the other day:
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(kind of explains the price they charge for their planes etc!)
During the section in the foundry, after decanting molten metal into the first intermediate container, you see a chap throw a block of something into the mix which results in lots of "activity" and bright white light production from it? What is all that about?
Also at various stages they also seem to chuck tubs of a power or similar in...
I had 16 years in different foundries until they (nearly) all shut down.
It's a magnesium compound to change the plain cast iron into spheroidal graphite (SG) iron. The way they do it in the video is a bit old fashioned, we've been doing it during the pouring of the metal here for
30+ years ( I think it was a patented method when first brought in). It makes the cast iron far more ductile, with up to 25% tensile stretch before it breaks. Chucking in on top of the ladle does not always distribute the magnesium evenly.
Rice crispies, (or snap crackle and pop). Sounds just like it when you throw it on. The slag (impurities) are molten on the surface of the iron, you sprinkle this powder on, and it coagulates the slag, and sometimes sets it into an easy to remove lump. The extra flashing is the magnesium being mixed in that was floating on the surface.
Afair, it's a lump of calcium carbonate, which forms a slage on the top, which you see the bloke raking off. Used to see them doing that down the steelworks every day.
Depends on who is doing it. One place I worked, there would be an intermediate spout, a bit like an elongated, flat, funnel. Before the metal was needed, 20kg or so of magnesium mix was put inside the funnel (for 500kg of cast metal), then the metal poured from the furnace, through the funnel, and into the pouring ladle. It mixed as it went through, and was pretty much fool proof, as it would be mixed again as it churned about when entering the large ladle, ensuring all of the metal had its influx of magnesium.
You'd then put the collagulant on, scrape off the slag and impurites, then pour it when the temperature was right.
You had a limited time to do it, 5 minutes or less, as the magnesium burned off. With full treatment, all of the carbon in the metal would be in spheroidal shapes. As it went off, the carbon would gradually return to its natural state in cast iron, looking like corn flakes when viewed through a microscope - this is why grey cast iron is brittle - it snaps along the lines of graphite. SG does away with this weakness, adds tensile strength, but loses some hardness, but is far cheaper than steel to produce. Each pouring had to have a sample taken, and polished and viewed through the microscope. Something like less than 75% spheroidal would result in all of that batch being quarantined, possibly scrapped if the castings were no better than the sample.
Another place did it with the magnesium in the mould, in a little chamber, which swirled the metal round as it was poured in. This got away with the problem of the magnesium buring off, but there was the added problem of impurities in the finished casting due to the swirling and splashing inside the mould, which could be negated slightly by designing some curves and stops on the runnings bars (where the molten metal runs to get to the finished piece mould).
Thanks for that Mr. Lee, I'd always wondered how they made grey cast iron into the SG stuff but had never tried to find out.
Presumably, a few of the eye-wateringly expensive planes might shatter if dropped?
A lot of the cast iron stuff you see sold in the UK (wood burning stoves, typically) is obviously made in some Asian sweat shop foundry. I believe that most of it is grey cast iron but don't know for certain. Would you expect it to be the grey, brittle variety? Is the magnesium additive stuff very expensive?
Yes, I would guess that most of the cast iron stoves, garden seat ends etc would be grey cast iron. Anything with a precision use (in the loosest terms), such as exhaust manifolds, pump bodies etc would be of one of the ductile alloys, making them easier to machine, and harder to break.
For cheap consumer goods with little problem if they do break, then yes it would be expensive. The expense would be mainly in the quality control of the metal - it doesnt work with too high carbon, sulphur, silicon, manganese and other elements in it. To get a good alloy, you need to use a good starting point, roughly 50% good quality steel scrap, and 50% old iron castings to give a good alloy.
For cheap castings, they would be likely to melt down anything that came in, giving an alloy that would not be suitable for any ductile treatment, but fine for big lumps that are not likely to be machined or bashed in use.
Does not the inclusion of scrap steel in effect mean the new alloy is also a form of steel rather than iron in the strict sense? Or is it a percentage thing when it comes to classification?
Percentage. Steel has a low carbon content, up to 1%, cast iron can go up to 5%. Other elements come into it as well, so it is very rare to be able to melt down old iron castings, and use it again, you usually have to add some things, and take away silicon and other elements.
Ductile irons need a balanced amount of each element, say 0.6% copper,
2.7% carbon, 1% silicon etc. Melting down old scrap cannot give this, so a mix of good quality steel (with its high iron content, and low other elements) dilutes the mixture, allowing you to adjust the alloy to get the result you want. You could add carbon, manganese and copper and other elements if needed.
When I visited Hepworth and Grandage???'s foundry about 40 years ago they used a mix of scrap in their electric furnace. they then blew oxygen through it and then sent a sample to the labs whilst th melt was on hold. When the results were returned they then threw in selected scrap and other additives to make the carbon (and other alloying elements) up to the required amount.
Presumably experience determined the mix of washing machines, old engine blocks, tin cans, girders, irn bru etc. used in the original mix.
Exactly the same process on a larger scale took place in the steelworks where I worked almost exactly 40 years ago on leaving school. Fascinating stuff, at the time. It occurred to me what a splendid way of getting rid of evidence it was.
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