05/19/2008 (A long entry with very few pictures)
|I thought it would be interesting to talk about iron. You know the stuff we make our tools from. This is sort of an overview to make sense of the terms that are used to discuss iron as related to toolmaking.|
Iron starts out as some sort of ore. Somewhere in my house I have a big chuck of Magnatite ore from Chester, New Jersey. Magnatite is really high quality stuff, with so much iron in it magnets stick to the surface of the stone. But it's really hard material to mine and in spite of the quality when the Mesabi range in Minnesota. was discovered in 1866 the lower quality ore was softer, easy to strip mine, and by 1900 the mines in Chester were mostly abandoned.
Anyway, once you have the ore, which is basically a lump of rust (iron oxide) in one concentration of iron or another you have to separate the iron from the oxygen in a furnace. In the real old days the way you did this was in what was called a bloomery where you chop up the ore, mix it with charcoal, set it on fire, and blow air through it to get it hot. Bloomery iron doesn't get hot enough to really melt the ore and you end up with a mass of iron and impurities. The mass can be beaten further to force the impurities out (forming wrought iron - see below). This practice hasn't been done on any large scale in the West for hundreds of years, although very small bloomeries operated in the US in the 19th century as a local source of iron. In Japan swordmaking steel that is made from iron ore the traditional way is made starting with using a form of the bloomery process.
The usual method for making iron in the industrial age was using a blast furnace. Basically you build a large chimney, fill it with charcoal and ore, set it on fire, and blow air in the bottom,. The ore filters down, losing it's oxygen atoms to carbon monoxide, and pure iron, which has a lower melting point, drifts to the bottom of the furnace and gets tapped off as a liquid. Except there is a problem: while the iron was drifting down the inside of the furnace it was collecting some carbon, as big weak flakes of graphite - about 4%. This makes the iron brittle. and we get pig iron from the furnace. Pig iron, also know as gray iron, is recast into good, solid, brittle cast iron which is great stuff for table saw tables, and anywhere else you want a big heavy chuck of stable material.
Except cast iron, as cast isn't stable. As the liquid iron cools, depending on how it cools, and the shape of the mold, you can get all sorts of stresses in the iron, which the second you start machining the iron, come out and your nice casting warps all over the place. The solution is to age the castings. Basically, toss the casting out in the yard for a few months or years until it naturally stabilizes. This works but takes too long - so another method needed to be found. What is done is to anneal the casting. Basically heat the casting up in an oven and let it cool very, very slowly so all the stresses in the steel can be worked out. Kind of like a spa treatment for metal.
This works, and you get a pretty stable casting, which will machine nicely but remain brittle (the 4% of carbon/graphite). If you still want a cast shape and you need to get rid of the brittleness there are two approaches you can use: The first is malleable iron. Lots of iron alloy has silicon in it so when you smelt it the first time you get iron carbide instead of graphite in the iron (known as white iron), but if you stick the white iron in a sealed retort and keep it hot the iron carbide turn from nasty flakes to little graphite spheres, and the soft iron can flow around the graphite. This type of iron won't break if dropped, but it will deform. This is how modern plane companies such as Lie-Neilsen make plane bodies that will drop without breaking. Malleable iron has been around since the middle of the 19th century.
Another way around the breakage problem is called Ductile Iron and it's a much more recent invention. There are a bunch of ways to do it but the most interesting is a slow continuous annealing of the iron in an oven.
It's time consuming and expensive, takes two days in an oven and then the item has to be ground slowly to avoid heat which would change its characteristics. The advantage is that not only is the casting much stronger, it doesn't deform on impact. This is how Clifton treats the casting in it's planes.
However neither method turns iron into something that can be forged. Up until 1855 when the Bessemer process was invented the only way of getting iron in a form you could forge into horse shoes, tongs and other useful things was to take the ball of iron and slag from the Bloomery or blast furnace and heat it up and beat it with sledges until the slag and graphite crystals was squeezed out of it and you had wrought iron. In theory wrought iron is pure iron - which is why it is so soft and welds so easily, but in fact the process left a good many impurities in the iron. By the middle of the 18th century (I think) you could buy rolled sheets and rods of wrought iron ready to make into whatever. The plane pictured above is by Christopher Gabriel and dates from about 1790. This is when infill planes first begin appearing in quantity in the UK. The sides of the plane are wrought iron and the sole (which you can't see) is blister steel. You can see in the striations of the wrought iron the "grain" that resulted from fragmented impurities left in the iron and it was heated, rolled, folded over, and re-rolled, to remove impurities. In Japan old iron from this period is forged into the upper layer of chisels and plane irons and then the impurities are eaten away with acid, leaving a "mokume" decorative texture that looks like tree bark. In 1855 Bessemer (concurrently with the American Kelly) figured out that if you took melted iron and blew air into it you could get rid of all the impurities very quickly and then by adding a controlled amount of carbon back in you could make wrought iron - which we now call mild steel.
The next topic is how to take iron and convert it to steel with a cutting edge.
Note: Most of the iron information in this entry Ilearned form a pamphlet by Jack Chard called "Making Iron & Steel The Historic Porcesses: 1700-1900", published by the Roebling Chaper of the Society for Industrical Archiology" 1986
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