When a Metallurgist met a Blacksmith

Meera -   For this week's Naked Engineering, I have a new partner in crime.  It's John Aveson, a research student from the materials science department at the University of Cambridge.  Now John, this week we're going to explore iron works and metal working...

John -   That's right.  Even though iron work has been done for about 4,000 years now, it's only the last 100 years or so that scientifically, we've kind of understood exactly what's going on during all of the processes.  So the art of the blacksmith is really to take a piece of metal and do a series of processes to it to give you the shape that you want, but also the properties that you want, and that really happens by controlling the structure really, down to the atomic level.

Meera -   So to see this in action, this week, we've come along to meet Gordon Bevan, a blacksmith based in Eltisley in Cambridgeshire.

Gordon -   We make gates and railings, fire grates, anything to do with ironwork.

Meera -   Well today, you're going to be making us a chisel.  What are the main requirements of a chisel?

Gordon -   We're going to form the material into a chisel shape and then we're going to harden it and then temper it.

John -   So basically, once we've heated it, then it gets really easy to form into a certain shape that we want because materials tend to get softer the hotter they are.

Meera -   Moving over to the forge now, what's the actual material burning underneath it.  What's the fuel?

Gordon -   It's coke.

John -   Coke is just a sort of coal that's been treated to remove things like sulphur.  Putting a load of sulphur in the iron will be really, really bad news for the properties of the work piece.

Meera -   You've got a piece of metal heating up here in a bright orange flame, but what metal is this?

Gordon -   It's a piece of tool steel.

John -   Tool steel is just an alloy of iron and carbon, fairly small amounts of carbon in there.  What makes it particularly good is that if you keep carbon contents low in iron, then it's possible to harden it really, really well without having the sort of brittleness of cast ions and other higher carbon content steels.

Meera -   So Gordon is just pulling the metal out now and the end of it is bright orange, so it's just burning hot.  What temperature must it be reaching, John?

John -   It must be about 900 Celsius.  Really, really high temperatures.

Meera -   And so, what are you able to do with the metal at this high temperature, Gordon?

Gordon -   The oranger it is or white, the softer it goes, so it's easier to hammer.  Now when I hammer it, it's going to be easier to handle.

Meera -   And John, what about the metal makes it extra bendy and malleable at such a high temperature?

John -   Well there's two things.  Atoms are just moving around a lot faster at high temperatures, so it's really easy to slide them past each other and permanently deform the material.  But also, irons got this really, really unique property that at about 700 Celsius, it transforms from being one crystal structure to another and the higher temperature crystal structures are a lot easier to form.  So that's two real reasons why it's so good to work when it's so hot.

Meera -   Hence, why I guess when Gordon is hammering it and shaping it, the end has become quite nice and flat and sharp now.

John -   Yeah, exactly.

Meera -   What's the next step?  You've heated it and you have bashed it pretty much into the shape you want.

Gordon -   We're now going to harden it.  We're going to quench it in some water.

Meera -   So you're just dipping it into what just looks like regular - it's a bit dirty water, it's a bit murky, it's brown, but it's not like it's even ice cold water.  It's just simply room temperature water that you're dipping this into?

Gordon -   That's right.  It's probably warmer than room temperature because it's standing by the forge.

Meera -   So a minute ago, this was reaching up to what, a thousand degrees or something and now, you're just handling it.

Gordon -   It's now hard and cold.

Meera -   So what has made it become hard simply by dipping it in this water, John?

John -   Iron, especially carbon irons have this remarkable thing that when you quench them down from really high temperatures like 1,000 Celsius that we saw before, they form this crystal structure called martensite.  It only happens when you cool things down really, really quickly and this structure is really, really hard, really hard to deform, hard to bend, but it's also is ridiculously brittle.  So if we were to hammer it or to bend it or something, it would just shatter straight away.

Meera -   So Gordon, you've made it malleable, bendy, and knocked into shape, hardened it by dipping it in the water.  What's next?  I mean, it's starting to really look like a chisel.

Gordon -   I'm just going to clean it up now on the linisher, sharpen the end, and then it will be down to getting it hot again and tempering it.

Meera -   You've placed it back into the forge now and one of the reasons you've cleaned it up is so you can see the colours at which it's burning?

Gordon -   That's right.  So I can see the colours, it should go straw.  If I go past straw, it will go to the blue which will be too soft so I need to stop it before it gets to the blue stage.

Meera -   And this is all the end of the chisel, so the bit that you've actually worked.

Gordon -   Yeah.

John -   The fact there are different colours gives us like a natural thermometer to tell us just by eye, how hot the steel is.  We want to get a mixture, between the hard, brittle martensite with the much softer, but much more ductile and malleable low temperature iron structure.  Just by judging the temperature, we can choose the mixture that we get between those two different types of crystal. That locks in the final properties of the tool.

Meera -   So as Gordon mentioned, he doesn't want it to get to the blue colouring which is far too hot and so, is that therefore too malleable?

Gordon -   Yeah.  That would end up too malleable.  Too much of the martensite would disappear and so, it wouldn't be hard enough to be a really useful tool.  It's this last step that really locks in the properties that give the tool what it needs to do its job.

Meera -   So looking at it now, it's got that straw colour which is what you wanted, but there is a bit of blue and so, you've pulled it out now.  Is that now just to cool it down a bit to stop it being that blue?

Gordon -   Yeah.  I'll stop it when the chip gets to the straw colour like it's getting straw now, so now I've got to cool it off.

Meera -   Interestingly this time to cool it, you haven't dipped it into room temperature water.  You've dipped it into a box of oil.

Gordon -   Used oil.  It's slightly better than water.

John -   The oil will take the heat out of the tool a lot more slowly and so, we can really lock in the crystal structures that we put in by just picking the colours perfectly on the tool before we quenched it.  It's a lot more gentle and it's a lot nicer process to the poor bit of metal.

Meera -   Okay so Gordon you're just pulling that out now and I don't really want to touch it.  It's covered in oil.

Gordon -   Just got to be wiped off and then we'll try it.  Yeah, it's cold now so we can try it on the side of the anvil to see if it will cut.

Meera - Is it any good?

Gordon -   It's not shattered and it's not dented.  It's perfect.

John -   So just using the skills of a blacksmith, Gordon's managed to control the atomic structure of a piece of iron to be able to cut another piece of metal perfectly. It's really, really amazing skills.

Chris -   Cambridge researcher student John Averson and blacksmith Gordon Bevan explaining the art of metal working to Meera.