Super Steel

Steel has a huge variety of uses, from armoured cars to and inside jet engines. Do we still have more to learn about steel?
30 June 2014

Interview with 

Lucy Fielding, Cambridge University


Continuous casting of steel bars

Steel has a huge variety of uses, from armoured cars to and inside jet engines. Do we still have more to learn about steel? To find out, Ginny Smith talked to Lucy Fielding, a PhD student in  Cambridge University Department of Materials Science and Metallurgy, who works on super-high strength steels for armored cars etc.

Ginny -   Now, steel kind of feels like it's been around forever.  Are we really still finding out new things about it?

Lucy -   Well, as my supervisor used to say, the universe has also been around forever and we're still finding out new things about it.  So, you should never underestimate the new things that you can still discover.  One of the wonderful things about steel is that because we've known about it for so long, because it has been around for so long, and I think the oldest steel artefact that's ever been discovered is about 3,500 years old - so that's older than Stonehenge - is that we've had a long time to try and understand it.  That means that we're much more advanced in terms of where we are because we've had all of that preliminary work done for us back in the last century.

Ginny -   So, what exactly is steel?  I know it's a metal, but it's not on the periodic table.  It's made up of some other things.

Lucy -   No, exactly, so steel is actually a mixture of iron and carbon.  So, it's an alloy.  You can also add in other elements to do different things, but in reality, what you have is, you have pure iron that contains 0% carbon and then you add a little bit of carbon in, so maybe, anything from very small amount to moderate amounts like maybe 4% carbon, you might get away with calling that steel.  If you keep adding carbon, it goes back to being iron again which is a bit odd, but we've got so small window, iron with a little bit of carbon in is steel.

Ginny -   Why is it so useful?  Why do we use it for so many different things?

Lucy -   Well, there's two reasons really.  One, it's cheap and two, it's everywhere.  Iron is pretty abundant in a lot of places around the world, and that is what in a sense makes it cheap.  It's relatively easy now to extract.  It's easy to work with and it's also able to be recycled.  So, we want to try and use it for as much as possible because of its cheapness, because we don't have a problem with running out of it.

Ginny -   Now, you said you work on super strong steels.  What do you have to do to this stuff to make it that strong?

Lucy -   Well, you can do quite a lot of things fundamentally and we have to understand what strength means when we talk about metals.  When we say strength, what we mean is, how much force can we apply to it before it permanently deforms.  So, we have to understand what happens when that metal deforms.  On an atomic level, what is essentially going on, is the atoms are moving relative to one another.  Now, in a regular metal, well, in any metal really, those atoms are bonded together.  So, if you're going to start making them move past one another and change their position, you're having to break through all those bonds.  And that's what requires you to apply a force.  That's what requires you to put energy in.  So, what we try and do as metallurgists, is we try and find ways to stop the atoms being able to move past one another so easily.  So, one method might be to actually try and deform the material a lot because then the atoms move around a lot.  They get all jumbled up.  They get in each other's way and it becomes progressively more difficult.  Another method might be to mix in other elements, so other atoms of different shapes and sizes.  So, carbon is one.  Typically, high carbon steels will be stronger than low carbon steels, but you can also add in other elements like aluminium, like silicon, like chromium.  Suddenly, you've got atoms all in different shapes and sizes everywhere and again, that makes it difficult.  We can even do something called precipitation and that's when we try and make a second compound appear inside the first compound.  So, you might be familiar with precipitation.  We dissolve salt in water while it's hot, cool it down and the salt starts crystallising out.  And you can do something a little bit similar with metals and these blobs of precipitate of a second compound will get in the way of the atoms and stop them moving.  I think Dave has got some little experiments to illustrate how we can change the behaviour of steel with some fairly simple techniques.

Dave -   So at the moment, if you're in the audience, we should have some paper clips being handed out.

Kate -   So, these aren't special paper clips.  They're not special science paper clips.  They're just normal ones that anyone could buy.

Dave -   These are just perfectly normal paper clips.  What I want you to do is just sort of stretch your paper clip out a bit so you can get at it, forming a bit of a kind of quadrilateral.  So, just stretch it out so it's not overlapping itself anymore.  Now, what I want you to do is we would think about distorting this material.  So, Lucy was talking about how materials get stiffer as you distort them more often.  What I want you to do is pick a straight bit which hasn't been bent already and then just between your fingertips, bend it about 90 degrees and make a really nice sharp bend around with your fingernails and try and feel how hard that was.  Remember how hard that was.

Kate -   Just remember to do it with your fingertips because I was doing it earlier, sort of brute force because I need that for a paper clip and I couldn't feel anything.

Dave -   So, just bend it around one of your fingernails.  Remember how much force that required.  Now, bend it back again and see if it's harder or easier.

Boy -   Quite a lot harder.

Kate -   So Dave, should that have been harder or are we just getting weaker with every bend?

Dave -   So, that should be getting harder.  The more you bend steel certainly to start with, there are little intersections which the steel uses to bend more easily and they all kind of get locked up.  As you bend it, it gets stiffer and stiffer, and stiffer.  If you keep one going, it gets stiffer, and stiffer, and stiffer, and it will break 'brittely' and snap like Ginny's...

Ginny -   Mine broke.

Dave -   So, if you do it too often, and the material just gets weaker and weaker, and as it gets more and more brittle, the only way it can change shape is to crack, and that's really bad because your steel gets weaker.  So, that's kind of dull.  But one of the other reasons why steel is really, really important is, it's not just the material itself and how you bend it which affects the properties.  It's how you heat it.

Kate -   Yeah, I can see you're getting a blow torch here, Dave.  And we haven't got a blow torch under everybody's chair.  I'm very sorry.  I wanted to, but health and safety got in the way.  So, we'll light up the blow torch.

Dave -   And so, I'm going to heat up this paper clip.

Kate -   The flame is going a different colour.  Should that be happening?

Dave -   So basically, what we're doing here is flame test.  So, there's a little bit of sodium from my sweat on my fingers and that makes the flame go orange.  But the steel itself is now going orange.

Kate -   So, it's glowing sort of red-hot as I'd call it.

Dave -   And I'm just going to let it cool down.  It's actually orange hot.  I'm just letting it cool down to room temperature very slowly.  If you do this at home, beware that it stays hot longer than you think it is because it looks like it's not red anymore, but it doesn't mean it's not hot enough to hurt you.  I'll just put in some water to make it cold enough.  And now Kate, try bending that.

Kate -   It's really, really soft and malleable and sort of squidgy.  I've got also burned bits all over my hands now.  So, if I compare it to the other bit, it's a lot more effort whereas this bit is really soft and squidgy, should that have happened or you've just destroyed the paper clip?

Dave -   When I heated it up and let it cool down very, very slowly, that means that all of these imperfections, as they get hot, it kind of relaxed back their normal state and then they can move really easily for a while.  As you bend it, it should get stiffer, and stiffer, and stiffer until it gets really hard to move again.

Ginny -   So Lucy, you work on super strong steels.  So, you're not going to want to do what Dave just did and he start to make it really soft, but what kind of applications are you using these steels for?  Why do you want to make such strong steels?

Lucy -   Well first off, you're a little bit wrong in that, we always want to heat up the steel to make it soft.  This is a very common first step in steel production, getting it hot because when you see steel products, they are not in ingots - the shape that you might get them when they're being cast.  You often get steel in the form of plates in rods and I'm sure that Sinan is familiar with I-beams and other structural forms of steel.  So, you actually want it to be soft so that you change it into those shapes very easily.  The other reason that we want it to be soft is because it's a good first step when we're producing more complex structures to just heat it up so that it's nice and soft, and uniform, and then we can cool it down at different rates, and we can create much more complex structures.  These kinds of structures are immensely varied.  Honestly, I mean, you can use it for quite a lot of applications.  So, we see applications in oil pipelines, oil rigs, in nuclear submarines.  In the nuclear industries, there's a lot of steel.  I work on high strength armour steels at the moment.  Another very common application for steels is bearings.  So bearing steels are very, very common, some of the most efficient, high performance bearings in the world are made from steel.  And lastly, jet engines, which is a fact that not many people know.  So, you're very familiar probably with nickel super alloys and things like that that are used in the most advanced jet engines.  But many people are unaware including sadly some people who actually make jet engines that the central shaft of the jet engine that you mount all of the turbine blades on is still commonly made from steel which needs to be extremely high performance material.  So, there's a huge range of applications.

Ginny -   So, who's got some questions for Lucy about steel?  Yeah, we've got one at the back there.

Sam -   My name is Sam.  I'm from St. Ives and I'm 12 years old.  I might have misheard this in like a science lesson, but if you mix like, I don't know what element is, would you make like a steel called Damascus steel.

Lucy -   So, you're asking about Damascus steel.  That's quite interesting.  Now Damascus steel is a very high performance steel that used to be manufactured quite a long time ago.  If I'm perfectly honest with you, nobody really knows how it was made, but it is remarkably hard.  It holds a very good edge.  So, it was often used for cutting blades and in fact, I think that Damascus steel was paid as tribute to Alexander the Great at one point because of such a finely prized material.  Now, there probably wouldn't have been a lot of control over the elements that went into that steel because typically, in the ancient world, when they made steel, it wasn't very good.  They couldn't control the quality of it.  They didn't necessarily know about getting elements like vanadium or chromium that we would add into steel today.  So, a lot of the old steel makers, they had to work with what they had.  Nevertheless, they managed to come up with some quite extreme examples of high performance materials and Damascus steel has often been associated with carbon nanotubes.  There is some speculation which is questioned, it's not certain that actually, the way in which carbon was added into the steel and the manufacturing process was able to generate carbon nanotubes which are present within the Damascus steel.  I'm a little bit unsure of the veracity of that and I'm not sure how much of an effect that would have in reality, but it's a bit of a gimmicky claim.

Dave -   I've read recently that it was also associated with a certain ore body, a certain form of ore which they're digging up to make the iron from and that ran out and then they couldn't make any more Damascus steel.

Lucy -   Yeah, that's quite possible.  I mean, a lot of the steel around the world obviously was just made from what they can dig out of the ground.  So there would be very different composition or variations just dependent on the geology of the area.

Ginny -   Kate, do we have a question from Facebook?

Kate -   Yes, talking of carbon nanotubes, Jaehyeon Hwang asks, "Carbon fibre and carbon nanotubes are materials with lots of potentials.  I wonder how those kind of materials are being applied to your area you study for and if you can think of any commodities, let's say, 20 years from now, that might have been made from carbon nanotubes and carbon fibre."  So, are carbon nanotubes and carbon fibre going to take over from steel?

Lucy -   I have to be careful not to be too rude about this because it's well-known in my field that if you insert the phrase 'nano' into your research, you get a lot more grant money which is why the steels that I work on have now been renamed nanostructured steels.  Now, carbon nanotubes did indeed attract a lot of excitement, a lot of interest when they were made because they drew some exceptional claims.  People were claiming that they had this strength of 3,000 gigapascals.  Now, just to put that into context for you, the strongest steels commonly in use today, excluding the kind of very specialist high-end steels are maybe about 2.5 gigapascals.  So, that's a factor of over a hundred in terms of difference.  Now, what their paper about carbon nanotubes didn't mention is that the carbon nanotubes that they use to measure the strength were 300 nanometres long.  That's not very big.  In a structure like that, we have something called a perfect crystal.  So essentially, there are no defects in the crystal whatsoever.  There are no missing atoms.  It's a perfect structure of atoms.  Each links to one another in a regular arrangement.  Now, steel derives its strength as I explained earlier from atoms all being higgledy, piggledy and not really being able to move for that reason.  And that means that you can achieve this strength state in steels of any size.  The problem with carbon nanotubes is they derive their strength from being perfect.  So essentially, the strength of a carbon nanotube is related to the strength of the covalent bonds between the carbon atoms which are incredibly strong.  So, you can obtain very high strengths from very small carbon nanotubes.  However, as a result of entropy which says that we will get disorder in a system, if you make a carbon nanotube bigger than about 2 mm, it starts to get defects in it and its strength absolutely plummets.  Now unfortunately, there was a lot of excitement around saying, "Space elevators.  Let's make space elevators out of carbon nanotubes..." to which the response of the steel community was, "We can build a space elevator out of carbon nanotubes as long as space is only 300 nanometres away."  Carbon fibre is different. Carbon fibre is an exceptionally interesting material because it's what's known as a composite material.  So, it's made up of woven fibres of carbon which are very, very strong, very tough, and when mixed with epoxy resin to make a composite, provide a very lightweight, very strong, very tough material.  But it is extremely expensive and that at the moment is its downfall.  It's also a lot harder to recycle than steels, but we are seeing it used in applications where lightweight strength is required such as racing cars and so forth.  Another similar material is Kevlar which I wanted to mention today because its inventor, Stephanie Kwolek died last week.  But that's become a very, very useful example of a composite material used in armour technology.  Where carbon nanotubes really come into their own is with their electrical conductivity process.  But it's thought that carbon nanotubes offer a lot a very promising potential for electrical transmission, electricity distribution and so forth.  But in terms of strength, I disdain it slightly.

Hannah -   My name is Hannah and I'm from Cambridge.  I was just interested.  You keep talking about recycling.  Is there a limit to the amount of recycling for the types of special steels that you're working on?

Lucy -   Well, I suppose there is in terms of composition.  In that, it takes quite a lot of work to get the composition right and a lot of these steels are very sensitive to composition.  So I suppose, if you were to sort of check everything into a big vat, you basically have lost control of your composition to an extent.  So certainly, we're looking at tightly control in the composition, it would be difficult to recycle them.  But there are a lot of applications for steel where the composition doesn't matter so much.  So yeah, I'm not aware too much about that.  I imagine there might be a problem, but recycling steel is still very feasible.


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