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  • Steel is the most important material to human society.

  • It has been the skeleton of human industry for centuries, and the advent of the methods

  • to mass produce it from iron ore was the impetus to transform human society from a mostly agricultural

  • lifestyle to urban industrialisation.

  • It forms the backbone of our skyscrapers, it paves the way for our railways, it shapes

  • the engines that powers our society, and the very tools that forge these objects are made

  • from steel.

  • We have spoken before about how the refining processes of iron determines whether the resulting

  • material will be steel or iron and how the exact percentage of carbon present in the

  • material has dramatic results on the final material properties, and how the production

  • methods through time has taken steel from an expensive material reserved for swords,

  • armour and toolmaking, to one that has permeated into nearly every technology we use in our

  • lives.

  • But I failed to tell why that simple addition of carbon has such a huge effect on the iron,

  • turning it from a relatively weak material to one capable of launching an industrial

  • revolution, which is what we are going to learn about today.

  • Much of our knowledge in crafting steel was passed down over the centuries from blacksmith

  • to blacksmith, creating tools for their communities, so to learn more about this amazing material

  • and how blacksmiths carefully tailor it's material properties, I visited Alec Steele's

  • workshop to create my own knife from scratch.

  • We started our forging process with round stock 1055 steel with 0.55% carbon content,

  • placed it in the forge and gradually shaped it with a power hammer to a rectangular bar

  • that could be more precisely shaped into the shape of our blade using a 3 pound hammer

  • that my wee arms struggled to swing after an hour.

  • Once we had roughed out the shape we began to grind and refine our blade.

  • Eventually producing our blade blank that would later be grinded to it's final shape,

  • but before that could happen we needed to perform some metallurgy wizardry through the

  • heat treatment process.

  • To drive home how important this step is, we tested 4 samples of the same material at

  • different stages of the heat treatment process.

  • This was our sample before the heat treatment process, which you will quickly see was the

  • weakest of the bunch.

  • This is the normalised sample, which was ductile with a low yield stress.

  • It took several hammer blows, which it absorbed through plastic deformation, not ideal characteristics

  • for a sword or knife.

  • Next we tested our quench hardened sample, which is stupid and dangerous and should not

  • be tried at home.

  • This fractured explosively and tore a hole through Alec's reflector.

  • Finally we tested the tempered material, which absorbed every hammer blow with minimal plastic

  • deformation and only broke when we cut a notch into the material to create a stress concentration

  • point.

  • This material is tough, capable of absorbing energy without deforming permanently, and

  • hard allowing it to resist damage to the cutting edge.

  • It is the ideal material for a blade.

  • If any of these terms confused you, I created a video called Material Properties 101 that

  • you can check out to get a better understanding of material property terminology.

  • So how can the same steel alloy change so radically by simply applying heat?

  • Well this is the magic of the iron carbon alloy.

  • We can careful control how the internal metallic crystal structure forms with heating and cooling

  • cycles.

  • First let's see how adding carbon to iron affects it's crystalline structure.

  • With no carbon present pure iron will form a crystal structure called body centred cubic

  • with an iron atom at each of the eight corners and another in the centre.

  • Each crystal structure has a direction it most easily wants to deform, called a slip

  • plane.

  • For body centred cubic the slip plane occurs along this planes.

  • Metals with Body centred cubic crystals like iron and tungsten tend to be harder and less

  • malleable than metals with face centred cubic crystals like aluminium, lead and gold.

  • When a metal is cooling, these crystals grow from individual nucleation points and form

  • grains where each grain has the same orientation of slip plane but neighbouring grains may

  • not have the same slip plane.

  • Let's think about this 2 dimensionally, when a force is applied, the grain wants to

  • slip in a particular direction, and passes the force onto the next grain in that direction

  • too, but this grains slip plane is at a different angle, so that force needs to be greater in

  • order to cause deformation.

  • It's like trying to push a train down a railway track, by pushing on its side.

  • It's not going to go anywhere easily.

  • So smaller and more numerous grains results in a stronger material.

  • Pure iron tends to always has the same crystal structure as it cools and it's crystal structure

  • doesn't change meaningfully with heat treatment.

  • This is where alloying with carbon comes in.

  • To explore this let's look at our phase diagram for carbon steel.

  • On this diagram we have our carbon content percentage on the x-axis and the temperature

  • of the metal on the y-axis.

  • This tells us the crystalline structure of the metal at various temperatures and carbon

  • contents.

  • On the left hand side we have pure iron, which as we explained earlier forms only one crystal

  • structure, called ferrite.

  • As we move across the diagram to the right hand side, less and less of the crystal structure

  • forms ferrite, and more forms an iron carbide alloy, commonly called cementire.

  • Now if we move up in temperature we start to see these lines that represent transitional

  • temperatures, where the crystal structures of the steel begin to transform into a new

  • crystal structure called Austenite.

  • Moving further up again we see lines representing the transition of the material to a liquid

  • state.

  • Austenites primary difference to ferrite is that it forms that face centred cubic crystal

  • structure that we saw early, while ferrite is body centred cubic.

  • And while this packing pattern is denser than body centred cubic, it does open up spaces

  • in the crystal structure that interstitial carbon atoms, which are smaller than iron,

  • can snuggly fit.

  • Allowing austenite to have a higher solubility to carbon over ferrite.

  • Using all of this information, let's take our 1055 steel with 0.55% carbon content and

  • see how it transforms from the start of our heat treatment cycle to the end.

  • The first step is called normalisation.

  • Normalisation is primarily functions to relieve internal stresses and strains that formed

  • during the forging process and return the material to its original crystal structure

  • before forging began.

  • It's effectively a reset button for the steel and creates nice even grain size and

  • distribution, increasing its strength.

  • Here we placed the knife blank inside a steel tube to prevent the metal from receiving heat

  • directly from the flame, but instead a more even radiative heat from the tube.

  • Once it reaches this transition temperature we let it soak to give the crystal structure

  • time to settle.

  • The next step in normalising is to allow the steel to slowly air cool.

  • What happens now depends on the carbon content.

  • If we take a 0.8% carbon steel, no hang on we're gonna need a bigger graph for this,

  • If we take a 0.8% carbon steel and cool it to it's through transitional temperature

  • the austenite and the interstitial carbon will slowly transform to a mixture of ferrite

  • and cementite, which takes this laminar structure called pearlite.

  • Pearlite only forms at a 0.8% carbon solution

  • Now if we take a 0.55% 0.2% steel, like the one we used.

  • And slowly cool it, to its first transitional temperature here.

  • Where ferrite first begins to form, ferrite is pure iron so as it forms the carbon percentage

  • begins to rise, this will continue to happen until the remaining austenite has a carbon

  • percentage of 0.8% and it will then form pearlite from this point on.

  • This forms a crystal structure dominated by ferrite, showing here as a lighter colour,

  • surrounded by the darker pearlite.

  • Comparing these microstructures to another 2 we can see the effect carbon has on the

  • microstructure.

  • Here we have pure iron, with 100% ferrite, showing as this light colour, you can even

  • see the grain boundaries.

  • This is a 0.5% carbon steel similar to ours, where a very small amount of ferrite formed

  • before we reaching the point of pearlite formation, and this is 0.8% carbon steel where the entire

  • structure is pearlite, with our previous example showing pearlite under 500 times magnification

  • where you can readily see that laminar structure.

  • How Pearlite strengthens steel is not well understood.

  • It has little effect on the steels stiffness, but increasing the pearlite content has dramatic

  • effects on the materials yield point, making it much more capable of absorbing energy without

  • permanently deforming.

  • But we can increase the materials stiffness and hardness with our next step.

  • If we heat the metal back up to form austenite once again, but this time instead of letting

  • it cool slowly, we rapidly cool it in oil, while casually burning your arm hair off and

  • barely flinching, the carbon atoms that spread out throughout the hot austenite structure

  • cannot diffuse out of the crystal lattice to form cementite, and instead gets stuck

  • in solution, creating a new crystal structure called martensite.

  • This crystal structure has a huge amount of internal stretching.

  • In part because the carbon trapped within the crystal structure causes the crystal lattice

  • to deform, but also because during the rapid cooling the surface cooled much faster than

  • the internal material.

  • This causes internal tension in the material.

  • These internal strains make it harder for additional deformation to occur, but this

  • does not make the material stronger.

  • It simply means it will not stretch and bend before breaking, and when it finally does

  • fracture all of this internal tension is suddenly released in an explosive expansion.

  • If you have watched Smarter Every Days video on the Saint Rupert's Drop, you will have

  • seen this principle demonstrated in incredible slow motion.

  • This material property is called hardness, and we want our cutting edge to be hard to

  • resist damage when cutting, but we do not want our entire blade to be hard as it will

  • not be able to absorb a lot of energy.

  • It needs to be able to dissipate some of that energy through heat and deformation.

  • This is where the final step of the process comes in, called tempering.

  • Tempering raises the temperature enough to allow the carbon trapped in solution to escape,

  • we just used an oven set at 200 degree, the carbon then coalesces to form cementite once

  • again, but instead of forming pearlite like before.

  • It gathers in globules surrounded by ferrite.

  • Tempering also relieves some of that internal tension caused by the rapid cooling.This reduces

  • the hardness, but increases toughness.

  • This produces a material that has the perfect balance of characteristics between the normalised

  • material and the hardened material.

  • It is ductile enough to absorb hammer blows without shattering, but strong enough to not

  • permanently deform, and with enough hardness to ensure it doesn't gather damage on the

  • cutting edge.

  • This process gave our steel the perfect material properties and a lot of that is thanks to

  • the quality of the steel we started off with.

  • Learning these skills with Alec was a lot of fun and I highly recommend you check his

  • channel out.

  • His incredible attitude to work really inspired me to start learning some new hands on skills

  • this year and I am hoping to start that off by learning how to programme and create robotics

  • with an arduino.

  • This course I found on skillshare is the perfect jumping off point for anyone looking to learn

  • the same skills.

  • Skillshare is home to thousands of other classes in graphic design, animation, web development,

  • music, photography, video game design and more.

  • These days you can teach yourself pretty much any skill online and Skillshare is a fantastic

  • place to do it.

  • With professional and understandable classes, that follow a clear learning curve, you can

  • dive in and start learning how to do the work you love.

  • . A Premium Membership begins around $10 a month

  • for unlimited access to all courses, but you can get your first 3 months for just 99 cent

  • if you sign up with this link.This offer was supposed to be only valid until the end of

  • Jan, but I talked to Skillshare and was able to get this extended for ye until Feb 15.

  • In those 3 months you could easily learn the skills you need to start a new hobby or business.

  • So ask yourself right now.

  • What skill have you been putting off learning.

  • What project have you been dreaming of completing, but you aren't sure if you have the skills

  • to do it.

  • Why not start right now and sign up to Skillshare using the link below.

  • You have nothing to lose and a valuable life skill to gain.

  • As usual thanks for watching and thank you to all my Patreon supporters.

  • If you would like to see more from me, the links to my twitter, facebook and instagram

  • pages are below.

Steel is the most important material to human society.

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B1 US carbon material steel structure iron heat

Heat Treatment -The Science of Forging (feat. Alec Steele)

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    joey joey posted on 2021/06/09
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