Molten Salts and Plasma Turn Cheap Iron Into Strong Steel

This video made possible by Incogn. How do you make your tool hard? No, not that. Straight to horny jail with all of you. , when you want a workshop tool, you don't want a floppy noodle. You need it to stay firm and in its proper shape. No one wants a chisel

proper shape. No one wants a chisel that's useless after only a few blows. You need something that can take a pounding for hours. And generally, knives and other cutting tools work best when they're sharp. So, you want them to be made of a hard material that won't

be made of a hard material that won't bend or chip. No butcher wants a dull tool when they're handling their meat. But manufacturing a tool in the first place often requires the starting material to be in a softer, malleable state. So, you need to be able to make it go soft or hard at different points

in the manufacturing process. Humanity's answer to this is the wonder material we call steel. At its simplest, steel is a mixture of iron with a very small amount of carbon. But modern steel often has a variety of other elements mixed in for added flavor and to tweak its

added flavor and to tweak its properties. Today, we're going to be taking a trip through time and learn first how we convert iron into steel and then how we can make steel hard. from the oldest known methods using clay and charcoal to the most modern that use

molten baths of boiling salt or vacuum systems and plasma. And by the end, we'll have converted dirt cheap iron into a hard steel knife and show you how you can even harden stainless steel, which is normally impossible. But before we get to steel, let's take one step

back and look at pure iron. Iron as a pure metal is honestly mediocre. It can't hold an edge and it's quite soft and bendy. And worse than that, it's pretty difficult to make. The issue is that its melting point is

really high, well over 1500° C. So to take iron ore and convert it to iron requires a system that can get at least up to its melting point, though the hotter the better. Originally, this meant building a bloomer, which is just a big tube made out of

just a big tube made out of clay or mud with a pipe at the bottom you can blow air through called a tuier. Now, wood doesn't burn hot enough to make iron, which is one of the many reasons that it took so long for humans to master. Before you can even start up

the bloomer, you get to spend days and days cooking wood into charcoal, which burns much hotter. Then, you fill up the bloomer with your charcoal. And once everything is burning and hot, you start feeding in iron ore and more charcoal in

the top. There are many, many videos of making iron this way. And it's simple enough that even nearly naked in the jungle, you can make it work. The end result is a spongy mass of iron and slag that then has to be worked to collect all the little molten bits of iron and

all the little molten bits of iron and make them stick together into a cohesive chunk. That chunk then has to be worked, folded, welded, folded, and welded some more to get all of the junk out of it. And the result is what was called rot

iron. However, rot iron is just iron. It's not steel. Steel originally was just a lucky bonus item that came as part of this process. Some of that spongy initial mass of metal will have dissolved the perfect amount

will have dissolved the perfect amount of carbon into it and become a hardenable steel. So the blacksmiths would have to isolate those bits to be used for making tools while the rest was converted to rot. Obviously this is not efficient and the yield of steel is

efficient and the yield of steel is terrible. So you need a way to convert the bulk rot iron into proper steel so it can be used for tools too. By far the easiest way to do this is with a process called case hardening. Essentially, the idea is to treat a piece of iron such

idea is to treat a piece of iron such that the outer surface is converted to a hardenable steel skin. So, you preform your tool, be it a file or a chisel or whatever else. Then, you case harden it to make the working edge hard. This leaves the core still relatively soft

leaves the core still relatively soft and ductal, so the piece won't snap, but the cutting edge will now stay sharp. This is also the oldest way to make steel on purpose. We're not really sure who first figured it out, but people most likely noticed that pieces of iron

left hot and buried in coals for a while got strangely hard when dunked in water in a way that normal iron didn't. Eventually, this process was refined such that you bury a piece of iron in charcoal in a flameproof container and

charcoal in a flameproof container and then bring it up to a red heat and bake for a while. This will cause the charcoal to release things carbon monoxide, which can then diffuse slowly into the iron and raise the carbon content. This, as you might imagine, is

not a fast process. Since the iron is still solid, the diffusion rate is very slow. Between 0.1 and 0.2 mm of depth per hour, but since we only need a relatively thin, hard skin, that's fine. Originally, they would

fine. Originally, they would have used clay to make a vessel for this, but pretty much any flame proof box works, though, we don't want it to be totally sealed. Gases do need to be able to escape, so even a clay box would have a small hole in it. Or you can weld up a box to fit your items. We made lots

up a box to fit your items. We made lots of these test samples out of some quarterin square bar made out of mild steel, which is the closest modern equivalent to pure iron. It can't harden, and it's the cheap iron that you would find at a hardware store. good enough for rebar or fence, but not

good enough for rebar or fence, but not something that you're going to make a cutting tool out of. We're going to do three runs to show you the difference between a control piece and 15, 30, and 60 minutes cooking at 950°. Now, a wood fire would be

Now, a wood fire would be perfectly fine for this, but my landlord probably wouldn't appreciate me setting up a bonfire in the lab, so we're going to use our much more accurate kiln for this. When it comes out, it's still ripping hot. But to keep the timing consistent, we opened the still hot box

to fish out the bars. This usually meant that the charcoal was red hot and would burn a little bit. And the bars were red as well. We allowed these to slowly cool. So, they don't harden yet because remember, just because there's some carbon in here now doesn't mean they're

carbon in here now doesn't mean they're hard yet. And the way you can tell is similar to how you pick good bread from the sound. How do you tell a good bread is without tasting it? Not the smell, not the look, but the sound, the crust, the

symphony of crackle. Only great bread sound this way. To demonstrate this, I've got two test pieces. This first one is our control, and it's dead soft. If I take a file to it, it makes a low pitch sound from the metal being cut. And you can see that the file very

And you can see that the file very easily leaves a mark on the metal as it's starting to reshape it. Now, let's compare that to a hardened piece. Whoops. Sorry, that must be the wrong end. Let me flip this around and try that again. Okay, now you should have heard the

Okay, now you should have heard the difference. It's a much higher pitch noise and the file is hardly able to scratch it. What you're hearing is the file skating and bouncing over the metal rather than cutting it. So now the question remains, how do we go from soft

question remains, how do we go from soft to hard? Well, that can be a very personal question. But no matter the case, we're going to need some lube, or rather coolant. If you've watched blacksmithing content before, this is the most exciting part of the process. The piece of steel has to be heated

above 800° to a nice orange color. Then, while it's still hot, it's plunged into coolant, which is often oil, meaning a big burst of flame as the sudden flush of oil vapor burns. We'll do a comparison of different quenching fluids in a minute, but for the sake of our case hardening test, we're just going to

use canola oil. Most oils can work, but canola is cheap and it doesn't release as many toxic fumes. While we get the forge up to temp, have a quick look at the four test pieces fresh from case hardening. You can see a distinct color difference as the layer of carbides and

difference as the layer of carbides and oxides on the surface grew thicker with increasing cook time. Anyway, once the forge was hot, we popped in the test pieces, being very careful to keep track of which one was which. Then, when they're good and hot, each was quenched in oil. So, how'd they turn out? This

time, rather than just any old file, we picked up a set of hardness testing files. These are each calibrated to a specific hardness. So, if the file can cut the metal, it must be less hard than the file, and vice versa. Our control piece, as we expected, fared poorly.

piece, as we expected, fared poorly. Even though it's been heated and quenched, it's still just iron, so it didn't harden at all. Even the lowest file cut it with ease. But the case hardened pieces are very different. Even after 15 minutes, there's now a very

after 15 minutes, there's now a very hard skin on the outside, and it takes a 55 Rockwell file to cut it. As we go up in time, this only gets better. And by the 60thminute sample, only the highest 65 Rockwell file got any purchase, and even then only barely. So, as you can