The Blade's primary function Part Two: Heat Treatment and hardness

CONAN, not Cohen...oh well.

Treating something hot

It's not exactly like what you picture

We've all seen scenes in Conan the Barbarian, The Hunted and many other films where they take a
red hot blade and stick it in a bucket of icewater, or a bank of snow or something along those lines.
We've heard (horror) stories about persians who quenched their blades in the bellies of slaves to make them better and the magical properties of moonlit forging.

As you know however Hollywood has a tendency of hearing something....and then completely screwing it up in their films. So that, while the basis of what they're showing is true, they're completely missing a LOT of important details.

Because four hands on a keyboard
are WAY faster than two
everyone knows this!

They do this with hacking, crime solving, war fighting, engineering and even basic physics. So why not for blademaking too.

Recap, steel

So a brief recap, steel is iron mixed in with carbon and often some other elements like chromium to give it the desired properties.
The main interesting one is Carbon though. And whether it contains carbon or not determines whether the metal/steel can be hardened.

Iron balls?

The reason this is needed is that iron itself, is rather ductile....that means it bends when put under
stress. Granted, iron is still better than a lot of other materials, copper for instance is softer still. But when using a knife it's kind of nice to not have to sharpen it again everytime you make the initial new slice in a tomato. Or when cleaning a carcass when hunting you have to sit down and resharpen it after making the first cut.

Offcourse you could just use a harder material like stone...but that comes with a whole host of other problems. Because since it's so hard when having it in thin slices (like on the cutting edge of a knife) it becomes brittle. So a knife needs a combination of hardness (so it stays sharp) and flexibility (so that chunks don't break out of it while using). It's like a balancing act.

And so far, the best material at this balancing act is hardened steel.

Heat treatment, how does it work?

Sooooo metal!

The technical side of heat treatment is rooted in the science of metals. Also known as metallurgy. Now metallurgy is a real sciency subject and is pretty difficult to follow. Even for me when they start getting all technical. However the basis for it is fairly easy to understand.

Iron atoms have a crystal type structure. In other words, a very organised structure. The comparison doesn't work 100%, but picture honeycombs. You know the hexagonal shape that they have and how it's repeated all throughout the beehive. Iron is like that when it's cold.

It's like a 3d honeycomb structure. Like a rubber ball with hexagonal sides all around.

Bounce bounce bounce

If you've ever handled one of these then you know you can squeeze and manipulate the shape of it all over the place.

Now steel however it like lots and lots of these all stuck together. With harder pieces in between the balls. Just for picture's sake let's say there's little black balls stuck in between the red rubber balls that are exactly big enough to be on the inside of the red balls.

What we WANT is to get the black balls inside of the red structures. This will make the steel harder (because of the black balls on the inside the red balls won't be able to compress anymore) but still somewhat flexible (the red balls can still stretch).

Now the black balls are bigger than the hex openings in the red balls....so something needs to happen to make those bigger.

To make the openings bigger we're going to heat up the material. When materials get hot the atoms and molecules start moving around more. When they move around more they require more space. So we heat up the material past a certain point and the red balls get slightly bigger, just big enough to let the black balls into the structure of them.

Now if we let them cool down slowly, the red balls will squeeze out the black balls again. And essentially nothing will have changed.

However if we go FAST and cool the material fast enough then (somehow) the red balls get smaller so fast that the black balls don't have the time to move out of them. And we have hardened steel.

That's the first step, we call this "Quenching" the steel.

You can imagine that something that happens so quick...might not be the most stable of structures. In fact, most of the time. This structure is so hard that it's become brittle. Now remember, brittleness is something we've wanted to avoid all along. Otherwise we'd have been better off with something like glass or stone.

The only acceptable brittle

So there's at least one more step in the process. We have to stabilise the material. And we call this process "Tempering" the steel. When tempering the steel the hardness is brought down a little bit so that it can become more stable, tougher and more flexible.

For example, if you take a really hard metal like a file (steel files are extremely hard, they have to be because they have to CUT other steel) and throw it on the stone tiles, it'll break into two or three pieces.
Take that same file, temper it a bit more and do something similar and it'll end up scratched, but still in one piece.

Heat treatment practises

So how does this go in practise? Well. You heat up the steel to the appropriate temperature (anywhere from 820° celcius up to 1200° celcius) and keep it there for the appropriate time. Remember, for perfection you have to give the red balls enough time to absorb the black balls. For some steels (simpler steels with less carbon and little/no other elements) this goes almost instantaniously when it hits the right temperature. Other steels need to be kept at a certain temperature for a certain time to ensure the transformation of the whole steel.

For bonuspoints....do this kind of stuff
WITHOUT the gloves...

Then after that it can be quenched in a medium appropriate with the right cooling off rate.
Some steels require a FAST cooling like water or salt saturated water.
Other steels work best with oils.
Other again work best through cooling them with cold metal plates (aluminium or steel) and others again just need to cool down in "normal" temperatur air.

All of this depends on how hard you want the steel to be, the kind of structures you want the steel to have and what ingredients are in the steel.

After hardening tempering can correct the hardness and the grain structures that you didn't want. Tempering is done on much lower temperatures than quenching temps. More between 180° celcius and 300° than the extreem red hot temps.

Hardness and grain structure

The eventual goal offcourse of all this hard work is to get the steel hard enough and flexible enough. Hardness can be seen/measured by hardness testing. In working with steel we generally work with the Rockwell C scale. It measures hardness by clamping the steel in something. And then pressing a tiny diamond or other hard point into the steel with the same force every time. Depending on how deep the dent goes you can see how hard something is.

In general, the harder a steel is. The longer it will stay sharp when using.

Yes....smiths are nerds and they LOVE diagrams
and charts and graphs

Then there is the grain structure. This one is harder to measure. The only way is to break the piece in two, polishing the pieces, etching them and then checking under a microscope. Now you'll understand that you can't do this for everything you make. So knifemakers generally do "practise pieces" and check with those so that they can get the right "recipe" down for their actual knives.

While we all agree about one thing, "finer grain is better". We don't really have an ideal hardness.
That one depends on all sorts of stuff, intended use, type of steel, type of knife, thickness of the grind and all factors like it.

For a big wood chopping knife you don't generally want it to be too hard. It might break, or pieces might snap off.

On the other hand on a pocket knife, that's only used for softer materials (meat/vegetables/fruit etc) maybe you'd want as hard as possible since there's very little risk in it breaking.

A lot of thought goes into the end result And if done wrong the results can be pretty bad....if done right the results can be amazing.

The Blade's primary function Part one: Steel Type

What makes a knife....a knife?

It's been a while since I wrote anything. Basically I did not feel "triggered" for a while as well as being rather busy with other things in my life. But recently something triggered some thoughts of mine and I thought I'd do another writeup.

The War Wizard and his edict.

As an early 20 something I really got into the writings of Terry Goodkind. Somehow his books really spoke to me. What helped was the fact that the main character was a wizard/swordsman who when he fought had the the following directive:

"It means only one thing, and everything: cut. Once committed to fight, cut. Everything else is secondary.

Cut. That is your duty, your purpose, your hunger. There is no rule more important, no commitment that overrides that one. Cut." (source)

Only way that could've been cooler would  if he used a chainsword

To me, this was just plain cool. No other reason than that. It's raw, and pure and the core of what a

Awesome indeed...I would probably still do it if it were lik this.

fight should feel like. As a teenager I was a fencer and that was how it felt for me (except we didn't cut each other...we poked at each other with pointy weapons)

Now that I make knives my the edict for my blades is pretty much the same. That's what they should do and in general...they do it pretty well.

But....why? And how can you recognise something that will cut and cut well?

What influences the edge?

And with some knives...the design is so impractical
that it really doesn't matter......

Now if you'd go on to knife forums like the ones that I sometimes frequent you'll find a LOT of talk about several factors. The most that is talked about when it comes to how well (or long) a knife will cut are a couple of things namely:

• Steel type
• Heat Treatment (hardness of the blade)
• Edge angle

All of these definately have an impact and I'll explain a little how. But there's at least  2 more factors that will play a HUGE influence on how well (and how long) a knife will be effective as a knife.

•  Stock or spine thickness (how thick the knife is at it's thickest point)
• Grind/Geometry of the blade. (What shape/thickness the blade is going from the spine or back of the knife to the edge

Today's Topic: Steel Type

When we look into fiction (books/ Comics) we always see some "Ultimate steel". Looking at the most recent Marvel films for instance they talk about Vibranium (Capt. America's Shield for instance) a lot. Apart from that metal there's mention of Adamantium (Wolverine's Metal Claws and infused skeleton). Then in the movie Avatar they talk about "Unobtanium". And in decades past Titanium was seen as the Unobtanium (different link than the last one) that everyone wanted. So much in fact that right now there's still a bit of a Titanium craze going on.

But fictional or hyped metals aside....what makes a metal suitable to make a knife out of. And for that...we have to go down to Chemistry. And what the difference in between Iron (an element) and Steel (An alloy).

We all know about Iron. Iron is the stuff that we see all around us. The stuff that rusts when it gets wet. The stuff they make ships out of.

Or is it? See it turns out..that Iron on itself is pretty soft and rusts easy. So for a lot of purposes it's not that good after all.

But we CAN make it better at certain things. This is when we start making alloys. The romans and those before them had already discovered that certain metals like Copper were pretty good...but if you mixed them in with other metals you could make things like Bronze. Which has some other desireable properties.

In the air? Yup....in steel? Well I wouldn't want my
blade to smell like a fart....but I'm pretty happy with carbon
being where it is.

The same can be done with Iron. For instance you can add some Carbon to the mix (yes, carbon, the stuff that we all worry about in emissions is a very usefull additive to iron). And when you start doing that you create what we know as "Steel".

Steel is a lot stronger than iron. Meaning it'll resist deformation (It won't bend as easily).

For steel to be usefull as a knife steel (because we use steel for lots of other purposes too, like ships, pots, pans etc) it needs around 0.6% of carbon in the mix.

As a general rule (but not a golden one, this is a bit of a simplification but a useful one) the more carbon you add to a steel the harder you can make it with the heat treat. (We'll talk about that in the next blog)

No...no that kind of Chrome.

Then there are other ingredients you can add to the steel. One of the most important ones is Chrome. Which is used to make a regular steel resist corrosion (Rust) better. The more chrome in the mix, the less likely the steel is to rust.

Who knew you could burn a knife too?

Then there are things like Vanadium and Tungsten which can make for very small...very HARD pieces in the steel. (Carbides) Offcourse the harder something is, the less likely it'll be to wear down (get dull) so lots of small extremely hard bits in a metal could mean that the knife made from it stays sharp longer.

There are quite a few other things than can be thrown into the mix. But what it comes down to is, like with a cake...where the ingredients will for a large part determine what the end result tastes like.....the ingredients in the steel determine for a large part how the knife will behave when finished.

Like with a cake though....if you build it wrong, or overheat it, or underheat it you can still easily ruin it no matter how nice the original ingredients were.

What can we conclude?

A little bit more about steel. And offcourse why knife collectors rave about it sometimes. In general the more expensive the steel is. The better it'll be at certain things. A steel with a lot of ingredients might stay sharp for a very long time. But you might not be able to GET it as sharp because at a microscopic level there are bigger particles in the mix. (The smaller something is...the better it'll cut things,)

This is why for a long time a lot of people thought Stainless steels were no good for knives. Stainless steels have more ingredients and therefore a courser grain (bigger pieces), bigger pieces cut less efficiently. But most carbon steels have relatively few ingredients and therefore smaller grain /particles so they cut better.

Technology has since caught up quite a bit and we can make steel do all sorts of funky stuff....like, have a steel that doesn't rust and has hardly any cabon in it or stainless steel that has as fine a grain as most cabon steels  or steel that can be hardened so much than almost nothing but diamonds will sharpen it. 

The picture has to be compl

But when it comes down to it. Even the cheap steels will "make a decent cake" if the proper recipy is followed.

Will a more expensive steel make a better knife? Sure...if all the other factors are also done right. But if not....tough cookies. Your expensive $500 knife with unobtanium steel might just be outcut by the simple and humble Opinel that can be bought at your local hardware store for $15. (Not affiliated with Opinel....just owned one as a boy (still do) and they're great knives).

Next installment:  Hardening and basics about Heat Treatment

Ofcourse this blog wouldn't be complete without some bonus knife pictures of the stuff I make.