Heat Treating Primer

As you read about making knives you'll often see the term "heat treating". It's an integral part of knifemaking. If you make knives, you will heat-treat (or send your blades to someone who does).

When you're just starting out in knifemaking (and especially if you are just considering taking the plunge), heat treating can be something of a mystery. I can remember, not so long ago, when the term gave me only vague images of flames and red-hot metal. So I decided to add this primer to explain a little about the process.

What is heat treating?
Simply put, heat treating is the magic that turns plain steel into knives. More scientifically, it's a metallurgical process that uses heat to alter the internal structure of the steel, making knives that cut better and last longer. Under the hood, heat treating is a complex process dealing with altering the crystalline structure of the metal, grain size and growth, etc. As you develop your skills as a knifemaker, you'll learn more about what's going on at the microscopic level. But for this short article, we'll keep things simple.

For knifemakers, there are four stages of heat treating:

Annealing - This is for softening hardened metal. If you want to make a knife from an old file, you'll need to anneal the file first so you can work it more easily.

Normalizing - This is Valium for your steel. It relieves the stress in the metal that results from being abused during forging.

Hardening - This is to make your steel a hard as it can be.

Tempering - This is for mellowing out your hardened steel to keep it from being too rigid and brittle.

Makers who forge their knives often make use of all these stages. Those of us who focus on stock removal mostly use Hardening and Tempering.

In the next instalment, I'll look at how you can heat-treat your own blades.

 

Thanks! I'm trying to keep a balance between simple and useful.

 

Heat Treating Primer Part 2

Now that you have some background, it’s time to heat treat your own blade. The two processes that are common to all makers are hardening and tempering. Once you understand them, annealing and normalizing are simple.

First though, it's important to know that heat treating is different when working with different steels. For some it’s a pretty straight forward process, while some steels require a progression of timed steps at different temperatures that can only be attained if you have a computer controlled oven. For this article, I’ll focus on simple carbon steel that anyone can heat-treat at home.

When to harden?
For me, it’s time to harden when the blade is completely shaped, the initial bevels are ground, and the whole thing has been sanded in stages up to at least 400 grit. Any scratches, pits, or flaws should be gone. Once it’s heat treated, it is a whole lot harder to get them out.

There are some who wait until after heat treating to start grinding the bevels. I think it’s madness, but you’ll have to experiment to see what works best for you.

The steps
Hardening a carbon steel blade is quite simple. You heat it then quench it. You’ll need a method of heating the blade and a container (with a lid) containing the quenchant.
To heat the blade, you can use a forge, if you have one, or a simple pile of firebricks and a decent propane torch. A forge is more efficient and a simple two-brick one can be built for next to nothing. (See the post here.)

Obligatory Warning: Playing with torches, forges and hot steel can be hazardous. Be aware of fire risks, carbon monoxide, and toxic fumes. Always work in a well ventilated area and have a fire extinguisher handy.

There are alternative methods for heating your knife blades, such as a digital heat treating oven like an EvenHeat or Paragon. Or if you’re more adventurous, a high temperature salt bath.

By heating, I mean bringing the blade up to the steel’s “critical temperature”. For most carbon steels, that’s somewhere in the area of 760C or 1400F. The actual temperature varies depending on the amount of carbon in the steel. Critical temperature is when magical things happen.

Warning! Science content
I’m no metallurgist, but in a nutshell, here’s what happens when you heat your blade to critical.

Carbon steel is basically a mix of iron(Ferrite) and iron carbide(Cementite). In it’s annealed state this mix is called Pearlite and the Cementite runs through the Ferrite like microscopic ribbons. When you heat the steel the Ferrite starts to combine with the Cementite to form Austentite. The steel reaches critical temperature when all of the Pearlite has been transformed into Austentite.

Whichever method you use, try to heat the blade evenly. Keep the torch moving slowly, or the blade moving in the forge, to avoid hot spots. The colour of the steel as it heats will tell you how well you’re doing.

When the blade reaches a nice even cherry red, it should be at critical temperature. And for those of us who don’t have the ability to see perfect colour, there’s an easier measure. When carbon steel reaches critical temperature, it becomes non-magnetic. Just keep a magnet handy and as you heat the steel, touch the magnet to it occasionally. When it no longer sticks, you’re there.

Note: Just like baking cookies, you can overcook your blade. Test often. You want to reach critical temperature on the way up, not overheat it and find critical as it cools. With experience, you’ll be able to tell when the blade is getting close by looking at it.

Once the blade reaches critical temperature, let it “soak” for a minute or so to dissolve any leftover carbides. Look at the colour and brightness of the blade, then move the torch around or move the blade in and out of the forge, trying to keep the blade at that same colour. When the soak is finished it’s time to quench.

If you’re a fan of old westerns, you’ve seen that part in every other movie showing the blacksmith at the forge making horseshoes. When he’s got the hot iron all pounded into horseshoe shape, he sticks it in a bucket of water with the dramatic hiss and accompanying cloud of steam. That’s quenching.

What's the point of quenching?
Remember that when the steel reaches critical temperature, the Pearlite structure is all converted to Austentite. That Austentite only exists at high temperature. If you allow the steel to cool down on its own the Austentite will revert back to the original Pearlite structure, leaving you back where you started.

To harden the blade, you need to keep that from happening by cooling the steel so fast it doesn't have time to revert. When done right the iron and carbide will be locked together in a very hard, fine-grained structure called Martensite. That's where the quench comes in.

Unlike the old blacksmith, you won’t be using a bucket of water, unless you really want to hear that nasty pop as your blade cracks. In most cases you’ll be using some sort of oil or oil mixture (There are a few steels that really do prefer a water quench).

There are as many “secret recipes” for quenchant as there are knifemakers. You can buy a commercial quenching oil or make your own. Engine oil(new or used), transmission fluid, or a mixture of both, are popular choices. I like to use 100% Canadian Canola oil. It’s cheap, works, and the smell reminds me a bit of fries.

Whatever you choose, you’ll need to put it in a metal container that is deep enough and/or long enough so the quenchant will easily cover the entire blade. It’s also important to have a good lid for your container in case the quench oil ignites.

The key to getting a good quench is getting the heated blade emersed in the quench tank as quickly as possible. That said, it does require a bit of technique. Don't just drop the hot steel into the quench tank. Instead, keep holding the blade with your tongs or pliers while you slide it into the quenchant, making sure it's completely covered. Then slowly move the blade in the oil. That ensures the hot steel is always staying in contact with cooler oil. Keep it moving for a minute or two to give the blade plenty of time to cool, then leave it in the quench tank until it's cool enough to hold in your hand.

That's it for basic hardening of carbon steel blades. As you develop as a knifemaker, you'll start to learn more techniques, such as differential heat treating and edge quenching.

In part 3 I'll look at tempering.

 

Thanks. It's one of those things that is quite simple to do but complex in how it works. Your recipe for quenchant sounds good too -- it just wouldn't work well on hot steel.

Oh, and the link wasn't broken. I just forgot to put it in.:blushing:

 

RE: Heat Treating Primer Part 3: Tempering

Welcome back to episode three in this increasingly long primer. This time the focus is Tempering.

When you heat and quench your carbon steel blade, it becomes hard. Really hard. Surprisingly enough, that’s not a good thing. If the blade is too hard, it won’t hold an edge as well. As you use it, tiny flakes will break off the sharpened edge, leaving you with a dull, rough blade. Even worse, the blade can be so hard and brittle. You can actually break your knife blade by using it, or even dropping it on a cement floor.

That’s why you need to temper. Tempering, as the name suggests, involves talking away some of the hardness of the steel. So why not just make it less hard in the first place? That would be easier, but it doesn’t work that way. In the hardening process you’re changing the molecular structure of the steel. Hardening is an all or nothing kind of thing. (Note: With the right equipment you can do something called Austentempering, which is like hardening to a lesser degree, but most of us lesser mortals harden, then temper.)

In tempering, the goal is not to use heat to change the whole structure of the steel as in hardening. If you recall, when hardening and quenching, the steel is changed from it's soft pearlite form into a very hard martensite form. That martensite is also very brittle and combined with the internal stresses that build up because of the the rapid cooling of the quench, creates a surprisingly fragile piece of steel. By heating the steel to a lower temperature for a longer period, you can make small changes to the structure. Tempering removes the internal stresses from the hardening process and allows just a small amount of the martensite to revert to pearlite, softening the steel.

So after you’ve made the blade as hard as it can be, you need to draw out some of that hardness. If you do it right, you’ll end up with a blade that has both strength and resilience. That’s a knife that will take and hold an edge, and last a lifetime or two.

To temper, you follow the same steps as hardening. Heat the blade up, then cool it down. With tempering, however, you use much less heat for a longer time and no quench. All you need to temper is a toaster oven or the one in your kitchen (Not recommended if you used a mix of old motor oil and transmission fluid as a quenchant for hardening. The smell will linger).

Temper immediately after the hardened blade is cool enough to handle with your bare hands. The actual tempering process will vary from maker to maker, with each swearing his gets the best results. I heat my blades in a toaster oven at 205C (400F) for two hours. I temper twice, letting the blade sit in the oven until it’s cold both times.

Once you've gotten the hang of it, you can try experimenting to see if you get better results. Instead of 400F for two hours, try 450 for 1 hour. Or try tempering once, then storing the blade in the freezer overnight before the second temper. I can't say I've noticed any real difference when trying some of these things but some variation may work for you.

In the final instalment, I'll cover normalizing and annealing.

 

Heat Treating Primer Part 4: Normalizing & Annealing

Welcome to the final part of this series. This time I’ll cover the last two stages in heat treating carbon steel, normalizing and annealing.

Once you’ve mastered the art of hardening and tempering, these two are pretty simple. Both start out the same way as hardening; you need to bring the steel up to critical temperature, using your torch, forge, oven, etc. Unlike with hardening, however, you don’t need to quench the hot steel.

Normalizing couldn’t be simpler, once it’s heated to non-magnetic, just set the steel somewhere it won’t burn anyone or cause a fire and leave it. As I mentioned in the first installment, the point of normalizing is just to take out the stress that you put into the metal through the forging and shaping process. Letting it cool down on it’s own allows the internal structure of the steel to relax. If you make knives using stock removal, you’ll likely never need to normalize, though it doesn’t hurt the steel if you do.

In annealing, you’re starting out with a piece of steel that has already been hardened and tempered, with the goal of making it softer and more malleable. Earlier, I gave the example of making a knife from an old worn-out file. The file is much too hard to shape as it is, but after annealing, it’s just like any other piece you work with. Likewise, you may have an old kitchen (or other) knife that has been sharpened to death. By annealing the steel you can reshape the blade and give it a brand new life.

Again, there’s no need to quench when you’re annealing. In fact, you want the steel to take as much time as possible to cool down. By slowing down the cooling process, you give the hardened steel as much time as possible to revert back to it’s softer, pearlite structure. As in most things in knifemaking, there’s no one way of doing it.

One way is to turn off the forge when the steel reaches critical temperature, then leave the piece sitting in the forge. Another option is to bury the blade in a container filled with preheated material. It could be sand, ashes, or kitty litter. (Note: the preheating usually involves burying a piece of hot scrap steel in the medium first.) Of course, if you have a digital heat-treating oven, you can simply ramp the temperature down over many hours until the blade is cool. Any way you do it, the goal is to slow down the rate at which the metal cools, producing a nice piece of steel ready for working.

With that, we come to the end of this heat treating primer. As you may have guessed, there is a whole lot more learn about the process. There is plenty of good material in books and on the web to give you more in depth information about all the stages and what really happens to the metal. Hopefully, this brief overview has given you enough get you started.

 

A very good question, Edmundo. I'm sure someone with more knowledge of metallurgy than I have could give a better answer, but here's my thoughts.

A blade that has been taken to its maximum hardness is under a great deal of stress and becomes very brittle. Even just sitting on a bench, it can develop stress fractures which may reduce the life of the blade. Following the hardening with a tempering cycle greatly reduces the stress in the steel, improving the quality and lifespan of the blade.

After one tempering cycle, it is safe to let the blade sit for some time. Some makers will keep the blade in a freezer for a few days before the second temper.