How to Heat Treat a Knife [simplest Method Possible]





Introduction: How to Heat Treat a Knife [simplest Method Possible]

Heat treating is undoubtedly the most important part of knife making. It alone is what determines whether or not your knife will hold up under abuse, shatter when dropped, or bend like a pretzel when hacking on an unusually rebellious piece of wood. Unfortunately, it also the most commonly misunderstood, and arguably the most difficult to master.

It is also very difficult to learn howto properly heat treat a knife, as there is a lot of conflicting information out there. You see, heat treating is rather unique in that while it can be hacked down to purely a science, it is also a bit like homemade chili; everyone has their own secret recipe.

There are those who want you to believe the only way to achieve a good heat treat is by using a temperature regulated heat treating oven, soaking for 15.7 minutes, normalized 2.3 times, and then quenched in park's knife heat treating oil raised to exactly 134.6 degrees Fahrenheit. And if you find you were half a degree off, well then, your knife is simply sub-standard.

And then there are those who firmly believe that a knife can only be properly heat treated at midnight exactly, underneath a full lunar eclipse, quenched in a tank of boiling dragon's blood when the knife glows cherry red (with the blade pointing due north), and then tempered by holding above burning coals (made of carbonized diamonds) until the metal turns golden-brown. Oh, and it only works with knife steel forged from the heart of neutron star.

The fact is, heat treating is often much simpler than it is made out to be. (Man, am I going to be raked over the coals for saying that)

But it is. Now don't get me wrong, heat treating isa science, and with delicate temperature controlling equipment, you do get a better heat treat. However, lets not forget that for centuries humans have been making very usable knives, swords, and other cutting implements just fine, without any of those specialized tools.

Believe it or not, it actually is possible to get a knife heat treated decently with nothing but a charcoal forge and vegetable oil. (and a few other things)

That is what this Instructable is about. How to Heat Treat a knife in the simplest and easiest way possible.

Without any further ado, let's get started.

Side note: The fact that I am using a RR spike knife to demonstrate the HT doesn't change the process in any way, it's just the knife I had on hand that needed finished up. The process is the same for any simple high carbon steel.

Step 1: Tools and Materials

For this simple heat treating method, you won't need much. Here is the main stuff you will need:

1.Charcoal. Here, I am using 'natural' hardwood lump charcoal. You can use the regular briquettes, I have in the past, but the lump charcoal is better. It gets hotter, heats up faster, and is easier to use. If you are feeling especially redneck, I'll mention that you can actually use regular wood (anything will get hot enough with a lot of air on it), but it is more difficult and burns faster, so you'll need a lot more of it.

2.Air source. Here I'm using a semi-broken blow dryer my Mom had thrown away. The heating element is busted, but as we just want the air it doesn't matter. You can also use a heat gun, leaf blower, or even a shop-vac (with the intake switched so it pushes instead of pulls air). All you want is to put feed large amounts of oxygen into the fire. Be creative.

3. Quenching Oil. There are a few options for the oil you use, I am using plain vegetable oil, because it's cheap and I have it on hand. Realistically, just about any kind of oil would work for this. Vegetable oil, olive oil, peanut oil, motor oil, used motor oil, etc. It will all work. You do not want to use water, however. It is too harsh and will crack most steels.

4. Magnet, for testing the temperature of the steel. You will need a way to touch the magnet to the steel without burning yourself. I am using a broken telescoping-magnet-thingy.

5. Something-with-which-to-take-the-knife-in-and-out-of-the-fire-with-without-losing-any-fingers. I am using some long handled pliers and welding gloves. Trust me, you will be wanting them. A charcoal forge like this will get upwards of 2000 degrees Fahrenheit.

6. A Heat Resistant Container (to hold the oil for quenching). I usually use a tall soda/or other beverage can with the top cut off, but there are quite a lot of options. Just make sure it's able to take the heat.

Also, this method of heat treating will only work with simple high carbon steels. 1090, 1080, 1095, 5160, 1075, 1085, O1, etc. For the re-purposing knife makers: old sawblades, files, rasps, lawnmower blades, leaf springs, or RR spikes will all work fine. Sorry, but this method will not work with any kind of stainless steel, or any of the more complicated "super steels". Don't even try.

Step 2: Forge

Quite obviously, you are also going to need a forge of sorts. For this bare bones heat treat, there is no need to over-complicate things. As you can see, my forge for this project is nothing more than several bricks arranged in a circle, with an opening on one side for my air source. No need to get fancy. (although I recommend you get creative)

Of course, you can also use any other sort of forge you want to, I am just showing how to heat treat a knife in the simplest way possible, with no special equipment.

Alright, now on to the actual thing.

Step 3: The "Actual Thing"

The first step would be to start up your forge. I loaded up my brick box (well, really more of a circle) with a sizable mountain of lump charcoal, got my hair dryer in place, and lit her up.

You will need to give it a few minutes to heat up, I generally wait till most of the charcoal has turned to white/gray color. At that point the forge is upwards of 2500 degrees Fahrenheit, and almost too bright to look at directly. Yes, you actually canmelt steel in a crude forge like this. You will not be able to get your hands anywhere near it, which is why I recommend welding gloves.

While you are waiting for the forge to reach temperature, its a good time to get your quench ready. Get your oil in your heat resistant container, and pre-heat it to about 130 degrees (Fahrenheit). You can heat it up on your stove if you like, the way I did it was by heating a piece of rebar in the forge, and then dunking that into my quench. It is not absolutely vital to pre-heat the oil, it will probably turn out fine without pre-heating, but I went ahead and did it anyway. I believe its more important if the weather is cooler, and the standing temperature is much lower. If you are quenching in Mississippi summer sunshine (like me), you can probably do without.

Step 4: Heating Things Up

After you have heated up your forge, go ahead and stick the knife in. Now comes the most difficult part, where a nice beautiful temperature regulated HT oven would come in handy; heating up the steel evenly. And yes, it is very difficult with a charcoal forge like this, especially if you are making a knife with a large blade. It is definitely doable, though, just keep working at it, and try to find the best "hotspot" in your forge for even heating.

You will also need to be careful of overheating the steel. A little bit over and you'll be fine, just let it cool back down a bit. However you don't want to let it get too far above critical temperature, as you will risk cooking the carbon out of the steel, as well as just melting the steel dead away. Don't underestimate the power of this forge. It will melt your blade if you leave it in for too long.

As you are heating it up, watch the color of the steel. After you have heat treated a few knives, you will be able to tell roughly what temperature the steel is based on the color. Until then, you'll just have to trust me; 1500 degrees (what you are aiming for), is a cherry red color which comes just after the metal first starts to glow. This is where the magnet comes in. It is an interesting property of steel that it ceases to draw a magnet when it hits 1345 degrees Fahrenheit. The critical temperature of high carbon steel is around 1475. As the steel heats up, touch it to the magnet every few seconds. When it stops sticking to the magnet, you know you are close. A few seconds longer, or until the steel has shifted color a notch brighter, and you know you have hit critical temperature (approximately 1475).

Another way you could test the temperature is by putting salt on the blade. Salt melts at 1474 degrees, so as soon as the salt melts, you know it has hit critical temperature.

But as I said, getting the steel to critical temp isn't very difficult. Getting the whole blade evenly heated to critical temperature is the challenge. After all, you don't want half the blade hardened and have the tip bend when you look at it wrong.

Step 5: Quenching

When you are satisfied you have got the blade to critical temperature, you need to move fast. You don't want to lose too much heat. Quickly move the knife out and plunge it into the oil. I generally agitate it back and forth inside the oil, to break up any air bubbles that may have caught in there and make sure that blade is cooling evenly. In the video you can see me pulling the knife out of the oil and putting it back in several times. That is called an interrupted quench. Don't do that. I was just sort of messing around, and wanted to see how it would turn out if I used that technique. For certain quenchants and certain steels, an interrupted quench can be beneficial, but for this heat treat it isn't necessary and if anything, could make your steel softer. Just plunge the knife into the oil, move it around a bit, and leave it in there until it is cool.

To test the steel and see if has hardened correctly, take a file and scrape the corner of the file across the knife. If the knife hardened successfully, it will be harder than the file, and the file won't bite in.

Step 6: Tempering

After quenching, the steel is extremely hard. So hard that it is about as brittle as glass. If you were to drop it or subject it to any amount of force at this point, it would be liable to shatter or crack. In other words, it is way too hard to be a knife.

The next step is to heat the blade up again, this time to 400 degrees. This relieves the stress built up in the steel, and softens it down a bit, and you end up with the perfect hardness for a knife blade. Its called tempering.

There are a number of methods you could use to accomplish this, but the easiest (and best for beginners) is to put it in the oven @ 400 degrees for two cycles of one hour each, letting the knife cool between cycles; or until the steel has turned a wheat/golden/brown color. Yes, steel changes color as it is heated. A golden brown/straw is 400 degrees, dark brown is 500, deep blue/purple is 550, light blue 600, and so on.

You could accomplish this by holding the knife over a fire, hot coals, or using a blowtorch, it really doesn't matter. When the steel hits that golden straw color, you know you nailed it.

Step 7: Finishing Up

At this point, the heat treat is completed and your knife is fully hardened. All that it lacks is the finishing up.

My least favorite part. Quenching has a nasty habit of producing this material known as scale all over the blade. In technical terms, it is commonly known as CRUD.

Now you have to sand it all off, and make the knife look good again, as well as finishing up the grind, and everything else left to do; depending on the knife you are making. Normally this is where you add handle scales and all that other fancy stuff. Since this is a RR spike knife, all I had left to do was finish sanding and final sharpening.

Lucky me.

Step 8: Knife!!

To sum up:

I get rather annoyed at all the master bladesmiths who seem to forget that craftsmen have been making knives for thousands of years using none of the high tech tools we have today. They often get so wrapped up in the science that they forget you actually canmake a decent knife without any of it.

Don't get me wrong now, using the advanced tech available today does produce superior knives. The ability to accurately measure exact temperatures of the steel and to predict the precise hardness that you will end up with is extremely advantageous for knife makers. Using those tools they are able to produce the best knives possible. However, that doesn't mean you are dependent on them to end up with a good knife. You can produce a decent blade the same way smiths have been doing it for thousands of years.

And really, while you can argue that a couple points more or less on the hardness scale determines a good knife from a bad one, lets be honest; in actual use the difference is minimal to unnoticeable.

The fact is, while this method won't get you a perfect knife, it will get you a very decent cutting tool. That's good enough for me.

Step 9: Video

Hope ya'll enjoyed this Instructable, and I'll see you next time.

Jake out.



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    Please be positive and constructive.




    Not a bad 'ible...

    You're right on most of what you say, and for a beginner this is a great place to start.

    However, used motor oil?

    Personally, I think there are enough people in this world suffering from cancer and all kinds of lung diseases, so please forget everything about motor oil, used or new!

    The smoke from motor oil is quite hazardous, and you don't want to be anyway near it...

    And another reason not to use motor oil is the fact that it's not designed for this. Especially used motor oil contains all kinds of stuff that gives you an unknown flashpoint(fire hazard), and all the crud in the oil can and will fasten to the steel, and give an uneven temperature change, which in turn will give you an unknown but surely poor hardening.

    Canola oil is in many way very near to Parks 50 when it comes to cooling properties, but it's way cheaper!

    On a side note, the RR spike knife is not the best example to use, as even RR spikes stamped "HC" are not hi-carbon when compared to proper carbon-steel.

    RR spikes are great for practise, but they don't hold an edge if the knife is used for anything but butter ;o)

    Please take this as positive ctriticism to a pretty good 'ible.

    And stay safe ;o)

    Thanks! (for the 'pretty good ible' part ;)

    Yes, very true about RR spikes. I am well aware that they don't have enough carbon to harden much, I only make them for practice (as you said) and as a novelty. Would have been better to use another knife to demo a ht like this, but it was the only unfinished one I had laying around at the time I did this 'ible. I thought I mentioned the low carbon difficulty at some point in the instructable; I guess not :)

    Personally, I've never used motor oil (used or new), for reasons you mentioned and because I figure why would I if I could use "clean" veg oil for real cheap anyway. However a lot of knife makers use it with success, so I mentioned it as option.

    Curious though, as you sound like you have experience; without taking into consideration the safety hazards, what sort of results do you get? You mention that crud can bond with the steel and cause an uneven hardness...... is it really that much of an issue? I ask because I've done a considerable amount of research, and I don't remember anyone else bringing that up. Thanks for the compliments. And the criticism. Good criticism is how to learn. It's becoming increasingly rare.

    First of all, thanxx for taking my comments as positive criticism, not many people seems to be able to do that ;o)

    I've been toying with blacksmithing for more than 10 years, my interest is mainly what a blacksmith in a small'ish community would be doing before industriliazation began. From time to time I volunteer at a Medieval museum centre.

    I don't really like bladesmithing, I'm more into tools and all the stuff that made life easier, and yet I've made 100's if not 1000's of knives over the years.

    The first few times, I was using used motor oil simply because I didn't know better.

    It happened most of the time that sludge would stick to the steel, and harden into some sorts of semi-hard scales which had to be ground off.

    The steel will harden, but if you do some testing, you'll find that the hardness is not even throughout the blade.

    It does make sense in the way that motor oil is designed to do a very different job than proper quenching oil. I sometimes ask newbee blacksmiths if they would use quenching oil or veggie oil in their cars or bikes, and when they roll their eyes and tells me that would be a stupid idea, I'll ask them why they think that it would work the other way round...

    There are a few websites left, dedicated to blacksmithing, and there are some very good groups on FB, and from time to time, the discussion pops up when some one is claiming that motor oil is OK for quenching. There are chemists that will explain in great detail why it's not a good idea, and there'll be a lot of guys telling about their own experiences. Obviously there'll be a lot of guys claiming that they've used it for a long time, and that it's OK. But apart from the health issues (which are quite serious), it's just not a good idea.

    I don't have any links handy, but I've seen some research that suggests that Canola oil is almost frightenly close to Parks 50 parameters, and that's why it's recommended for the backyard blacksmiths.

    I'm using Canola oil, or water for certain alloys, even on stuff I'm selling.

    There'll be a lot of guys that will tell you that I'm dead wrong, but do your own research.... Don't just think that my, or somebody elses words are gospel...

    Talking quenching oil amongst blacksmiths is almost like asking bikers what lube oil would be the best.... It's religion to some people ;o)

    Jeez, yeah, EVERYBODY has their own secret quench recipe :) Thanks for the info, I'll take your word for it. No real reason i'd use motor oil anyway. Thanks man.

    I didn't know about te salt 'trick'. Interesting . Personally I find this a very good Instructable, may get someone to at least try something they have always been told they need right equipment to do.

    Thanks! Yes, I hope this will get some people out into the shop!