Steel: Heat-Treatment

Steel, as you find it, can be anything from annealed (as soft as it'll get), to already heat-treated (springs, some tools). Some already heat-treated things can be ground to shape and use, as long as you don't overheat it. But if you have annealed stock, or forge a piece to shape, you'll have to heat-treat it if you want it to do the job at its best.

I'm going to stick with what I know, which is the basic heat-treatment of carbon steels, with a small bit on the more highly-alloyed stuff. Heat-treatment of steel to harden it is a two-step process:

1. Hardening: bring it up to critical temperature, then quench it (cooling it fast enough that it freezes the structure of the piece in a highly-stressed state).
2. Tempering: heat it up again, but to a much lower temperature. It's a balancing act, getting it hot enough remove enough stress to keep it from being brittle, but low enough that sufficient hardness remains for the desired purpose.

Hardening
With carbon steels, the critical temperature is generally ~1500° Fahrenheit. That's an important number, because if you get much below it, the steel won't be in a condition to harden completely; get much above it, and the heat causes the grain structure to enlarge, which weakens it.

How do you tell when you're at that point?

Experience via Trial and Error
Learn to see exactly what shade of red is that temperature range. The usual description is 'cherry red', but seeing people see colors differently, the best way I can describe it is 'watching the shadow'. I described it in my Introduction to Blacksmithing post and I'll repeat it here:

Take a piece of steel of small size - no more than 1/4" thick, better about 1/8" - and a propane torch. Start heating the piece with the flame directed at a point about 1/2" back from the end, shifting it back & forth just a bit so it heats a wider area, and watch. As it hits about 1000 F you'll start to see a visible heat glow, and as the temperature rises it'll get brighter. You'll notice - at the risk of sounding poetic - a shadow inside the steel, inside the red; as the heat rises the shadow will get fainter and fainter, and right at critical temperature the shadow disappears. That is the point at which you'd quench it to harden. May take some practice to see it, but it works.

You will screw up a few times doing it this way, but it's the original method and requires no special tools.

Use a Magnet
The critical temp for these steels is also their Curie point: they go non-magnetic.  Get one of those magnets on a extendable shaft, and as the piece gets toward that color you can start touching the magnet to it; when the magnet no longer sticks, you're there.



Use a Tempilstik
A Tempilstik is basically a temperature-sensitive crayon -- draw a line on the piece, and as it hits that temperature, the mark will melt. They're quite accurate, and can be bought for temperatures useful for lots of other things (annealing brass, tempering, etc.)

Nowadays you can pick up a thermometer that lets you read temperatures on something a few feet away; I'm told that, used carefully, they work pretty well.

Quenching
As noted in my previous article, carbon steels come in types marked 'oil-hardening' and 'water-hardening'. Generally speaking, water-hardening steels need a faster quench than oil-hardening. 

Usually.

The big "however" is because it depends on what you've made with it. Something with a pretty thick cross-section (like a hammer head) has a lot of heat to lose for it to harden deeply, and may require water, but something like a knife or other cutting tool is usually much thinner than that. If you cool steel too slowly, it won't fully harden; cool it too fast, and it may well crack. With blades I've always had better results using oil; when reshaping a hammer head, I used water.

Oil
Quenching oil should be warm: 100°F or a bit more. Warm oil flows better, which means a more even quench and less thermal shock. That 100°F may not sound like much when the steel is at 1500°, but it does make a difference.

Almost any oil, from light motor oil to corn oil to olive oil to specifically-made quenching oil, has been used successfully. The advantage to a quenching oil is it's a bit less likely to have the smoke flash-ignite, and if it does it goes out much sooner. A good quenching oil can also be reused many times with no problem.

Water
If you use water, I have two suggestions to make: Heat the water and add salt. Add a lot of salt. Salt water has a higher boiling point, which means somewhat fewer bubbles, and therefore a more even quench.

Lowering It
How you lower a blade into quench depends on the style of blade and how you want it hardened. You can start fights over whether to lower a blade into the oil point-first (vertically) or edge-first (horizontally). I've generally had better results with edge-first, but that's with single-edge knives; with a double-edge, like a dagger, you need the whole piece up to critical temp, and it all fully hardened. With an edge quench you can do a thing called differential hardening: you get half of the blade from the edge back up to critical temperature, the other half is a bit lower, and you quench. That gives you a fully-hardened half backed-up by a back that's not fully hardened; that back is going to be softer, but also tougher and springier, which reinforces the cutting area. This isn't a big deal on small knives, but on a big, heavy, chopping-capable blade it can be a very good thing.

Note that to put a knife in point-first, you've got to have something like a piece of pipe deep enough for the whole blade to be lowered into; for edge-first, you need a trough long enough.

These suggestions are for blades. Let's say you want to make a chisel or centerpunch. With either of these, you only want the edge/point and maybe 1/2" back to be fully hardened; the softer shaft means much less chance of pieces cracking and spalling off when you hit it when a hammer. Bring up to heat so that 1/2" or so is up to full heat, and quench vertically.

There are lots of other things that would need heat-treating (springs for instance), but I'll stop here.

Tempering
After the piece has cooled to ambient temperature, it needs a tempering heat*, and that heat will depend on what kind of steel you used, and what it'll be used for. Take that chisel, for instance; it has to be hard enough to actually cut into metal without dulling quickly, so it has to be hard.

• Clean it off thoroughly and shine it up (sanding with fine paper will do) for at least an inch above the hardened area.
• Keep some oil or water handing to cool it with. 
• Grab it by the back end with pliers or tongs, put the torch on low, and start heating it about 2" above the hardened end. Here you need light to watch the colors run, but not direct sunlight; that's too bright.

Colors?
Yep. This is how everything was tempered for a long, long time. As the heat nears about 350°F, a bright, shiny piece of steel will start showing a touch of yellow. As it heats the yellow will deepen (temperatures approximately 400-425°F or so), then start darkening into a bronze at around 450°F. As the heat rises, that will finally darken into a purple around 500°F, and then blue. Beyond that, it'll go to a grey, which means you've removed pretty much all the hardening.

The color you're going for depends on the steel and use. On a knife where edge-holding is a critical factor, or a wood-carving chisel, you might only take to a medium/dark yellow (which is where that chisel needs to be). Most knives should be more of a bronze. A prick punch, which is only struck lightly to mark the spot for a center punch to deepen, should be a light to medium yellow. Something that needs a lot of spring, like a sword, ought to be light to medium-purple (again, all this depends on the steel being used). A spring is probably a dark purple/light to medium-blue.

In the example of a chisel, as the body heats, watch for the colors to start. You'll be able to watch them moving down along the polished area. As the yellow approaches the hardened area, move the torch closer or further to keep the colors moving, but not too fast. Get it right, and you should be able to get that desired shade of yellow from the edge back a ways. As it reaches the edge, put it in the coolant to stop the tempering. For knives and springs, the easiest way is to use the oven**. 

Example: Knife

Say I'm making a knife from O1 tool steel, it's got a 5" blade an inch wide. I know from past experience that about 450°F will be what I want for the edge. 

1. After hardening, wash all the oil off so it doesn't smell up the place, and use a grinder, sander or just sandpaper to shine it. 
2. To prepare the oven, either use a baking stone or a piece of clean steel plate (thin stuff will do) if it's an electric oven, and then bend some wire to make a rack that'll hold it horizontally with the edge up or down. 
3. Place it in, turn the oven to the desired temperature, and let it heat. You are certain the oven settings are calibrated properly, don't you? If you're not sure, set the temperature 25 degrees below the desired temp.
4. Leave the blade in for an hour (you can get away with less with a small blade; with a bigger, thicker one give it the full time), then take it out and let it cool to room temp, and check the color. If it's good, put it back in for a second heat, give it the hour, then turn the oven off and let it cool down. If it's not quite to the color you want, turn the oven up 25° for the next heat.
5. If it's darker than you planned on, there's no going back. If it's softer (relatively speaking) than you wanted, the only way to get that hardness back is to harden it again... but not immediately, because you need to test it for proper hardness before you head back to the fire or start polishing.

Testing Hardness
There are two ways to do this on a knife.

First Method

1. Brace it solidly on something. 
2. Take a sharp, new file, and work it on the edge with medium pressure. 
3. If it just skates off without marking the steel, it's too hard for anything other than a scalpel. 
4. If it ALMOST bites, you should be good (yes , I know that's subjective) for a general-purpose knife. 
5. For something big like a brush knife, you might want to be able to sharpen it with a file, so you'd have to temper that down a bit more.

Second Method

1. Sharpen it to a good edge, then go to the vise. 
2. Put a piece of brass rod in the vise. 
3. With good light, lay the edge bevel flat on the brass, lift the back of the blade a bit so the edge itself is on it, then press and watch that edge. 
4. If it chips, it's too hard. 
5. If it flexes, and returns to true when you release pressure, should be perfect. 
6. If it bends, may well be too soft (depending on intended use). 
7. You have to watch CLOSELY here, as it's only a very slight deflection from true you're looking for.

Too hard? Increase the temperature by 25°F and give it another tempering heat. If it's too soft after some cutting, you'll have to start over with hardening.

Example: Springs
These can be tricky. You won't generally have to mess with coil springs made from music wire; they're hardened to start with, and the work of coiling will actually make them even more so. Flat springs, however, are where it gets interesting.

1. With a flat or V-spring, you need to finish the shaping, and clean and polish it well. 
2. Harden it (it's easier to clean and polish before hardening). 
3. Clean the surface again, as you'll have to see colors. 
4. If the oven won't get quite hot enough, the best way I know is to get a piece of steel plate -- clean, so it won't put out any smoke or fumes -- and put it over a heat source like a forge fire, torch, range burner. 
5. Put the spring on it and start heating as evenly as possible, turning it regularly, and watch the spring closely. 
6. With most steels suitable for this, watch for a light- to medium-blue; have that oil handy to cool it down soon as it gets there. 
7. Then test it carefully; if a bit too hard, you can take it to a slightly higher temp.

Example: Hammer Head
I've modified several hammers when I needed one face with a certain shape. Quenching was done in heated water (a BIG can of it was needed for a 4-pound slug of steel). To temper:

1. I cleaned it and polished both faces and a little ways back nice & bright. 
2. Took a piece of steel bar that would fit in the handle hole, got it to bright red, stuck it in a vise and set the hammer onto it. (This takes a while, but the heat soaks in and moves out.) 
3. You may have to re-heat the bar a couple of times, depending on the hammer size, but you can watch for the color to move to the faces, and when they're right quench it to stop the process. 
4. I actually did that twice, just to make sure it was even all the way through. that gave a nice, hard face with a progressively softer body to help absorb shock.

That's the basic of it.  This can get a lot more involved, but people were turning out every kind of cutting tool and spring known for centuries using these methods.



*A lot of knifemakers will do two tempering heats, letting it cool completely between.  I've known a couple of people who do three, depending on the steel.

**Some ovens can't get high enough to properly temper some springs.  You're talking 500°F or more.