Step eleven is gut check time. Proper heat treatment is what makes the difference between a good blade and a piece of broken steel. In the first pic you can see the blade being brought up to critical, the point at which carbon steel is no longer magnetic, in my little homemade charcoal forge. Youíll have to forgive that I canít show the actual heat treat actions. My camera crew has the day off and it kind of takes two hands to get it right.
At this point itís good to talk about how to heat the steel. And forgive meÖthis takes the longest to explain and is the quickest to happen. Heating 10 series high carbon steels to anything above 500 degrees and quenching it will cause it to harden to some degree. Or soften if itís already hard. But full hardening and optimal grain growth will not occur until the steel is heated to critical and quenched. If the temp is too low the steel will not fully harden, to high a temp and the crystalline/grain structure will be destroyed.
Before I start heating the blade I have the oven preheated to 375-400 degrees. Donítí heat the blade and think, ďOh crap, I forgot to heat the oven.Ē Youíll have to pull the blade out and start over. It is critical that 10 series steels go into temper as soon as itís quenched. I also like to use a 2 or 3 inch deep pan filled with sand for temper. But Iíll talk more about tempering in the next step.
I like to heat the steel by nesting it in the coals with the edge down. Heat it until a point about 3/4 the way up the grind and before the grind meets the spine becomes nonmagnetic. The reason for this is that if you have the ability to heat your forge to exactly 1750 degrees and hold it there, the steel will heat through, not overheat and not be damaged. However, in most homemade forges it is impossible to keep the temp form spiking way over critical temp. Maybe as much as 3000 degrees, depending on the fuel and air supply you use. That means the thin grind section will over heat while waiting for the thick spine to reach critical. This WILL damage the molecular structure of the steel and ruin the knife. On top of that, there are benefits to only heating the grind to critical. My method will get the grind/cutting edge part of the blade extremely hard for edge retention and leave the spine and tang softer and therefore tougher and more flexible. Basically what is known as deferential quenching.
As I said, heating the blade up and quenching it will harden it to some degree. Just because the spine does not reach critical does not mean it isnít getting very hot. Then during the quenching process, a great deal of the heat that is being driven from the grind is being pushed into the spine and tang. That means that they too are going to get a lot harder than the annealed state.
In the second pic you can see the scale and discoloration left by heating and quenching the steel. So the process is, heat the steel to critical and plunge it edge down into oil thatís heated to about 160 degrees. This is just a quick pic I snapped as I ran past to get the blade in the temper oven. And I mean ran past.
Step 12 is tempering the blade. As I said, the oven has to be preheated to 375 or 400 degrees before I even start to heat the blade for quenching. I also like to use a 2 or 3 inch deep pan thatís big enough to hold the blade an filled with sand for temper. I stick the quenched blade in the sand just like I put it in the coals to heat for quenching. The sand retains heat and levels out the highs and lows of the oven kicking on and off. The reason I say 375-400 is because no two ovens will heat exactly the same. Mine is about 25 degrees high. But check each oven with a oven safe thermometer.
The goal is 400 degrees maximum temp for at least 2 hours for most knives. Then turn off the oven and let it cool to room temp before the oven is opened. Very important. Never open the oven after the blade is in for temper until the cycle is complete and the oven and knife are back to room temp. A lot of makers like to double temper blades. That is to run the blade through another temper cycle after itís cooled from the first one. I generally donít on thinner blades, but it wonít hurt the blade and may be of some benefit. And if the blade is still too hard a second cycle at a higher temp will soften it so that it wonít chip.
Just a note on tempering. Why temper? If quenching gets the steel as hard as it can be and you want a knife to have a hard edge so it wonít get dull, why temper? Well temper is not just softening the steel. But it does. And it is necessary because the edge will be brittle and chip at full hardness. It also relieves those crazy stresses that are set up during the violence of quenching. If stresses inside the steel arenít relieved by temperingÖit will crack. Yes, I mean split open. Within as little as 2 minutes or up to 24 hours latter it will crack somewhere in the blade and ruin it. Just depends on how itís quench, blade thickness, blade shape, surface blemishes and several other variables, but it will crack if itís quenched to full or very near full hardness! Thatís why I run to get it in the temper oven.
This next pic shows the knife after itís gone through the temper cycle. The pic after it shows the cheap little telescoping magnet I use to check when the steel has reached critical. Thatís the point at which I check it. When this point is no longer magnetic, I quench the blade without wait for the rest of the blade to reach critical.
Blade and Magnet
Step 13 is when the fun starts. There are deposits of carbonization, sometimes called scale or decarb, and pitting left by the heat treat process. These can be pretty heavy depending on the fuel source and speed of heating before quenching. They can be from a few thousandths to a couple hundredths thick. Itís why we left the dime thickness in the grind.
I like to take the blade to my belt sander to remove the worst of it. Then I go to my rotary tool to refine the lines and reestablish the shape of the blade. I use 1/2" drums on a dremmel style rotary tool to get this done. I also use a little fiber reinforced cutting wheel on the same tool to shape the parts I have avoided till now. That can be seen in the pic. The sharpening relief has been cut in and the shape where the tang meets the blade has been refined.
Oh, now is a god time to mention a common problem. Sometimes the blade will take a bend during heat treat and temper. If that happens it can be clamped in a vise and gently tugged on to straightened itÖif the spine and tang were left softer during quenching as this one was. But the blade should be heated to 200 to 300 degrees to do it. It can be heated and straightened with a hammer on a block of wood.
Or the sensible choiceÖtake advantage of that extra material we leave for heat treat and use a table top 4x36 sander or something similar to grind it back straight. Then re grind in the grind lines to match the new grind on both sides. Try to remove most of the bend from the material on the tang first. It should be tapered toward the blade anyway. Then the blade should require only minimal grinding.