Tempering The Blade
The first thing to remember is the principle I brought up in the very first article of this series:
Steel can be thought of on a sliding scale of Toughness/Softness to Hardness/Brittleness.
If hardening jumps us to the extreme hard and brittle end of the spectrum, tempering is the process to bring it back down to the ratio we want. That said, tempering is really a very simple process.
The basic idea is to thoroughly heat the blade at a fairly low temperature, ranging from 350 F - 450 F. The hotter the temperature, the softer the steel. Many folks use a kitchen oven (use a thermometer; the built in one is usually fairly inaccurate), but a toaster oven is plenty ideal.
Now many people use a method called "torch tempering", which technically works okay, but it's inconsistent and risky. Torch tempering is using a torch to heat only the spine of the blade anywhere from a straw yellow to a blue color (called "temper colors"), thus toughening the spine and leaving the edge hard. However the problem with this is that it's incredibly easy to heat the cutting edge, softening it too much and thus ruining it. This would result in having to re-harden the entire thing. The other issue is that the steel isn't completely "soaked" in the heat for a sufficient time, and it can still remain very brittle.
So, the ideal way to temper is for several hours. Various makers will temper anywhere from 3 to 6 hours, ensuring a thorough heating. A common method is to temper in several cycles: for instance a few hours at 420 F, cool in water, then a few hours at 375 F.
Now remember, the temperature you use depends on the steel. This can be found by googling different heat treating charts for the steel type. 1095 steel tempered to 400 F will not act the same as 1084 steel tempered to the same temperature.
Not just this, but a sword will require a higher temperature than a paring knife, for example. A sword needs to be much tougher than a paring knife does.
So, that gives you quite sufficient to perform your own heat treats. The key to getting excellent results (chopping boards of wood while retaining the edge, bending without snapping, etc.) is constant testing, noting, modifying, and retesting.
Walter Sorrels has an excellent video on testing, I highly recommend checking it out.
Now, as tempering is indeed fairly simple, I'll move on to the subject of warps, and more specifically, removing them.
Warps and Straightening
Especially with longer blades, it's very easy to get a warp during the quench. This is usually due to uneven cooling in the quench, mostly something like stirring side to side rather than in a cutting motion. In any case, a warp is always possible, even when you think you've done everything to prevent one.
So, straightening the blade. There are several methods to doing this. A common one, is to flex the blade in the opposite direction of the warp directly out of the quench, while the blade is still steaming or smoking. It's generally able to take a change in direction before it's completely cool and set, but this is incredibly risky. One pound too much force and snap goes your blade. This can be done effectively, however it's best left to those with a lot of experience and have a deep intuition to know the breaking point.
Another method is to try putting pressure on after tempering. Assuming you've tempered the steel soft enough to take a set rather than spring back in blade, you can set up a jig in a vise using a few pegs to straighten it. Two pegs on one jaw of the vise, on the inside of the warp's curve, and a third on the other jaw, on the outside and center of the warp's curve. When you tighten the vise, this will bend the blade in the opposite direction, hopefully enough to take just enough of a set to straighten it.
However, if you didn't temper hot enough to take a set, one of two things will happen. First, you'll have to bend it very far to take a set. Second, it may snap before it gets quite that far. Tough though blades may be, they're not meant to be flexed that far.
The two methods above are known as cold straightening. I've snapped many a blade trying these and it's not pretty, though it does enable you to see the grain structure to check your normalizing job.
So, since then I've elected to take a much safer, and in my opinion, more effective, of an approach.
Quite simple, really. I've found that if you straighten the blade while it's tempering, it's in a hot state, and changing, and is more susceptible to taking a set. My preferred method for doing this is get a flat bar of fairly thick steel and gently clamping the blade along it using a c-clamp (inside of the warp's curve against the steel). Sometimes if this still springs back I'll put two pegs between the blade and the bar, each peg at opposite ends of the warp. The C-clamp's pressure in the middle causes a reverse flex, and while not enough to take a set cold, it should do so nicely while tempering. Be careful not to over tighten before it's begun tempering; hardened steel is extremely brittle. I like to sometimes just get the blade "taught", then tighten at several intervals throughout the tempering cycle.
This technique has produced the highest rate of successful straightening for me. Another method that I've found to be effective without risk of snapping is when cooling in between tempering cycles. When you cool with water between cycles, if you cool in a sink or something and spray the water on the outside of the warp's curve, that will cause that side to contract slightly faster than the opposite side. With experimentation, proper application, and a dash of luck, you can bring the blade back to perfectly straight using just water. This of course depends how serious the warp is.
So, tempering is the process of heating hardened steel anywhere from 375-450 degrees fahrenheit. This softens and toughens the steel just the right amount, to bring the steel to the desired balance between toughness and hardness. Warps can occur during the quench, and I've found that straightening is best done during tempering, both for effectiveness and to lower the risk of snapping.
With tempering, we've covered just about everything in basic heat treating. Next week, I'll delve a little bit into selective hardening and hamons. Note however that I'm not an expert on clay heat treating, but I've done my bit and will pass on what I've found most helpful.