An Intro to Heat Treating, Part IV

Hardening

After several cycles of normalizing, and after the blade has been ground to rough shape, you still don't have what many knifemakers will call "a knife".

The more philosophical among our craft will call it a "knife shaped object" at this point. 

The quench is where the steel begins to become a proper and capable blade. 

So, let's get down to what happens

If you remember the very first article in this series, in blade terms there are two basic qualities that are on opposite ends of a sliding scale. The first is toughness/softness, and the second is hardness/brittleness. A very tough blade is rather soft and will not hold an edge, but a very hard (edge retaining) blade is rather brittle and will break easily. 

The final heat treating in simple terms is broken down into two basic parts. Hardening and Tempering. Hardening comprises of hardening the blade (who would thunk?) and Tempering softens it sightly, to the exact hard-tough ratio you like. Today we're talking about hardening. 

The smith begins by heating up the blade as evenly as possible in a furnace, kiln, or forge, to reach critical temperature. Professional smiths will use a very controlled heat treating kiln, to reach just the right temperatures and hold those temperatures (soak) for just the right amount of time. These times and temperatures of course vary from steel to steel, but for basic terms in the scope of this article we're just concerned with getting a nice even hardening. 

In something like forge with a single area of heat that might not reach the entire length of the blade, the best method is to slowly draw it back and forth to get an even heat over the entire length. If possible, keep the blade out of the direct flame, this helps to reduce warping, scale buildup, and uneven heat. A common technique is to use a scrap steel pipe as a "mini kiln", which is place under the flame. You can then draw the blade back and forth inside this pipe to get a near perfect, even heat. 

In the absence of a thermometer there's one way to test whether you've gotten up to critical temperature or not. Once the phase change happens (atomic structure goes from BCC to FCC), the steel loses it's magnetism. So, you can touch a magnet to the steel as a test: if the magnet sticks, it's not hot enough. Put the steel back in the forge and heat it up some more, then remove it and test again with the magnet. 

An excellent technique is to always heat treat at night or in relative darkness. That way, after you've heat treated a few blades using a magnet to test, your eye will grow accustomed to the glow and color of the hot steel, and you'll know exactly when you're at the right temperature without having to magnet test. If you heat treat at various times in various lighting, the steel will appear different each time. 

Now that the steel is up to temperature, the next step is the quench

The basic idea behind hardening is this. The steel, at roughly 1600 Fahrenheit, switches atomic structure. If you can cool the steel down to a certain temperature within a certain timeframe, the steel convert to a hard, almost glass-like state. Cooling in plain air is too slow for most steels (there are exceptions!), and so a liquid is the best bet for an even, fast quench. 

That said, let's have a word about different quenchants. Plain water is a common quenchant, traditionally used in Japanese bladesmithing, but it's often too violent of a quench. Both because it's extremely fast of a quench, and because the vapor bubbles create a very unstable environment for the now-glassy steel, and the shock often results in a sharp *PING* noise, indicating a fracture in the blade. 
Because of this, various oils are used. The standard professional oil, Parks 50, is of course ideal, but other oils such as motor (not healthy when burned, so be warned) oil, peanut oil, olive, canola, etc. work very well. Often using a brine helps eliminate the violent bubbles in a water quench. Preferably do some preheating (120 F or thereabouts being ideal) for the quenchant, this lowers the viscosity and makes for a more thorough quench. 

Now sometimes there can be issues with using just oils (too slow), as well as just water (too fast or too violent). This is where you can experiment with interrupted quenches, but that'll be a future article. 

The actual quench must be very quick and very precise. Have the quench tank nice and close to the forge but give yourself plenty of room. 

You can quench either horizontally (edge first) or vertically (tip first), but for our intents and purposes it doesn't really matter all that much. Quenching horizontally, if you can, gives you the ability to edge quench (again, more on that in future articles) and observe different factors that may lead to a failed heat treat. 

Completely submerge the blade in the liquid. For constant cooling, move it back and forth in a cutting motion: don't go side to side, as the blade is still in a plastic state and this can cause warping which is a nightmare to get rid of. 

If you quench in oil, there can be flame, but really you don't actually want this. Any portion of the steel hot enough to cause a flame should be completely submerged. To put it simply, fire needs three things. Oxygen, Fuel, Heat. If the hot steel is completely submerged, there's no oxygen immediately adjacent to the oil that's actually hot enough to catch fire. 

Continue to stir back and forth until the smoke is nearly gone, and the steel will be cool enough. I like to remove it, wipe off with a rag, file test, check for warps, examine for any cracks, then immediately go to tempering. 

The File Test

There are numerous factors that could lead to a failed quench. Whether too cold of steel or oil, uneven heating, too slow of transfer, even things such as atmosphere humidity can affect a quench. So, it's important to check whether the steel hardened up properly. 

The way to do this is with a file. Files are heat treated to a very high hardness; the teeth need to be extremely hard to stay sharp, but a file isn't put under much stress so it can be fairly brittle. Of course, they are still tempered (softened slightly), so a file is the perfect thing to test hardness. 
It's quite simple. 
If the blade hardened successfully, it will be harder than the file. If it failed, it will be softer than the file. So, if the file is harder than the blade, then when it's drawn across the blade's edge, it will cut in and drag. If the file is softer than the blade, then the file will simply skate over the surface as if it were glass. 

Examining for Cracks

Possibly the most heart dropping sound in bladesmithing is the tiny *ping* or *tink* that comes with a fracture forming in the quench. The fractures that come from this rarely form across the entire blade, rather being anywhere from a few millimeters to more than a centimeter in length, perpendicular to the blade,  and starting at the cutting edge. 

Unless you're making yourself a scrap blade, if you find a fracture, it's time to scrap it or see if there's enough steel left to chop it into a smaller blade. Never try selling a blade with a crack in it, even if it's practically invisible. 

Now the blade is usually covered all over with black oxides from the quench and it's often hard to see if what you're looking at is a crack, or something like a scratch filled with oxide. So long as you see a matching mark on the other side, indicating it went all the way through, then you can be sure it's a fracture. Scrap it.

So, you've heated, quenched, tested for hardness, and examined for cracks. The final step before tempering is to check for warping. It is very easy to do this. Hold the blade up so you're looking down the length of the spine, as if you were looking down a rifle or shotgun barrel. This enables you to see very clearly any wobble or warp. If the spine looks straight, flip it over and look down the cutting edge. If they check out, it's on to tempering.

If not, if you see a warp or wobble, then there's an extremely tense straightening process to do simultaneously to tempering. Most blades are ruined during the quench itself, but a close second is when you try to straighten and don't do it just right. 

I'll cover correct warps as well as tempering in next week's article. See you then!