Summary and Miscellanous
This is the last of the articles series on basic heat treating. I'll be putting them all together in a PDF for E-book form shortly, and I'll keep you updated on the progress of this. With the close of this series, I'll be starting a new one next week, and haven't yet settled on a subject. If there's anything you're curious about and would like to have addressed, shoot me an email!
Today we're running through a basic summary of the past six articles.
The first element to understanding heat treating is that of hardness, and toughness. A hard knife will stay sharp (hold an edge) but is brittle, a tough knife will not break but will easily dull. The harder a knife is, the less tough it will be. The tougher a knife is, the less hard it will be. The point of heat treating is to get at just the right point between the two extremes. Coincidentally, this is often yielding in a springy steel, one that will not take a set when bent.
Now the second element in understanding heat treating is at the molecular level.
Iron is an element, carbon is also an element. Steel is an alloy; a mix of the two, and often other elements. Steel requires a certain amount of carbon in order to be properly heat treated, generally between .40% and 1% Carbon by weight. This is known as high carbon steel.
High carbon steel, at room temperature, is generally in a small, tight cubic formation called Body Centered Cubic (BCC). When heated past critical temperature, which can be tested by using a magnet (it loses magnetic attraction at this point), the atoms switch formation into face centered cubic (FCC) which is also a cube formation but slightly larger and more open. Carbon atoms, which interspersed between BCC cubes, now move inside the FCC cubes.
If cooled extremely slowly over many hours, the FCC slowly changes back to BCC and the carbon atoms slip out into nice, easy locations, and the entire piece of steel moves to the easiest, most "relaxed" state. This is known as annealing, getting the steel dead soft.
If cooled at a normal pace (air cooled), much the same happens but the steel retains some hardness. Doing this several times (heating then cooling to black), each time will refine the grain structure of the steel.
If cooled at a very rapid pace, using a quenchant liquid such as oil or water, the carbon does not slip out in time. The structure becomes very rigid, tough to move in relation to one another but when they do it's at the breaking point.
Normalizing is done, as mentioned above, by heating a bit above critical and allowing to cool until it's completely lost its color. Why? Steel is made up of "grains"; granules of atoms that are all linked together in a cubic formation oriented the same way. A bordering grain will have its atomic orientation a slightly different way. This border is a weak point. The larger the grains, the weaker the steel. You can see the grain structure in the broken surface of steel, ranging from milky looking to coarse sand.
Normalizing then, is to refine; that is, multiply and shrink these grains. The more a steel is kept at a malleable temperature and forged, the larger these grains grow. Now, when the steel is heated past critical, FCC grains begin to form at the borders of BCC grains. Once it's been completely converted to FCC, the steel is left to cool and now BCC grains form at the border of the FCC ones. This is repeated several times, each time because there is little time to keep growing, the grains get smaller and smaller.
Hardening is done by heating to critical and cooling very rapidly, using a quenching like water or oil. The rapid cooling entraps carbon atoms inside cubes too small for them, and a lot of rigidy is put over the structure of steel. The atoms are hard to move in relation to each other (thus it will not dent or bend) but when they do, it's by breaking.
Tempering is done by heating anywhere in between 350 F to 450 F or so for an extended period of time; this eases up on the steel's pressure, toughening it and making it less brittle.
Often during a quench, the steel becomes warped due to uneven cooling or stirring in the quenchant. It is unwise to try to straighten it directly after a quench, or even putting pressure after tempering. I've found that by far the most effective and least risky way is to make a jig to bend it slightly in the opposite direction of the warp, and temper like this. The heating makes it slightly more plastic and more likely to take a set, and it is far more controlled then trying to bend it after tempering.
Differential Heat Treating
As mentioned above, a good blade is one that can hold an edge and that is tough enough to withstand shock in use. One way to get the best of both worlds is to leave a spine that is soft and tough, and so can take much stress, and an edge that remains hard, although brittle, so it can keep an edge.
This can be done in one of two stages. During tempering, and during the quench.
If done during tempering, the process is simple. Using something like a torch, the smith heats up the spine only, to a bluish temper color, avoiding heating the edge as much as possible.
If done during hardening, we can again break it down into two methods, both based on a simple principle which is this: if the steel is heated to critical temperature, and cooled within a certain time frame (i.e., if the steel cools within 3 seconds, it hardens, if it takes 4 or more, it remains soft), it will harden.
Based off that principle, the first method is by heating the edge only to critical, then quenching. Thus, only the edge hardens.
The second, you can simply heat up the entire blade but only quench the edge in the quenching liquid. This means the edge cools within the required timeframe but the spine does not. Similarly, a method was developed by the Japanese called Yaki Ire- the clay quench. An insulating clay is applied over the blade's spine, and the blade is heated and quenched in full. The clay slows the cooling rate of the steel underneath it, and so that steel cools too slowly to harden. The benefit of a clay quench is that when polished and/or etched, the steel exhibits a border line between the soft and hard steels, which is often breathtakingly beautiful.
This covers and summarizes all the articles so far. I apologize for today's article being rather late, but I'm glad it's finally out. Once again, suggestions for future articles are welcomed, shoot me an email, or if you have any questions I would be glad to help out!