The knife is an interesting tool and it serves many purposes. Before you heat treat the blade, you have to know what characteristics you would like to see in the knife. It is convenient to set a priority list and then choose the right steel and heat treatment to achieve those results. This will vary with each style of knife, a hunting knife and a dagger have much different requirements. For example, I will go through the heat treatment of a camp knife.
My priority list for a camp knife would be as follows:
The knife must be tough. Since a camp knife is expected to perform a wide range of chores from chopping wood to cutting meat and may also serve as an emergency tool, it must be tough enough to withstand extreme usage without breaking.
It should take a good edge. This may seem like a given, but not all steels will take a good edge or it is impossible to maintain that edge in the field.
Edge holding. There are many viewpoints on edge holding. Some folks want a knife that they never have to sharpen, others, usually professionals, would prefer a knife that they can easily maintain the edge they want.
Since what we are dealing with here is the heat treatment, blade style, geometry, weight and balance are not being considered. Also you will note that maintenance is not on my list. For some folks, stain resistance is important. The problem with stain resistant steels is that that quality is the primary characteristic and it compromises all other attributes.
There are degrees of toughness. I was asked to make a working knife for an expedition across the arctic. The order was prefaced by the fact that no knives they had taken to date had survived the winter. The knife was expect to chop frozen meat at 80F below. My choice was L-6 because of its extreme toughness. The knife performed well and is still in use on the expedition. L-6 would then be a good choice for our camp knife.
Another way to add toughness to the blade is to differentially heat treat the steel. Differential heat treating can be done in simple carbon steels a number of ways, the method I like best is using the traditional Japanese clay coating. By applying a layer of insulating refractory along the spine of the blade, you inhibit hardening of that area of the blade yielding a knife that will have a soft tang, a tough back and a hard edge. If done properly it is possible for the blade to break in the hardened area and still hold together and be functional. It is virtually unbreakable.
As I mentioned earlier, the heat treating starts with the forging of the blade. The initial breaking down heats should be quite high, around 1800F. At these temperatures the steel flows more readily and it also helps to put all of the carbides into solution. As the forging progresses, careful attention to the heats will help to establish a fine grain structure. I like to finish forging at and around critical temperatures. When the blade has been forged to shape, I cycle the steel from just above critical down to dark heat several times, this will establish the finest grain size possible. The last heat I let the blade normalize by allowing it to cool in still air. To add further toughness and make the steel more workable, the normalizing will be followed by a sphereodizing heat.
In the sphereodizing cycle, the blade is brought back up to a point just below Ae1 and held for at least an hour. This is the maximum drawing temperature and it will soften the steel, but it also what happens is that the carbides will begin to aglomulate or pool and it helps to evenly disperse then throughout the matrix. By evenly dispersing the carbides, they help to pin the grain size and restrict their subsequent growth and thereby adding toughness.
The blade is then ground and shaped. When it is ready for hardening, I check the blade over for any coarse scratches or sharp angles. Ideally there should be no shoulders on the portion of the blade to be hardened. I have seen blades crack along makers marks. This occurs because stressed build up along sharp transitions and creates a weak point.
Clay is mixed and applied to the areas of the blade that you do not want to harden. Ideally leaving at least a third of the edge surface exposed. I have described this process in the Sword forging notes. There are many clay formulas, but the best I have found is a high temperature mortar mix called Satanite from A.P. Green. I mix the dry clay with distilled water adding a bit of carbon black to it as I stir. The clay should have a fairly thin consistency and be applied with a spatula. The temperline or hamon is the transition line from the hard edge and the soft back and it can be controlled by how the clay is applied. Varying the thickness and pattern will determine the look of the line. The clay is completely dried and then the blade is ready for hardening.
In my shop, I heat treat swords as well as smaller knives. For the small knives, up to fifteen inches, I use high temperature salt. A clay coating will stay on the blade if it is thoroughly dried before immersing it in the salt. Howard Clark bakes his blades at 500F for an hour before hardening them. If you do not dry the clay completely, the moisture will convert to steam immediately and blow the clay off and spit hot salt. I set my controller so that the temperature of the salt is 1500F and ease the blade down in. There will be a drop off in temperature as the blade cools the salt, but it will recover quickly. I let the blade come up to 1500F and fully austenize. The liquid salt makes for a very fast heat transfer and the blades will come up to temperature much faster than in atmospheric.
Once the blade has fully austenized, I remove it and quickly quench. I use Tough Quench quenching oil from Brownell's. The temperature of the oil should be warm. I keep my oil at ambient shop temperature 78F. I hold the blade in the oil until it has stopped bubbling. The clay will hold the heat longer than a bare blade, but you can feel the vibration from the cooling and when all vibration has ceased it has cooled enough to remove from the quench.
The next sequence needs to be done quickly. You have a window of about 5-10 minutes as the blade cools down before it needs to be put in the tempering oven. First, I knock off any residual clay, then site down the blade and check for warping. I rarely get warping, but it is possible to make minor corrections by bending in a padded vise before the blade cools below Mf. I prefer to straighten my blades after tempering and with the soft back it is much easier than when the blade has been fully hardened. I made a handy straightening jig that works well.
The next check is with a file to make sure that blade has fully hardened. Use a fresh file for this test and remember the feel for future reference. A fresh files has a hardness of around 65RC. Depending on the steel you are working with, you should be able to skate the file across the edge if it has hardened.
Because I am working with temperlines, I like to take the blade to the disk grinder and clean it up so that I can see the temperline in the blade. If I have any doubts about how the blade has hardened, I will check the line with a wipe of ferric chloride. This will color the hardened and unhardened areas of the blade so you can see your heat treating results. If all looks good, I rinse and neutralize the ferric with ammonia, dry the blade and pop it in the oven.
I do a double draw. The first draw is usually at 400F for one hour. I check the blade with a fresh file after the first draw. I expect to feel some bite with the file this time since I have softened the blade, but it should not bite aggressively, that would be too soft. If it is still too hard, I will increase my temperature by 25F and retemper for another hour.
Randal Graham wrote a wonderful description for water quenching which I have posted on its own page.
After the blade has been hardened and tempered it is sharpened and tested. Since blades are tools, they each have a particular task for which they were designed and consequently the tests need to be both generalized and specific to the function each piece. For general testing I use a series of standard tests. First I sharpen the blade and check the edge first by feeling it bite against my thumbnail. A sharp edge will bite into the nail and throw up a whisper of material when dragged perpendicular to the nail. If it feels sharp I then test it by cutting thin paper. Cigarette paper is good or even thin newsprint like Shotgun News. The blade should slice it cleanly and any burr or dullness will show up immediately.
If the blade is sharp, I then will cut into a 2x4. It should bite cleanly and push easily into the wood. Most 2x4's have knots and for some reason knots are a great test to see if the edge will chip. I begin by chopping into a knot gradually increasing the strength of my blows until I am convinced that it will not either bend or chip. If I testing a chopping tool or sword, I then will go into the woods and limb and hack down alders and small trees. Carpet tube, rolled up newspaper and almost anything else you can think of will work. You should work the piece without temerity, if it will break, you want it to happen in your hands and not after it leaves your shop.
For cutting and edge holding, I like to use manila rope. This is a material that will dull a knife fairly quickly and you can get a sense of how a blade will perform by doing it. Like all tests, it is only a reference and it will be more useful to you if you standardize all tests so you will have something to compare one blade to another. I usually cut rope until it begins to resist cutting or loses its shaving edge.
It should be noted here that how the blade is ground has a lot to do with how it will perform. I would say that it is at least as important as any other factor. There are many different edges that can be applied and each will have different performance.
It is really important to test your blades not only for quality control, but also as a reference for future work. If you introduce a new steel or technique to your process, I recommend destruction testing so you know what the limits of the steel are and hopefully find ways of improving it.