Blade Hardness By Prof. Roland Phlip

Here is the blade from a beautiful balisong by Terry Guinn. You can read more about it by clicking on the picture.

Notice the dimple between the two tang pins. It's closest to the tang pin on the left.

That dimple is the scar left behind by a Rockwell hardness test.

The Rockwell Surface Hardness Test for Metallic Materials, ASTM standard 18-84, is conducted but pressing a super-hard tipped instrument (often diamond-tipped) into the surface to be tested using a standard amount of pressure and then measuring the depth of penetration. The test is actually done twice, once with a lower pressure and then again at the same point with a much higher pressure. The actual measurement is how much deeper the second test penetrated.

Newage Testing Instruments is a major manufacturer of equipment for surface hardness testing. You can see the equipment used and read more about the process on their website.

Obviously, the Rockwell test leaves a scar, a little dimple, on the surface of the material tested.

Why Bother testing? Why is hardness important?

If a knife blade is not hard enough, it won't take and retain a good, sharp, durable edge. But if it's to hard, then it will be brittle and can shatter under ordinary use. Exactly how hard a blade is is determined by the material used and by the process used to harden it.



Modern knives have blades made of steel. When we say "steel" we usually think of something very hard and very strong. But the strength of steel actually varies greatly depending on alloy and depending on how it's processes.

That variability is a good thing. We want a knife blade to be very hard. But machining hard steel, cutting it, grinding it, shaping it, etc., is not easy or economical. Knife makers prefer to do the majority of the cutting and shaping while the steel is still relatively soft and easy to work and then harden the finished blade before final shaping and sharpening.

There are several ways to harden steel. They all involve some sort of process that is variable. So, for quality control purposes, it is important to test the final product to assure that the hardening process has worked properly and has not hardened the blade to much. Remember, we're trying to maintain a balance here. If the blade isn't hard enough, it won't take and retain a good edge. If it's to hard, it will be brittle and break or even shatter under use.

Consider a simple paper clip. It's probably made of some sort of steel. But you can bend it and shape it by hand. You can even break it with your bare hands Try bending, much less breaking, the blade of a well-made knife and you'll find that it's not so easy. Unless you're very strong, you won't be able to do it with just manual pressure. The difference is that the steel used to make the paper clip isn't very hard. In fact, if you have a good knife, you can probably use it to cut through a paper clip. Obviously, the steel in the knife blade is stronger (actually harder) than that of the paper clip.

There are several ways to make steel stronger and harder. Perhaps the simplest and most economical is heat-treatment. The process is really quite simple. The steel is alternately heated and cooled. This encourages the formation of a tight crystal structure within the metal.

A typical heat-treatment process is to heat the metal quickly to 1975F (1079C), leave it at that temperature for 40 minutes, cool it very quickly to room temperature (about 70F or 21C), heat it quickly to 950F (510C), maintain it at that temperature for two hours, allow it to cool quickly to room temperature, then reheat quickly to 950F again, maintain at that temperature for two hours again, and cool slowly to room temperature. Believe it or not, this simple process of heating, cooling, heating, cooling, heating and cooling again will dramatically increase the hardness of the steel. Some heat treatment processes involve more or fewer cycles. (For an exact heat-treatment protocols refer to the steel maker's literature.)

The heating can be done by putting the metal into a hot fire, a forge for example. You can judge the temperature by the color of the metal. The cooling can be done by plunging the material into oil or even water. But these methods, while still used by many knife makers, are poorly controlled and produce inconsistent results. Modern heat-treatment is done in strictly-controlled industrial ovens. The temperatures, the rates of heating and cooling, and the times are all exactly controlled. This gives very good and very consistent results.

If you make a large batch of blades, maybe a hundred, from the same material, and then run them simultaneously through the same heat-treatment process in a large oven, all of those blades will achieve approximately the same hardness. It's only necessary to test, and deface, one or two blades as representative samples for quality control.

If you take several blades from the same material that are about the same size and run them separately through the same well-controlled heat-treatment process, they will all come out with about the same hardness. As a result, many custom knife makers do not test each individual blade.

Steel is not the only material that responds favorably to heat-treatment. Many metals do, though the exact procedures differ for different types of metal.

And heat-treatment isn't a new idea either. Men have been heat-treating metals for well over a thousand years.

It's only recently, though, that modern science has come to understand exactly what happens during heat treatment, about the crystal structure of the metal and so forth. Our modern understanding of heat-treatment, metallurgy, and crystallography have enabled us to formulate steel alloys that respond especially well to specific heat-treatment processes. Our modern facilities allow us to confect those steel alloys accurately from highly pure materials. And our modern process control capabilities allow us to heat-treat that steel with great accuracy and repeatability.

For our ancestors, heat-treatment must have seemed like magic: you apply a little fire, a little water, and a piece of metal dramatically changes its character. That must be magic, eh? And to further contribute to the mystery, the magic didn't work very well. It could have been inconsistent composition of the metal or impurities in the metal, poor control of temperature, or poor control of the rates of temperature change. All of these factors affect the outcome of the heat-treatment process and none of these factors were understood or well-controlled by our ancestors. For them, the only available explanations for failures were, "The gods didn't favor us that day," or maybe, "the eye of newt wasn't newty enough." And the only explanation for successful heat treatment was that, "the gods were with us," or "we performed the magic spell correctly." As a result, many myths and legends and superstitions arose surrounding heat-treatment, especially for edged weapons.

Imagine an ancient blacksmith who, on a Monday, hardens a sword using the process his father taught him and that usually works well. But it doesn't work very well that day. Why? Maybe because the metal contained some impurities. Maybe because he got the fire just a bit to hot this time. Maybe he was distracted by that attractive lady in the short dress who walked past the shop and so he ended up leaving the blade in the fire a bit longer than usual this time. Who knows? He certainly doesn't. All he knows is that it didn't work very well that day and the resulting sword isn't very good. On Tuesday, as he's about to heat treat his next sword, he feels the call of nature and decides instead of wasting time running out to the outhouse, to relieve himself in the water in which he'll be quenching the blade. For whatever reason, better steel, a slightly cooler fire, better timing, whatever, the process goes especially well that day and the sword is superb. But, on Wednesday, for whatever reason, the process fails again. He may sit down and ask himself, "Why did Tuesday's sword turn out so much better?" He may remember that on Tuesday, he urinated in the water. And without a better understanding of metal and heat-treatment, he may conclude that there's something magic about urinating in the water insist on doing so from now on. And he'll teach his son to as well.

Ancient documents are full of stories and formulas about adding magical elements to the water or the fire, about magical songs or incantations to sing or say or dance during the process (which may actually help by giving more consistent timing), about how heat-treatment must only be done by the light of a full moon (which may add consistency to the judgment of metal temperature by color), and so forth. There are even stories, probably with some basis in fact, about swords, still glowing-hot from the fire, being quenched by being plunged into living humans, usually prisoners or slaves (which may give some consistency to the cooling process).

Today, our modern, consistent, pure alloys and our carefully-prescribed and fully-controlled heat-treatment processes, we get much more consistent results. But, you know what? There's still a certain element of what might still be called, "magic." It's not really magic. It's just variablilty that remains in the process. For example, a sudden power surge could cause the oven to go a little over the desired temperature or to rise use a little faster than expected. So, there is still some variability in the heat-treatment results and still the need to test at least some blades occasionally.

Many modern steels are formulated specifically to respond especially well to specific heat-treatment protocols. Many modern alloys are very sensitive to heat-treatment processes. If they're heat-treated properly, the results are exceptionally good. But, if the heat-treatment process is off even a little, the results may be exceptionally poor. So, control of the process becomes very important and quality control through testing becomes even more needful.

While many knife makers still heat treat steel in open air, modern heat treatment is often done in an inert atmosphere or even a vacuum to remove the atmosphere as a variable. This helps achieve more consistent results.


These websites have more technical information about heat treatment of steel:

Here are some websites for companies that make huge furnaces for heat treating steel on an industrial scale -- amazing:

Those machines are huge! And they're very expensive both to buy and operate. Here are two companies that offer smaller, even table-top, furnaces more suitable for custom knife makers:

Even the small furnaces offered by companies like Paragon and Evenheat cost a thousand or more dollars and require heavy-duty electrical service. Furthermore, as simple as it may sound, heat treatment is still a very skilled task. As a result, many smaller custom makers simply send their blades out to one of several heat-treating services such as Paul Bos mentioned above.

The small furnaces offered by companies like Paragon and Evenheat are way to small for production quantities of blades. On the other hand the large furnaces offered by companies like Dynatech/VacuumFurnace or Lucifer Furnaces are so expensive and require so much energy to operate that if you can't keep them constantly busy, then you can't afford to own them. As a result, most large knife manufacturers such as Benchmade and Spyderco end up sending their blades to outside services.


By the way, two other word for hardening metal by heat-treatment are "annealing" and "tempering." Steel which has not been hardened is often referred to as "mild steel." Steel which has been hardened is often referred to as "tempered steel," or "annealed steel."


Knife blades are generally heat-treated to a Rockwell hardness between about 50 and 70. The higher the Rockwell number, the harder a material is. Harder blades retain a sharp edge longer, but they are more brittle and easier to break. As a result, hardening to higher Rockwell numbers is generally reserved for thicker blades.


What about "cryoquench techniques?"

Some heat-treatment processes call for the metal to be cooled very quickly, faster than can be achieved with oil or water. Those processes may cool, or "quench," the blade in a cryogenic material such as argon or liquid nitrogen. Sometimes, the steel is cooled to low temperatures and held there. The example heat treatment formula I gave above is almost Paul Bos's formula for ATS-34. Paul Bos of El Cajon, California is a very famous expert in heat treating knife steel. And ATS-34 is a very popular steel alloy for knives. Mr. Bos's exact formula for ATS-34 is:

Heat the metal quickly to 1975F (1079C), leave it at that temperature for 40 minutes. Quench with liquid argon to -120F (-84C). Straighten the blade if necessary. Freeze the blade to -220F (-140C) and leave it at that temperature for eight hours. Heat the blade to 950F (510C) and maintain it at that temperature for two hours, allow it to cool quickly to room temperature, then reheat quickly to 950F (510C) again, maintain at that temperature for two hours again, and cool slowly to room temperature. This will result in a hardness of 59-60 Rockwell.

So, that's an actual heat-treating formula with a cryoquench and everything. This should give you a good idea of exactly what's involved. Mr. Bos has written a nice article about heat treatment. You can download it in pdf format from this site:


Heat treatment, with its alternating hot/cool cycles is only one way to harden steel. Extreme cooling, putting the metal into liquid nitrogen for example, can also encourage the formation of exceptionally tight crystal structures. If you think about it, this is really just a form of heat treatment, an alternating heat/cool cycle. You heat the blade to 70F (room temperature), then cool it to about -200F, then heating it back to about 70F. You can read more about this type of "heat" treatment on Cryotron Corporation's website,


What about "differential heat treatment?"

These techniques can harden different parts of the blade differently. The metal near the edge may be very hard so that it takes and holds a very sharp edge. The rest of the blade, the spine, for example, may be left softer. That makes it less brittle. This gives you the best of both worlds. This is often done by covering part of the blade with some material to insulate that part of the blade during the heat-treatment process and to absorb some of the heat. Clay is often used and so such blades are often called "clay-tempered blades." These techniques often leave a visible line on the blade where the two regions meet. The Japanese name for this line is "hamon." You can see "hamon lines" in this blade.

It's a Damascus steel blade, so you see the the beautiful wavy lines of the Damascus. But you can also see the straighter, horizontal "hamon" lines. You can also see how the steel seems a bit darker toward the edge, below the hamon lines. This is a "differentially heat-treated" blade. It's harder toward the edge.


What are some of the other ways to harden steel?

Running an electric current, usually a huge current, through it can harden steel. Exposing it to strong magnetic fields can too. And there are chemical methods. These other methods, though, are either very expensive or don't produce hardness in the range desired for knives. These other methods are generally reserved for situations when there's some special reason not to heat the metal. Blade steel is usually hardened with a heat-treatment process.


So, if it's not "magic," then what exactly does happen during hardening?

Not surprisingly, that's a difficult and technical question that gets deep into crystallography and metallurgy. But you can basically understand it at least at a high level by filling a tall, narrow jar, perhaps a glass "mason" jar, with sand. Lift the jar up and then tap it down onto the table repeatedly. As you do this, two things will happen. First, the apparent level of sand in the jar will go down. Second, larger grains, small pebbles, larger rocks, etc. will come to the top. Of course what you're doing here is packing the sand into the jar. You do that by applying a mechanical shock to the jar. The analogy is far from perfect, but hardening steel is sort of like this. By applying some kind of shock, thermal in most cases, you encourage the steel molecules to pack or organize themselves into a tighter (and therefore stronger and harder) crystal structure.


Knife owners are often quite concerned about heating their blades fearing that they'll somehow destroy the heat treatment. And it's true; you can un-harden steel -- "detemper it" is the common term -- by exposing it to high temperatures. To do that, you have to heat the steel to a temperature above the final heat-treatment temperature. Paul Bos's formula for heat-treating, ATS-34, which is a fairly typical example of a heat-treating formula, is given above. The final temperature is 950F (510C). To detemper a blade hardened with this process, you'd have to heat the blade to above 950F (510C). The lowest final heat-treatment temperature in common use today is over 300F (177C). Leaving the knife on the dashboard of your car even on a very hot day will not detemper it. Sterilization by boiling will not detemper it. Even running it through a dishwasher won't detemper a blade. But putting it into a fire very well might.


There are, of course, many other aspects of heat-treatment and blade hardness. But this should give you some background to start with.


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