Originally posted be dePaul:
Do I understand this matter correctly, claiming that an optimal heat treatment would be shooting for the best combination? Or am I completely confused? And if it were so, would it mean that the variations for specific steel are very limited?
Yes, heat-treatment is all about finding the best combination of properties for working (grinding and shaping), hardening, and tempering (softening) a blade. So no, you are not completely confused. This is a very difficult subject however, and one for which there are many conflicting opinions about what combinations of properties produce the Best blades.
And No, the potential variations of physical properties within any given steel alloy are very large indeed. Any steel can be rendered completely useless for making blades (being too soft, or, too hard) by improper heat-treatment. Even at the same degree of hardness, blades made of the same steel alloy may not perform identically, depending on the specific parameters of their heat treatment steps. Further, the shape of the blade and the edge geometry may play an important role in determining the 'optimal' heat treatment for any given blade should be.
Heat treatment is a process which changes the crystalline matrix of the iron based alloy. The chemistry of this is a little too complex for a post, but simply put, there are a number of different ways that the elements in an iron alloy can combine. Some of these combinations are soft and pliable. Some are very very hard. The temperature of the steel and how rapidly changes in temperature are made strongly effect these crystalline rearrangements. The optimal matrix for any steel alloy must be determined by trial and error. It is a balancing act, trying to maximize wear resistance with toughness (flexibility).
Heat treatment actually means any of several different steps with different purpose. Annealing, or softening, is done by heating to the critical temperature (maybe 1800 degrees) and cooling very slowly. This results in the softest steel that can be cut and shaped more easily.
Normalization is a process whose goal is to remove internal stresses after grinding and shaping.
Hardening is done near the end of blade production by heating to the critical temperature and cooling (quenching) very rapidly. How fast to cool the hot blade depends greatly on the alloy. The goal here is to make the blade as hard as the alloy will allow, to maximize the formation of martensite crystals (the really hard stuff).
The final heat-treatment step is tempering. This step involves lowering the hardness by heating the blade at lower temperatures (a few hundred degrees) to melt down Some of the martensitic matrix into a more pliable (tougher) material that will hold an edge but is not to brittle. You could think of this sort of like having diamonds imbedded in some sort of hard but pliable plastic. The diamonds do all the cutting. But the plastic matrix helps the blade withstand sudden impacts and lateral stresses.
Heat treatment is the Most important aspect in making blade steels that perform their intended function well. It is far more important than the mixture of elements in the steel (the alloy). Unfortunately, it is also the hardest thing to measure in a finished blade. The RC (hardness) will give you some indication of how well a blade was heat-treated, but that is a very imprecise measurement of blade performance. Some makers also use cold treatments (cryo) on some blade steels to refine the crystalline nature of the hardened steel.
Case hardening is completely unsuitable for knife blades.
The best way to heat any particular steel is a matter of opinion. Different makers will heat treat the same steel in different ways. And the proper heat-treatment for various steel alloys can be quite distinct. Some steels benefit from rapid quenching from the critical (non-magnetic) temperature. Others need to air harden. Some makers like those in the American Bladesmith Society favor differential heat treatments for some steels. This means selectively making the edge hard, and the spine soft, in order to balance edge holding and toughness within the blade. Many others (including some ABS mastersmiths) prefer to make their small blades 'all hard'. That is, the same matrix, or hardness, everywhere in the blade.
Anyway, that is a quick review of the major heat treating steps and their purpose. Hope that helps
Disclaimer: I am not a bladesmith. If any knowledgable folks would like to add to, clarify, or correct any errors of fact or ommission in my post, please post your thoughts here. That way I can learn more, and everyone else can as well.
Paracelsus
[This message has been edited by Paracelsus (edited 12-06-2000).]
