There are several heat treatment processes applicable to knife making and the intention of this article is to debunk some of the myths that surrounding this aspect of our art. The processes that we need to understand are stress relieving, annealing, normalising, hardening and tempering.
Stress relieving is a procedure to remove induced stresses from (mainly) blade material without changing the microstructure. Some of the blade material available is manufactured and shipped as a coil (big roll). Before purchase the material is de-coiled (straightened and flattened) so that we can buy a flat (or sometimes not so flat!) strip.
This bending and un-bending induces stress and makes the material harder (exactly as bending a paper clip back and forth will harden it till it breaks). When we then start to heat up the material for hardening these stresses relieve themselves and the blade bends. We then blame, quite wrongly, the heat treater for a bent blade. If your heat treatment process does not use a system of press quenching to keep the blade straight, this problem can be reduced to a minimum by bending your blade blank straight and heating to 600-6500C, air cool and then, if necessary, re-straighten it before hardening.
Annealing and normalising are processes that are used to a) soften a hardened blade (when you realise you’ve forgotten to drill a hole in it!) and b) to “normalise” a deformed microstructure, eg after forging. Annealing means heating the steel to the hardening temperature and cool it slowly enough, in the furnace or in vermiculite, to prevent it hardening. For stainless steels it may be necessary to have a furnace that can cool at a very slow (programmed) rate, say 10 or 200C per hr.
Normalising is similar to annealing except that after heating the steel is usually air cooled and it doesn’t matter if it hardens slightly (or even fully). Here we are trying to “normalise” the structure of the steel, not soften it. All steels can be normalised but if any machining is required afterwards then normalising is the wrong treatment and annealing is required. For those who forge blades normalising is not optional but essential. I’ll cover the “best practice” procedures in a later article.
Hardening means to heat the steel to the recommended hardening temperature and cooling it fast enough to prevent the normalising or annealing process described above and forcing the microstructure to change to a very hard (martensitic) state. This required cooling rate can be as fast as iced brine for very low C plain carbon steels to still air for high alloy stainless steels. “Recommended hardening temperature” means that dictated by the material specification or the steel manufacturer. You cannot improve on this so please don’t try. Quench rate will also be recommended – quench too fast and you will probably crack the blade. Too slow and it won’t harden properly. Again details on how to achieve this in subsequent articles.
Once properly hardened the blade is very highly stressed and if used as such will suffer edge chipping at best to outright breakage at worst. All blades must be tempered (and possibly require multiple tempers and/or sub-zero treatments) and again follow the steel manufacturer’s guidelines.
I’ll go into the detail of all these procedures, with do’s & don’ts, in subsequent articles.