Quenching Methods and Steels

If you get 20 bladesmiths in a room together you'll likely discover at least 20 different methods of heat treatment, with lots of reasons why they work just fine for the people using them.  What we're describing here is not necessarily in conflict with what others have found.  What one finds often depends on what one looks at or is looking for.  For a long time we used a differential tempering process for everything but found that we were sometimes unhappy with the result. So we started looking for a better method for the cases where the result was not as good as we wanted.

We've heard it said that edge quenching produces a blade that has a hard edge and is unhardened everywhere else.  That is certainly true if you only austenize the edge of the blade to prior to quenching and this is generally done to produce exactly the stated effect.  The statement is also true for steels that don't contain a significant amount of chromium and/or manganese or where the blade cross-section is quite thick and a shallow edge quench is done.  In our experience it is untrue for 5160 and 52100 steels where the entire blade is austenized and the cross-sectional thickness of the blade is no more than 1/8-3/16 inch.  There's a range there because geometry, the width of the blade and the location and width of the thickest section in it have an effect on this.  In our experience, for 5160 and 52100 in a certain set of blade geometries and sizes (amounting to a large percentage of the blades we make), edge quenching produces exactly the performance characteristics we want: a blade with a good hard edge that is tough, as well as springy enough, everywhere else.  Using a differential hardening process instead of a differential tempering process on these blades is desirable because it's all too easy to get parts of a relatively thin blade too soft when differentially tempering it.

As we've said in Our Thoughts on Knives and Steel, our primary blade steels are 5160 and 52100, though we also make a fair number of 1084/1080/1075 blades (see Steel Information).  The results of edge quenching straight-carbon steels are markedly different from the results for 5160 and 52100.  Looking at that from a performance point of view, the result is generally better for 5160 and 52100 than the others in larger blades and blades that might be flexed in use.  This is due to the chromium content of the steels and the fact that the portion of the blade above the quench medium air-hardens to some extent.  We usually edge quench with the entire blade austenized.  With 5160 and 52100 this results in a graduated hardening over much of the blade's depth.  The portion of the blade immersed in the quench medium is fully hardened while the remainder is progressively less hardened as the distance from the quench medium increases.  The straight-carbon steels have little or no chromium.  When edge quenched in this manner a straight-carbon steel blade does not appreciably harden above the quench medium.  However, the straight-carbon steels can be heat treated to be springy in the right places so that the blade won't take a set if flexed.  Because of this we sometimes don't edge quench straight-carbon steels, choosing instead to fully quench, and differentially temper the blade.  The process we use for a given blade depends on the blade design and how the blade is to be used.

It's worth noting that we don't differentially temper blades strictly in the traditional way which is done by heating the blade (spine first) in the forge (or with tempering tongs or a large piece of heated steel or, in a modern variant, with a gas torch) two or three times while observing the oxide coloring of the steel to determine the temperature and thus the tempering effect achieved.  We do use one of these methods once after quenching but follow that with the same triple-temper sequence we use for edge quenched blades.  More specifically, we don't temper the edge in the traditional way.  The differential temper method is applied only to achive the desired toughness in the non-edge portions of the blade.  The edge temper is achieved in the same process used for edge-quenched blades

Information about our heat treatment processes and the rationale behind our choices of steels and blade design can be found in Forging, Heat Treatment and Finishing and Our Thoughts on Knives and Steel.