Heat Treat Question

Feb 26, 2003
I have seen pics of swords and Bowies where the wavy line of the heat treat was obvious. I assume this is what is called the "diffenential" heat treat. I quess only the edge part of the blade is heated by what...torch, oven, and the rest of the blade retains its native qualities. So now what about the rest of the knives, lets take the sebenzas for example. Since there is no little wavy line along the edge, they must put the whole blade in the oven and cook it to whatever hardness. Am I correct then that there is no differentiation in the blade between harder/softer metal in most knives? If so, what makes the Boz (sp) heat treat so special?
Most blades are heat-treated in an oven which results in a more-or-less even hardness throughout the blade.

Differential heat treatment, which results in the so-called "hammon line(s)" that you've observed, is accomplished either by selective heating often with a torch, or by shielding part of the blade from the heat by covering it with an insulating material. Clay is often used and so you often hear this described as "clay tempering."

The gentleman you're referring to, I suspect, is Paul Bos. He is quite famous for achieving excellent heat treating results. What's magic about his process? Well, nothing except that he's got it all dialed in and he's very experienced with it. He consistently achieves excellent results.

Heat treating is very much science at its core, but in practice it remains very much an art to get everything exactly right to achieve the best possible results consistently.

Heat treatment of many modern alloys is quite touchy. Many modern alloys are quite sensitive to heat treatment. The difference between a great knife and a piece of junk can be just a few percentage points on a temperature or a time. So, to achieve excellent results, you have to have your process well-controlled.

Such carefully-controlled processes require expensive equipment and also a lot experience and experimentation. Most small-scale knife makers choose not to invest either the money or the time and, instead, look to an outside service. Mr. Bos has been offering that service for some time and is famous for delivering consistently-excellent results.
Heat treatment is a lot like cooking. In cooking you can burn things or undercook things or turn them into rubber. In heat treatment you can get things too hot or leave them hot for too long during some of the initial stages and sort of make the internal structure of the steel lumpy. If you cool things too fast you can leave internal stresses or microcracks in the steel. If you don't warm the steel up long enough during the tempering process you can leave the steel brittle. If you heat it too much or for too long during tempering you can leave the steel too soft. Some steel will benefit from chilling it with liquid nitrogen after quenching.

So what Paul Bos does is figure out a way to do everything right for particular steels to make them hard, tough, and with a fine grain structure for a sharper edge. He also figures out how to get it right every time. He also helps Buck with their production processes. There are extra problems when you deal with large batches of knives in big ovens.

Differential blade hardening can only be done on certain of the non-stainless steels. It makes sense on a sword or bowie which need extra toughness. Smaller knives are less subject to heavy impacts and don't benefit much from differential hardening.
HERE is an article written by noted balisong authority Professor Roland Phlip at the Institute for Advanced Balisong Studies which gives an overview of hardness, heat-treatment, and related subjects. It's certainly not intended as a deep, technical treatise, but just as an introduction and overview for the average knife user and collector.