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Heat Treating 101

Heat treatment procedures are usually straight forward in terms of procedure and application. But, sometimes descriptions are inadequate because the same technique may be used to obtain different objectives.The best example for this would be stress relieving and tempering. Both are  accomplished with the same equipment and use the identical time/temperature cycles however, the objectives for the two processes are inherently different. With that in mind, the principal heat treating processes are described below...

Classification of Heat Treating Processes

Normalizing

Normalizing steel allows the crystalline structure to be reset and resets the carbides back to uniformity. Steel doesn’t like uneven structure. If the carbides have gotten all bunched up and oversized from forging, the steel won’t hold an edge as it potentially could. Bar stock from the mill probably shouldn’t have to be normalized, especially if it has been annealed. Material that has been forged could probably benefit. Basically, heat it in your forge or oven and let it air cool. Done.

Annealing

Generally, most people heat to temperature in their forge as the last heat of the day, turn off the forge and let the steel cool in the forge overnight. If you have other work you want done, you can use a half size trash can full of vermiculite and heat a couple large bars of scrap steel to add mass/heat. Heat the steel to temperature and put it all in the vermiculite to cool down slowly using the vermiculite as an insulator. Cooling slower in the forge works better but cooking in the vermiculite works fairly well also. Some people prefer to use lime instead of vermiculite.

Hardening

Generally, in a forge, this means heating it until a magnet doesn’t stick and then “just a little more” to get the extra heat into the steel past non-magnetic. A few minutes at this temperature will not grow grain but does allow the carbon to get “into solution”. Overheating steel into the 1550F to 1600F range and beyond and soaking it will grow grain. Simple, heat it to non-magnetic, give it another minute or so to heat a bit more, quench. 

Quench

Quenching oil is very sophisticated in design and application. If you can’t afford it, use low viscosity motor oil or even canola, vegetable or peanut oil. It won’t be perfect but, it will work. Veteran knife makers will tell you to use dedicated heat treat quench oil to get better results.

Most agree that the steel really needs to be cooled off at a high rate, like 1 to 2 seconds and, that is absolutely true. However, that doesn’t mean you only have 1 second to get from your heat source to your quench. Moving a blazing hot piece of steel from heat to oil is where a lot of fires get started. Knock over the oil, drop the red hot steel into the oil and instant fire! So, be careful! The steel will retain heat and survive a few seconds in the air as you move from heat to quench. Do this safely and be prepared for a flare up of fire and a large amount of smoke. And, always be prepared for a fire.

Tempering

If you did everything right quenching, your steel is fragile as glass. If you drop it now, it will shatter. You want to temper it as soon as it gets to room temperature. Temper twice at 2 hours each allowing the steel to cool back to room temperature between cycles.

  • 450F yields approximately 62RC
  • 500F yields approximately 59-60RC which is where most will want this steel.
  • 600F yields approximately 58RC

Here are some more detailed guidelines: General Heat Treating Data for Various Metals

Cryo Treatment

Cryo treatment is soaking steel after hardening but before tempering at temperatures at least minus -90F (dry ice range) to minus -290F (liquid nitrogen range) for eight hours. Most high carbon steels are not cryo treated as the benefit to carbon steels is usually not as significant as it is with the newer super stainless steels. Soak it in liquid nitrogen overnight or even a cooler full of dry ice. Some people add acetone to the dry ice. Acetone is crazy flammable, and it would not be a recommendation. Use Kerosene or even diesel fuel if you feel you need a liquid medium but, just putting your blade under a block of dry ice is sufficient. Some chose to do one low temper heat at 300F to 350F, called a “snap temper”, to take some of the stress out of a hardened blade before cryo. Adding this step lowers the chance of cracking the blade.

If you are looking for more detailed information, on these processes as well as the terminology associated with them, please refer to this guide: Practical Heat Treating (2nd Edition) by Jon L. Dossett and Howard E. Boyer.

 

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