Heat treating is an indispensable process that transforms a blade from a soft piece of metal to a hardened steel possessing proper strength and flexibility. Proper heat treating imparts the required performance attributes of the finished product. A number of characteristics can be altered or improved via heat treatment: various procedures can increase hardness of steel, decrease brittleness, eliminate stress or impurities in the metal.
During heat treating, metal is heated to a critical temperature then cooled, to alter its structure and impart some desirable characteristics, so that the finished product has the perfect balance between hardness and plasticity. An exact extent of hardness needed for a blade will depend on the intended purpose of knife. Metal must be hard enough to keep an edge well, yet flexible enough to withstand intense or even rough use. Accordingly, the entire procedure, and tempering temperature in particular, will be different in each particular case, subject to the hardness level desired in the finished blade.
Once hardened, the steel will not be as workable as before. It will be harder to file and sand the blade after the heat treatment, so only minor finishing polish will be added afterwards. Therefore, prior to heat treating put the bevel on the blade and bring the surface to its preferred finish.
Prepare the required tools:
Typically, heat treatment includes four stages, which are:
The forging process brings much warping into the metal; in particular, it causes the carbides to bunch up. Such steel will not be hard and strong to retain an edge properly. The first step of heat treatment, which is hardening, is required to recreate a uniform state in the metal to make it strong and workable.
During the hardening phase, the steel is heated to a critical temperature of around 1600 to 1750°F, or even 1900°F (depending on the type of steel), to be then quickly cooled down (quenched). You can use various heat sources to heat your blade to a critical temperature. If using a torch, make sure you don’t focus on one side only, remember to rotate your metal regularly over the flame. You can assess the proper temperature with a magnet: steel loses its magnetism at around 1425°F, so once it’s non-magnetic; it means you’ve almost reached the point. Heat the steel a bit more and you should be at the correct heating point.
Quenching or cooling is required to transform rapidly the metal’s crystal structure from austenite into martensite. Martensite is a very hard state of steel that is the goal of this phase. When the steel is rapidly cooled after hardening, the molecular structure is transformed to a fine grain structure with maximum hardness possible.
Prepare an oil quench tank with a lid. Nontoxic mineral or food grade oil will do fine for an amateur. It’s better to preheat the oil to 120 degrees. You may also warm up your oil by heating up a piece of steel and dipping it in the oil prior to quenching. Be cautious when heating the oil – it is flammable and may cause burns.
Once the oil is preheated to the required temperature and the blade has reached non-magnetic status, do the quenching. Do it as quickly as possible after removing the blade from the forge, without letting it to cool down. It is highly recommended that you use welding gloves and long handles tongs during this phase. Take the blade and quickly plunge it into the oil tank. Move it forward and backward to prevent formation of air bubbles around the steel. Then lay the blade on the regulator block submerged in the oil tank – it will ensure that both sides of the blade are evenly cooled.
Wait about 10 to 15 seconds, and then get the blade out of the tank. You can check the hardness of the blade by scraping across the steel with a file. Instead of using a Rockwell Hardness machine (which might be too expensive for a hobbyist knifemaker), you can use a file tools, or even a set of files with sequentially marked hardness. After removing the blade from the oil tank, try each file until one pecks into the steel. While not giving you the exact hardness, this method will let you know a hardness range in which your blade lies.
After the hardening, the steel becomes much harder, but also brittler: it can shatter like glass if dropped. It must be tempered before use. A properly tempered blade will keep an edge while retaining strength and plasticity.
Annealing or tempering the steel involves heating it to a non-critical temperature (around 400°F) to slightly soften the metal and relieve built-up stresses. A variety of equipment, including an ordinary kitchen oven, may be used to this end. It is crucial to provide a reasonable time after hardening (at least an hour or so): the blade should be cooled down to room temperature before tempering. This is required for the proper transformation to martensite, hence for the hardening properties and the quality of the final product. The annealing procedure removes the stresses and induces plasticity to make steel durable and capable of withstanding strains during operation. The steel is brought down from the highest hardness to the proper hardness for an optimum balance between edge retention, grindability and toughness.
Heat up the blade again, this time to 400°F, then let it cool very slowly for four or more hours. You may leave the blade in the forge (or in the oven) after turning it off. An alternative process involves two one-hour cycles, letting the blade cool down between each one.
A number of options exists during this stage, depending on the hardness level desired in the blade. Different hardness levels can be achieved by varying the temperature during the annealing stage. Tempering for 2 hours at 350°F may result in a hardness as high as 63 HRC, while tempering at 660°F will bring your blade to about 53 HRC. The hardness-and-toughness compromise must be considered: a higher tempering temperature will yield a softer steel with higher toughness, whereas a lower tempering temperature will produce a harder and brittler steel. The choice depends on the intended purpose of your knife: for example, a survival knife that is intended to be sturdy may be tempered at the highest temperature of 660°F to make it very tough and able to withstand rough handling. On the contrary, if a sharp and resilient edge is expected to be the forte of knife while toughness can be compromised (for example, for a fine surgical instrument), such blade can be tempered at a low temperature for maximum hardness.
After completing the three stages of heat treating, all is left to do is sand away any scale that may have amassed on the blade. Use a belt sander with a fine abrasive to clean carefully the surface of the blade until it is perfectly clean. Then you can apply an additional bevel, if desired, and apply a fine grit for final sharpening and polishing.