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Heat Treatment for Knives and Blades

Written by:
Aleks Nemtcev
Updated:
January 7, 2024
heat treat

Noblie: This platform caters to collectors, enthusiasts, and professionals, encouraging their participation in the custom knives community and exploration of the extensive knife universe, with a special focus on the art and science of heat treatment.

heat treat

What is a Heat Treatment

Heat treatment is a controlled process used to alter the physical and sometimes chemical properties of a metal. This process involves heating the metal to a specific temperature, holding it at that temperature for a certain period, and then cooling it down, either rapidly or slowly. The primary aim is to improve the metal’s mechanical properties, making it stronger, more durable, or more flexible, depending on the desired outcome. Common heat treatment processes include hardening, tempering, and annealing, each tailored to achieve specific characteristics in the metal such as increased hardness, improved elasticity, or enhanced resistance to wear and tear.

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. Several characteristics can be altered or improved via heat treatment: various procedures can increase the hardness of steel, decrease brittleness, and eliminate stress or impurities in the metal.

Heat treatment of steel: the main stages

The primary stages are austenitizing (heating the steel to a specific temperature), quenching (rapidly cooling the steel), and tempering (reheating to a lower temperature to achieve desired properties).

During heat treatment, metal is heated to a critical temperature and then cooled, to alter its structure and impart some desirable characteristics, so that the finished product has the perfect balance between hardness and plasticity. The exact extent of hardness needed for a blade will depend on the intended purpose of the 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. 

blade hear treat

Getting prepared

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 afterward. Therefore, before heat treatment put the bevel on the blade and bring the surface to its preferred finish. 

Prepare the required tools: 

  • Heat source (e.g. mini-forge, or gas torch)
  • Fireproof quench container with lid
  • Regulator block (e.g. aluminum tube)
  • Oil for quenching
  • Magnet 
  • Tongs or vise-grip pliers
  • Kitchen oven 

Typically, heat treatment includes four stages, which are: 

  • Hardening 
  • Quenching 
  • Annealing (or Tempering)
  • Final polishing

 

custom damascus knives

Heat Treatment, step 1: Hardening

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 enough 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 (approximately 871 to 954°C, or even 1038°C), 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 (about 774°C), 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. 

Heat Treatment, step 2: Quenching

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 which is the goal of this phase. When the steel is rapidly cooled after hardening, the molecular structure is transformed into a fine grain structure with the 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°F (49°C) degrees. You may also warm up your oil by heating a piece of steel and dipping it in the oil before 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 cool down. It is highly recommended that you use welding gloves and long-handle tongs during this phase. Take the blade and quickly plunge it into the oil tank. Move it forward and backward to prevent the 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 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 the 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. 

heat treat metal

Heat Treatment, step 3: Annealing (or Tempering)

Annealing or tempering the steel involves heating it to a non-critical temperature (around 400°F or approximately 204°C) 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 (about 204°C), 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. 

Several options exist 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 (about 177°C) may result in hardness as high as 63 HRC while tempering at 660°F (about 349°C) 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 (about 349°C) 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 the knife while toughness can be compromised (for example, for a fine surgical instrument), such a blade can be tempered at a low temperature for maximum hardness.

Final polishing

After completing the three stages of heat treating, all that 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.

Video credit: Outdors55

FAQ

 

Why is heat treatment important for knives?

Heat treatment refines the structure of the steel, enhancing its characteristics like hardness, toughness, and wear resistance. This process ensures that a knife can maintain a sharp edge, resist damage, and have a longer lifespan.

How does quenching affect steel?

Quenching rapidly cools the steel from its austenitizing temperature, forming a hard but brittle structure. The specific medium used for quenching (water, oil, air) can influence the final properties of the steel.

Why is tempering necessary after quenching?

Tempering is done to reduce the brittleness induced by quenching. It allows the steel to achieve a balance between hardness and toughness.

Can the same heat treatment be applied to all types of steel?

No. Different steel compositions require specific heat treatment processes to optimize their properties. Factors like heating temperatures, cooling rates, and tempering durations must be adjusted accordingly.

What is differential heat treatment?

Differential heat treatment involves treating different areas of a blade to varying hardness levels. This can create a blade with a hard edge for sharpness and a softer spine for flexibility.

Does heat treatment affect corrosion resistance?

Indirectly, yes. While the primary purpose of heat treatment isn’t to enhance corrosion resistance, certain processes can influence the steel’s microstructure, which can impact its resistance to rust and corrosion.

Conclusion:

In the vast world of blades, heat treatment stands as a pivotal process that defines the very essence of a knife’s performance. It’s not just about shaping metal; it’s about refining and optimizing it for tasks ranging from everyday slicing to specialized artisanal crafting. Through heat treatment, a simple piece of steel transforms into a resilient, sharp, and durable tool, embodying the delicate balance of hardness and flexibility. For knife enthusiasts, understanding this process deepens their appreciation for the art and science of blade-making. Whether you’re choosing a knife for your kitchen, your workshop, or your outdoor adventures, recognizing the significance of heat treatment ensures you’ll value not just the blade’s edge but also the journey it took to achieve its excellence. Remember, a blade’s soul isn’t just forged by its maker, but it’s also tempered by fire.

Author: Aleks Nemtcev | Connect with me on LinkedIn

References:

https://en.wikipedia.org/wiki/Heat_treating

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comments

  • K-E Nylund

    Clear and good — but used Celsius for 2x hell.

    Noblie

    OK.Information updated.

  • Arturo

    Excellent presentation of the procedures.

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