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What is Heat Treating a Blade?

What is Heat Treating a Blade?

Few of the gods in Greek mythology are as interesting as Hephaestus. Virtually all of 12 Olympian deities possessed beauty and skill in equal measure, although they also had moral foibles aplenty, from Zeus’ rapine lust and Poseidon’s indiscriminate promiscuity to Dionysus’ debauchery and Athena’s manipulation. Hephaestus, though, was different due to his physical deformity, a condition that was either congenital or owed to injury thanks to a very early childhood spat with his thunderbolt-slinging father. Various versions of the myth differ about his origin, but what they don’t disagree about is his dedication to the creative arts, particularly the discipline of forging. Dubbed both “the lame one” and the “sooty god,” Hephaestus was synonymous with the symbols of tongs, anvil, and hammer. However, the act of forging has advanced quite a bit since the time of the ancient Greeks, and given the addition of heat treating, we’d have to add a pot of water or brine to the renowned artificer’s tools if Hephaestus was still a force today.

Despite the fact that heat treating plays an essential role in knife making, many blade aficionados lack even a passing knowledge of what it is and what it does. This article will define heat treating, discuss its scientific niceties, and address some practical questions regarding it.

What Is Heat Treating?

Though we will talk about heat treating as relates to knives, know that heat treating occurs in many industries with many materials such as glass. For our purposes, though, we will be discussing only the heat treating of steel, which undergoes the process for a variety of reasons. One common reason cited for heat treating steel is that it makes the metal harder. While true, hardness isn’t an unalloyed good when it comes to knives as we will see in the next section and it isn’t the only reason why someone might turn to heat treating. The process can also make metal softer or increase its ability to mechanically deform without cracking, breaking, or shattering (i.e., its ductility).

Ironically, the best definition of heat treating notes that heat needn’t necessarily be involved. Why? Well, heat treating involves exposing a material to extreme temperatures in order to effect a change in it. While those temperatures are typically hot, intense cold can produce similar results. Some of the most commonly sought characteristics of heat treating and most common techniques employed include …

  • Hardening: When one heats steel to a particularly high (or low!) temperature, the result is a much harder metal. Hardened steel has drawbacks, though, such as increased brittleness, which is why some seek out the next kind of heat treatment.
  • Case Hardening: One way to manage brittleness is through case hardening. By hardening only the outside layer of a piece of steel, one can leave the internals relatively soft, combining exterior robustness with flexibility.
  • Normalization: Bringing steel to a certain predetermined temperature and holding it there for an extended period produces more uniform metallic structures that exhibit greater strength and greater ability to mechanically deform without breaking.
  • Annealing: Annealing sounds very similar to normalization since it involves heating metal at a certain temperature for a particular length of time. However, to properly anneal steel, one must also slowly air cool it for a certain time at a certain rate.
  • Tempering: Another way to reduce brittleness in an already hardened blade is to heat it again to a lower temperature and allow it to cool.
  • Quenching: To greatly increase the hardness of steel or iron, one can heat it to above the particular metal’s critical point (the temperature where it starts to shift from one phase to another) and then quickly cooling it in a liquid. Water is the most common material used to quench hot steel, but brine or oil are also employed and produce different qualities in the metal.

What Happens During Heat Treating?

We’ve talked a lot about how to perform heat treating and what results heat treating might cause in knife steel, but we haven’t discussed what actually happens during heat treating. Far from being merely an intellectual exercise, knowing the chemical process will help you understand some common heat-treating pitfalls.

The first thing to understand about heat treating is that steel isn’t a singular sort of material. Steel is an alloy that contains more than merely iron. It also has carbon, and many varieties will also include chromium, manganese, nickel, and/or molybdenum. Each of these metals possesses its own crystalline structure, and that structure changes when exposed to heat and force. Naturally, each substance reacts at different temperatures, and the interactions between them can lead to the formation of various compounds. On an atomic level, atoms may slip past one another or combine, creating new materials. This process will vary based on the intensity of the heat (or cold), the length of exposure, the method of cooling, and the medium facilitating the cooling.

This leads us back to the various heat-treating techniques mentioned in the prior section. Each creates different qualities in steel, and it’s rare for any single method to provide the metal with every quality it will need to adequately work as a knife. That’s why the heat-treating process usually includes several, such as normalizing, quenching, and tempering.

Should You Heat Treat Finished Knives?

All knives undergo some sort of heat treating during their forging, but some knife aficionados suggest performing a kind of at-home “spa treatment” for one’s blades, buffing and grinding them and sometimes intentionally heating them as a kind of “reconditioning.” But does this make sense for your average knife owner and are there any risks involved?

The best answers to these queries are “not so much” and “absolutely.” While one *can* recondition a knife that has sustained hard wear and lost its edge, a better solution is to exercise care and perform regular maintenance. The downsides of DIY heat treating are quite significant, such as color alteration, increased softness, or increased brittleness.

Another way to avoid the need to recondition a knife is buy quality right from the start. TacKnives sells top-notch OTFs at reasonable prices, and you can find our selection here.

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