Introduction

The term "heat treating" is usually used to describe the process of hardening steel, though it can mean several things, including hardening, softening, and stress relieving.

In this article, we'll use it to describe the hardening and tempering process for steel. This article includes some common terms as well as Liquid Hardening, Plate-Quenching and Air Hardening practices.

If you're curious about how to temper and harden a certain type of steel, check out our article on steel types and how to use them.

Common Terms

Liquid Hardening

The most common hardening technique for blacksmiths and bladesmiths is liquid hardening; the vast majority of steels that forge easily are oil or water hardening steels, most of which are oil hardening. This means that steel that taken above the critical temperature will harden when quenched. Oil is the more common of the two, as it is a more gentle and controllable process than water quenching, especially on a blade.

Water quenching is more violent due to the much faster cooling rate than oil. As a 1500°F blade is quenched in a liquid, the liquid converts into its gaseous state, forming a "vapor jacket" around the hot blade. The thinner the liquid, the faster the vapor jacket is shed from the blade, due to having less resistance to rise to the surface, and the quicker it cools the blade. A quicker quench usually results in a harder blade, though it will have more internal stress, which can manifest through warping, twisting, splitting, or cracking. This is one of the reasons why we recommend preheating most quenching oil to a specific temperature; the warmer the oil, the faster it cools the blade. Water will always be far thinner than oil, and as such, the vapor jacket (steam) sheds much quicker than in oil, resulting in a speedier quench.

A technique commonly used by bladesmiths and blacksmiths is to move the workpiece up and down or back and forth in the oil to help release the vapor jacket to cool the blade evenly. If the blade is moved side to side, it's very likely that it will warp, as it doesn't fully harden until it cools past 400°F (204°C), and the pressure of moving the oil side to side can bend it, as well as the vapor jacket shedding on one side and not the other, causing uneven cooling. The cooling from 1475°F (802°C) or critical temperature to 900°F (482°C) in less than 2 seconds is what causes the steels grain structure to form into martensite, the hardened grain structure of steel.

Each carbon steel has a specific temperature and soaking time for ideal hardness and toughness. This often isn't possible to control without a digital controller. It can be estimated, but for exact control of temperature, a heat-treating oven, kiln, PID controlled forge, or salt/ sand pots are necessary. We prefer heat treating ovens since they're precise and versatile, while still being very safe.

For the hobbyist blacksmith or bladesmith, a heat-treating oven generally isn't a viable option due to cost and or space, so a forge is often the tool used for heat treating and thermal cycling. Most forges can be controlled very precisely by adjusting the fuel and oxygen intakes. By turning down the amount of air allowed in and adjusting the amount of fuel going in, it's possible to run a forge in the 1500°F-1550°F (816°C-843°C) range (You can verify the temperature by using magnets to see how much hotter the steel is getting past the point of non-magnetism). As long as the steel gets just past that point, it will usually be close enough to harden a blade sufficiently. 

Plate-Quenching

The second most common hardening technique for bladesmiths is plate-quenching; this is the technique used to harden thin pieces of martensitic-stainless steels. Stainless steels are primarily air-hardening steels, meaning that they don't need to submerge in a liquid, the shock would be too much and would likely crack most thin pieces.

Once a blade has gone through the proper thermal cycles and soak times, it is taken from the heat source, placed between two aluminum plates, and clamped. The aluminum sucks the heat from the blade and allows it to form martensite, and the straight aluminum plates help keep the blade straight as well. Compressed air is often blown onto the blade while it is between the plates, which allows for faster cooling. Stainless steels oxidize (form forge scale) differently than carbon steel, and need to be protected from oxygen while above critical-temperature.

There are two common ways of doing this; the most common way is to seal the blade in a packet of stainless steel foil, and the other is to purge the heat treating oven with argon. Plate-quenching isn't generally feasible for those just working with a forge, as the steels that require plate quenching need a precise soak time at a specific temperature. An excellent example of the precise control required for the heat treatment of martensitic stainless steel is AEB-L, which needs to be taken to 1900°F (1038°C) and held for 10 minutes, then have a fast ramp-up to 1975°F (1079°F) for another 10-minute hold before being plate quenched. Custom makers commonly use a heat treating oven for hardening stainless steels, but a salt pot would work as well.

Most frequently forged steels are oil hardening, rather than air-hardening, except for some tool-steels such as H-13 and S-7.

Air-Hardening

The final type of heat treatment is a non-plate assisted air-hardening and is the least common type of heat treatment for blacksmiths and bladesmiths, but is more common in industrial settings. The process of air-hardening is essentially bringing the steel up to a specific temperature (for H-13 it's in the 1800°F range) and letting it cool in room air or with a fan blowing on it, that cooling rate is enough to harden the steel.

Conclusion

Now that you've got some of the basic of heat treating, check out our article on how to work with and heat treat some of the most common steels you'll use as a blacksmith or knifemaker.