Heat treating (if done in-house) is a critically important step in the knifemaking process. It’s also one that when it goes wrong it can go wrong very quickly, resulting in cracked or warped blades that either get chucked into a corner of the shop or need careful straightening post heat treat.
Far better in my opinion is simply to take that stress out of the equation altogether and go with plate quench for a 100% bulletproof setup, with repeatable results.
Why would you want to use quench plates?
Plate quenching is the recommended method for heat-treating as it will give a more optimal hardening response than air quenching (faster) and will keep blades straighter than oil quenching.
There are two main reasons to use quench plates in the home shop:
- Quenching Air hardening steels
- Keeping knife blades straight (reduce warping)
But if you need more reasons:
- It’s cleaner (no oils to clean off before tempering) Your wife will approve 😛
- It’s a less aggressive quench than oil (for those intermediate steels).
- You don’t have to remove the blades from the foil.
What steels can I plate quench?
As you already know, not all steels are created equal and as such, they require different heat-treating regimens to get the best results. With air-hardening steels, as the name suggests, you would simply let the knife cool slowly in the atmosphere or blast it with compressed air.
I’ll admit that I’ve had blades warp when doing it this way, which is a massive annoyance and incredibly stressful when trying to straighten the blade post heat treat.
You can use any air-hardening steels, stainless and or high alloy steels eg: A2, D2 (see my guide), D3, M2, M4, 10V, 15V, 3V, Vanadis 4E, K390, 440C, N690 (see my guide), AEB-L, 154CM, S30V, M390, Elmax (see my guide), However, it is always best to check with the manufacture if in doubt.
Are plates fast enough to obtain maximum hardness?
Aluminum (conducts heat very rapidly) quench plates are used as they provide cooling rates that are significantly faster than required whilst helping to maintain flatness.
How fast steel must be cooled to form martensite can be found by looking at CCT diagrams. Continuous cooling transformation (CCT) diagrams are used to represent which types of phase changes will occur in a material as it is cooled at different rates.
For knifemakers, the critical cooling rate aims to avoid the “pearlite nose” of the austenite transformation diagram to obtain fully martensitic structures.
Typically, aluminum plates can have the temperature down to handling temperature in 30 seconds or so, which for most air-hardening steels, is fast enough to ensure full hardness.
A note on advanced heat-treating processes. If you complete grain refining processes prior to heat treatment, just know that it will reduce the hardenability of the steel. Not to say it won’t get hard, but in the context of this article, finer grain shifts the “nose” to the left so the steel will require a slightly faster quench than normal.
What equipment is needed?
The main piece of equipment that you need is two big pieces of aluminum. 6061 is a popular choice as it is readily available and cost-effective. Copper is arguably a better conductor of heat, but is far more expensive.
How big is big? There are no hard and fast rules here but regarding thickness, aim for roughly 1” / 25mm or thicker and as long wide as your biggest knife. Another rule of thumb is 20-30 times the volume of the blade. This is to ensure the aluminum can absorb the heat from the blade and maintain cooling rates across multiple blades/quenches.
If you are batch processing blades, the plates will warm as they absorb heat from the blades, and you’ll need to regulate the plate temperature to maintain the cooling rates as they approach room temperature. To get around this, some people cool the plates in water in-between quenches (which I do) or take it a step further and run a water-cooled setup (most advanced).
Another nice to have includes mounting the plates in a Woodworkers/carpenters vice for a quick clamp solution. This can be done either vertically (preferable) or horizontally if you want. The setup allows you to move it quickly from your kiln to the plates.
Clamps are ubiquitous in the knife-makers shop and they are all you really need to ensure straightness.
How to plate quench:
- Place your knife hot from the oven, between the plates (don’t remove it from the stainless steel foil pouch).
- Clamp shut with firm pressure (vice or with clamps) to avoids warpage and ensure an even contact across the blade’s surface.
- Shoot compressed air all around in between the cracks. (optional)
- Blades are typically cool enough to handle within a minute and can then be given a cold treatment or tempered.
Tip: I have thinner aluminum plates that I clamp the blades in for both the cold and Tempering cycles. In short, they remain clamped at all times and come out perfectly straight.
Note: If you pre-grind your knives, it’s best to keep full-thickness along the spine, to ensure it can make as much contact with the plates as possible.
Warning: Keep your plates dry. If you get condensation on the plates and then place a burning hot packet of steel in contact with the water, you are going to have dangerous outgassing of scalding steam, which could burn you.
Plates for Carbon Steels
The 10xx series steels like 1080, 1095, have a pearlite nose of less than one second….so plate quenching is out, at least for the initial quench. However once the temperature has dropped below the Perlite “nose” (an oil or water quench), then the plates should be used during Ms-Mf to help minimise warp issues.
The best system is to agitate the blade for five to eight seconds in oil/water until black, followed by immediately placing it between the plates.
I hope you have found this article useful and do feel free to shoot me an email if you have any questions.