Rob and Marilyn Ridley have been heat treating for many years and they have seen it all! They continue to offer heat treatment services. We asked Rob and Marilyn to give us their "Tips on Heat Treating" and here is what they said.
Heat treat is the soul of the blade and much like the soul within us, it’s largely invisible except through experience. Does it sell knives? Probably not. Striking handle materials, sexy blade shape, engraving and presentation will clear your table at a show faster than a perfect heat treat. Why then, does it matter? It matters for the following reasons.
- Edge retention: Without heat treatment, your knife would be useful for little more than a letter opener. In hunt camp, you don’t expect to be constantly sharpening and if you are a chef, a dull knife is both inconvenient and dangerous
- Return to true: Without heat treat, a flexed blade just stays bent.
- Corrosion resistance: A stainless blade will not be stainless until it is properly heat treated.
- Differentiation from commercial offerings: Most manufacturers are deeply motivated by ease of machining and cost of tooling and abrasives. With careful heat treat, these considerations can be tilted from the manufacturer to the user – where they belong!
This isn’t an article about recipes. There are thousands of those both in print and online (including Knifemaker.ca). Notwithstanding a few nut cases, almost all of them will give you reasonable results. The bullets below are things we learned through many years of experience heat treating knife blades. Experience is what you get when the first try didn’t work out and you keep trying until it does.
- Blades at high temperature need protection from oxygen to prevent decarb, or less serious, oxides forming. A knowledgeable bladesmith can adjust the forge flame for an appropriate flame to accomplish this. Others need tool wrap, anti-scale, vacuum, inert gas or salt bath – all of which have strengths and weaknesses. We use anti-scale (ATP641 or Turco) for carbon steel blades that have to go immediately into quench. We use tool wrap foil for stainless and some others that have slow quench requirements. The result is much cleaner if done properly.
- Don’t remove blades from the foil when plate quenching. They still quench more than fast enough. When a hot blade is exposed to air, oxides begin to form immediately.
- Plate quench. The thick aluminum plates are pricey, but they drain heat faster than open air and help to reduce warping.
- Get the top plate on quickly. Uneven cooling encourages warping.
- Having the blade in the envelope always in the same orientation, reduces the chance of cutting off the thin tip of the blade when opening the envelope.
- Cryogenics used to be seen as voodoo. There was only an effect if you believed there would be. We now know the good effects cryo can have on retained austenite and, accordingly, performance. Particularly with stainless blades, cryo is not required for your blade – but it is required for your blade to be the best it can be.
- Don’t overthink temperature. The thermocouple in your kiln is very accurate. However, it only measures temperature at one spot. A pyrometer in the blade rack will likely show a surprisingly different temperature than at the thermocouple. Also, the thermocouple feeds information to the computer (PID) which often responds slowly to that information – resulting in significant fluctuations even after the kiln ‘stabilizes’. This isn’t a fault – just the nature of the beast. If you think choosing 1465 instead of 1475 makes a difference, you either have an active imagination or OCD. You do need to hold the temperature inside a reasonable range – sometimes for extended periods.
- Hardness testing tells much of the story but has limitations. Hardness files are often subjective with makers expressing widely varying interpretations. ‘Skating a file’ is binary – hard or not hard. A proper Rockwell hardness tester is but it can give erroneous readings from burrs, engraving, mill finish, welded tangs and contaminants. Did you know that a human hair between the workpiece and the anvil can show an error of four points?
- Just because you can, does not mean you should. By way of example, you can get 440C to RHC 62 – resulting in a weak blade and a grainy edge.
If any of this sounds too complicated or equipment intensive, we’d be happy to offer you heat treat services promptly and at reasonable cost.
Rob and Marilyn Ridley
www.HeatTreat.ca
Rob@HeatTreat.ca
403-556-1113
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