Making Kitchen Knives – Part 13 – Headscratching Curls

These are the three blades that were quenched by using protective stainless steel foil. The function of the foil is to prevent decarburization during the extremely high temperature at which this steel needs to be held at for prolonged time in order to get all carbides into solution.

My initial thoughts were that the blades warped because they are ground too thin. Well, that is not true. Today I have measured the thickness and they are indeed way thinner than I should have made them – all are just 0,35-0,45 mm thick at the cutting edge – but three of the remaining blades are even thinner, three are in the same range and only four are thicker. And of those thinner or just as thin as these, one has very, very slight bend towards the tip that should be possible to correct, and the rest is straight.

So the blade thickness is not the cause. I cannot imagine what else could it be, I do not believe that the foil could have such impact, not to mention that these blades were pulled out of the foil prior to quenching.

My second guess would be decarburization, maybe the experimental protective coating did not work as well as it should and the steel has lost some of its carbon, making it less prone to warping in the quench. But it should also leave it much softer post quench, and I just do not see that.

I have tried my hardness assessing gauges on bought kitchen knife – that big fat stainless steel overpriced junk to be precise – and I got the same result as for the softest one of these – that is, approx 52 HRC.

This means that the blades where my 62 gauge does not scratch are definitively the hardest blades and harder than the store-bought one. And the 62 gauge scratches all these three, but it does not scratch 3 of those where I used the experimental protective coating. And to add to the confusion, one of those three hardest ones is also one of the thinnest. This to me rules out decarburization as the deciding factor for the warping, although it might have caused the high variation in hardness.

I do not believe it is due to my grinding skill, because that should distribute the warping randomly and not only on the three blades that were quenched with foil.

Currently, I am just scratching my head. Any opinion is welcome.

My next step can be either to make these blades circa 5 mm narrower by grinding away the curly parts or trying to re-harden them with the protective coating and maeybe even trying plate-quenching instead of oil. I have never done plate quenching, maybe this could be a good opportunity to try it out…


  1. says

    The edge is definitely too thin, and you’ve got tuem all jammed in the forge so they can’t possibly heat evenly. I know you’re trying to save gas and time but I’d suggest quenching them one by one and visually inspecting them for even heating -- I’d keep the blade in motion in the forge and not touching anything -- raise it slowly to an even temperature. The protective covering gets in your way there. If you’re worried about decarb, run the forge with a reducing gas mix instead, or use clay on the blade and be prepared to grind the blade’s surface down.

    I don’t recall what steel you said this was, but also be careful to check what knifemakers recommend for quenching it (as opposed to the manufacturers) manufacturers are often recommending what works for large blocks of the stuff not a thin edge.

    If the edges weren’t so thin you could bring it up to normal temperature for a while and hammer the edges straight then quench it again, then fix it in the grind.

    Mass production, which you are attempting, requires very precise and reproducible process after you have nailed down the process completely. Try grinding thicker and quench one at a time without anything touching the blade to make it slump or act as a heat sink, and I bet it’ll work better.

  2. says

    It is N690, and there is plenty of recommendations and comments on the web from different knifemakers on how to heat treat it, some very complicated and some with even simpler setup than mine. As far as I was able to find the manufacturer has simplified the datasheet significantly, originally it said things about ramping up the temp slowly through stages, but that version cannot be downloaded anymore. Now it says only heat to 1030-1080°C and dunk in oil.

    Before dunking the blade in oil, I took it out of the foil and I moved it for five minutes through the forge until I got even heat (bright orange color).

    I will definitively grind thicker next time, It is a skill problem -- this ist the thinnest stock I have (2 mm). I will need to improve my magnetic chuck for that, it was a bit wobbly and I slipped more than a few times. And I will only use the protective coating next time, and not the foil, since not only is the foil more expensive, it didn’t even work well this time (although I used it for this steel in the past without problems). The protective coating seems to work better than clay -- the blades are hardened well and it cleaned off easier than clay does.

    As far as the number of blades in the forge is concerned, I do not see how that could be a problem and I even think it actually helped to keep the heat more even, because after half an hour I had a glowing chunk of steel in there that functioned as IR radiator. If that were the cause, it would affect all blades, or a random sample, not only those that were covered with foil.

    I cannot run my forge with reducing mix, I have no power over the mix -- I have two settings only. And if I reduce oxygen with this torch by constraining the air intake, I would automatically reduce the temperature. To have that kind of control, I would have to build a completely new forge with a special burner, and I do not have time, resources and above all know-how for that. I will try to do it at some point, but it won’t be any time soon.

  3. dakotagreasemonkey says

    Can you anneal those wavy blades, straighten them, and heat treat them again?

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