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…