I do heat treatment of mostly tool steels for complex injection molds. Disclaimer... I have never heat treated a sword blade. However we noticed that we were warping our mold components a lot ( this was with s-7 steel) so I looked up the heat treat info sheet from the manufacturer and found that when hardening the components a pre-heating cycle should be used. I had previously thought that this ment that you put the parts in the oven at room temperature and then let them heat as the oven warmed... this was erroneous.. what it ment is that I needed to put the parts in at room temp and let it heat to 1400 deg F then let them soak for 1 hour per inch of thickness and then boost the temp to the critical temp for hardening which is 1750 deg F then let the temp heat the part evenly and then soak for another hour per inch of thickness. this solved all of our warping issues with this steel.. So to sum up this is not a totally similar case but I think the principles may apply. Look up the manufacturer's data and see if it recommends a preheat cycle. This cycle will be significantly lower temp than critical temp for hardening. What it does is it causes less stress in the metal when it goes through its phase changes. This should not cause any decarburization either. Obviously the times of heating the blade will not be the same as mold parts but again I think this would be worth pursuing.