I would recommend that you heat them to just above non-magnetic and let them air cool. Tempering to 400° would still leave the steel rather hard to grind and machine. After you have shaped your blades from the files and ground your primary bevels in them you will want to repeat the process again then heat them a little hotter and quench in warm canola oil. If that doesn't make them hard enough that a file won't bite into them then you might need to go to a warm brine solution. After you harden the blade to where a file will skate off the edge then you can proceed to temper at around 400° and proceed to finish grinding and sharpening.
Doug
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