WNC.....have you accounted for the layer of decarb that forms during heat treating? Not to be confused with scale, decarb is a layer of steel under the surface that has been robbed of it's carbon because of the heat treating. (the carbon that comes out forms the scale). A file will always bite into the decarb layer, and it must be sanded thru in order to reach the hardened steel.
Can't tell you how many times this has been the culprit in instances like this. It's nearly always decarb. Then temps being off. Then wrong steel ID.
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