There is nothing about flame hardening, just quench hardening mainly because I dont know anything about flame hardening. so I asked, My pinned post does explain how hardening happens, so changing the method of how we trap carbon in the iron matrix does not make more carbon appear, either its there in the first place or it isnt, so unless other things are added as in a surface treatment of more carbon or nitriding, then as quenched is as hard as it is going to get which iirc was also covered in the pinned post