Preheating your weld area is good. I don't remember my metalurgy lessons that well please let me know if I have this wrong. If you water quench your parts after the weld your steel will remain the same hardness as it was prior to welding. If you air cool or just let the part cool on its own you weaken the metal around the weld. The problem is in the Filler. Unless matched to each gun it will almost never be exactley the same hardness.

Jeff Hren