welding heat treated steel

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Any advice for welding heat treated gears? Having trouble with MIG fillet welds breaking off the gear like there is no penetration and TIG butt welds like to crack at the beginning of the weld along the joint. Doesn't happen on every weld which is sorta weird.
Reply:Gears are often made from low carbon alloys then carburized and hardened.  Carburizing diffuses extra carbon into the surface of the steel so it will harden, resulting in a tough part that will accept a shock load without breaking, yet have good surface hardness to resist wear.  The high carbon layer can be anything from .005 to .050 thick and would explain the "no penetration" look of your MIG welds ... the high carbon zone is cracking from rapid cooling inherent with MIG welding.  TIG has a greater heat input, so the base metal gets hotter and therefore takes longer to cool causing cracks at the beginning of the weld while the base metal is still relatively cold.The simplest thing to try is preheating and slow cooling.  The other possibility is grinding/machining the surface to remove the high carbon layer so you would be welding to a low/medium carbon material.Dynasty 300DXSmith He/Ar gas mixerMM350PHobart Handler 120Smith LW7, MW5, AW1A