In Reply to: Re: Cast Steel Repair posted by John S. on 12/14/01 at 9:32 AM:
The only way to really get around the problem of stress cracking is to cause the part to shrink more slowly as it cools. This can only be done with proper preheat temps and maintaining the temp during the weld procedure. I would heat it to at least 300°F and would prefer 800°F. Post welding procedure would be to peen the daylights out of the part and bury it in sand, vermiculite, redi-mix powder, Kao wool, etc. The center bore will shrink as much as .020" to .050" in diameter during the cooling process.
To achieve the preheat needed to repair a part that size w/o a furnace you could try a couple of different methods. An oak wood fire would create sufficient heat from the ash as the fire burned down and welding would take place with the part placed in the ash and then the entire thing would be smothered with dirt and left to cool. The old timers repaired large casting using this method.
A more modern method would be to introduce heat by utilizing an airless gouging electrode or copper coated carbon electrode at around 150 to 200 amps on DC- polarity. Ac polarity could be used if you wish to evenly divide the heat between the electrode and the part. A long arc will preheat w/o melting the base material if manipulated properly. Perhaps you would want to use this method in conjuction with a rosebud torch.