Just successfully completed a repair on a broken feedbox lever (cast iron). I have used this technique a number of times on different lathes of different origin, including chinese / taiwanese to repair aprons, feedbox levers, covers etc. (the stuff that gets broken if someone lets it fall over!). much more success that I have ever had with brazing.

With the MIG set for completing around a 6-8mm fillet weld in MS, turn the argon supply right up. Mate the broken pieces together start a spot / tack. There will be lots of spitting and blowing but keep going until you get one established. Complete spot / tack on the opposite side and check alignment.

Welding: position the gun usual distance at right angles to the break. Start weld, hold gun in one position, keep feeding wire, initially it will blow in quite a bit, keep going and the weld pool will begin to fill up, stop when you have slightly overfilled. Repeat at different positions in rapid succession until you have completed welding along the break.

Making a V in the parts would seem like a better practice, but from what I have seen, you get a lot of penetration without doing so, in addition, the more of the original break surface their is, the better the alignment will be, you can also have problems filling up a large V.

The completed weld will be as hard has anything, you will need to use a grinder or dremel with grinding stone to bring it down to flush.

If the break is oily / greasy, you should heat it with a blowtorch or oxy-acetlyene until all the oil and residue has burnt off.

Note that I haven't found any pre-heating to be necessary to weld successfully, usually by the time you get the tacks done, things are pretty hot anyway. Important to let the part cool down very slowly, away from wind and definitely away from water! I haven't had any problems with post-welding cracks by this method as well, even on large cover plates, but in those cases I covered the part with dry sand to let it cool down very slowly.