Mig Welding Cast Iron

I've used mild steel wire and argoshield with excellent results, plus it means many repairs can be done in situ.
What worked for me was...
1/ grind out vee 2/ weld in stitches no more than 2cm long and at least that far apart 3/ use the lowest current / feed setting 4/ pause between stitches
the idea, contrary to traditional methods with stubs high nickel rods, is to ensure the cast iron never gets hot, so you never get carbon migration and austentic / martensic (sp?) problems.
Weld it like steel and even if it doesn't crack on cooling it will dull a file it will be so hard.
NB I would NOT advocate this on anything other than baths, engine block water jackets, manifolds etc, eg stuff not subject to stress.
I know of NO WAY to satisfactorily weld a cast iron flywheel for example.

You will still get iron carbide formation in the HAZ. It may not be an issue, depending on the use. I've migged up an Escort exhaust manifold well enough to stop it leaking and taken the scars out of a drill table. The first was left as-is, the second needed surface grinding. If the weld needs tidying up afterwards, then use a grinder. I'll save the cutting tools.
Forget about peening the weld. With a low current MiG weld, you'd have to be hitting it within milliseconds of the weld going on to have any effect, it cools so fast.
Nickel rods do give a nice result, but they're not much good if all you've got is MIG :-(
Mark Rand RTFM

Thanks for the replys guys, had a bit of a play today but unfortunately with very little success, so guess i will have to find somebody amongst my mates who can arc weld or braze it for me for a few beer tokens.
Paul.

--You also might want to post over on sci.engr.joining.welding. Can you TIG?

I'm having a problem with the first line of your query when you mention "stainless" wire. If by "stainless" you mean stainless steel, then I'll run away from any job you've done - and fast! The composition of stainless wires include nickel and chromium. Cast iron comprises iron but, more importantly, free carbon. Molten cast iron changes its structure as it slowly cools until eventually it cannot hold the excess carbon, which forms areas of free carbon throughout the structure. If the molten cast iron is cooled very rapidly, this transformation cannot occur, resulting in a structure that is so hard as to be non-machinable,and intensely brittle (white cast iron),
MIG welding is a process where, under the heat of the arc, both the cast iron and the filler wire are melted - and herein lies the frightening result. Upon cooling, much of the free carbon combines with chromium from the filler wire that results in the formation of hard, brittle chromium carbides in the structure. This is highly prone to cracking and is also non-machinable.
In fact, there is a heat affected zone at the joint interface which can have a thin white cast iron crack-prone structure.
Using a stick electrode with a Nickel or Nickel/iron core wire is an important step forwards because the melting point of such material is much lower. Being chromium free it does not develop chromium carbides and the heat imparted into the cast iron can be minimized by a proper stitch welding technique which involved peening the weld runs whilst they are cooling in order to relieve contracting stresses on the heat affected zone (HAZ).
Personally, I would use powder welding for small repairs. On larger jobs I would prefer to "butter" the prepared joint with powder welding and finish off with Nickel electrodes. The reason is that powder welding can be performed without raising the the cast iron in the joint area above its transformation temperature (the carbon remains free). Carefully filling a large joint with electrodes uses the powder weld layer as a heat insulator, so the cast iron is relatively unaffected.
Most powder welding consumables are Nickel alloys and so are 100% compatible with the Nickel and Nickel/Iron electrodes.
Used properly, Nickel Iron electrodes can match the elongation properties of spheroidal graphite and nodular cast irons.