Model Forum / General / Models / October 2008

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.

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.