I have done a small amount of welding cast iron with success.
I use an arc welder with a high frequency attachment.  I have used
a Eutectic Xuper 2240 rod and a Nickel-arc 99 rod I believe was made
by Alloy Rods.

I had a welder with a lot of cast iron experience tell me his secret
was to keep pounding on the weld with a chipping hammer for some time,
starting immediately.  He said the length of time depended on how fast
the item cooled.  On something like the strut of a plate I would
probably pound on it for at least 1 to 2 minutes then wrap as much of
the area in fiberglass bat insulation as I could and leave it alone
until it was cool to the touch.

I do no preheating but I do grove out the crack completely.  Sometimes
I grove it from one side, halfway through, and weld then immediately,
turn it over and grove and weld from the other side, then 'pound the
stuffings' out of it.

My guess is that the pounding of the hot weld super-packs the weld so
that when it cools and shrinks the tension is much less.  Like I said,
I have only used this a few times but it has worked for me.  It may
or may not work for someone else and before trying it on a good piano
I would find a junker to experiment on.

John Dewey
Penfield, Illinois