Repair of cast iron is very possible, and is done by a few shops that
specialize in it.  Welding with nickel rod will work in some cases,
but there is the risk of a crack opening next to the weld.  If the
weld fails on something that is common, you just throw it away and get
another one.  If the article is a cylinder block from a Duesenburg,
or a rare and desirable instrument, the risk is unacceptable.

As was already mentioned, the plate must be heated to extreme
temperatures, and welded with a torch and cast iron rod.  Afterwards,
it is cooled at a controlled rate in order to equalize the stresses
throughout the piece.  It was unlikely that the harp in any piano was
stress-relieved after casting, so it would actually be better than when
it was new.

I had an engine block welded in a shop in Nevada, Iowa.  The weld was
so good that the original sites of the cracks were not even detectable.
However, I'm not sure that the furnaces that are commonly used are
large enough for a piano harp.  I suspect that Hemmings Motor News
carries ads for shops that do iron repair, and these shops could be
contacted if you are working on an instrument that would justify the
expense.

Larry Mayo