One of the problems with welding cast iron is that the repair usually
"holds" but the process sets up so many other stresses that the piece
can crack somewhere else, many times next to the weld.

The only way that seems effective is to pre-heat the piece being welded
and after welding it to put it in an oven and raise the temperature up
to a fairly high level then let it cool very slowly.  I must admit,
however, I never tackled any piano plates.

Jim Moore