Welding cast is difficult because of the impurities. These impurities make the weld unable to melt the base metal well, and melting the base metal is key to a good weld. The best way is to make a slight groove along the crack with an angle grinder, and fill the groove in with your weld material. This will give more surface area to the crack. The weld sort of binds to, not melts with the base material, but it is better than nothing. Bronze braising works better because the bronze gets into the pores of the cast better.