It would be nice to see some photos of the wrecked castings. I've had good results repairing broken cast iron castings using a cast iron repair method called Metalock. Basically Metalock is a cold repair solution which requires a lot of drilling, tapping and the insertion of threaded rods. You have to clamp the cracked parts together an then drill and tap a hole, preferably right through the casting cracked part. Then you insert the threaded rod and cut it off flush. The next hole is drilled half way through the cast iron and half way through the threaded rod, tapped and another threaded rod is screwed in and cut off flush. Repeat, repeat, repeat until the whole crack has been drilled, tapped and filled using the threaded rod.
This repair method has been successfully used to permanently repair cast iron 1500 KW marine gearbox feet (thrust supports), 16,000 HP low speed diesel engine diaphragm plates and steam pump castings, to marine classification society approval standards. I have never personally done this repair myself, but as a marine surveyor, have monitored this type of repair over a number of years on many vessels and am very impressed with this type of repair.
In my opinion it's worth doing some research on Metalock repair methods, as welding heat tends to cause castings to warp and a weld repair may just end up cracking right next to the weld, as has happened to me on numerous occasions. I do believe that you can weld almost any metals, but your procedure has to be spot on to have a lasting repair solution.
About using grease on machines, this is a no no! The grease and swarf mix well together and make a good grinding paste, which rapidly wears a machines sliding ways. Only use way oil or if you have no way oil, a gear oil can be used. (Gear oil has a sticky composition.) Basically way oil is designed to stick onto metal and resists being washed off by the coolant, giving a lasting protection to your machine ways. Speak to your local oil distributor about this and consult the machine manufacturers to find out the best lubrication oil to use on your machine.
Enjoy the rebuild. Geoffrey.