Hi
Assuming that your cast steel is a cast Carbon or Carbon Manganese steel, it should weld well with the E7018 electrode. You will just have to ensure that you give it the correct pre-heats, based on the material thickness. (In castings this can often be rather substantial.)
In addition, you must check your material specification for the heat treatment condition of your valve. You may need to perform a PWHT, although this will probably not be essential if it is a carbon steel.
Regards
Niekie Jooste
Fabristruct Solutions