Confusing questions. Cast Iron? 4140? Which is it? Seeing as how a Metallurgist is involved I assume you are not a hobbiest.

What is a WRP? Yes there are compatable GTAW (TIG) fillers for E8018-C3 but thats not to say you can just grab one up and start using it. Do you have a WPS for this application?

There are lots of references available but its really not that simple off the top of your head. You need a writen procedure (WPS) backed by a Procedure Qualification Record (PQR). Either one can answer your questions.

If you want to learn more (and I do mean more) scan and read through this forum, you can learn more here than you ever imagined.

Ron is totally right....4140 is an HTLA steel (Heat-Treatable Low-Alloy). This is no where close to a cast iron. I have serious doubts that you will be using an 8018 series welding rod to weld a cast iron. With all due respect, it seems that there is a big miscommunication between you and the Metallurgist.
Ron is also right about reading through these forum subjects....You can probably find the answer to about any question about filler metals you might have.
In case you would like to get a piece of literature that will help you with your questions about filler metals for cast iron, stainless, HTLA, Cr-Mo's, and just about all the rest of the different grades of steel, may I suggest you buy the AWS Welding Handbook, Vol. 4, Materials and Applications--Part 2, Eighth Edition.

Chuck

Thanks for all of your help. The 8018 C3 was recommended because they did not want to do a post heat treat, and supposedly the higher nickel content helped with this. You all were right, it should not have been classified totally as a "cast iron", but I was told tha it is classified as a "hardenable cast iron" that is used for dies (information may be faulty). I will pick up volume 4 and probably answer all of my questions. Again, thank you.

By the way, a WRP is a Welding Repair Procedure.....