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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.