does some of the carbon burn during welding of cast iron (gray, ductile)?
It’s not decarb, you get a faint white line at the edge of a gas or arc fusion welding with cast iron rods or mild steel electrodes.
I just used to use whatever worked, cast iron rod, steel/brass/bronze or electrode. Prolly still have 40-60# of Kastweld 111 & some Postile cast$$$. If you gas weld it well you can’t find the fix.
An example below