When smelting iron ore directly into steel plates, I cannot add productivity modules; the game doesn't recognise the recipe as an intermediate product. I've only tested this on the Hell Oven

https://imgur.com/zqdOIJd
by default, all the vanilla module effect are enabled. And so does the crafting categories. it defaults to be able to smelt all the vanilla recipes. if you can't find that recipe. maybe you are using a mod that adds custom recipes?

It's not a vanilla recipe; it's a custom recipe that has been added with your furnaces. The in-game tooltip says the recipe comes from this mod.

https://imgur.com/a/Ej2m6N1

In another furnace I am using the vanilla recipe for Iron Plates and that works fine with Productivity Modules, it's just this specific recipe that has an issue.

oh, I don't know what this mod originally intended to do with this recipe. but for now it seems to take no effect (no other recipe relies on it) so I just ignore it. and leave it there haven't delete it. so, you can safely ignore it as it doesn't produce anything.

It's a simple way to make steel rather than smelting Iron Plates and then Smelting those Plates into steel. It cuts out the middle section and crafts steel plates directly from Iron Ore. It works perfectly, but the game doesn't recognise it as an Intermediate Product, so I can't use Productivity Modules.

okey, i will take a look into that when i have time, busy perpairing for exam right now.