https://www.homebrewtalk.com/threads/welding-question-cast-iron-to-black-pipe.240502/

Cast iron is difficult to weld without cracking. Pipes can be difficult to weld into holes. Why? Thermal expansion/contraction coefficients.

Mixing metals and welding them together is difficult. Why? Different alloys with different chemical compositions will form that have different thermal expansion coefficients, different thermal conductivities (will cool at different rates), different oxidation characteristics, etc.

There can be a lot of black art involved with welding. Unless you're willing to devote a lot of time, and a lot of mistakes, to it, you're not going to get there just puttering around.

3M makes all kinds of adhesives, but I don't know if they will have the temperature range you need. You would have to look.

I think this is another case where you should pay the man. (There are welding outfits mounted on big trucks, so they could come to you.)

Short of that, use steel rather than cast iron. That's easier to weld, but the hole / pipe welding issue may still bite you.

If you want to go forward with learning welding, make sure you get high quality eye protection. (My dad lost his right eye, and eventually his life, to uveal melanoma, which he thought he got from the UV light from arc welding (even with a good welding eye shield).)

Hope this helps a little.

Best wishes,
Scott.