I was wondering too if this was a masonry fireplace or a wood stove.

Its a fire place with a metal insert surrounded by masonry

The one I am working on was built in when the fire place was built

Are you putting a steel plate vertical in the back or an over head steel plate. Is there a burn through spot you are trying to cover?

Ahhh, we'd call that an insert or inset stove. Over here a fireplace is an open fire.

When that was made it was built from 3 or 4mm mild steel, nothing special I'd imagine. If it was really expensive maybe 5 or 6mm but still only regular mild steel id imagine

Yes, it does.
But it rust's like cast iron does.

Ummm no it does not, I can prove it... come over and I will show you on my container.

Replacing the back and sides, back and one side were shot, finally got them burned out today and new peaces made, had to use a torch and skim the old liner our because of another plate behind them. Replacing them with 1/4" plate

You gotta love taking out a layer with a torch with another behind .

Ought to work.:)