catalytic converters get vey hot, 400 C I was told. Given, they are not water cooled, only air cooled. But. If I took out the cat and replaced with steel tubing, would it also reach 400 C? If not then it would mean the cat converter is actually "burning" something present in the exhaust fumes.

From what little I can remember it uses some metals/alloys/ceramics to react chemically with the exhaust fumes and trap some CO or NO2 gases??