What are the catalyst casings made of?

[WIL35]

As in the title really! While fitting some new centre pipes, I noticed some black 'soot' on one of the chassis rails and it has come from the joint that goes around the middle of one of the catalyst. It looks like it just needs welding. I can do steel welding with my MIG, but are these stainless steel or another metal?

Thanks

[Storer]

Mine are stainless.