[Phil Brown]

A lot of different grades of stainless steel all with differing degrees of corrosion resistance. The cracking of the welded joints that you see from time to time is most likely from a lack of back side air purging during the welding. The molton weld puddle on the back side gets contaminated with with oxygen instead of a cover gas like it does on the top side.
Can you weld stainless exhaust or headers without back gas purging, sure, but it will most likely eventually crack.