Stainless exhaust vs mild steel?

modulus wrote:When 304 and 316 grade stainless steels are held at elevated temperatures for long periods (say 500C for 100 hrs or so), the crystalline structure can change to a form referred to as "martensitic". This structure is much more brittle than the original and can lead to cracking failures which are sometimes experienced in e.g. oil refinery column trays which operate under exactly these conditions.

If a 304/316 stainless exhaust is of adequate thickness and properly supported, it would be unlikely to experience such a failure in a reasonable exhaust lifetime; however, some systems are made of very light materials and (especially on air-cooled VWs) are poorly supported. In such circumstances, martensitic embrittlement failures would be expected.

I have no experience testing 409 grade SS, so I don't know how that grade would react to long-term elevated temperatures. No direct equivalent embrittlement process exists for mild carbon steeels, though as noted by other posters it is prone to rusting, which will determine the effective lifetime of a carbon steel exhaust earlier than anything else.

A decent quality stainless or carbon steel system will give an acceptable lifetime if properly supported, and *that* is the big issue with air-cooled VWs, of far more importance than the choice between SS and CS.

hth