Depending on the welding conditions, some stainless exhausts can become sensitized after welding. A sensitized weld is susceptible to brittle cracking in corrosive environments. One of the auto companies paid me good money to study this phenomenon on some warranty returns.

If the weld is done on clean surfaces with little carbon pickup, and is not held at certain temperatures (around 900°F) for too long, then it should be okay.

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95 E320 Cabriolet, 159K