There is no hard and fast rule, just a rule of thumb. I do not know of any code or regulatory requirement that places a limit on the number of repairs to a austenitic stainless steel weld. Repairs to austenitic stainless steel may expose the heat affected zone to enough heat to cause chromium carbides to form along grain boundaries (sensitization). Repairs can involve a number of scenarios of either removing weld metal only, weld metal and a portion of the HAZ, or the entire weld and HAZ. So, the real answer is "it depends". It depends on where the repair is, it depends on what the total heat input is, it depends on cooling rate, it depends on total time at sensitizing temperatures (800 F to 1500 F), it depends on base metal grade (e.g. 304 vs. 304L, 321, 347 316NG), it depends on carbon content of the base metal, it depends on whether solution annealing is performed after welding, .... A qualification test for a repair procedure might be to "qualify" the total time at sensitizing temperatures on representative material using representative welding parameters and perform sensitization tests under ASTM A262 Practice A or E.
I have seen this before, but only in the customer specifications. I do not believe code addresses it, but this would not be the first time I have been mistaken.
jrw159 :-)
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