[updated:LAST EDITED ON Nov-22-05 AT 11:15 AM (CDT)]From your question it is hard to say because like a couple of others said you are missing some important info (pressure and temp.). One can assume that because C-.5Mo was used that these exchangers are in high temp service (plus you said H2S vapor service). I would guess because the service for H2S vapor and that the stainless steel (308L elecrode is sufficient) cracked in the HAZ when welding or shortly after, that there was hydrogen trapped in the base material. The cracking that occured in the C-.5Mo might of been initiated by SOHIC (if gasses were condensing) - my best guess. Couple of questions: 1. did you apply a bakeout before welding (500F or 260C for 2 hours prior to welding)? 2. has there been and SOHIC inspection done on these exchangers or follow-up checking other welds? 3. was the cracking on the inlet or outlet side? 4. Is this stream just H2S or H2S/H2? Scott123