"postweld heat treatment"

Question:

"I am searching for information on the recommended practices for stress relieving steel for sour service and/or caustic service in a refinery environment."

Answer:

Post weld heat treatment (PWHT) and maximum hardness guidelines for sour service can be found in NACE MR0175. MR0175 can be purchased and downloaded from the NACE website at http://www.nace.org. PWHT for caustic service is typically done at temperatures and times recommended for materials requiring PWHT by ASME Section VIII, div. 1. As NACE MR 175 the max hardness to avoid SCC by h3S is 22 HRC,if the stream has a content of h3S with partial pressure more than is provided in NACE standard. For CS,in refinery piping sistem,only in HAZ that is very narrow and only in external surface is possible to have, as welded, values near 22 HRC. As my research in CS (ASTM A 106B, API 5L B)welded with E7018 with adeguate heat input do not reaches this hardness even in HAZ. Post weld heat treatment is required from codes, but my experience is that even with PWHT and hardness low than 22 HRC we had SCC by h3S and welds of piping without PWHT in h3S are in operation after 20 years.

 

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