Debasis World over refineries, petro-chemical, fertilizer plants use low alloy steels - P11, P 22, P 5, P 7, P 9 materials - for the last 6 decades in high temperature hydrogen service. These are susceptible for %E2%80%9Ctemper embrittlement%E2%80%9D after prolonged exposure at 371 to 593 oC (a well known phenomenon to all users) Of these, P 22 or P 5, the most commonly used materials; do undergo specifically the %E2%80%9C885 oF - 454oC embitterment in heavy wall pressure vessels or piping components. In most plants, world over, these materials have performed well without major problem %E2%80“ if precautionary gradual heating is done after a major plant outage and during re-start. i.e., these should not be subjected to sudden temperature excursions and impact shocks. Gradual start up and heating is done %E2%80“ mostly a "built-in%E2%80%9D operational procedure in all plants (including in plants where sub-zero temperature conditions are likely to prevail). Most maintenance and inspection practices adopt the code stipulated heating and cooling cycles for PWHT for P 22 or P 5 materials. This includes any modification or repair welding in piping or new vessel addition that may be needed from expansion or for plant thru-put increases. Segregation of P, Sb, Sn, As, Mn, Si is believed to occur in grain boundaries and in weld metal after prolonged exposure at the temperature range at 371 to 593o C but need not play any active role during PWHT cooling cycles from 700 to 300 o C to meet code stipulated cooling rates. With original P.O, (clear in specification details), supply of materials with controlled chemistry and mechanical properties meeting to ASTM standards, with care in quality control during fabrication stages to code stipulated WPR, PQR, and PWHT practices plus good operating practices for re-start, temper embrittlement problem can be over come in P 22 or P 5. However, Japanese researchers / manufacturers for heavy wall pressure vessels using P 22 recommend and adopt the step cooling for PWHT to improve the shock resistance after prolonged exposure. We have successfully repair welded P 5 or P 22 in high temp hydrogen service, using conventional code stipulated PWHT without any attendant problem of temper embrittlement after prolonged exposure with service life > 25 years and probably still working well (25+15 years). Well thought and sequenced operation and maintenance practices with P 22 or P 5 may not give problem with conventional PWHT heating and cooling cycles even with exposure in the range of 371 to 593 o C in many plants %E2%80“ if due care is taken during re-start of the plant after outages. It is imperative, however, that impact loading of long term high temp service exposed P 22 or P 5 should be avoided during shut down or during re-start operations in any refinery or petro-chemical plants. Hope this helps you C.V.Srinivasan Nishi Engineers Pvt Ltd India August 18, 2007 E-mail: nishi@vsnl.com >Temper Embrittlement of Heat treatable Low alloy Mn-Cr-Mo >Steel depends upon presence of certain elemnts like P, Sn, >Mn, Si, Sb, As and this embrittlement happens during PWHT, >or SR, or in-service exposure within temperature zone of 450 >- 550C preferably due to preferntial segregation of tramph >impurities at the prior austenite grain boundary which >reduces the impact performance of the material significantly >by shift of DBTT and FATT. > >It has theoritically adviced to redesign the PWHT process >cycle by increasing the cooling rate below 600C upto at >least 300C in order to avoid formation of intermetallic >segregations at grain boundary and thus to reduce the >embrittlement effects. > >My question is (1) for in-service PWHT does any one >conducting this step cooling PWHT Practice ? (2) If by >default we donot follow the practice of step cooling PWHt >does anyone have any breakdown experience of such component >( P11, P22 etc. ) during s/d maintenance due to any >mechanical impact/ loading etc ? (3) Any other related >discussion/ shares pl. > >regards