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It has been known that application of distinctive version UNS S34751 with extra low Carbon and Higher Nb/C ratio was effective to prevent Polythionic Acid Stress Corrosion Cracking (PTA-SCC) without Post Welding Heat Treatment (PWHT) and neutralization treatment in several plants of petroleum refineries. On the other hand, due to lower creep strength, it is difficult to apply UNS S34751 to high temperature application such as Delayed Coker operated over 700 oC (1,292 oF). In order to suppress possibility of PTA-SCC occurrence, improvement of creep property for UNS S34751 is required for high temperature application. In addition, stress relaxation cracking susceptibility should be considered at such high temperature. It was found that Copper addition contributes to strengthen creep property of UNS S34751, and the allowable tensile stress is higher than that of S34709. In this study, PTA-SCC tests and stress relaxation tests were conducted to clarify the effects of Copper content on both resistances. New developed material UNS S34752 shows excellent resistance of PTA-SCC and stress relaxation cracking.
Key words: Polythionic acid stress corrosion cracking, Sensitization, Austenitic stainless steel, Delayed coker, UNS S34709, UNS S34752, Creep property, Stress relaxation cracking
Polythionic Acid Stress Corrosion Cracking (PASCC) of conventional austenitic stainless steel is serious problem at desulfurization units in petroleum refinery.Post Weld Heat Treatment after welding and/or neutralization treatment using alkaline solution are required during each turn around to prevent PASCC according to NACE SP0170-2012.It has been known that proprietary version Type 347LN has superior PASCC resistance even if without Post Weld Heat Treatment (PWHT) compared with Type 347(H) or Type 321(H). Although the proprietary version Type 347LN is lower C content compare with standardized C content range (0.02%) to suppress precipitation of M23C6 carbide it is not reported that the effect of the slight variation of C content to PASCC resistance.Therefore the relationship between C content and PASCC resistance was investigated by using long term aged samples with simulated welding Heat Affected Zone. As the result it was clarified that the strict control of C content less than 0.015% was effective to keep superior PASCC resistance under extra low C content range less than 0.02%.Key words: Polythionic acid stress corrosion cracking Sensitization Proprietary version Type 347LN Hydroprocessing C content matching filler weld metalBest regardsTakahiro Osuki from NSSMC
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In this study, the effect of Mo, Cu and W contents in stainless steels on both NAC and PTA SCC resistance are investigated. The purpose was to optimize a proprietary version of UNS S34751 (TP347LN) with excellent PTA SCC resistance.