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