Properties of Sumitomo 347AP Steel tube for Use in Desulfurizing Plants in the Petroleum Refinery Industry

摘要

Austenitic stainless steels such as TP321 and TP347 are widely used to make equipment for use in desulfurizing units such as Hydrotreaters and Hydrocrackers in the petroleum refinery industry. When these steels are sensitized during operation, intergranular stress corrosion cracking (SCC) occurs in polythionic acid, which is a serious problem in heating furnaces and reactors. To address this problem, Sumitomo 347AP(UNS S34751) tube has been developed and widely used in the petroleum refineries in Japan. This paper describes the characteristics of 347AP especially excellent resistance to intergranular SCC in welded joints even without any post-weld heat treatment. Sumitomo 347AP is an austenitic stainless steel which contains optimum amount of carbon, nitrogen and niobium for SCC, mechanical properties and welding. A low carbon content of 0.02% prevents the grain boundary precipitation of chromium carbonitride in welding, which causes intergranular SCC. Sensitization property was evaluated by ASTM A-262. As a result, 347AP was not sensitized after aging at 550deg.C for more than 10,000h. Microstructure was observed by TEM, which shows the amount of Chromium carbides precipitation was depressed apparently compared with the conventional TP347. PTA (Polythionic acid) SCC examinations ASTM G35 reveal that SCC did not occur even after 10,000h aging at 550deg.C. The addition of nitrogen compensates for the reduction in strength due to the lower carbon content of 347AP steel. The high temperature strength of 347AP is almost the same as that of conventional TP347. Varestrain test adding 2% strain shows that crack length was almost same as TP304 and was much less than that of the conventional 347. The weldability of 347AP is better than conventional TP347 due to the lower niobium content.