Austenitic stainless steels such as TP321 and TP347 are widely used to make equipment for use in desulferizing plants in the petroleum refinery industry. For these steels intergranular stress corrosion cracking (SCC) by polythionic acid is a serious problem in heating furnaces and reactors. To rectify this problem Sumitomo introduced its 347AP (UNS S34751) steel tube in 1985, which offers resistance to intergranular SCC in welding joints without any post-weld heat treatment. Widely used in the petroleum refineries in Japan since 1985, the Sumitomo 347AP is an austenitic stainless steel which contains an optimum of carbon, nitrogen and niobium. A low carbon contact with 0.02% prevents the grain boundary precipitation of chromium carbonitride in welding, which causes intergranular SCC. 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 the conventional TP347.
