Development of UNS S 32760 super-duplex stainless steel produced in large diameter rolled bars

Abstract

Nowadays super-duplex stainless is an important material for the Oil and Gas industries, especially for off-shore production. In deep water exploitation the reliability of production system is very important. Corrosion resistance for pitting of the high alloyed duplex stainless steels with high Mo and N content has to be achieved even in large diameters bars. Therefore, the present work deals with an improved super-duplex stainless steel for the production of large diameter rolled 6bars up to 152.40 mm (6 inches). Among the main improvements, the corrosion resistance evaluated both by the chemical method according to the ASTM G-48 method, as well as electrochemical methods, was achieved. During the production of such large dimensions, the precipitation of inter-metallics and nitrides after cooling from high temperatures was studied by changing the chemical composition using Thermo-Calc and evaluating the new proposed chemical compositions. Several alloy compositions were laboratory scale cast, and the austenite/ferrite balance as well as PREN pitting resistance equivalent number content was correlated to the microstructure and the corrosion properties obtained. It was thus possible to determine the ideal chemical composition and define the new processing parameters to produce the UNS S32760 grade (4501) according to the Norsok standard. The new material properties produced in a production full scale heat are also presented.