Influence of Cold Deformation on the Behavior of Short Time Low Temperature Aging of Superduplex Stainless Steel UNS S39274

The main characteristics of a superduplex stainless steel (SDSS) are the biphasic microstructure composed by ferrite and austenite, and the pitting resistance equivalent (PRE) higher than 40. The mechanical properties and corrosion resistance are optimized with austente:ferrite proportions close to 1:1, and the absence of other phases. The UNS S39274 grade is a W-alloyed SDSS used in critical services in the Exploration & Production of the Oil & Gas Industry, such as in Oil Country Tubular Goods (OCTG). This material can be used for tubes that work with high pressure (until 138MPa) and corrosive environments (with CO2 and H2S). To achieve a high mechanical strength, the seamless tubes are cold drawn. Short duration and low temperature aging can further improve the mechanical resistance of SDSSs. This work was focused on the investigation of short duration aging at 400 and 475oC of the UNS S39274 steel, comparing two initial conditions: cold worked and solution treated. The objective and main contribution of this study was to determine the effect of work hardening on the low-temperature aging, specifically the ferrite phase decomposition by δ→α+α' reaction, where α’ is a nanometric Cr-rich precipitate. The effects on mechanical properties (hardness, tensile, and impact toughness) and pitting corrosion resistance were determined. The activation energy for precipitation (Ea) was obtained through Differential Scanning Calorimetry (DSC). In addition, Transmission Electronic Microscopy (TEM) images show the presence of α' inside ferrite grains of specimen aged at 475oC for 8 hours.