CHARACTERIZATION OF CHROMIUM NITRIDE PRECIPITATION IN THE HEAT AFFECTED ZONE OF THE SUPERDUPLEX STAINLESS STEEL UNS S32750: AN EXPERIMENTAL AND NUMERICAL ANALYSIS

It is well known that pitting corrosion resistance of duplex and superduplex stainless steels strongly depends on microstructural characteristics such as ferrite/austenite proportion, presence of intermetallic phases and elemental partitioning between the austenite and ferrite phases. In particular, during the welding operation, very fine chromium nitrides may precipitate within ferrite grains of the heat affected zone drastically reducing the corrosion resistance of welded joints of duplex and super duplex stainless steels. However, due to their small size and low distribution, analyzing the chemical composition and crystallography of chromium nitrides is quite difficult and only a restricted number of advanced techniques of investigation may discriminate their signal from the surrounding matrix. This work is aimed at supporting the microstructural characterization of a welded joint of a superduplex stainless steel by means of a field-emission gun scanning electron microscope. Sub-micron chromium nitride precipitates, identified within the ferritic grains of the heat affected zone, are recognized to be the main reason for the reduced pitting corrosion resistance of the analyzed welded joints. The results are supported by a multi-pass welding process numerical simulation aimed at estimating the cooling rates promoting chromium nitride precipitation in the heat affected zone.

Constitutive Equation Development to Model the Hot Forging of ZERON®100 Super Duplex Stainless Steel and Associated Microstructural Evolution

A constitutive equation of flow stress based on the Norton-Hoff equation has been developed for the high chromium and nitrogen containing super duplex stainless steel, ZERON® 100 (UNS S32760). This was then used to model, using the finite element method, the strain distribution within a uniaxial compression test under typical two-phase forging conditions. Predictions from the model were used to correlate deformation history to microstructure morphology. The microstructure consisted of austenite, γ islands (both primary and secondary) within a ferrite, δ matrix that contained chromium nitride precipitates. For deformation temperatures of 1050°C and 1120°C, the small secondary austenite was equiaxed, whilst at 1280°C the secondary austenite exhibited a Widmanstätten morphology. The highest level of nitride precipitation occurred at the highest deformation temperatures, i.e. highest volume fraction of ferrite, independent of strain rate. This suggests that nitride precipitation appears to be driven to a greater extent by thermal factors than deformation substructure.