Effect of Sigma Phase in Wire Arc Additive Manufacturing of Superduplex Stainless Steel

In the present study, the thermal program in wire and arc additive manufacturing has been varied in terms of heat input and interpass temperature. Three walls were completed with subsequent Charpy V impact toughness and crack-tip opening displacement fracture toughness, together with a detailed microstructure characterization using light microscopy and scanning and transmission electron microscopy. The results clearly demonstrate that the formation of sigma phase may deteriorate the toughness of superduplex components. Such formation may take place under prolonged cooling time, which may occur when subsequent passes are deposited with too high interpass temperatures. This transformation behavior may limit the productivity in additive manufacturing of such steels and care must be taken in selection of proper combination of arc energy and interpass temperature.

Strength Enhancement of Superduplex Stainless Steel Using Thermomechanical Processing

The impact of microstructure evolution on mechanical properties in superduplex stainless steel UNS S32750 (EN 1.4410) was investigated. To this end, different thermomechanical treatments were carried out in order to obtain clearly distinct duplex microstructures. Optical microscopy and scanning electron microscopy, together with texture measurements, were used to characterize the morphology and the preferred orientations of ferrite and austenite in all microstructures. Additionally, the mechanical properties were assessed by tensile tests with digital image correlation. Phase morphology was not found to significantly affect the mechanical properties and neither were phase volume fractions within 13% of the 50/50 ratio. Austenite texture was the same combined Goss/Brass texture regardless of thermomechanical processing, while ferrite texture was mainly described by α-fiber orientations. Ferrite texture and average phase spacing were found to have a notable effect on mechanical properties. One of the original microstructures of superduplex stainless steel obtained here shows a strength improvement by the order of 120 MPa over the industrial material.

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.

Modelamento computacional das transformações de fase durante os ciclos térmicos de processamento de um aço inoxidável superdúplex

This project aims to perform the computer simulation of the transformation’s kinetics and phase evolution during the thermal processing cycles of a superduplex stainless steel, considering the stages of heating, hot working and cooling, using DICTRA® software. The input data for the simulations were chemical composition and phase size, desired simulation temperature, and heating and cooling rates when necessary to describe the thermal cycle. TCFE9 thermodynamic database and MOBFE4 atomic mobility database were used in order to obtain results for different models, determining the one that best describes the phase transformation kinetics. Different rates were simulated during the heating of the material from 950 ° C, considering the initial microstructural condition of 50% of ferrite [alpha] and 50% austenite [gamma], up to 1250 ° C, typical forming temperature. In heating, a maximum fraction of 66.6% [alpha] was obtained at a rate of 0.30 ° C/s, [alpha] value close to the 70% expected by the equilibrium simulation in Thermocalc®. After 1000 s of plateau at 1250 ° C and cooling to the solubilization temperature, 1090 ° C, at the rate of 0.30 ° C/s, the fraction of [alpha] reduced to values of 58.7%. In the sequence, different cooling rates were also simulated with or without the presence of solubilizations plateau. Considering 3600 s of plateau at 1090 ° C, it was possible to recover the desired duplex condition, reaching 55.5% [alpha], but not reaching the 50% expected by the equilibrium balance, since there is still a composition gradient in [alpha] and [gamma] by DICTRA® simulations. Seeking the maintenance of the duplex microstructure, a cooling was performed from 1090 °C to 790 ° C at a critical rate of 3.0 ° C/s, obtaining volumetric fractions of 56% [alpha], 43% [gamma] and sigma fractions equal to or less than 1%. If the plateau at 1090 ° C was not considered, that is, promoting cooling from 1250 °C, where the condition of 58.7% [alpha] was reached, to 790 ° C at the rate of 3.0 ° C/s volumetric fractions of 59.6% alpha], 39.4% [gamma] and 0.9% [sigma] were obtained. DICTRA® was unable to simulate the precipitation of chromium nitrides (Cr2N) during cooling, either because there was no nitrogen supersaturation (N) in [alpha] or because it was unable to predict this supersaturation. From the results of kinetics and evolution of the phases’ volumetric fraction obtained in the thermal cycle of steel processing UNS S32750, it was possible to obtain the computational model that best describes the real behavior of the studied steel

Effect of Post-Weld Heat Treatment on Hardness in Superduplex Stainless Steel ASTM A890/A890M - Grade 6A

Superduplex stainless steel alloy exhibit high mechanical and corrosion resistance, which main industrial application is in the petrochemical industry. The manufacture and maintenance of such equipment usually involve welding processes, followed by post-welded heat treatment and it often becomes impossible to apply heat treatments. Thereby, the purpose of this work is to verify the effect of a post-welded heat treatment on shielded metal arc welding in steel grade ASTM A890/A890M - grade 6A. The microstructure in the as-welded condition consisted of austenite, secondary austenite, and ferrite phases and, the post-welded heat treatment condition exhibited only austenite and ferrite. The hardness in the melt zone reached values of 300 HV after welding and, the value was reduced to 260 HV in the post-welded heat treatment condition.