The main goal of this work was to study the effect of plastic deformation on weldability of duplex stainless steel (DSS). It is well known that plastic deformation prior to thermal cycles can enhance secondary phase precipitation in DSS which can lead to significant change of the ferrite-austenite phase ratio. From this point of view one of the most important phase transformation in DSS is the eutectoid decomposition of ferrite. Duplex stainless steels (DSSs) are a category of stainless steels which are employed in all kinds of applications where high strength and excellent corrosion resistance are both required. This favorable combination of properties is provided by their biphasic microstructure, consisting of ferrite and austenite in approximately equal volume fractions. Nevertheless, these materials may suffer from several microstructural transformations if they undergo heat treatments, welding processes or thermal cycles. These transformations modify the balanced phase ratio, compromising the corrosion and mechanical properties of the material. In this paper, the microstructural stability as a consequence of heat history due to welding processes has been investigated for a super duplex stainless steel (SDSS) UNS S32750. During this work, the effects of laser beam welding on cold rolled UNS S32750 SDSS have been investigated. Samples have been cold rolled at different thickness reduction (ε = 9.6%, 21.1%, 29.6%, 39.4%, 49.5%, and 60.3%) and then welded using Nd:YAG laser. Optical and electronical microscopy, eddy’s current tests, microhardness tests, and critical pitting temperature tests have been performed on the welded samples to analyze the microstructure, ferrite content, hardness, and corrosion resistance. Results show that laser welded joints had a strongly unbalanced microstructure, mostly consisting of ferritic phase (~60%). Ferrite content decreases with increasing distance from the middle of the joint. The heat-affected zone (HAZ) was almost undetectable and no defects or secondary phases have been observed. Both hardness and corrosion susceptibility of the joints increase. Plastic deformation had no effects on microstructure, hardness or corrosion resistance of the joints, but resulted in higher hardness of the base material. Cold rolling process instead, influences the corrosion resistance of the base material.
Surface quality and dimensional tolerances of the selective laser melting (SLM) process are not good enough for many industrial applications and grinding as a common finishing process introduces many surface modifications. Investigation on the effect of grinding induced surface residual stress (RS) on early stages of stress corrosion cracking (SCC) of SLM manufactured 316L austenitic stainless steel was conducted. Potentiodynamic and galvanostatic tests in a 3.5% NaCl aqueous solution, XRD, SEM and energy-dispersive X-ray spectroscopy (EDX) analysis were performed. For annealed and specimens with a low RS magnitude, the dominant observation was pit initiation from existing pores and growth in the build direction. For specimens with medium RS level, SCC initiation from pore sites and propagation along melt pool boundaries and for specimens with the highest detected RS, crack initiation from melt pool boundaries, grains, machining marks, and pore sites were observed. Cracks propagated in different directions, i.e., along melt pool boundaries, near-surface transgranular, and transgranular through columnar microstructure. Galvanostatic tests showed three distinctive regions that corresponded to crack and pit initiation and growth. The synergistic effect of high dislocation density along melt pool boundaries, stress concentration in pore sites, molybdenum segregation, and surface RS was the cause of SCC susceptibility of specimens with high RS magnitude.
Duplex stainless steels (DSSs), a particular category of stainless steels, are employed in all kinds of industrial applications where excellent corrosion resistance and high strength are necessary. These good properties are provided by their biphasic microstructure, consisting of ferrite and austenite in almost equal volume fractions of phases. In the present work, Nd: YAG pulsed laser dissimilar welding of UNS S32750 super duplex stainless steel (SDSS) with 316L austenitic stainless steel (ASS), with different heat inputs, was investigated. The results showed that the fusion zone microstructure observed consisted of a ferrite matrix with grain boundary austenite (GBA), Widmanstätten austenite (WA) and intragranular austenite (IA), with the same proportion of ferrite and austenite phases. Changes in the heat input (between 45, 90 and 120 J/mm) did not significantly affect the ferrite/austenite phase balance and the microhardness in the fusion zone.
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