CONICET | Buscador de Institutos y Recursos Humanos

The studied duplex stainless steel (DSS) was the standard type UNS S32750 DSS in the form of hot-rolled plate. The material has a similar volume fraction of austenite and ferrite and does not reveal a completely anisotropic microstructural morphology. Indeed, the lamellar structure of the austenite in the ferrite matrix, typical of rolling process, is missing in certain areas of the plate and the average austenitic grain size is appreciably small. It is widely accepted that [1-4], for the typical rolling morphology (in bars or plates) of DSS and cyclic deformation at low plastic strain ranges, microcracks initiate in the austenitic phase on {111} slip traces and can propagate to the ferrite along slip marking associated with persistent slip bands (PSB). On the contrary, at high strain ranges, both phases are active and a structure of slip markings is observed as part of the surface damage in the steel. Moreover, due to the wavy character of the ferritic matrix, slip markings result in a more intense surface damage [5]. Most of these studies have been carried out with specimens machined from the bulk of the bar with a coarse lamellar and grain structure and have not taken into account the surface effect of the hot-rolling process. The present work focuses on the fatigue performance of S32750 DSS in the form of 3mm plate and on the microcrack nucleation due to surface microstructure formed during rolling. Fatigue tests have been performed until fracture on flat specimens of S32750 DSS cut parallel (RD) and transversal (TD) to the rolling direction to compute the life time for different total strain ranges. In addition to that, fatigue tests until about 90% of the fatigue life were carried out in RD specimens to evaluate the surface damage accumulation and the dislocation structures. The fatigue surface damage of the specimen was analyzed by Optical Microscopy with Differential Interference Contrast (OM-DIC) technique. Slides parallel to the tensile axis have been taken from the specimens to obtain thin foils for TEM. Some of them come from the near-surface region and others from the interior. Thin foils of the surface region have been produce using a single-side thinning technique. The dislocation structure was observed by Philips mod. EM 300 Transmission Electron Microscope (TEM) operated at 100 kV. The cyclic response at the different deformations of RD specimens is characterized by an initial cyclic hardening, whose level depends on the applied total strain range, followed by a continuous cyclic softening. That of TD specimen tested with a total strain range of Äåt=0.8% shows similar softening but does not present an initial cyclic hardening. The microstructure developed in the flat RD specimen cycled at Äåt = 0.8% shows slip lines distributed mainly in the austenitic phase in a lamellar á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. in the austenitic phase in a lamellar á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. cyclic hardening. The microstructure developed in the flat RD specimen cycled at Äåt = 0.8% shows slip lines distributed mainly in the austenitic phase in a lamellar á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. in the austenitic phase in a lamellar á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. Äåt=0.8% shows similar softening but does not present an initial cyclic hardening. The microstructure developed in the flat RD specimen cycled at Äåt = 0.8% shows slip lines distributed mainly in the austenitic phase in a lamellar á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. in the austenitic phase in a lamellar á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. Äåt = 0.8% shows slip lines distributed mainly in the austenitic phase in a lamellar á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes. á-ã structure and some slip lines in the ferritic phase when the microstructural morphology is not completely anisotropic. At higher deformations, the slip markings appear homogeneously distributed in both phases. The study of the dislocation structure in the bulk and near the surface have shown that in the bulk of the rolled material no differences have been observed in comparison with other authors [8] [10]; however, in the near-surface, the analysis has revealed the presence of martensite laths in the austenite and micro-twins in the ferrite as a consequence of the fabrication process, and that this structure affects the mode of crack nucleation in the ferrite in which microcracks forms in the trace of the {112} planes.

enviar mensaje