Characterization of the High Temperature Strain Partitioning in Duplex Steels

Abstract

International audienceA microgrid technique has been developed for the analysis of the high-temperature micro-scale strain distribution between ferrite and austenite into duplex stainless steels. The local strain is measured by micro-extensometry using square microgrids engraved on flat specimens by electro-lithography. The sample with microgrids on the surface and preliminary imaged with high definition scanning electron microscope (SEM), is inserted in a plane strain compression specimen to be deformed under conditions representative of hot rolling. After deformation, the sample is extracted from the compressed block and the surface is again analyzed by SEM and image processing to determine the strain field. The strain is heterogeneously distributed with a strong localization of the deformation, in the form of shear bands located within the ferrite and at the vicinity of the austenite/ferrite interphase boundaries. These strain maps provide useful informations about the rheology of the phases as well as about the local conditions at the origin of the damage proces

New Experimental Procedure for the Analysis of Micro-Scale Surface Damage at High Temperature

“…4), therefore enabling the characterisation of local strain distributions in relation to the early stages of damage development in hot deformed austenite. Moreover, gold microgrids do not influence damage or microstructure evolution unlike etched microgrids used in previous studies [22][23][24]37]. The sharper definition of the microgrid gold lines as opposed to etched grooves also reduces the error in manually locating microgrid intersection points.…”

“…Although an optimal microgrid pitch of 20 μm was selected in this work due to the relatively coarse grain size of the investigated material, results obtained in this study demonstrate that a higher spatial resolution could be achieved at temperatures up to 1000°C by changing the scanning parameters during irradiation of the electro-sensitive resin in order to reduce the pitch size of gold microgrids. Gold is indeed commonly used for electron-beam lithography and, for instance, pitch sizes down to 40 nm have been reported in the literature [38] as opposed to a minimum pitch size of 5 μm achieved with etched microgrids in [22][23][24]37].…”

“…The technique has also been successfully applied at temperatures as high as 1050°C in a plane strain compression test carried out on a duplex stainless steel using a specimen geometry modified to lay microgrids at the surface of an insert welded to the rest of the specimen [22]. This experimental procedure was later further developed and applied at 850°C in [23] and at 950°C in [24]. However, during these tests, microgrids were engraved at the surface of the metal, through etching, and the minimum achievable pitch size was about 5 μm, therefore limiting the spatial resolution for local strain measurements.…”

“…They have also been proven to survive temperatures up to 1000°C as demonstrated in a recent study under plane strain deformation conditions [25]. In addition, the inserts used in [22][23][24] needed to be sealed to the rest of the specimen, through welding, to avoid oxidation of the microgrid surface. The microgrid technique is therefore not suitable for surface crack studies when tests are conducted in an unprotected environment.…”

Local Plastic-Strain Heterogeneities and Their Impact on the Ductility of Mg

“…The CorrelManuV (CMV) DIC software [17][18][19][20][21][22] was used to correlate the grids from undeformed and deformed regions using a 30 9 30 pixel correlation subset and a 15 9 15 pixel search domain. Bilinear interpolation of grayscale was used to allow an accuracy of 0.1 pixel in displacement.…”