Strength Enhancement of Superduplex Stainless Steel Using Thermomechanical Processing

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Article Strength Enhancement of Superduplex Stainless Steel Using Thermomechanical Processing

M. A. Lakhdari 1,2,*, F. Krajcarz 2, J. D. Mithieux 2, H. P. Van Landeghem 1 and M. Veron 1

1 Univ. Grenoble Alpes, CNRS, Grenoble INP, SIMAP, F-38000 Grenoble, France; [email protected] (H.P.V.L.); [email protected] (M.V.) 2 Aperam Research, rue Roger Salengro, 62330 Isbergues, France; ﬂ[email protected] (F.K.); [email protected] (J.D.M.) * Correspondence: [email protected]; Tel.: +33-4-7682-6624

Abstract: 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. Citation: Lakhdari, M.A.; Krajcarz, F.; Mithieux, J.D.; Van Landeghem, Keywords: superduplex stainless steels; thermomechanical treatment; morphology; texture; mechan- H.P.; Veron, M. Strength Enhancement ical properties of Superduplex Stainless Steel Using Thermomechanical Processing. Metals 2021, 11, 1094. https://doi.org/ 10.3390/met11071094 1. Introduction Among the different types of stainless steel, duplex grades, which are two-phase alloys Academic Editor: Andrey Belyakov consisting of equal volume fractions of austenite and ferrite phases in their microstructure,

Received: 1 June 2021 provide the best compromise between mechanical properties and corrosion resistance [1–4]. Accepted: 6 July 2021 These materials have attracted great interest due to their outstanding properties, which are Published: 9 July 2021 desirable in offshore, construction and energy generation applications [1–4]. Superduplex stainless steel is, for instance, the material of choice for offshore rig umbilicals and for heat

Publisher’s Note: MDPI stays neutral exchangers in power generation. with regard to jurisdictional claims in As with any other metallic alloy, the mechanical properties of duplex stainless steel published maps and institutional affil- (DSS) grades are intimately related to their microstructure [5]. Previous studies [6] proved iations. that thermomechanical processing, which combines heat treatments and rolling, is con- sidered an effective approach to tailor the microstructure of DSS. In the literature, two thermomechanical treatments can be identified: the first, as shown in Figure1a, is based on an industrial development [7] that consists of a succession of both hot and cold rolling

Copyright: © 2021 by the authors. steps followed by annealing. As a result, a strongly oriented microstructure aligned along Licensee MDPI, Basel, Switzerland. the rolling direction is obtained. This article is an open access article The second treatment, as described by T. Maki et al. [6], is shown in Figure1b. It distributed under the terms and differs from the ﬁrst process by the application of a solution treatment at high temperature conditions of the Creative Commons in the ferrite single-phase region after the hot rolling and annealing steps are performed, Attribution (CC BY) license (https:// which leads to the formation of an equiaxed microstructure. creativecommons.org/licenses/by/ 4.0/).

Metals 2021, 11, 1094. https://doi.org/10.3390/met11071094 https://www.mdpi.com/journal/metals MetalsMetals 20212021,, 1111,, 1094x FOR PEER REVIEW 22 ofof 1515

Figure 1.1. Schematic illustration of the two two types types ofof thermomechanicalthermomechanical treatmentstreatments leadingleading toto thethe formationformation of of microduplex microduplex structure structure in in duplex duplex stainless stainless steel. steel. (a) IndustrialIndustrial processing and ( (bb)) thermomechanicalthermomechanical treatmenttreatment describeddescribed byby MakiMaki etet al.al. [[6]6]..

The mechanical propertiesproperties ofof duplexduplex steelsteel 22052205 (EN(EN 1.4462)1.4462) asas aa function ofof annealing temperaturetemperature were investigatedinvestigated by De LacerdaLacerda etet al.al. [[8]8].. InIn thisthis study,study, duplexduplex steelsteel 22052205 waswas elaboratedelaborated byby industrial processingprocessing withwith threethree finalfinal annealingannealing temperatures:temperatures: 10601060 ◦°C,C, 12001200 ◦°CC andand 13001300 ◦°C.C. ItIt waswas shownshown thatthat thethe tensiletensile propertiesproperties decreasedecrease asas thethe annealingannealing temperaturetemperature increases due toto thethe variationvariation ofof differentdifferent factorsfactors includingincluding phasephase volumevolume fractions,fractions, graingrain sizesize andand nitridenitride precipitation.precipitation. This study did notnot considerconsider thethe effectseffects ofof either morphologymorphology oror texturetexture evolution.evolution. Two recent recent studies studies have have been been carried carried out out in order in order to investigate to investigate the mechanical the mechanical propertiesproperties of superduplex of superduplex stainless stainless steel steel as a as function a function of the of evolutionthe evolution of its of microstructure.its microstructure. In theIn firstthe work first work[9], the [9 authors], the performed authors performed several annealing several treatments annealing at treatments low temperatures at low ◦ ◦ betweentemperatures 400 betweenC and 600 400 C°C after and the600 hot°C after rolling the step. hot rolling It was step. shown It was that shown the two that most the influentialtwo most influential occurring phenomena occurring phenomena are represented are represented by the stress by relief the stress and the relie secondaryf and the phasesecondary precipitation. phase precipitation. In the second In the one second [10], theoneeffect [10], the of controlledeffect of controlled cold rolling cold on rolling the mi- on crostructurethe microstructure and mechanical and mechanical properties properties of the rare of earththe rare Ce-modified earth Ce-modified superduplex superduplex stainless steelstainless was steel investigated. was investigated. It is shown It that is shown with the that increase with the in deformation,increase in deformation, the strength the of Ce-modifiedstrength of Ce superduplex-modified superduplex stainless steel stainless increases steel rapidly increases and the rapidly elongation and the decreases elongation dramatically.decreases It dramatically. should be noted It should that in be both noted works, that the in grain both morphology works, the grain is mainly morphology elongated, is asmainly well aselongated additional, as phaseswell as besidesadditional ferrite phases and besides austenite ferrite being and present. austenite being present. The texture texturess o off the the different different phases phases obtained obtained after after the the thermomechanical thermomechanical processing process- ingcan be can mainly be mainly described described by a rolling by a rollingtexture texturethat is characteristic that is characteristic of cubic structures of cubic [6,11 struc-– α tures13]. On[6 ,one11– 13hand]. On, ferrite one texture hand, ferriteis described texture by is α- describedfiber orientations by -fiber and orientations especially by and the especially by the {100}<011> orientation. On the other hand, austenite texture is described {100}<011> orientation. On the other hand, austenite texture is described by the Goss by the Goss ({011}<100>) and Brass ({011}<112>) orientations. ({011}<100>) and Brass ({011}<112>) orientations. Hutchinson et al. [14], and Komenda and Sandström [15] reported anisotropic tensile Hutchinson et al. [14], and Komenda and Sandström [15] reported anisotropic tensile properties in duplex structure 2205 produced by rolling. They compared the mechanical properties in duplex structure 2205 produced by rolling. They compared the mechanical responses in different directions in the plane according to their morphologies. It was demon- responses in different directions in the plane according to their morphologies. It was strated that the observed anisotropy is controlled mainly by the crystallographic texture, demonstrated that the observed anisotropy is controlled mainly by the crystallographic while the morphology showed no measurable impact on the overall mechanical response. texture, while the morphology showed no measurable impact on the overall mechanical However, there are few reports in the literature regarding the relationship between response. microstructural evolution and mechanical properties in superduplex stainless steel 2507 However, there are few reports in the literature regarding the relationship between (EN 1.4410). Those properties are a critical advantage of this alloy in its various applica- microstructural evolution and mechanical properties in superduplex stainless steel 2507 tions over competing materials, and it is thus highly desirable to understand how these properties(EN 1.4410) can. Those be tailored, properties notably are through a critical microstructure advantage control. of this With alloy this in in its mind, various it is applications over competing materials, and it is thus highly desirable to understand how Metals 2021, 11, 1094 3 of 15

important first to understand the effect of the various microstructural parameters of the alloy on its mechanical properties. The objective of the present work is to investigate the impact of microstructural evolution on mechanical tensile properties in flat superduplex stainless steel. To this end, samples of the same superduplex stainless grade were treated according to a set of thermomechanical schedules specifically designed to produce clearly distinct microstructures. The crystallography, phase morphology and mechanical properties were thoroughly characterized. The results were analyzed to identify the relationship between the microstructural features and the mechanical properties.

2. Experimental Procedures 2.1. Material The material used in this study is UNS S32750 superduplex stainless steel, usually known as 2507. It is provided by Aperam as hot-rolled, annealed and pickled (HRAP) plates of 5.5 mm thickness, and cold-rolled (CR) or cold-rolled and annealed (CRA) sheets of 1.5 mm thickness. The chemical composition of the material is given in Table1.

Table 1. Chemical composition of UNS S32750 superduplex stainless steel (wt %) measured by combustion (infrared absorption for C), combustion (conductimetry for N), and by X-ray ﬂuorescence for the rest.

Fe Cr Ni Mo Mn C N Other 62 26 7 3.8 0.8 0.017 0.27 <0.051

2.2. Thermomechanical Processing The base material presented in Section 2.1 is subjected to different thermomechanical schedules in order to produce the five microstructures on which the study is based: the industrial microstructure (IM), the modified annealed microstructures (MAM1 & MAM2), the ultrafine microstructure (UM) and the equiaxed microstructure (EM).

2.2.1. Industrial and Modified Annealed Microstructures The microstructure obtained by industrial processing is called the industrial microstructure, which corresponds to the microstructure of the final industrial product. As described before, the industrial processing is mainly composed of five steps. The first step is continuous casting, during which superduplex, as with all types of DSS, solidi- fies in ferrite, followed, in the solid state, by the precipitation of austenite in the form of Widmanstätten patterns. In the 1370–1050 ◦C range, ferrite (δ) and austenite (γ) are the only thermodynamically stable phases in SDSS. These temperatures are representative of hot-rolling, i.e., conditions under which the lamellar duplex microstructure is obtained through the plastic deformation of Widmanstätten austenite. After first annealing operation aiming at recrystallizing this microstructure, leading to the so-called HRAP state, a cold rolling step with 72% thickness reduction is performed, as shown in Figure1a, providing the CR state. The last step consists of continuous annealing to recrystallize the two phases of SDSS and to obtain equal volume fractions, corresponding to the CRA microstructure. From the CR microstructure, modified annealed microstructures are obtained by applying two final annealing conditions. In this study, heat treatments at 1100 ◦C for 0 s (MAM1) and at 1180 ◦C for 300 s (MAM2) are carried out on a Gleeble 3500(VPG Brand), as shown in Figure2. The annealing treatments are carried out in air and with a heating rate of 100 ◦C/s. To ensure temperature homogeneity, three equidistant thermocouples are used during the annealing treatments, where one is placed on the center of the sample and the two others on both sides. At the end of the treatment, the samples are water cooled to freeze the microstructure. Metals 2021, 11, x FOR PEER REVIEW 4 of 15

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Figure 2. Final annealing treatments carried out on a Gleeble 3500 to obtain modified annealed microstructures at (a) 1100 °C for 0 s (MAM1) and at (b) 1180 °C for 300 s (MAM2).

◦ Figure 2.2. Final annealing treatmentstreatments2.2.2. Ultrafine carried carried out andout on on Equiaxed a a Gleeble Gleeble 3500M 3500icrostructures to to obtain obtain modified modified annealed annealed microstructures microstructures at ( aat) 1100(a) 1100C for°C for 0 s 0 (MAM1) s (MAM1) and and at (bat) ( 1180b) 1180◦CThe for°C for u300ltrafine 300 s (MAM2). s (MAM2) microstructure. (UM) is elaborated according to the thermomechanical treatment described by Maki et al. [6]. The main task is to determine the conditions appropriate2.2.2. Ultrafine Ultrafine for aand solution Equiaxed treatment M Microstructuresicrostructures and cold rolling in the case of SDSS. As shown in Figure 3a, followingThe ultrafineultrafine the HRAP microstructure step, the sample (UM) i iss first elaboratedelaborated solution according according-treated in to tothe the the temperature thermomechanical thermomech rangeanical of thetreatment ferrite singled describedescribed-phase by by region Maki Maki, i.e.et , al.alabove. [[6]6].. 1370 The The °C, main and task water isis to toquenched determine determine in order the the conditions conditions to obtain supersaturatedappropriate for for ferritea asolution solution at room treatment treatment temperature. and and cold The cold rolling solu rollingtion in the-treated in case the of casespecimen SDSS. of SDSS. As is shownthen As cold shown in -Frolledigure in with3Figurea, following a 90%3a, following thickness the HRAP thereduction, HRAPstep, the step,and sample subsequently the sampleis first solution is annealed first solution-treated-treated at 1100 in °Cthe for temperature in 4 themin temperaturein the range ferrite of +therange austenite ferrite of the singletwo ferrite-phase-phase single-phased regionregion ,to i.e. recover region,, above the i.e., 1370 duplex above °C, and 1370microstructure water◦C, and quenched water with quenched ain phase order ratio to in obtain orderclose tosupersaturatedto 50/50. obtain supersaturated ferrite at room ferrite temperature. at room temperature.The solution-treated The solution-treated specimen is then specimen cold-rolled is withthen Asa cold-rolled 90% shown thickness in withFig reduction,ure a 90% 3b, an thickness andadditional subsequently reduction, step is annealed added and subsequently after at 1100 cold °C rol for annealedling, 4 min which in at the consists 1100 ferrite◦C of+for austenitea4 flash min inannealing two the-phase ferrite treatmentd +region austenite to at recover high two-phased temperature, the duplex region microstructure 1320 to recover °C, for the 10 with duplex s, followed a phase microstructure ratioby water close quenchingtowith 50/50. a phase , resulting ratio close in the to 50/50.formation of the equiaxed microstructure. As shown in Figure 3b, an additional step is added after cold rolling, which consists of a flash annealing treatment at high temperature, 1320 °C, for 10 s, followed by water quenching, resulting in the formation of the equiaxed microstructure.

FigureFigure 3. 3. ThermomechanicalThermomechanical treatments treatments used used to to elaborate elaborate an an ( (aa)) ultrafine ultraﬁne microstructure microstructure and and ( (bb)) equiaxed equiaxed microstructure microstructure ofof superd superduplexuplex stainless steel 2507.

2.3. MicrostructureAs shown in FigureCharacterization3b, an additional step is added after cold rolling, which consists Figure 3. Thermomechanical treatments used to elaborate an (a) ultrafine microstructure and◦ (b) equiaxed microstructure of superduplex stainless steelof 2507. aThe flash microstructures annealing treatment resulting at high from temperature, each of the 1320 aboveC, for-listed 10 s, thermomechanicalfollowed by water processquenching, are characterized resulting in the on formation sheet cross of section the equiaxeds (RD-ND). microstructure. 2.3. MicrostructureSamples for light Characterization microscopy are first ground with silicon carbide papers from P320 to 2.3. Microstructure Characterization P2400.The They microstructures are then polished resulting using diamon from d each pastes of with the particles above-listed of 3 µm thermomechanical then 1 µm. They The microstructures resulting from each of the above-listed thermomechanical process areprocess finally are etched characterized with Beraha on ’ssheet reagent, cross composed sections (RDof 0.27g-ND). of 퐾2푆2푂5, 20 mL of HCl and 100 are characterized on sheet cross sections (RD-ND). mL ofSamples 퐻2푂, to forreveal light the microscopy two phases are of first SDSS. ground This withetching sili leavescon carbide ferrite papers grains from in dark P320 and to Samples for light microscopy are first ground with silicon carbide papers from P320 austeniteP2400. They ones are in thenlight .polished The samples using are diamon observedd pastes using with an Olympus particles DSX500 of 3 µm light then microscope. 1 µm. They to P2400. They are then polished using diamond pastes with particles of 3 µm then 1 µm. are finallyElectron etched backscattered with Beraha diffraction’s reagent, (EBSD composed, EDAX of )0.27g in the of scanning퐾 푆 푂 , 2 0electron mL of HClmicroscope and 100 They are finally etched with Beraha’s reagent, composed of 0.27g2 2 of5 K S O , 20 mL of HCl (SEMmL of, Zeiss퐻 푂, Ultrato reveal 55) ithes used two to phases determine of S DSS.the crystallographic This etching leaves texture ferrite of 2bo grains2th phases.5 in dark In order and and 1002 mL of H O, to reveal the two phases of SDSS. This etching leaves ferrite grains toaustenite improve ones the in specimenlight2 . The samples surface are quality observed for EBSD using an analysis, Olympus samples DSX500 were light ground microscope. and in dark and austenite ones in light. The samples are observed using an Olympus DSX500 Electron backscattered diffraction (EBSD, EDAX) in the scanning electron microscope light microscope. (SEM, Zeiss Ultra 55) is used to determine the crystallographic texture of both phases. In order to improve the specimen surface quality for EBSD analysis, samples were ground and Metals 2021, 11, 1094 5 of 15

Electron backscattered diffraction (EBSD, EDAX) in the scanning electron microscope (SEM, Zeiss Ultra 55) is used to determine the crystallographic texture of both phases. In order to improve the specimen surface quality for EBSD analysis, samples were ground and polished, as described for light microscopy, with an additional ﬁnal step of mechanochemi- cal polishing using colloidal silica in order to remove the residual deformation or stress resulting from mechanical polishing. The acquired EBSD maps are 2 mm long and cover the whole thickness of the sheet samples. They were recorded in a Zeiss Ultra 55 SEM using an EDAX EBSD detector and the following settings: voltage: 20 kV; step size: 150 nm; acquisition speed: 250 fps.

2.4. Morphological Analyses The aim of morphological analyses is to describe the shape evolution of both phases across the various SDSS microstructures and to determine their volume fraction. In order to carry out this task, the morphological measurements are performed using the public domain programm ImageJ (NIH) and Aphelion software packages(ADCIS, 4.4.0). To do so, light micrographs are binarized using the “Minimum” thresholding method before the surface fractions of phases are measured with ImageJ. Two parameters are used to characterize the austenite-ferrite microstructure: the mean chord size and weighted average Feret’s diameter. The Feret’s diameter is weighted using the particle surface areas. The ﬁrst parameter is measured with the Aphelion software package and the second one with the ImageJ software package. The size of the measuring frame is 4 mm long over the entire thickness. The chord size, which is the length of an object intersection with the test line, is used to measure the characteristic length for both phases of SDSS [15,16]. The measurement of the chord size in the normal direction (ND) is performed by counting the intersections between the particles and the test grid, which consists of an equal number of lines and pixels, determined by the length of the light micrograph. This measurement makes it possible to calculate the layer thickness of both phases of SDSS, as described by the ASTM E1268 standard [17]. Feret’s diameter along the rolling and normal directions are used to characterize the austenite particles’ length and shape (aspect ratio). To this end, after binarizing the light micrograph, the austenite particles are identiﬁed using the “Analyze particle” macro before Feret’s diameter is measured by ImageJ.

2.5. Tensile Tests The mechanical characterization is carried out by tensile tests with image correlation. The experimental device consists of a servo-hydraulic Zwick (ZwickRoell) machine with a CCD camera (6 MPixel) pointing at the area to be observed. The tensile tests are performed in the rolling direction at room temperature, and with displacement control, at a rate of 2 mm/min. Sample elongation is measured with a virtual extensometer using Digital Image Correlation. GOM Correlate 2.0.1 software (GOM GmbH, ZEISS company) is used to process the images taken during the test. The uniaxial tensile tests use rectangular cross-section samples machined by electrical discharge machining to minimize plastic deformation. Their dimensions comply with ASTM 8 [18] (length = 12.5 mm, width = 3.1 mm, l/w = 4) with thickness that varies between 0.5 and 1.5 mm. For each microstructure, ﬁve to seven tensile tests are performed in order to ensure the reproducibility of their strength properties.

3. Results 3.1. Microstructure Evolution The industrial microstructure and modiﬁed annealed microstructures are strongly oriented and aligned with the rolling direction, as shown in Figure4. This morphology results from the rolling processes. A characteristic pancake microstructure is observed, consisting of ﬂattened, widened and elongated ferrite and austenite bands, whose mean thickness (dimension in ND) is between 2 and 4 µm. Figure5 presents the grain maps for the industrial microstructure. Metals 2021, 11, x FOR PEER REVIEW 6 of 15

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It should be noted that the ferrite and austenite layers in microstructures of this type are Metals 2021, 11, x FOR PEER REVIEW 6 of 15 essentially one grain thick, which makes it possible to consider the layer spacing in ND as a good approximation of the grain size.

Figure 4. Industrial (a) and modified annealed (b) 1100 °C —0 s and (c) 1180 °C —300 s microstructures where ferrite grains appear in dark and austenite ones in light.

A characteristic pancake microstructure is observed, consisting of flattened, widened and elongated ferrite and austenite bands, whose mean thickness (dimension in ND) is between 2 and 4 µm. Figure 5 presents the grain maps for the industrial microstructure. It

should be noted that the ferrite and austenite layers in microstructures of this type are ◦ ◦ Figure 4. IndustrialIndustrial ( (a)) and and modified modiﬁedessentially annealed one grain ( b) 1100 thick, °CC—0 —which0 s s and and makes ( (cc)) 1180 1180 it possible°CC—300—300 s sto microstructures microstructures consider the layerwhere where spacing ferrite grains in ND as appear in dark and austenite ones in light. appear in dark and austenitea onesgood in approximation light. of the grain size. A characteristic pancake microstructure is observed, consisting of flattened, widened and elongated ferrite and austenite bands, whose mean thickness (dimension in ND) is between 2 and 4 µm. Figure 5 presents the grain maps for the industrial microstructure. It should be noted that the ferrite and austenite layers in microstructures of this type are essentially one grain thick, which makes it possible to consider the layer spacing in ND as a good approximation of the grain size.

Figure 5. ((aa)) F Ferriteerrite and (b)) austenite austenite grain maps of the industrial microstructure withwith a minimum misorientation ofof 1515°.◦.

TheThe m mainain differences between these three microstruct microstructuresures are the lengthslengths of austenite layerslayers,, the the thickness thickness of offerrite ferrite layers layers and andthe phase the phase volume volume fraction fractions.s. As reported As reported in Table in2, anTable increase2, an increase in the ferrite in the volume ferrite volume fraction fraction and ferrite and ferritelayers’ layers’thickness thickness by 13% by and 13% 100% and, 100%, respectively, and a decrease in the austenite layers’ length by 91% are observed with respectively, and a decrease in the austenite layers’ length by 91% are observed with increasingincreasing of of the annealing annealing conditions. conditions. This This evolution is governed governed by by thermodynamic thermodynamic Figure 5. (a) Ferrite and (b) austenite grain maps of the industrial microstructure with a minimum misorientation of 15°. considerationsconsiderations and and the the surface surface energies energies of of the the interfaces interfaces and and grain grain boundaries boundaries [19 [19]. ]. In the case of the ultraﬁne microstructure shown in Figure6, austenite particles in the formThe of relatively main differences short bands between are precipitated these three alongmicrostruct and withinures are ferrite the length grains,s asof shownaustenite in layersFigure, 6the. As thickness in previous of ferrite cases, layers and as and can the be phase noticed volume in Figure fraction6, theys. areAs reported oriented alongin Table the 2, anrolling increase direction in the and ferrite are onevolume grain fraction thick. Morphologicaland ferrite layers measurements’ thickness by in 1 ND3% and show 100% that, respectivelythe mean thickness,, and a decrease taken as the in the mean austenite chord sizelayers along’ length ND, is by about 91% 2areµm. observed Furthermore, with increasingthe mean lengthof the of annealing austenite conditionsparticles measured. This evolution in the rolling is governed direction by is thermodynamic about 40 µm. considerationsFrom Figure and7, the the surface comparison energies between of the ferrite interfaces grain and maps grain with boundaries minimum [19 misorienta-]. tion of 5◦ and 15◦ reveals a structure of subgrains in the ferrite matrix. In addition, it can be seen that the ferrite matrix grains are much larger, about a hundred times, than in the industrial microstructure. Metals 2021, 11, x FOR PEER REVIEW 7 of 15

Table 2. Microstructures morphological characteristics of superduplex stainless steel.

Modified Modified Industrial Annealed Annealed Ultrafine Equiaxed Parameters Microstructur (1100 °C —0 (1180 °C —300 Microstructure Microstructure e s) s) Ferrite volume 50% ± 1% 49% ± 1% 62% ± 1% 50% ± 1% 50% ± 1% Metals 2021, 11, 1094 fraction 7 of 15 Mean Chord size in ND 2.4 ± 0.2 µm 2.0 ± 0.2 µm 4.0 ± 0.2 µm 2.3 ± 0.2 µm 5.1 ± 0.2 µm Table 2. Microstructures(ferrite) morphological characteristics of superduplex stainless steel. Mean chord Industrial Modified Annealed Modified Annealed Ultrafine Equiaxed Parameters Microstructuresize in ND(1100 ◦2.3C—0 ± 0.2 s) µm 2.1(1180 ± 0.2◦ C—300µm 2.6 s) ± 0.2Microstructure µm 2.0 ± 0.2 µmMicrostructure 5.0 ± 0.2 µm (austenite) Ferrite volume 50% ± 1% 49% ± 1% 62% ± 1% 50% ± 1% 50% ± 1% fraction Feret’s diameter in 291 ± 5 µm 1633 ± 58 µm 140 ± 1 µm 39.4 ± 0.4 µm 46.0 ± 0.2 µm Mean Chord size in ND 2.4 ± 0.2 µmRD 2.0 ± 0.2 µm 4.0 ± 0.2 µm 2.3 ± 0.2 µm 5.1 ± 0.2 µm (ferrite) Aspect ratio 18 ± 1 25 ± 1 15 ± 1 3.3 ± 0.7 2.3 ± 0.1 Mean chord size in ND 2.3 ± 0.2 µmIn the case 2.1 ± of0.2 theµm ultrafine microstructure 2.6 ± 0.2 µm shown 2.0in Figure± 0.2 µ m6, austenite 5.0 particles± 0.2 µm in the (austenite) form of relatively short bands are precipitated along and within ferrite grains, as shown in Feret’s Figure 6. As in previous cases, and as can be noticed in Figure 6, they are oriented along the 291 ± 5 µm 1633 ± 58 µm 140 ± 1 µm 39.4 ± 0.4 µm 46.0 ± 0.2 µm diameter in RD rolling direction and are one grain thick. Morphological measurements in ND show that the Aspect ratio 18 ± 1mean thickness 25, taken± 1 as the mean chord 15 ± 1size along ND 3.3, is± about0.7 2 µm. Furthermore 2.3 ± 0.1 , the mean length of austenite particles measured in the rolling direction is about 40 µm.

Metals 2021, 11, x FOR PEER REVIEWFigureFigure 6.6. Ultrafine microstructure of superduplex stainless steel. Ferrite grains appear in dark8 of and 15 Ultraﬁne microstructure of superduplex stainless steel. Ferrite grains appear in dark and austeniteaustenite onesones appearappear inin light.light.

From Figure 7, the comparison between ferrite grain maps with minimum misorientation of 5° and 15° reveals a structure of subgrains in the ferrite matrix. In addition, it can be seen that the ferrite matrix grains are much larger, about a hundred times, than in the industrial microstructure.

Figure 7. Ferrite grain maps with minimum misorientation of (a) 55°◦ and ( b) 15° 15◦ of the ultrafine ultraﬁne microstructure.

InIn order order to to obtain obtain an an equiaxed equiaxed mi microstructurecrostructure of S SDSS,DSS, it proved necessary to modify modify thethe thermomechthermomechanicalanical treatment treatment described described by Maki et al.al. [[6]6].. For For this this reason, a ﬂashflash annealing treatment at high temperature is added after the cold rolling step. The resulting microstructure is illustrated in Figure 8, where austenite particles in the form of equiaxed grains are precipitated in the ferrite matrix. Morphological measurements show that the mean length of austenite particles is about 45 µm and the mean chord size in ND measured is about 5 µm.

Figure 8. Equiaxed microstructure of superduplex stainless steel. Ferrite grains appear in dark and austenite ones appear in light.

The ferrite matrix grains, as presented in Figure 9b, have a size similar to those of the ultrafine microstructure but with a more equiaxed morphology. Contrary to the ultrafine case, however, there are much fewer differences between ferrite grain maps, presented in Figure 9, with minimum misorientations of 5° and 15°. Metals 2021, 11, x FOR PEER REVIEW 8 of 15

Figure 7. Ferrite grain maps with minimum misorientation of (a) 5° and (b) 15° of the ultrafine microstructure. Metals 2021, 11, 1094 8 of 15 In order to obtain an equiaxed microstructure of SDSS, it proved necessary to modify the thermomechanical treatment described by Maki et al. [6]. For this reason, a flash annealingannealing treatmenttreatmentat at highhigh temperaturetemperature is is added added after after the thecold cold rolling rolling step. step. TheThe resultingresulting microstructuremicrostructure isis illustratedillustrated inin FigureFigure8 ,8 where, where austenite austenite particles particles in in the the form form of of equiaxed equiaxed grainsgrains areare precipitatedprecipitated inin thetheferrite ferritematrix. matrix. MorphologicalMorphological measurementsmeasurements showshow thatthat thethe meanmean length length of of austenite austenite particles particles is about is about 45 µm 45 and µm the and mean the chord mean size chord in ND size measured in ND ismeasured about 5 µ ism. about 5 µm.

FigureFigure 8.8. EquiaxedEquiaxed microstructuremicrostructure ofof superduplexsuperduplex stainlessstainless steel.steel.Ferrite Ferrite grainsgrains appearappear inin darkdark andand austeniteaustenite onesones appearappear inin light.light.

TheThe ferriteferrite matrixmatrix grains,grains, asas presentedpresented inin FigureFigure9 b,9b, have have a a size size similar similar to to those those of of the the ultraﬁneultrafine microstructuremicrostructure butbut withwith aa moremore equiaxedequiaxed morphology.morphology. ContraryContrary toto thethe ultraﬁneultrafine Metals 2021, 11, x FOR PEER REVIEW 9 of 15 case,case, however,however,there there areare muchmuch fewerfewerdifferences differences between between ferriteferrite grain grain maps,maps, presentedpresented inin FigureFigure9 ,9 with, with minimum minimum misorientations misorientations of of 5 5°◦ and and 1515°◦..

FigureFigure 9.9. FerriteFerrite graingrain mapsmaps withwith minimumminimum misorientationsmisorientations ofof ((aa)) 55°◦ andand ((bb)) 1515°◦ of of thetheequiaxed equiaxed microstructure.microstructure.

3.2.3.2. TextureTexture EvolutionEvolution TheThe textures textures ofof ferritic ferritic and and austenitic austenitic phases phases in in three three microstructures microstructures of SDSS—SDSS— industrial,industrial, ultrafineultrafine andand equiaxedequiaxed microstructures—aremicrostructures—are presented presented inin FiguresFigures 10 10 and and 11 11.. It It shouldshould bebe notednoted thatthat thethe texturetexture ofof modifiedmodified annealedannealed isis notnot shownshown herehere asas itit isis highlyhighly similarsimilar toto thatthat of of the the industrial industrial microstructure. microstructure. Textures Textures appear appear with with a marked a marked intensity intensity in ferrite,in ferrite, mainly mainly described described by byα-fiber α-fiber (<110>//RD) (<110>//RD) spread spread from from {001}<110> {001}<110> toto {112}<110>. {112}<110>. FerriteFerrite textures textures along alongα-fiber α-fiber show show a maximum a maximum on {001}<110> on {001}<110> in the industrial in the microstruc- industrial microstructure and on {011}<011>, {112}<110> in the equiaxed microstructure. An interesting feature is the weakness of {111}<110> in the case of all the components of the ϒ-fiber, which are normally the dominating textures for BCC structures after rolling and annealing [6,12]. In the case of ultrafine microstructure, ferrite presents strong {001}<110> and {111}<011> textures.

Figure 10. (a) Orientation distribution function section with select fibers and orientations in BCC metals. Ferrite textures of superduplex stainless steel are presented in the same section for the (b) industrial, (c) ultrafine and (d) equiaxed microstructures. Metals 2021, 11, x FOR PEER REVIEW 9 of 15

Figure 9. Ferrite grain maps with minimum misorientations of (a) 5° and (b) 15° of the equiaxed microstructure.

3.2. Texture Evolution The textures of ferritic and austenitic phases in three microstructures of SDSS— industrial, ultrafine and equiaxed microstructures—are presented in Figures 10 and 11. It should be noted that the texture of modified annealed is not shown here as it is highly Metals 2021, 11, 1094 similar to that of the industrial microstructure. Textures appear with a marked intensity9 of 15 in ferrite, mainly described by α-fiber (<110>//RD) spread from {001}<110> to {112}<110>. Ferrite textures along α-fiber show a maximum on {001}<110> in the industrial microstructure and on {011}<011>, {112}<110> in the equiaxed microstructure. An ture and on {011}<011>, {112}<110> in the equiaxed microstructure. An interesting feature interesting feature is the weakness of {111}<110> in the case of all the components of the is the weakness of {111}<110> in the case of all the components of the γ-ﬁber, which are ϒ-fiber, which are normally the dominating textures for BCC structures after rolling and normally the dominating textures for BCC structures after rolling and annealing [6,12]. annealing [6,12]. In the case of ultrafine microstructure, ferrite presents strong {001}<110> In the case of ultraﬁne microstructure, ferrite presents strong {001}<110> and {111}<011> and {111}<011> textures. textures.

Metals 2021, 11, x FOR PEER REVIEW 10 of 15

FigureAustenite 10.10. ((aa)) OrientationOrie texntationtures in distributiondistribution all microstructures function section of SDSS with, as selectselect shown ﬁbersfibers in andand Figure orientationsorientations 11, are mainly inin BCCBCC metals.described Ferrite Ferrite by textures Brass textures { 011of of superduplex}< superduplex211> and stainless Gossstainless { 011steel steel}< are100 are presented> presented orientations. in inthe the sameThese same section results section for for arethe the( inb) (agreementindustrial,b) industrial, ( cwith) ultrafine (c) ultraﬁneprevious and and ( dstudies) (equiaxedd) equiaxed carried microstructures. microstructures. out on duplex stainless steels. [7,14]

Figure 11. (a) OrientationOrientation distribution function section with select ﬁbersfibers and orientationsorientations inin FCCFCC metals. Austenite textures of superduplex stainless steel are presented in the same section for the (b) industrial, ((c)) ultraﬁneultrafine andand ((d)) equiaxedequiaxed microstructures.microstructures.

3.3. Mechanical Properties Mechanical properties for all microstructures of SDSS are characterized by uniaxial tensile test with digital image correlation. Figure 12 shows the true stress–strain curves obtained for the different microstructures of the study. The main mechanical properties obtained from the engineering stress–strain curves are presented in Table 3.

Table 3. Tensile mechanical properties for all microstructures of superduplex stainless steel 2507. 휎푌푆: Offset yield stress; 휎푈푇푆: Ultimate tensile stress; ε: Uniform elongation.

Annealed State Annealed State Industrial Ultrafine Equiaxed Parameters (1100 °C —0 s) (1180 °C —300 s) Microstructure Microstructure Microstructure

휎푌푆 (MPa) 710 ± 13 643 ± 3 653 ± 7 772 ± 13 695 ± 8 휎푈푇푆 (MPa) 920 ± 10 857 ± 7 952 ± 12 1000 ± 5 918 ± 15 ε (%) 20 ± 0.9 22.5 ± 0.9 20.2 ± 0.9 16.5 ± 1.6 17.7 ± 0.2

Mechanical properties of the industrial microstructure are characterized by a yield stress (ε = 0.2%) and an ultimate tensile stress of the order of 650 and 950 MPa, with a uniform elongation of the order of 22.5%. In comparison with other microstructures, the mechanical behavior of industrial microstructure is bounded at the top by that of ultrafine microstructure and at the bottom by that of equiaxed microstructure and modified annealed microstructure. The comparison between industrial, ultrafine and equiaxed microstructures in terms of mechanical properties shows a maximal difference of about 120 MPa regarding yield stress, 80 MPa regarding ultimate tensile strength and 3.7% regarding uniform elongation. In the case of modified annealed microstructure, the strength properties decrease as the annealing conditions, time and temperature increase. Metals 2021, 11, 1094 10 of 15

Austenite textures in all microstructures of SDSS, as shown in Figure 11, are mainly described by Brass {011}<211> and Goss {011}<100> orientations. These results are in agreement with previous studies carried out on duplex stainless steels [7,14].

3.3. Mechanical Properties Mechanical properties for all microstructures of SDSS are characterized by uniaxial tensile test with digital image correlation. Figure 12 shows the true stress–strain curves Metals 2021, 11, x FOR PEER REVIEW 11 of 15 obtained for the different microstructures of the study. The main mechanical properties obtained from the engineering stress–strain curves are presented in Table3.

FigureFigure 12.12. TensileTensile truetrue stress–strainstress–straincurves curves for for all all microstructures microstructures of of superduplex superduplex stainless stainless steel. steel.

Table 3. Tensile mechanical properties4. Discussion for all microstructures of superduplex stainless steel 2507. σYS: Offset yield stress; σUTS: Ultimate tensile stress; ε: Uniform elongation.The different microstructures elaborated in this study by thermomechanical treatments show that in the case of SDSS, the implementation of the thermomechanical Annealed State Annealed State Industrial Ultrafine Equiaxed Parameters processing described by Maki et al. [6] does not allow the obtaining of an equiaxed (1100 ◦C—0 s) (1180 ◦C—300 s) Microstructure Microstructure Microstructure microstructure. In fact, the resulting microstructure is characterized by austenite particles ± ± ± ± ± σYS (MPa) 710 that13 are oriented 643 along3 the rolling direction. 653 7 This remarkable 772 13 change can be 695 attributed8 to σUTS (MPa) 920 ±the10 presence of857 austenite± 7 after solution 952 ± 12-treatment, as 1000 sho±wn5 in Figure 91813, ±where15 the ε (%) 20 ±austenite0.9 volume 22.5 fraction± 0.9 measured 20.2 at± the0.9 skin and at 16.5 the± 1.6core are 2% and 17.7 ±30%0.2 ± 1%, respectively. This amount of austenite is related to the quenching rate, which is controlled by theMechanical heat diffusion properties through of the the thickness. industrial On microstructure the other hand, are it characterized is worth noting by that a yield this stressamount (ε =is 0.2%) the smallest and an amountultimate possible tensile stress given of the the capabilities order of 650 of and the 950 heat MPa, treatment with afacilities uniform used elongation for the solution of the order treatments of 22.5%. of the In present comparison study. with other microstructures, the mechanical behavior of industrial microstructure is bounded at the top by that of ultrafine microstructure and at the bottom by that of equiaxed microstructure and modified annealed microstructure. The comparison between industrial, ultrafine and equiaxed microstructures in terms of mechanical properties shows a maximal difference of about 120 MPa regarding yield stress, 80 MPa regarding ultimate tensile strength and 3.7% regarding uniform elongation. In the case of modified annealed microstructure, the strength properties decrease as the annealing conditions, time and temperature increase.

4. Discussion

The different microstructures elaborated in this study by thermomechanical treatments Figure 13. Microstructures of (a) the skin and (b) the core of a solution-treated sample. Ferrite grains appear in dark and show that in the case of SDSS, the implementation of the thermomechanical processing austenite ones appear in light.

The results of EBSD analyses show that in the case of microstructures obtained through the industrial process shown in Figure 5, ferrite grain size is similar to that of austenite. In the case of ultrafine and equiaxed microstructures, ferrite grain size is much larger than that of austenite, as shown in Figures 7b and 9b. This difference is introduced by the solution and flash-annealing steps, during which the absence of austenite enables rapid growth of ferrite grains. The large difference in ferrite grain sizes observed between these two groups of microstructures highlights the effectiveness of the duplex structure at inhibiting grain growth. During final annealing, the elastic energy stored in the material due to the plastic deformation from cold rolling leads to the recrystallization of both phases of the industrial Metals 2021, 11, x FOR PEER REVIEW 11 of 15

Figure 12. Tensile true stress–strain curves for all microstructures of superduplex stainless steel.

Metals 2021, 11, 1094 11 of 15 4. Discussion The different microstructures elaborated in this study by thermomechanical treatments show that in the case of SDSS, the implementation of the thermomechanical processingdescribed by described Maki etal. by [6 Maki] does et not al allow. [6] does the obtaining not allow of anthe equiaxed obtaining microstructure. of an equiaxed In microstructure.fact, the resulting In microstructurefact, the resulting is characterizedmicrostructure by is austenitecharacteri particleszed by austenite that are orientedparticles thatalong are the oriented rolling along direction. the rolling This remarkable direction. This change remarkable can be attributed change can to be the attributed presence ofto theaustenite presence after of solution-treatment, austenite after solution as shown-treatment in Figure, as sho13,wn where in Figure the austenite 13, where volume the austenitefraction measured volume fraction at the skin measured and at at the the core skin are and 2% at and the 30% core± are1%, 2% respectively. and 30% ± This1%, respectively.amount of austenite This amount is related of austenite to the is quenching related to the rate, quenching which is rate controlled, which is by controlled the heat bydiffusion the heat through diffusion the thro thickness.ugh theOn thickness. the other On hand, the other it is worthhand, notingit is worth that noting this amount that this is amountthe smallest is the amount smallest possible amount given possible the capabilities given the of the capabilities heat treatment of the facilities heat treatment used for facilitiesthe solution used treatments for the solution of the presenttreatments study. of the present study.

Figure 13. Microstructures of (a) thethe skinskin andand ((b)) thethe corecore ofof aa solution-treatedsolution-treated sample.sample. Ferrite grains appear in dark and austenite ones appear in light.

The results of EBSD analyses show that in the casecase ofof microstructuresmicrostructures obtainedobtained through the industrial industrial process process shown shown in in Figure Figure 55, ,ferrite ferrite grain grain size size is issimilar similar to tothat that of ofaustenite. austenite. In the In casethe case of ultrafine of ultrafine and and equiaxed equiaxed microstructures microstructures,, ferrite ferrite grain grain size size is much is much larger larger than than that that of austenite,of austenite, as shown as shown in Fig in Figuresures 7b 7andb and 9b9. b.This This difference difference is introduced is introduced by bythe the solution solution and flashand- flash-annealingannealing steps, steps,during during which whichthe absence the absence of austenite of austenite enables rapid enables growth rapid of growth ferrite grains.of ferrite The grains. large Thedifference large difference in ferrite grain in ferrite sizes grain observed sizes between observed these between two these groups two of microstructuresgroups of microstructures highlights the highlights effectiveness the effectiveness of the duplex of structure the duplex at inhibiting structure grain at inhibiting growth. grainDuring growth. final annealing, the elastic energy stored in the material due to the plastic deformationDuring from final annealing,cold rolling theleads elastic to the energy recrystallization stored in theof both material phases due of tothe the industrial plastic deformation from cold rolling leads to the recrystallization of both phases of the industrial microstructure. This is illustrated in the case of ferrite by the grain maps where changing the minimum misorientation between grains from 5◦ to 15◦ changes little. In the case of ultrafine microstructure, the EBSD observations presented in Figure7 reveal a structure of subgrains in ferrite formed by recovery. As for austenite, which has a low SFE [12,13], grains have been recrystallized. It is interesting to note that, despite the cold-rolling reduction in the ultrafine microstructure (90%) being larger than that of the industrial microstructure (72%), the ferrite of the former was recovered, while that of the latter was recrystallized during the final annealing treatment, which was carried out under the same conditions in both cases. This observation is justified by a pinning effect due to austenite particles [6]. In fact, during the final annealing treatment in the case of the ultrafine microstructure, it is characteristic that the recovery of deformed ferrite matrix occurs rapidly and the subgrain structure of ferrite matrix is formed readily prior to the precipitation of austenite phase in the earliest stage of aging. By further holding at 1100 ◦C, the volume fraction of austenite is increased to about 50%. The precipitation of austenite particles, which preferentially occurred in the subgrain structure, prevents the migration of ferrite subgrain boundaries and, consequently, the recrystallization of ferrite. Unlike ultrafine microstructure, the two phases of equiaxed microstructure are recrystallized. In the case of ferrite, this is illustrated by the grain maps shown in Figure9, where changing the minimum misorientation between grains from 5◦ to 15◦ changes little. This outcome is due to the flash annealing treatment at 1320 ◦C where the recrystallization of ferrite occurs in the absence of austenite precipitation. Another consequence of the Metals 2021, 11, 1094 12 of 15

flash annealing treatment is that the precipitation of austenite during the final annealing treatment occurs in a recrystallized ferritic microstructure, which leads to the obtaining of an equiaxed morphology. It is already known in the literature [11,12] that, on the one hand, in the case of ferritic materials, the recovery texture is described by deformation orientations which are α-fiber orientations, especially {100}<011> and {112}<011>, while recrystallization texture is described by γ-fiber orientations, especially {111}<112>. On the other hand, austenitic materials are likely to recrystallize and, as a result, they develop a particular Brass Recrystallization orientation {236}<385>. In this study, and from the orientation distribution functions shown in Figures 10 and 11, it can be seen that regardless of the outcome of the competition between recovery and recrystallization, the textures are mainly described by deformation orientations in all conditions. In fact, the ferrite texture in all microstructures of SDSS is mainly described by α-fiber orientations, and one can notice the absence of the Brass Recrystallization orientation in recrystallized austenite. All in all, this finding means that the duplex structure influences the development of the phase textures. As suggested in [12], texture development in austenite during annealing may be attributed to oriented nucleation or strain induced boundary migration, which leads to recrystallization textures similar to the deformation textures. From tensile tests shown in Figure 12, differences in mechanical properties between the microstructures of SDSS can be observed. In the following, an attempt is made to relate these differences to the microstructural parameters that were observed to vary across the microstructures, including austenite particle morphology, phase volume fraction, grain size and texture. The effect of morphology can be identified by comparing ultrafine and industrial microstructures. As reported in Table3, the strength properties of the former are higher than those of the latter. This outcome, associated with the fact that the aspect ratio value reported in Table2 is much lower in the case of ultrafine than industrial microstructure, suggests that the phase morphology of SDSS has no effect on its overall mechanical response. Indeed, according to the Shear-lag theory [20], the flow stress of the composite (σf ) is given in the form of: L σ = σ + σ V (1) f α 2d α L where σα is the flow stress of the matrix, and V and d are the volume fraction and the aspect ratio of the second phase. Based on Equation (1), a higher value of flow stress is expected in the industrial microstructure rather than in the ultrafine microstructure if phase morphology has an effect on mechanical properties in SDSS. This result is in agreement with previous works [14,16] carried out on rolled duplex stainless steels, which show that the phase morphology has no effect on their mechanical properties. It is already shown in duplex steels that ferrite is the resistant phase at the beginning of plasticity at room temperature [21,22]. Consequently, a larger ferrite volume fraction would be expected to result in a higher yield stress. Among the microstructures studied here, a maximum difference in ferrite fraction of 13% is found between the samples annealed for 0 s at 1100 ◦C and for 300 s at 1180 ◦C. However, it is found that the former, contain- ing less ferrite, is stronger than the latter. This suggests that limited variations around the 50/50 phase fractions characteristic of DSSs have a negligible effect on mechanical properties compared to grain size and texture. The difference in strength properties between modified annealed microstructures, which is about 70 MPa, can mainly attributed to grain size effect. It can be seen from this comparison that the grain size has an important effect on strength properties of SDSS, even with small variation. This result could be understood by the small grain size due to the duplex structure on one side, and the high Hall–Petch coefficient of ferrite and austenite materials on the other side. In fact, it is reported in the literature that the Hall– Petch (MPa. mm−0.5) coefficient of ferritic steel is of the order of tens [23,24]. In austenitic material, it is less than ten, which is clearly low compared to the Hall–Petch coefficient of Metals 2021, 11, 1094 13 of 15

ferritic steel. However, the study carried out by Norström [25] on the influence of nitrogen and grain size on yield stress in 316L austenitic stainless steel shows that the total effect of nitrogen on yield strength is composed of two different contributions: one independent of grain size and the other markedly dependent upon it. As seen in Table1, SDSS 2507 contains a significant amount of nitrogen that should lead to austenite contents in excess of 0.5wt %. In fact, based on the Norström equation, the expected hardening according to grain size in austenite should be of the order of 550 MPa in terms of nitrogen concentration in SDSS, which is six times higher than without it. This value is an extrapolation, as Norström’s coefficient was determined for much lower nitrogen contents, and should, therefore, be used with caution. However, it does reveal the strong synergy between austenite grain size and nitrogen content. However, comparing the 1180 ◦C—30 s modified annealed microstructure with the slightly coarser equiaxed microstructure shows that the latter is stronger, indicating that, on the one hand, the approximation of grain size in coarser microstructures by average phase spacing, which is measured by chord size, is a good way to characterize the size effect. On the other hand, it is clear that at least one other factor measurably influences the mechanical properties, and potentially more so than the microstructure size. Texture characterization of all samples shows that on the one hand, the texture of austenite, which is described by the Goss and Brass orientations, is the same in all microstructures. On the other hand, the main difference in ferrite texture lies in the intensity of the α-fiber and {111}<011> orientation. As presented in Table2, the ultrafine and the 1100 ◦C—0 s modified annealed microstructures show almost the same characteristic sizes, but the former is noticeably stronger, as shown in Figure 12. It was noted from Figure 10 that the ferrite textures of these two microstructures are markedly different, notably with a strong {111}<011> component in the ultrafine case that is entirely absent from the industrial and modified annealed microstructures. Major ferrite texture differences can also be noted in the above-mentioned case of the 1180 ◦C—30 s modified annealed microstructure and the equiaxed microstructure. Again, in these cases, the {111}<011> component is found in the stronger material, although far less so than in the ultrafine case. The equiaxed microstructure also shows a stronger {112}<011> component than that in the industrial and modified cases. Critical resolved stress shear (CRSS) calculations for ferrite oriented as {100}<011>, {112}<011> and as {111}<011> show that the active slip systems present similar Schmid fac- tors in both cases. This implies that it is not ferrite texture alone but its varying interaction with austenite that leads to pronounced changes in mechanical properties, as suggested by Hutchinson et al. [14]. Finally, it is worth noting that differences in the microstructural features of SDSS, induced by thermomechanical processing, can lead to a strength improvement of the order of 120 MPa over the industrial microstructure. The associated loss in ductility remains limited, with a minimum uniform elongation of 16.5%.

5. Conclusions The effect of microstructural evolution on mechanical properties was investigated in superduplex stainless steel UNS S32750. In order to modify the microstructure of SDSS, three main thermomechanical schedules were carried out to generate different morphologies ranging from banded to equiaxed. In all conditions, austenite was recrystallized and showed a combined Brass/Goss texture. Ferrite was either recrystallized or recovered and displayed a texture with α and γ ﬁbers of varying intensities. The main difference in ferrite texture lies in the intensity of the two orientations, {100}<011> and {111}<011>. The studied SDSS microstructures have clearly different morphologies. In particular, the aspect ratio values for austenite particles vary between 2 and 25. Despite these signiﬁ- cant morphological differences, the average phase spacing of all microstructures measured is less than 5 µm. Metals 2021, 11, 1094 14 of 15

It was observed that the insertion of a high-temperature annealing step in the thermomechanical schedule, where the austenite volume fraction is reduced to a few percent, leads to a large ferrite grain size. The large difference between the materials where the austenite fraction remained high and those where it was almost entirely dissolved highlights the effectiveness of the duplex structure in the inhibition of grain growth. Comparison of the mechanical properties of the different microstructures revealed that the two microstructural parameters that have the strongest effect on tensile strength are ferrite texture and average phase spacing. One of the original microstructures of SDSS obtained here showed enhanced strength compared to the industrial material with limited ductility loss, suggesting that SDSS thermomechanical processing can be tuned to tailor the properties of the resulting material.

Author Contributions: Conceptualization, M.A.L.; methodology, M.A.L.; software, M.A.L.; valida- tion, M.A.L.; formal analysis, M.A.L.; investigation, M.A.L.; resources, F.K., J.D.M.; data curation, M.A.L.; writing-original draft preparation, M.A.L.; writing-review and editing, F.K., J.D.M., H.P.V.L. and M.V.; visualization, M.A.L.; supervision, F.K., J.D.M., H.P.V.L. and M.V.; project administration, F.K., J.D.M., H.P.V.L. and M.V.; funding acquisition, F.K., J.D.M., H.P.V.L. and M.V.; All authors have read and agreed to the published version of the manuscript. Funding: The article is: ANRT-CIFRE 2016/1333. Part of this work was performed with the support of the Center of Excellence of Multifunctional Architectured Materials “CEMAM” No. AN-10- LABX-44-01 funded by the “Investment for the Future” Program operated by the National Research Agency (ANR). Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request. The data presented in this study are available on request from the corresponding author. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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