Spinodal Decomposition in Functionally Graded Super Duplex Stainless Steel and Weld Metal VAHID A. HOSSEINI, MATTIAS THUVANDER, STEN WESSMAN, and LEIF KARLSSON Low-temperature phase separations (T < 500 C), resulting in changes in mechanical and corrosion properties, of super duplex stainless steel (SDSS) base and weld metals were investigated for short heat treatment times (0.5 to 600 minutes). A novel heat treatment technique, where a stationary arc produces a steady state temperature gradient for selected times, was employed to fabricate functionally graded materials. Three diﬀerent initial material conditions including 2507 SDSS, remelted 2507 SDSS, and 2509 SDSS weld metal were investigated. Selective etching of ferrite signiﬁcantly decreased in regions heat treated at 435 C to 480 C already after 3 minutes due to rapid phase separations. Atom probe tomography results revealed spinodal decomposition of ferrite and precipitation of Cu particles. Micro- hardness mapping showed that as-welded microstructure and/or higher Ni content accelerated decomposition. The arc heat treatment technique combined with microhardness mapping and electrolytical etching was found to be a successful approach to evaluate kinetics of low-temperature phase separations in SDSS, particularly at its earlier stages. A time-temper- ature transformation diagram was proposed showing the kinetics of 475 C-embrittlement in 2507 SDSS. https://doi.org/10.1007/s11661-018-4600-9 The Author(s) 2018 I. INTRODUCTION transformations play a crucial role in processing and  applications of these steels. DUPLEX stainless steels deliver an excellent combi- A considerable variation of data reported in the nation of mechanical and corrosion properties, making literature and industrial datasheets points to a knowl- them an attractive alternative to austenitic and super edge gap about the kinetics of 475 C-embrittlement in  austenitic stainless steels. However, phase separations SDSSs. Hilders et al. found that 9 hours heat treatment occur relatively rapidly below 500 C, resulting in what of 2507 SDSS at 475 C did not decrease the impact is referred to as 475 C-embrittlement, which restricts toughness while it was signiﬁcantly reduced after 72  their application at temperatures between 250 Cand hours. Gutie´ rrez-Vargas et al. observed only minor [2,3] 500 C. Spinodal decomposition of d-ferrite to changes in microhardness after 1-h heat treatment of  Fe-rich (a) and Cr-rich (a¢) ferrite and the precipitation 2507 SDSS at 475 C. An industry datasheet for  of a¢ (by nucleation and growth), G-phase, R-phase, 2507 SDSS, in contrast, introduced a time-tempera- v-phase, nitrides, and Cu-clusters have been reported in ture transformation (TTT) diagram indicating that only this temperature range, which contribute to the embrit- 3 minutes aging at 475 C will result in a 50 pct [3,4] tlement. Super duplex stainless steels (SDSSs) have toughness drop in 2507 SDSS. high contents of Cr, Mo, and Ni and are prone to this Several indirect and direct methods can be used to type of embrittlement which is why kinetics of phase detect 475 C-embrittlement. Indirect methods monitor the changes in properties caused by the phase separa-  tions, which include hardness testing, impact tough-   ness testing, magnetic ferrite measurement,  electrochemical tests, alternating current potential   drop, electrical resistance measurement, scanning VAHID A. HOSSEINI is with the Department of Engineering Kelvin probe force microscopy and magnetic force Science, University West, 461 86 Trollha¨ ttan, Sweden and also with  Innovatum AB., Trollha¨ ttan, 461 29 Trollha¨ ttan, Sweden. Contact microscopy, small-angle neutron scattering  e-mail: firstname.lastname@example.org MATTIAS THUVANDER is with the (SANS), etc. Direct methods, on the other hand, Department of Physics, Chalmers University of Technology, 412 96 detect the phase separations by analyzing the Gothenburg, Sweden. STEN WESSMAN and LEIF KARLSSON are microstructure using scanning and transmission electron with the Department of Engineering Science, University West.   Manuscript submitted November 14, 2017. microscopy and atom probe tomography (ATP). Article published online April 17, 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2803 Especially, APT builds a 3D reconstruction of the on 475 C-embrittlement. The chemical composition atomic conﬁguration and provides high spatial resolu- of RBM was not analyzed but is expected to be very  tion allowing to measure nanoscale compositional similar to the BM except for nitrogen. Hertzman   ﬂuctuations. However, the direct methods are gener- et al. reported that autogenous TIG welding of ally more time-consuming, less easily accessible and 2507 SDSS using 2 pct N may result in a maximum expensive compared to the indirect methods. of 0.03 wt pct drop in the nitrogen content. Therefore, Functionally graded materials (FGMs) have been the RBM nitrogen content should not be less than developed through diﬀerent processes to tailor the 0.24 wt pct.   2 properties for speciﬁc purposes. Hosseini et al. Remelted weld metal (RWM) A8 9 20 mm groove recently developed a technique to produce FGM, aiming was machined in a 2507 SDSS plate and filled with 25 at facilitating and accelerating microstructural charac- passes of 2509 welding wire, from Elga, Sweden, using terization. In this novel technique, a steady-state tem- TIG welding with Ar-30 pct He-2N . This procedure perature gradient is produced in a disc by applying a was used to minimize dilution with BM in the weld stationary arc on the top side while water cooling on the metal. As a final step, the weld was TIG remelted back side. The arc heat treatment time can be controlled using Ar-30 pct He-2 pct N to remove possible and varied from a few seconds to several hours and the secondary phases and homogenize the chemical com- temperature range is from the material liquidus to position and microstructure. This sample was used to ambient temperature. study the influence of chemical composition on the This study aims at investigating the kinetics of 475 C-embrittlement. The chemical composition of 475 C-embrittlement of SDSS, as existing data are RWM was measured by LECO combustion technique conﬂicting. The inﬂuence of heat treatment times, initial and optical emission spectrometry, which is listed in microstructural morphology (hot-rolled and welded), Table I. and chemical composition (welding ﬁller wire and base material) have therefore been evaluated. Functionally graded SDSS samples, produced by arc heat treatment, B. Arc and Furnace Heat Treatment were screened with two indirect methods: electrolytical A novel arc heat treatment technique was used to etching and hardness mapping. The indirect methods create a steady-state temperature gradient in the were complemented with thermodynamic calculations, above-mentioned samples. The process and equipment x-ray diﬀraction analysis (XRD), and APT to charac- are schematically shown in Figure 1. A disc-shaped terize possible phase separations at short heat treatment sample (6 mm thick and B 99 mm) was mounted on a times (below 600 minutes). The observed kinetics of water-cooled chamber and a stationary arc was applied 475 C-embrittlement were summarized in a TTT dia- for a selected time. The steady-state temperature gradi- gram and compared with available data from literature. ent formed during the arc heat treatment is shown in Figure 1. An arc current of 100 A and an arc length of 3 mm were used to heat treat samples. More details about II. EXPERIMENTAL PROCEDURE the arc heat treatment technique are reported by A. Materials Hosseini et al. in Reference 18. Arc heat treatment times of 0.5, 1, 3, 10, 60, and 600 Hot-rolled 2507 SDSS plates and type 2509 SDSS minutes were selected to investigate the kinetics of welding wire were used to produce base material and 475 C-embrittlement in BM. To compare diﬀerent weld metal samples for the experiment. The chemical microstructural morphologies and chemical composi- compositions are listed in Table I. Three diﬀerent tions, 1 and 10 minutes arc heat treatments were applied materials were investigated: to RBM and RWM discs. As shown in Figure 1, each speciﬁc location in the cross section has a steady-state Base metal (BM) as-received hot-rolled 2507 SDSS temperature during the arc heat treatment. 6-mm plate, from Outokumpu Stainless AB, Sweden. In addition to the arc heat-treated samples, a separate Remelted base metal (RBM) weld metal produced by BM sample was heat treated in a furnace with argon Tungsten Inert Gas (TIG) remelting 2507 SDSS plate atmosphere for 600 minutes at 475 C to verify results material. Remelting with Ar-30 pct He-2 pct N from the 600 minutes arc heat treatment sample. allowed to produce a weld microstructure with a Microhardness and APT studies were performed on similar chemical composition as BM and to investi- the furnace heat-treated sample. gate the influence of the microstructural morphology Table I. Chemical Composition (Weight Percent) of 2507 Type Base Metal, 2509 Welding Wire and Remelted Weld Metal (RWM) C Si Mn P S Cr Ni Mo N Cu W Fe Base Metal 0.02 0.4 0.8 0.03 0.001 25.0 6.9 3.8 0.27 0.4 — bal. Welding Wire 0.02 0.4 0.6 0.01 0.001 25.5 9.2 4.0 0.26 0.1 0.04 bal. RWM 0.01 0.4 0.6 0.02 0.001 24.8 8.9 3.8 0.22 0.1 0.03 bal. 2804—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 1—Schematic illustration of arc heat treatment equipment and temperature distribution in the cross section. The circulating cooling water and the stationary arc made it possible to produce a steady-state temperature gradient. C. Characterization needle-shaped specimens suitable for APT by a standard  two-stage electro-polishing method. A local electrode Optical and scanning electron microscopy, micro- atom probe, LEAP 3000X HR, equipped with a hardness mapping, XRD, and APT were employed to reﬂectron for improved mass resolution was employed. study phenomena occurring during heat treatment at Analysis was done at 55 K with a 20 pct voltage pulse temperatures below 500 C. fraction, a pulse frequency of 200 kHz and an evapora- Cross sections of samples were wet grinded and tion rate of 1 pct. The software IVAS 3.4.3 was used to polished with standard procedures, electrolytically analyze the three-dimensional (3D) data. A line proﬁle etched using 10 pct NaOH at 4.0 V for 4 seconds and was extracted from a cylinder with a diameter of 2 nm and studied with an Olympus BX60M optical microscope a step-size of 1 nm. Wavelength and amplitude of and a Toshiba TM3000 scanning electron microscope spinodal decomposition were determined using the radial (SEM). Ferrite numbers of materials before arc heat distribution function (RDF) approach, which represents treatment were measured by a Fisher Feritscope. the average radial concentration proﬁle starting from Microhardness mapping was carried out with a each and every detected atom of the chosen elements. The Struers DuraScan 80 automated hardness tester on (bulk normalized) RDF can be expressed as polished samples following the ASTM E384-10 stan- dard. An indenter load of 200 g with a dwell time of 15 RDF ðÞ r ¼ C ðÞ r =C ¼ðÞ N ðÞ r =NrðÞ =C ; E E 0 A 0 seconds was used. The microhardness of ferrite and where C is the atomic composition of element E at a austenite in the furnace heat-treated sample was mea- distance r;C is the average composition of element E in sured with an indenter load of 10 g. the analyzed volume; N (r) is the total number of atom To investigate the possible formation of new phases, E at distance r, and N(r) is the total number of atoms of XRD analysis was employed on the as-received BM and all elements at distance r. The Cr amplitude was after 600 minutes arc heat treatment at locations heat estimated from the RDF at zero and the theoretical treated between 400 C and 550 C. A Bruker D8 Cr RDF assumed by a one-dimensional sinusoidal con- Discover with a Co-K X-ray source (k = 1.78901 A ) Cr pﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ was utilized with the following parameters: 40 mA, 35 centration proﬁle ðA ¼ C 2ðÞ RDFCr0 1ÞÞ. The kV, 2h =45tp114,a2h-scan step size of 0.02,an chemical composition of a and a¢ was measured within exposure time of 2.6 seconds for each step and a spot volumes deﬁned by iso-concentration surfaces with diameter of 200 lm. thresholds of 70 at. pct Fe and 30 at. pct Cr, respec- The furnace heat-treated sample was analyzed by the tively. The standard deviations were obtained from the APT technique. Rods with the dimension of chemical compositions of nine sub-volumes for each 0.3 9 0.3 9 15 mm were produced by low speed cutting. phase (austenite, ferrite, a, and a¢). More details about [15,20] Then, each rod was electro-polished to produce two the RDF have been reported by Zhou et al. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2805 D. Modeling and Thermodynamic Calculations has an as-rolled microstructure with elongated austenite and ferrite grains. The remelted base metal has a ferritic Temperature distributions and weld pool geometries matrix with grain boundary, Widmansta¨ tten, and intra- were stable and equal for diﬀerent arc heat treatment granular austenite. The remelted weld metal has a lower times, as was veriﬁed by thermocouples attached to the ferrite fraction compared to RBM, but a similar samples showing constant temperatures during the arc microstructural morphology. The ferrite numbers of heat treatments. To model the temperature distribution BM, RBM, and RWM, measured by Feritscope, were in the cross section of the arc heat-treated samples, a 2D 53 ± 4, 63 ± 4, and 50 ± 2 FN, respectively. As may be conductive heat transfer was considered and the model seen in Figures 3(d), (e), and (f), the highest overall was calibrated with data recorded by thermocouples hardness before arc heat treatment was found for RBM attached on the top and back sides of the sample. The and the lowest for RWM. steady-state temperature ﬁeld was modeled using the open source computational software OpenFOAM as reported in the References 18 and 21. Thermal and C. Temperature Distribution and Etching Behavior physical properties of 2507 and 2509 alloys, calculated The temperature distribution and corresponding by JMatPro, are very similar and the same model for the microstructure for a sample arc heat-treated 60 minutes temperature distribution was therefore used for BM, are displayed in Figure 4. It can be seen that the etching RBM, and RWM. response depends on the heat treatment temperature and Equilibrium phase fractions between 400 Cand in particular there is a distinguishable light etching band 500 C and kinetic of a¢ formation were calculated using corresponding to a temperature of about 475 C. JMatPro, Version 6.2.1, for the actual chemical com- Only ferrite and austenite were found by OM, SEM, positions of the BM, RBM (with 0.235 wt pct N), and and XRD in regions heat treated below approximately RWM (Table I). The equilibrium phase fractions and 560 C. The microstructure of as-received and 600 chemical composition of diﬀerent phases for the total minutes arc heat-treated BM (on the 475 C isotherm), ferrite chemical composition measured by APT were etched electrolytically with 10 pct NaOH, is shown in also calculated between 400 C and 600 C. Figure 5. Interestingly, less selective etching of the ferrite was observed after the 600 minutes arc heat treatment, resulting in a lower contrast between ferrite III. RESULTS and austenite. Furthermore, the maximum hardness in the as-received and arc heat-treated BM were 295 HV0.2 A. Thermodynamic Calculations and 340 HV0.2, respectively. These observations indi- Equilibrium phase fraction diagrams for 2507 SDSS cate that phase separations occurred during the arc heat BM and RWM compositions for the temperature range treatment, but on a scale too ﬁne to be identiﬁed with of 400 C to 500 C are presented in Figure 2. The optical and scanning electron microscopes. More details results of the calculations can be summarized as follows: about the characteristics of the phase separations were explored by APT and are presented in Section III–G. a¢ forms below about 480 C and 470 C for BM and RWM compositions, respectively. The fractions of a and a¢ increase with decreasing D. Functionally Graded Base Metal temperature. The fraction of a is lower for the Cross sections of the arc heat-treated BM samples for welding wire, while the fractions of a¢ are very diﬀerent heat treatment times are shown in Figure 6.A similar for the two compositions. bright band appeared after etching in the 3 minutes arc Austenite fractions are higher for the RWM. The heat-treated sample which became more visible and austenite fractions decrease slightly with decreasing wider for longer heat treatment times, which likely is temperature for both compositions. related to phase transformation taking place. As shown in Figure 2, Laves, Pi-nitride, and carbide Microhardness maps of arc heat-treated BM samples fractions are constant in this temperature range for are shown in Figure 7. The ﬁrst indications of a high both compositions, but G phase fraction increases hardness band around the 475 C isotherm were found with decreasing temperature and its fractions are in some regions of the 3 minutes arc heat-treated lower for RWM. sample. These high hardness regions were found at Cu is present in the BM, but not in RWM. mid thickness where the bright band is slightly wider in An equilibrium phase fraction diagram was also Figure 6. A band with the highest hardness was seen calculated for RBM, with a nitrogen content of 0.235 after 600 minutes. wt pct. It was found that diﬀerent RBM and BM nitrogen contents did not have any signiﬁcant inﬂuence E. Functionally Graded Remelted Base and Weld Metal on the phase balance in the temperature range Cross sections and hardness maps of RBM and RWM considered. arc heat treated for 1 minute or 10 minutes are shown in Figure 8. No bright bands were found in the etched B. Microstructure and Hardness Before Heat Treatment cross sections of RBM and RWM after the 1 minute The three diﬀerent initial microstructures and corre- heat treatment but they appeared after 10 minutes (see sponding hardness maps are shown in Figure 3. The BM red arrows). The microhardness maps show high 2806—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 2—Equilibrium phase fraction diagrams for compositions corresponding to 2507 SDSS BM and RWM between 400 C and 500 C. (a) Major phases in 2507 SDSS BM, showing a¢ forming at temperatures below 480 C. (b) Minor phases 2507 SDSS BM. (c) Major phases for RWM, showing a¢ forming at temperatures below 470 C. (d) Minor phases for RWM. hardness in the corresponding position after 10 minutes the phase separations of 20 C and a decrease of the arc heat treatment but not after 1 minute. minimum temperature of 30 C. Maximum hardness increased about 45 HV after 600 minutes arc heat treatment in the BM. F. Phase Separation Temperatures and Hardness Ranges The temperature ranges were quite similar in BM, In the present section, the observations made in the RBM, and RWM after 10 minutes arc heat treatment. microhardness maps and cross sections are quantiﬁed in However, the hardness was higher for RBM than for the terms of heat treatment temperatures and hardness BM and RWM after 10 minutes heat treatment. ranges. The temperature range was determined by As can be seen in the hardness maps and cross correlating locations of light etching bands to temper- sections, diﬀerent etching responses and higher hardness ature distribution maps. The hardness and temperature values are present in the arc heat-treated samples at ranges are listed in Table II. temperatures above 550 C, which is due to the precip- The temperature range, estimated from the width of itation of secondary phases such as r and v. However, the light etching bands, increased from 45 C after 3 the investigation of the above-mentioned observations is minutes to 95 C after 600 minutes in the BM. This was not the focus of this study and will be the subject of a result of an increase of the maximum temperature for other publications. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2807 Fig. 3—Optical micrographs and hardness maps of steel and weld metals before heat treatment. (a, d) Base metal (BM), (b, e) remelted base metal (RBM), (c, f) remelted weld metal (RWM). Overall hardness is highest in RBM and lowest in RWM. Fig. 4—Temperature distribution (left) and cross section of sample arc heat treated (right) for 60 min. Note the light etching band (spinodal band) at temperatures about 475 C. G. Phase Separation Studies concentration of Cr gradually increasing towards the center of each region as is characteristic of spinodal As explained in the experimental section, a separate decomposition. sample was heat treated in a furnace at 475 C for 600 Line proﬁles from a cylinder with a 2-nm diameter is minutes to verify the arc heat treatment results. The shown in Figure 10(a), illustrating phase separation into hardness of the ferrite before and after the heat Fe-rich and Cr-rich ferrite. Normalized Cr-Cr and treatment were 247 to 252 HV0.01 and 360 to 390 Cr-Fe RDF curves are shown in Figure 10(b). As can HV0.01, respectively, while the hardness of the austenite be seen, Cr-Cr shows a strong positive interaction, remained fairly constant at 250 to 280 HV0.01. In order whereas Cr-Fe has a negative interaction. The spinodal to characterize the phase separations occurring in the wavelength and amplitude of Cr-rich regions, calculated ferrite, APT was performed on two samples prepared to by the RDF method, are 10 ± 1 nm and 14 ± 1 at. pct, contain only ferrite and only austenite, respectively. respectively. Iso-concentration surfaces for high Cr (more than 30 The compositions of austenite, total ferrite, a¢, and a at. pct) and high Fe (more than 70 at. pct) regions in are listed in Table III. The a¢ had a Cr concentration of ferrite are illustrated by blue and red, respectively, in 41.3 at. pct, which dropped to 13.7 at. pct in a.In Figure 9(a). The APT results clearly show the separa- addition, a¢ and a have diﬀerent distributions of other tion of Cr and Fe with, as shown in Figure 9(b), the 2808—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 5—(a) Optical and (b) SEM micrographs of the specimen before arc heat treatment. (c) Optical and (d) SEM micrographs from the 475 C isotherm region of the specimen after 600 min arc heat treatment. Specimens were etched electrolytically with 10 pct NaOH. The selective etching of ferrite signiﬁcantly decreased after 600 min arc heat treatment. alloying elements, where a¢ is enriched in Mo and Si and of Fe and Cr concentrations. Therefore, the amplitude depleted in Ni, as also observed in a weld metal by Zhou and wavelength may be a better indicator of the degree  et al. of spinodal decomposition. In addition, the equilibrium Almost pure Cu precipitates containing some Ni were phase fractions will not be achieved after 600 minutes. also found in the ferrite (Figure 9(c)). The Cu-particle Consequently, the absence of other phases, predicted by 22 3 number density was about 1.6 9 10 m and they had thermodynamic calculations, was not unexpected. an average diameter of 4.2 ± 0.4 nm. No traces of the A minor degree of Cr and Fe separation has been [22,23] other phases suggested by thermodynamic calculations reported even in as quenched 2507 SDSS. How- were observed. ever, the Cr amplitude of 14 at. pct and wavelength of 10.5 nm for the spinodal decomposition measured in this study indicate that the spinodal decomposition was notably developed after 600 minutes heat treatment at IV. DISCUSSION 475 C compared to the initial state of the In the present section, the characteristics of phase microstructure. separations, the methods of monitoring, and kinetics are The increase in the hardness of ferrite conﬁrms that discussed. Finally, a TTT diagram for 2507 type SDSS phase separations causing 475 C-embrittlement at temperatures below 500 C is proposed. occurred during the 600-minute heat treatment of the BM. The absence of other known brittle phases such G, v, R, carbides, and nitrides, observed in other stud- A. Characteristics of Phase Separations [3,4,24,25] ies, suggests that spinodal decomposition is the The thermodynamic calculations predicted the stabil- main cause of earlier stage 475 C-embrittlement. This is  ity of a + a¢, Cu precipitates and austenite at 475 C, supported by Pettersson et al. who found that also a which were detected by a combination of APT and spinodal wavelength of 4 nm and Cr amplitude of 8.6 at. XRD. The comparison of the experimental data with pct, corresponding to much earlier stages compared to the calculated a¢ fractions is, however, not meaningful the 600 minutes heat treatment, resulted in 40 pct for spinodal decomposition as there is a gradual change toughness drop after 6000 hours aging at 300 C. The METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2809 Fig. 6—Electrolytically etched cross sections of BM samples arc heat treated from 0.5 to 600 min. The red broken lines show the position of the fusion boundary. A light etching band around the 475 C isotherm formed after 3-min heat treatment (red arrow) and become wider and more visible at longer heat treatment times (Color ﬁgure online). Fig. 7—Microhardness map of BM samples arc heat treated from 0.5 to 600 min. In each map, the left broken line shows the position of the fusion boundary and the right broken line the 475 C isotherm. The ﬁrst indications of a high hardness band at 475 C were seen after 3 min and the highest hardness after 600 min. few small Cu particles, however, would not be expected In addition to the equilibrium phase fractions pre- to cause signiﬁcant hardening compared to the spinodal sented in Section III–A, thermodynamic calculations decomposition. were also performed for the chemical composition of the 2810—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 8—Cross sections and microhardness maps of remelted base metal (RBM) and remelted weld metal (RWM) after 1 and 10 min arc heat treatment. The red broken lines indicate the position of the fusion boundary and arrows show the position of the light etching band. In the hardness maps, the left broken line shows the position of the fusion boundary and the right broken line shows the 475 C isotherm (Color ﬁgure online). Table II. Hardness and Temperature Ranges of Light supports the higher measured decomposition tempera- Etching High Hardness Band for Arc Heat-Treated Samples ture (500 C after 600 minutes arc heat treatment) compared to the previously calculated equilibrium Hardness Range in temperature (480 C) in Section III–A. These calcula- High Hardness T-Range tions also show that a¢ is enriched in Cr, Mo, and Mn, Time Sample Band by Etching but depleted in Fe and Ni compared to the total ferrite chemical composition, which is in good agreement with Before arc BM 275–295 — heat treatment RBM 275–300 — compositions presented in Table III. The measured and RMW 260–285 — calculated contents for each element are, as expected, BM no change — diﬀerent due to the much longer time needed to 1 min BM no change — approach the equilibrium condition. RBM no change — RWM no change — 3 min BM no change* 435–480 B. Monitoring Phase Separations 10 min BM 300 435–490 In addition to APT, electrolytical etching after 3 RBM 300–315 440–490 minutes or longer and microhardness mapping after 10 RWM 290–310 440–490 minutes or longer arc heat treatment revealed changes 60 min BM 300–310 415–495 600 min BM 300–340 405–500 that conﬁrm phase separations have occurred in the FGMs. However, SEM and XRD failed to detect these *Some locations showed high hardness but did not form a clearly even after 600-minute arc heat treatment. distinguishable band. The FGMs produced in this study provided an opportunity to compare the etching response of regions total ferrite detailed in Table III, only considering a, a¢, heat treated at diﬀerent temperatures in one single and Cu phase. The absence of other phases in the sample for each heat treatment time and made it calculations results in an increase in the predicted possible to characterize the phase separations based on spinodal decomposition temperature to 600 C, which changes in the etching behavior of ferrite as the austenite METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2811 Table III. Compositions Measured by APT of Austenite, Ferrite (Total Ferrite), and of a¢ and a Austenite Total Ferrite a¢ a Conc. Error Conc. Error Conc. Error Conc. Error Elements (At. Pct) (At. Pct) (At. Pct) (At. Pct) (At. Pct) (At. Pct) (At. Pct) (At. Pct) Cr 25.7 0.3 26.7 0.8 41.3 0.5 13.7 0.4 Fe 59.6 1.0 62.6 0.8 48.5 0.5 76.3 0.3 Ni 7.9 0.2 5.4 0.1 4.4 0.1 5.8 0.1 C 0.37 0.13 < 0.009 — < 0.006 — < 0.009 — N 1.49 0.12 < 0.008 — < 0.013 — < 0.016 — Mo 1.9 0.1 2.9 0.1 3.4 0.2 2.1 0.1 Si 1.21 0.05 1.05 0.03 1.14 0.04 0.85 0.05 P 0.03 0.01 0.12 0.05 0.13 0.05 0.11 0.04 Mn 0.72 0.04 0.60 0.06 0.66 0.06 0.46 0.07 Cu 0.32 0.06 0.24 0.04 0.13 0.01 0.23 0.02 was largely unaﬀected. The electrolytical etching same alloy. One reason could be diﬀerent levels of response is the result of changes in the electrochemical residual stresses in the as-received plates as this could behavior of ferrite during the spinodal decomposition, signiﬁcantly aﬀect the kinetics of the phase [27,28] which results in the absence of selective etching of separations. ferrite. This has earlier been observed by for example The maximum hardness after arc heat treatment and  Mraz et al. who reported the reduction of contrast percentage of maximum hardness increase are shown in between ferrite and austenite after the spinodal decom- Figure 11. The reason to use maximum hardness is to position in a cast duplex stainless steel. It has been see the maximum changes that occurred in the FGMs reported that ferrite and austenite had the same nobility compared to the condition prior to heat treatment. after 5-h heat treatment of 2205 grade DSS at 475 C, A larger temperature range of phase separations and a which was attributed to the presence of nano-sized a¢ larger hardness increase are to be expected for longer  regions enriched in Cr. heat treatment times as shown in Figure 11 and Hardness is another important easily observable Table II. However, the fact that phase separations were property that can be used to monitor 475 C-embrittle- observed in the BM down to 405 C after 600 minutes ment. However, small changes (about 5 to 10 HV) in (10 hours) heat treatment indicates a high susceptibility hardness are not a satisfactory evidence for phase to 475 C-embrittlement at relatively low temperatures separations. Therefore, early stage phenomena are not and short exposure times. easily studied by hardness testing as no signiﬁcant BM, RBM, and RWM had quite similar temperature changes will occur. In the present study, however, ranges where phase separations were observed, but the microhardness mapping showed small changes when corresponding hardness values were quite diﬀerent after compared to the microhardness before arc heat treat- 10-minute arc heat treatment. Three diﬀerent combina- ment and in surrounding regions heat treated at other tions of initial conditions can be compared as follows: temperatures. In addition, the diﬀerent etching A. Different initial microstructural morphology/similar responses around the 475 C isotherm facilitated mon- chemical composition RMB showed a higher hard- itoring earlier stage embrittlement by microhardness ness increase than BM, as shown in Figure 11, mapping. However, the electrolytical etching approach which implies that remelting accelerates the was more eﬀective in detecting 475 C-embrittlement at 475 C-embrittlement. Welding introduces residual the very early stages as it responded to smaller phase stresses which significantly accelerate phase separa- separations in ferrite than was needed to see a change in [27,28] tions. In addition, alloying element partition- hardness. ing may affect the kinetics of phase separation.  Hosseini et al. showed that autogenously remelt- C. Kinetics ing of 2507 SDSS decreased the partitioning of Ni One of the more interesting observations in the to austenite and Cr and Mo to ferrite compared to present study was the rapid development of phase the base material in as-received condition. separations in ferrite which were detectable after 3 B. Similar initial microstructural morphology/different minutes arc heat treatment. A TTT diagram presented chemical compositions The maximum hardness  increase is 3.0 pct higher for RWM than for RBM in the industry datasheet for 2507 SDSS suggests a 50 although the maximum hardness is higher for RBM pct toughness drop after about 3-minute heat treatment at 475 C, which is in good agreement with the etching than RWM. This implies that the spinodal decom-  response in this study. Hilders et al. and position is more rapid in the RMW. It has been  Gutie´ rrez-Vargas et al., however, reported minor suggested that the higher content of Ni in the RWM changes in microhardness after 1-h heat treatment and may promote phase separations in this temperature [24,27] a toughness drop only after 72 hours at 475 C for the range. This is in line with the fact that 2812—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 9—Atom probe tomography images from ferrite of BM specimen furnace heat treated for 600 min at 475 C. (a) Iso-concentration surfaces for high Cr, shown in blue (> 30 at. pct), and high Fe regions, shown in red (> 70 at. pct). (b) 2D image for Cr (blue) and Cu (orange), obtained from a 2-nm-thick slice (c) 3D image of Cu atoms clearly showing the presence of Cu-rich precipitates (Color ﬁgure online). thermodynamic calculations showed that the ratio for BM arc heat treated for 60 minutes. This of a¢/a is higher at equilibrium for the high Ni indicates that the combination of higher Ni content welding wire composition. and weld metal microstructural morphology signif- C. Different initial microstructural morphology/different icantly accelerated phase separations. This phe-  chemical compositions The increase in the maximum nomenon is in line with the study by Zhou et al., hardness of RWM is not only higher than for BM arc who reported that weld metal with higher Ni content heat treated for 10-minute arc, but also higher than is more prone to 475 C-embrittlement. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2813 Fig. 10—(a) Line proﬁles from a cylinder with a 2-nm diameter for the concentrations of Fe and Cr, illustrating decomposition into Fe-rich and Cr-rich ferrite, (b) Cr-Cr and Cr-Fe RDF diagram showing the normalized Cr and Fe concentration as a function of distance. The part of the diagram indicating where the Cr maximum, corresponding to the wavelength, appears is shown enlarged. D. TTT Diagram The TTT diagram of 2507 SDSS suggested in this study is in good agreement with that proposed in the A time-temperature transformation diagram for industry datasheet but they were determined using 475 C-embrittlement is presented in Figure 12. The diﬀerent approaches. As presented in Table II,an experimental TTT curves are from the present study, the etching technique was employed to ﬁnd the phase   industry data sheet and by Nilsson et al. and a separation temperature range in the present study but theoretical curve is calculated using JMatPro. As may be  a 50 pct drop in toughness was considered in the seen, JMatPro and Nilsson et al. suggest much longer industry datasheet for this alloy. It means that, in times for 475 C-embrittlement to occur. 2814—VOLUME 49A, JULY 2018 METALLURGICAL AND MATERIALS TRANSACTIONS A Fig. 11—(a) Maximum hardness and (b) percentage increase of maximum hardness in arc heat-treated samples. Remelted weld metal (RWM) shows the largest change in hardness after 10 min arc heat treatment while base metal (BM) shows the smallest. However, further investigations are needed to correlate welding thermal cycles and embrittlement to verify whether this is a practical concern or not. V. CONCLUSIONS Low-temperature phase separations (T < 500 C) and embrittlement of 2507 SDSS were investigated in func- tionally graded base and weld metals arc heat treated between 0.5 and 600 minutes. The degree and charac- teristics of phase separations were studied by atom probe tomography, electrolytical etching, and micro- hardness mapping. The main conclusions are as follows: Fig. 12—Time-temperature transformation diagram for 475 C- 1. Electrolytical etching and microhardness mapping embrittlement of 2507 SDSS based on the present study, literature, were successfully used to monitor phase separa- industry datasheet, and thermodynamic calculations. A good tions. The selective etching of ferrite in 10 pct correlation between results of this study and industry datasheet Ref.  NaOH decreased and hardness increased as phase for the studied base material is seen. separation occurred. 2. First indications of phase separations in base material were found at 435 C to 480 C after only contrast to increase in hardness, electrolytical etching 3-minute arc heat treatment. The temperature range behavior shows a good agreement with a 50 pct loss of increased to 405 C to 500 C after 600 minutes toughness approach for 2507 SDSS. This suggests that accompanied by a hardness increase of 14 pct. when the electrochemical behavior of ferrite is changed, Temperature ranges were quite similar for base the degree of spinodal decomposition is suﬃcient to metal, remelted base metal and remelted weld metal have a detrimental inﬂuence on the toughness. arc heat treated for 10 minutes, while the hardness A practical implication of the observed rapid phase increase was highest in remelted weld metal and separations could be embrittlement during multipass welding. As an example, thermal cycle analyses per- lowest in base metal. formed during single pass TIG remelting of SDSS 3. Thermodynamic calculations predicted the forma- showed that regions close to the weld zone were at 400 tion of a + a¢, Cu precipitates, carbides, G-phase, C to 500 C about 1 minute for a 6-mm-thick plate and Pi-nitrides, and Laves in ferrite. [29,30] welding heat input of 1.1 kJ/mm. During multipass 4. Atom probe tomography showed Cu precipitates welding it is therefore not unrealistic for some regions in and spinodal decomposition of base metal ferrite to the heat aﬀected zone to be subjected to temperatures in a + a¢, with a Cr amplitude of 14 at. pct and the critical range for times possibly causing embrittle- spinodal wavelength of 10.5 nm, after 600-minute ment. This could be the reason of unexpected loss of heat treatment at 475 C. toughness and failures in SDSS with no detectable mi- 5. A weld metal microstructure morphology and/or crostructural changes or measurable hardness increase. higher content of Ni accelerated phase separations. METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 49A, JULY 2018—2815 6. O. Hilders and N. Zambrano: J. Microsc. Ultrastruct., 2014, vol. 2, This was in line with thermodynamic calculations pp. 236–44. predicting higher ratio of a¢/a for higher Ni 7. G Gutie´ rrez-Vargas, V.H. Lo´ pez, H. Carreo´ n, J. Kim and A. Ruiz: contents. AIP Conference Proceedings, AIP Publishing. 2017, p. 110018. 6. A time-temperature transformation diagram, sum- 8. K.L. Weng, H.R. Chen, and J.R. Yang: Mater. Sci. Eng. A, 2004, marizing the kinetics of the 475 C-embrittlement, vol. 379, pp. 119–32. 9. S.S.M. Tavares, R.F. De Noronha, M.R. Da Silva, J.M. Neto, and was produced for 2507 SDSS based on electrolytical S. Pairis: Mater. Res., 2001, vol. 4, pp. 237–40. etching response and literature data. 10. K. Chandra, R. Singhal, V. Kain, and V.S. Raja: Mater. Sci. Eng. A, 2010, vol. 527, pp. 3904–12. 11. J. Ejenstam, M. Thuvander, P. Olsson, F. Rave, and P. Szakalos: J. Nucl. Mater., 2015, vol. 457, pp. 291–97. 12. C. Ornek, J. Walton, T. Hashimoto, T.L. Ladwein, S.B. Lyon, and D.L. Engelberg: J. Electrochem. Soc., 2017, vol. 164, pp. C207–17. ACKNOWLEDGMENTS 13. X. Xu, J. Odqvist, M.H. Colliander, M. Thuvander, A. Steuwer, J.E. Westraadt, S. King, and P. Hedstrom: Metall Mater Trans A, The authors would like to thank Dr. Dirk Engelberg 2016, vol. 47A, pp. 5942–52. and Mr. Pierfranco Reccagni for their valuable inputs 14. J.-O. Nilsson and P. Liu: Mater. Sci. Technol., 1991, vol. 7, pp. 853–62. to the project, Dr. Matthew Roy and Mr. Daniel 15. J. Zhou, J. Odqvist, M. Thuvander, S. Hertzman, and P. Hedstro¨ m: Wilson for their assistance with automatic hardness Acta Materialia, 2012, vol. 60, pp. 5818–27. testing, Mr. Kjell Hurtig and Mr. Jonas Olsson for 16. T.F. Kelly and M.K. Miller: Rev. Sci. Instrum., 2007, vol. 78, performing heat treatment, and Outokumpu AB for p. 031101. donating the plates. The ﬁnancial support from the 17. L.D. Bobbio, R.A. Otis, J.P. Borgonia, R.P. Dillon, A.A. Shapiro, Z. Liu, and A.M. Beese: Acta Materialia, 2017, vol. 127, pp. 133–42. KK-foundation for the research school SiCoMaP 18. V.A. Hosseini, L. Karlsson, K. Hurtig, I. Choquet, D. Engelberg, (20140130) is acknowledged. M.J. Roy, and C. Kumara: Mater. Des., 2017, vol. 121, pp. 11–23. 19. S. Hertzman, R. Pettersson, R. Blom, E. Kivineva, and J. Eriksson: ISIJ Int., 1996, vol. 36, pp. 968–76. 20. J. Zhou, J. Odqvist, M. Thuvander, and P. Hedstro¨ m: Microsc. OPEN ACCESS Microanal., 2013, vol. 19, pp. 665–75. 21. C. Kumara: Modelling of the Temperature Field in TIG Arc Heat This article is distributed under the terms of the Treated Super Duplex Stainless Steel Samples, Master thesis, Creative Commons Attribution 4.0 International University West, Sweden, 2016. License (http://creativecommons.org/licenses/by/4.0/), 22. N. Pettersson, S. Wessman, M. Thuvander, P. Hedstro¨ m, J. Odqvist, R. Pettersson, and S. Hertzman: Mater. Sci. Eng. A, which permits unrestricted use, distribution, and 2015, vol. 647, pp. 241–48. reproduction in any medium, provided you give 23. J. Zhou, J. Odqvist, L. Ho¨ glund, M. Thuvander, T. Barkar, and P. appropriate credit to the original author(s) and the Hedstrom: Scr. Mater., 2014, vol. 75, pp. 62–65. source, provide a link to the Creative Commons 24. B. Zhang, F. Xue, S.L. Li, X.T. Wang, N.N. Liang, Y.H. Zhao, and G. Sha: Acta Mater., 2017, vol. 140, pp. 388–97. license, and indicate if changes were made. 25. A. Weidner, R. Kolmorgen, I. Kubena, D. Kulawinski, T. Kruml, and H. Biermann: Metall. Mater. Trans. A, 2016, vol. 47A, pp. 2112–24. 26. L. Mraz,F.Matsuda,Y.Kikuchi, N. Sakamoto, and S.Kawaguchi: REFERENCES Trans. JWRI, 1994, vol. 23 (2), p. 10. 1. L. Karlsson: Weld. World, 2012, vol. 56, p. 6. 27. J.K. Sahu, U. Krupp, R.N. Ghosh, and H.J. Christ: Mater. Sci. 2. J.-O. Nilsson and G. Chai: In Duplex Stainless Steel Conference, Eng. A, 2009, vol. 508, pp. 1–14. Beaune, France, 2010. 28. F. Danoix, J. Lacaze, A. Gibert, D. Mangelinck, K. Hoummada, 3. C. Ornek, M. Burke, T. Hashimoto, J. Lim, and D. Engelberg: and E. Andrieu: Ultramicroscopy, 2013, vol. 132, pp. 193–98. Mater. Perform. Charact., 2017, vol. 6, pp. 409–36. 29. V.A. Hosseini, S. 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