F. TEHOVNIK et al.: NITRIDES AND CARBIDES IN 2101 LEAN DUPLEX STAINLESS STEEL 821–826 NITRIDES AND CARBIDES IN 2101 LEAN DUPLEX STAINLESS STEEL NITRIDI IN KARBIDI V LEAN DUPLEKSNEM NERJAVNEM JEKLU 2101 Franc Tehovnik, Barbara [etina Bati~, Franci Vode, Simon Malej, Jaka Burja Institute of Metals and Technology, Lepi pot 11, 1000 Ljubljana, Slovenia franc.tehovnik@imt.si Prejem rokopisa – received: 2018-07-13; sprejem za objavo – accepted for publication: 2018-10-18 doi:10.17222/mit.2018.145 The 2101 lean duplex stainless steel (LDX) was designed mainly for lightweight constructions and as a more economical substitute for the dominating stainless-steel grades. Compared with other duplex stainless steels, the corrosion resistance and costs of processing the 2101 duplex stainless steel are usually worse, as the Ni and Mo contents are lower; it is compensated by higher N and Mn contents to stabilize the austenite. At lower test temperatures the precipitation of different phases was observed, so different annealing experiments were conducted to further investigate the occurrence of precipitation. As the composition of lean duplex stainless steel differs from that of conventional duplex stainless steels, a different aging behaviour is expected. The embrittlement of 2101 lean duplex stainless steel occurs at approximately 700 °C to 750 °C, because of the precipitation of the deleterious Cr2N and M23C6 at the / and / interfaces, which begins after a few minutes of aging. These temperatures of the nitride precipitations are crucial to the cracking during the end of hot-working operations, which should stop at higher temperatures. The purpose of the research was to qualitatively analyse the phases in lean duplex stainless steel after thermal aging. Optical microscopy (OM), scanning electron microscopy (SEM) and electron-backscatter diffraction (EBSD) were used to investigate the structural stability and the chemical compositions of the phases. Keywords: lean duplex stainless steel, isothermal annealing, precipitation, Cr 2N, Cr23 C 6 Lean dupleks nerjavno jeklo LDX 2101 je bilo razvito predvsem za lahke konstrukcije in je ekonomi~en nadomestek za ve~ino nerjavnih jekel. V primerjavi z drugimi dupleksnimi nerjavnimi jekli je njegova odpornost proti koroziji slab{a, saj sta vsebnosti Ni in Mo ni`ji, s tem pa so ni`ji tudi stro{ki izdelave. Stabilizacijo austenita kompenziramo z vi{jo vsebnostjo N in Mn. Avtorji raziskave so ugotavljali izlo~anje razli~nih faz pri razli~nih pogojih `arjenja in ni`jih temperaturah preizku{anja. Ker se sestava preiskovanega nerjavnega jekla razlikuje od sestave klasi~nih dupleks nerjavnih jekel, se to jeklo med postopki `arjenja obna{a druga~e. Krhkost preiskovanega nerjavnega jekla se pojavi med 700 °C do 750 °C zaradi izlo~anja Cr2NinM 23C6 na mejah delta feritnih kristalnih zrn ( / ) in mejah zrn delta ferita in austenita ( / ). Izlo~anje se pri~ne `e po nekaj minutah `arjenja. Namen raziskave je bila kvalitativna analiza faz v preiskovanem dupleks nerjavnem jeklu med temperaturnim `arjenjem. Za raziskovanje strukturne stabilnosti in kemijske sestave faz so avtorji uporabili opti~no mikroskopijo (OM), vrsti~no elektronsko mikroskopijo (SEM) in uklon povratno sipanih elektronov (EBSD). Klju~ne besede: lean dupleks nerjavno jeklo, izotermno `arjenje, izlo~anje, Cr2N, Cr23C6 1 INTRODUCTION The 2101 lean duplex stainless steel (LDX) was designed mainly for lightweight constructions and as a more economical substitute for the dominating stainless-steel grades. LDX 2101 was developed with a low content of nickel in order to reduce and stabilize the costs. The low nickel content is compensated by an increased amount of gamma-stabilizing manganese and nitrogen to ensure a balanced microstructure with approximately equal amounts of ferrite and austenite. The strength of this material is comparable to DSS 2205 and the corrosion properties are generally better than for austenitic AISI 304. 1–4 Knowledge of the secondary-phase formation is necessary for an exploitation of the LDX, mainly in two practical situations: • cooling from high temperatures, as in the quenching treatment after solution annealing or after welding operations, • during long holding times at high temperatures, for instance, during service. On the other hand, LDX stainless steels have been found to be affected by nitride and carbide formation. 5 In any case, all duplex stainless steels are sensitive to the precipitation of nitrides, as they contain significant per- centages of nitrogen. 6–7 Nitride formation is strictly related to the variation of temperature and nitrogen solu- bility in ferrite, which increases at high temperature, thus bringing ferrite in supersaturated solid-solution condi- tions at room temperature. In the solution-annealed condition, these alloys retain almost all of the nitrogen in the austenite. The solubility difference of the nitrogen in the ferrite and the austenite is particularly high at low temperatures. 8 In LDX stainless steel, the ’ secondary austenite can be observed at the / interfaces, and the co-precipitation mechanisms M 23 C 6 + ’ and Cr 2 N+ ’ are assumed. 9 Materiali in tehnologije / Materials and technology 52 (2018) 6, 821–826 821 UDK 549.2:669.14.018.8:620.1 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 52(6)821(2018) The mechanisms of precipitation and phase transfor- mation can be described as follows. 10,11 Both carbides and nitrides precipitate on the / phase boundary and cause cracking (embrittlement) during hot deformation. 9 The M 23 C 6 exhibits a specific triangular morphology, while the Cr 2 N exhibits a rod-like morphology. The rapid precipitation of Cr 2 N and ’occurs after a few minutes of ageing. Cr 2 N and ’ grow together in the phase until the diffusion of the heavy nitride-forming elements, such as Cr, Mo and V, limits the Cr 2 N growth and allows the ’ to grow alone. In the vicinity of the / interphase boundary, the Cr depletion is located first in the be- cause Cr diffusion is faster in than in . In contrast, at the / interfaces only Cr 2 N precipitates and no ’is observed along the / interface. The precipitation of Cr 2 N occurs at the / interface after 10 h of ageing, as the diffusion kinetics are slower in the austenite. Due to the increased content of nitrogen in 2201, the precipitation of Cr 2 N-type nitrides increased in the temperature range from 700 °C to 900 °C. It is accepted that the embrittlement nose of LDX is approximately from 650°C to 750 °C, because of the Cr 2 N and M 23 C 6 precipitation at the / and / interfaces after a few minutes of aging. 6,10–12 The precipitation of nitrides and carbides was studied in the temperature range where hot rolling usually ends for most steels. Precipitates formed at the upper temperatures in the embrittlement nose (700–900 °C for 30 min and 120 min) were studied by qualitative phase analysis. 2 EXPERIMENTAL PART The lean duplex stainless-steel grade LDX 2101 with the chemical composition given in Table 1 was used in the experimental work. The specimens were cut from 20-mm-thick hot-rolled and solution-annealed plate. Table 2 shows the mechanical properties of the investi- gated steel. The initial microstructure is presented in Figure 1. At the 1100 °C annealing temperature both phases have an almost fully recrystallized microstruc- ture. Cylindrical specimens with a diameter 4 mm and a length of 10 mm were machined from flat specimens. Hot-ageing tests were performed on a TA Instruments dilatometer DIL805A/D, due to its good temperature and atmosphere control. The specimens were isothermally annealed at different temperatures from 700 °C to 900 °C in 50 °C steps under vacuum (5·10 –4 mbar). The specimens were heated at a rate of 10 K/s. The kinetics of the intermetallic phase precipitation was investigated by annealing for 30 min and 120 min and then quenching at the rate of 50 K/s. The samples were etched with a solution of 30 g KOH, 30 g K 3 Fe(CN) 6 and 100 mL distilled water. The etching causes the phases in the microstructure to colour differently: the -ferrite is dark and the -austenite is light. The specimens were inves- tigated by optical microscopy and scanning electron microscopy (SEM) with energy-dispersive X-ray spec- troscopy (EDS) and the electron-backscatter diffraction (EBSD) technique. A Jeol JSM 6500-F field-emission scanning electron microscope, equipped with an Oxford EDS system and a Nordlys II EBSD detector was used F. TEHOVNIK et al.: NITRIDES AND CARBIDES IN 2101 LEAN DUPLEX STAINLESS STEEL 822 Materiali in tehnologije / Materials and technology 52 (2018) 6, 821–826 Figure 1: Initial microstructure of LDX 2101 duplex stainless steel, with austenite (light) and ferrite (dark) in the as-rolled longitudinal and transversal directions Table 1: Chemical composition LDX 2101 in mass fractions (w/%) CS iM nPSC rN iC uM oA lNF e 0.03 0.68 4.9 0.024 0.001 21.4 1.46 0.27 0.32 0.015 0.226 Bal. Table 2: Mechanical properties of the investigated lean duplex stainless steel in solution-annealed condition at room temperature Direction of rolling Proof strength R p02 (MPa) Tensile strength R m (MPa) Elongation A (%) Reduction of area (%) Z Impact toughness average (KV) Longitudinal 464 696 45 69 291 for the electron microscopy. Post-processing of the EBSD data was performed with Channel 5 software. 3 RESULTS AND DISCUSSION The equilibrium phase composition for LDX 2101 was calculated using the Thermo-Calc TCFE8 database. The results are presented in Figure 2. As shown in Fig- ure 2, the equilibrium phase diagram calculated for the experimental LDX 2101 indicates that only -ferrite and -austenite are stable above 960 °C. The ferrite and austenite contents are almost equivalent in the tempe- rature range between 1100 °C and 1150 °C. The secon- dary phases are thermodynamically stable at lower tem- peratures, the nitrides below 960 °C, the carbides below 850 °C and the -phase below 750 °C. Figure 3 shows a series of optical micrographs of the specimens aged at different temperatures for 30 min and 120 min. The fine precipitates formed only between the austenite island and the ferrite matrix ( /)i nt h e samples aged at 700 °C for 30 min. The fine precipitates were also found at 750 °C for 30 min at the / and / the grain boundaries. The size and the amount of preci- pitates increased after 120 min of ageing in the tem- perature range from 750 °C to 850 °C. The nitrides formed along the / interface grew towards the ferrite, and the secondary austenite surrounding precipitate started to form. It is clear that the / interface moved away from the precipitate particles and grown into the ferrite phase, leaving the precipitates behind along the original interfaces. The SEM image of the microstructure and the EDS analysis are shown in Figure 4, where the sample was aged at 800 °C for 120 min. Our results are in agreement with Liljas et al. 1 and Fang et al. 13 The EDS analysis revealed that the precipitates at the / and / interfaces had high chromium contents. The region near the precipitates showed a lower chromium content compared to the ferrite phase, the austenite phase and the preci- pitates. This low chromium region was identified as secondary austenite ( ’). In the vicinity of the / inter- face, the Cr depletion first occurs in , because Cr diffu- sion is faster in ferrite than in austenite. The interfaces between the austenite islands and the ferrite grain boundaries are believed to be the preferential nucleation sites for the heterogeneous precipitation of Cr 2 N-type nitrides and M 23 C 6 type carbides. 14–16 The transfor- mation at the / interface is described as (1): = ’+Cr 2 N+M 23 C 6 (1) Compared to the normal austenite, secondary auste- nite is enriched in nickel and manganese, and depleted in chromium. The SEM image and the EDS analysis of the Cr 2 N are shown in Figure 5. These Cr 2 N precipitates have a high concentration of Cr and N and contain Fe, Ni and Mn. The precipitation of Cr 2 N is often accompanied by the formation of secondary austenite due to a de- F. TEHOVNIK et al.: NITRIDES AND CARBIDES IN 2101 LEAN DUPLEX STAINLESS STEEL Materiali in tehnologije / Materials and technology 52 (2018) 6, 821–826 823 Figure 2: Equilibrium phase diagram calculated using Thermo-Calc for the lean duplex stainless steel LDX 2101 Figure 3: Optical micrographs for the microstructures formed during isothermal annealing after 30 min and 120 min at different tempera- tures crease in the amount of Cr in the ferrite. Because stabilizer elements are also -phase stabilizers, the phase precipitates in the once the nitrogen, carbon and nickel contents in the are low enough. 17–19 In the inves- tigated LDX 2101 duplex stainless steel, this precipi- tation does not occur after 120 min of ageing. The kinetics of the ó-phase precipitation is expected to be more sluggish than in standard (SAF 2205) and super duplex stainless steel (SAF 2507) due to the lower Cr and Mo contents, 20–21 and the -phase is generally observed after several tens of hours of aging at 700 °C. The examination of LDX 2101 aged for 120 min (800 °C) revealed that the precipitates are located at the initial / , / , / interface and in the ferrite interior (Figure 5). Nitrides (Cr 2 N) and carbides (M 23 C 6 ) are observed on the grain / boundaries, at the triple-junction points and on the twin boundaries. Intergranular precipitation in the austenite does not occur after two hours of aging. To confirm the presence of the different phases, we per- formed spot EBSD analysis for each representative phase. As seen in Figure 6, the phases give distinctively different Kikuchi patterns and can be indexed as ferrite, austenite, Cr 2 N and M 23 C 6 . The crystal structures of the phases are given in Table 3. The austenite phase and the carbide phase have the same crystal structure (FCC), but the lattice parameter of the carbide phase is approxim- ately three times the lattice parameter of the austenite phase. The Cr 2 N phase has a hexagonal structure. Table 3: List of phases investigated in the lean duplex stainless steel using EBSD Phase Crystal structure Lattice parameters (nm) Ferrite Body Centred Cubic (BCC) a = 0.287 Austenite Face Centred Cubic (FCC) a = 0.366 M 23C 6 Face Centred Cubic (FCC) a = 1.056 Cr 2 N Hexagonal a = 0.481, c = 0.448 A detailed analysis showed that the carbides are fine particles between and ’, having the same crystallo- graphic structure as the austenite , and cube-to-cube orientation with the -phase. The co-precipitation of M 23 C 6 and ’ along the - boundaries is thermodyna- mically favourable, as M 23 C 6 is enriched in chromium, whereas the ’ is depleted. Particles with a hexagonal structure along the - ’, - and - grain boundaries were identified as chromium nitrides. The ferritic regions are characterized by the high diffusivity and low F. TEHOVNIK et al.: NITRIDES AND CARBIDES IN 2101 LEAN DUPLEX STAINLESS STEEL 824 Materiali in tehnologije / Materials and technology 52 (2018) 6, 821–826 Figure 4: SEM micrograph and EDS analysis for the microstructure after isothermal annealing at 800 °C for 120 min Spectrum (w/%) Si Cr Mn Fe Ni Spectrum 1 0.8 20.9 5.4 71.1 1.8 Spectrum 2 0.9 22.4 5.5 69.9 1.4 Spectrum 3 Cr 2 N 0.3 26.0 5.7 66.3 1.6 Spectrum 4 ’ 0.6 18.7 5.4 72.9 2.5 Spectrum 5 Cr 2 N 0.6 24.5 5.9 66.9 2.0 Spectrum 6 ’ 0.7 18.5 5.8 72.5 2.5 Figure 6: Electron-backscatter image after isothermal ageing at 800 °C for 120 min with marked phases and the corresponding Kikuchi patterns Figure 5: SEM micrograph and EDS analysis of austenite, ferrite and Cr 2 N after isothermal annealing at 800 °C for 120 min Spectrum (w/%) N Si V Cr Mn Fe Ni Mo Spectrum 1 Cr 2 N 13.4 0.5 1.5 50.3 3.7 30.2 0.4 0.0 Spectrum 2 0.0 0.8 0.0 20.5 5.5 69.9 1.5 0.5 Spectrum 3 0.0 1.0 0.0 23.0 4.7 69.7 1.1 0.6 solubility of nitrogen. Both these factors encourage nitride precipitation from the ferrite. Figure 7 shows higher-magnification EBSD images after isothermal ageing at 800 °C for 120 min. Figure 7a shows the band-contrast map, which reproduces the microstructure of the specimens. As observed, the microstructure consisted of a mixture of ferrite, austenite and a small amount of M 23 C 6 and Cr 2 N along the boun- daries. The matrix consists of ferrite with austenitic islands. Figure 7b is a representation of the phases, where we can clearly observe the presence of four diffe- rent phases. Figure 7c shows the orientation of the individual particles and Figure 7d shows a legend for the IPF-Z colour coding. A higher-resolution map (with a step size of 50 nm) allowed clear observations of the newly formed microstructure. The austenite grain at the interphase had convoluted morphologies, with newly formed secondary austenite protruding from the primary austenite islands (Figure 8). The secondary austenite seemed to have developed through grain growth from the ferrite, indi- cated by the white dashed line in Figure 8, showing the approximate position of the initial / phase boundary. From the results of the EDS and EBSD analyses in this paper, the precipitates were confirmed to the mainly Cr 2 N and Cr 23 C 6 , instead of the sigma phase. These are two main reasons for Cr 2 N becoming the critical preci- pitation in DSS 2101, instead of the sigma phase. Molybdenum controls the -phase formation. The content of molybdenum is only 0.32 w/% in DSS 2101. The high nitrogen content significantly increases the pre- cipitation trend of the Cr 2 N-type nitrides and also retards the sigma-phase formation. The results reveal rapid Cr 2 N precipitation during the aging of the DSS 2101. 9 4 CONCLUSIONS Isothermal ageing treatments were carried out over the temperature range from 700 °C to 900 °C for 30 min and 120 min. The conclusions are as follows: • The equilibrium phase diagram calculated using Thermo-Calc software for the experimental LDX 2101 indicated that -ferrite and -austenite are stable above 960 °C. The secondary phases are ther- modynamically stable at lower temperatures, the nitrides below 960 °C, the carbides below 850 °C and the -phase below 750 °C. • The temperature range from 700 °C to 900 °C is problematic for the formation of harmful precipitates. The Cr 2 N-type nitrides and a small amount of M 23 C 6 -type carbides were formed at the grain boun- daries, thus weakening them. The nitrides formed along the / interface grew towards ferrite, and the secondary austenite surrounding the precipitates started to form. These temperatures of nitride preci- pitations are crucial for cracking during the end of hot-working operations, which should stop at higher temperatures. • The newly formed secondary austenite ’ is poorer in chromium and enriched in nickel compared to the primary austenite. • M 23 C 6 has a triangular morphology, unlike Cr 2 N, which has a rod-like morphology and grows conti- nuously with time. • The LDX 2101 duplex stainless steel is resistant to the formation of the sigma phase in the 700–900 °C range for up to 120 min, mainly due to low Mo and high N. No evidence of sigma-phase precipitation was found, and the annealing time of 120 min was not long enough. 5 REFERENCES 1 M. Liljas, P. Johansson, H. P. Liu and C.O.A. Olson, Development of a lean duplex stainless steel, Steel. Res. Int., 79 (2008) 6, 466–473, doi:10.1002/srin.200806154 F. 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