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Evolution of the microstructure and mechanical properties of Sanicro 25 austenitic stainless steel after long-term ageing

Evolution of the microstructure and mechanical properties of Sanicro 25 austenitic stainless... A newly developed heat-resistant austenitic steel, Sanicro 25 is currently considered the leading candidate material for an advanced ultra-supercritical installation. The test material was subjected to long-term ageing (up to 30,000 h) at 700 and 750 °C, after which investigations into the microstructure, identification of precipitates, and testing of mechanical proper - ties were conducted. Sanicro 25 had an austenitic microstructure with annealed twins and numerous large primary NbX and Z-phase precipitates in the as-received condition. It was found that the long-term ageing of the steel resulted in numerous precipitation processes. For example, M C carbides, Laves, σ and G phases occurred at the grain boundaries. However, 23 6 Z-phase precipitates, ε_Cu particles, and Laves phase were observed inside the grains. At the same time, compound com- plexes of precipitates based on the primary Z-phase precipitates were revealed in the microstructure. The ageing process increased the particle size of M C carbides and the σ phase. After longer ageing times, a precipitate-free zone (PFZ) near 23 6 the grain boundaries was observed. The precipitation processes initially lead to an increase in the strength properties of the steel. However, after 5000 h, an over-ageing effect was observed at 750 °C, which was not observed at 700 °C. Keywords Sanicro 25 steel · Microstructure · Precipitation · Ageing · Mechanical properties 1 Introduction in modern conventional power industries, including steels with an austenitic matrix [1]. The overall development of the power industry stimulates The scope of using modern materials for boiler com- the growth of new creep-resistant steels. In addition, eco- ponents, including new-generation ferritic and austenitic nomic and environmental considerations and continuous steels and nickel superalloys, depends on the temperature improvements in the thermal efficiency of power units have and stress conditions [2]. Bainitic and heat-resistant marten- forced the introduction of newer and newer materials for use sitic steels of 9–12% chromium, currently most commonly used in the power industry, are designed for operation at temperatures not exceeding 620–630 °C. However, due to the low corrosion resistance of 9% Cr steels and the low * M. Sroka microstructural stability of 12% Cr steels, the use of creep- marek.sroka@polsl.pl resistant austenitic steels is required at higher parameters. Austenitic steels show higher creep strength than ferritic Department of Engineering Materials and Biomaterials, Silesian University of Technology, Ul. Konarskiego 18a, steels and higher heat resistance and can be used at tempera- 44-100 Gliwice, Poland tures up to around 700 °C [3]. Compared to the currently Łukasiewicz Research Network - Institute for Ferrous available austenitic steels and nickel alloys (which can be Metallurgy, K. Miarki 12-14, 44-100 Gliwice, Poland used at above 700 °C but are considerably more expensive), Department of Materials Engineering, Czestochowa the best alternative is Sanicro 25 stainless steel, which can University of Technology, Al. Armii Krajowej 19, significantly reduce the investment cost of the plant [4 ]. 42-200 Częstochowa, Poland Sanicro 25 steel (X7NiCrWCuCoNbNB25) was devel- Materials Research Laboratory, Silesian University oped under the European Therme AD700 program by AB of Technology, Ul. Konarskiego 18a, 44-100 Gliwice, Poland Sandvik Material Technology in Sweden. Like other aus- Department of Materials Engineering, University of Zilina, tenitic steels, Sanicro 25 has high plastic properties and Univerzitná, 8215/1, 010 26 Žilina, Slovakia Vol.:(0123456789) 1 3 149 Page 2 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 ductility with relatively low strength properties. These prop- of precipitation processes in Sanicro 25 for ageing /service erties are mainly linked to the solid solution-strengthening times longer than 25,000 h. Studies on the impact of longer mechanism of steel, primarily provided by nitrogen, tung- ageing times are essential to obtain the complete picture of sten, and cobalt atoms. In addition, the balanced chemical changes in the microstructure and understand the proper- composition of Sanicro 25 allows significant resistance to ties of steels intended for long-term operation. This article high-temperature corrosion and oxidation. It gives a higher presents the results of investigations on the microstructure creep strength compared to other austenitic steels, e.g., and properties of Sanicro 25 steel after 30,000 h ageing at Super 304H and HR3C [4]. 700 and 750 °C. The creep strength of austenitic steels is associated with two strengthening mechanisms: solid solution strengthen- ing and precipitation hardening. The precipitation harden- 2 Material and research methodology ing mechanism is dominant during service under creep con- ditions. It is related to the presence of particles (carbides, Investigations were carried out on specimens of nitrides, intermetallic phases) interacting with dislocations ϕ38 × 8.8 mm taken from superheater coils in the form of a in the microstructure provided by the so-called carbide- tube along the axial direction and were composed of creep- forming elements (i.e., Cr, niobium, Nb, and tungsten, W, as resistant austenitic steel Sanicro 25 (X7NiCrWCuCoN- well as the addition of copper, Cu, which is released as ε_Cu bNB25-23-3-3-3-2). Details of the chemical composition nanoparticles). The value of creep strength determined for a of the test steel in the as-received condition are presented 100,000 h service at 700ºC is 95 MPa, comparable to that of in Table 1. the HR6W nickel-base alloy. The high functional properties The investigations of microstructure and mechanical of Sanicro 25 make it one of the primary materials eligible properties were carried out on steel in the as-received con- for implementation in supercritical and ultra-supercritical dition at both 700 and 750 °C. Various ageing times were power units with an efficiency of approximately 50% [4 ]. investigated at these temperatures and were chosen to be In previous literature, two main directions of the research 1,000, 5,000, 10,000, 20,000, and 30,000 h. All tests were on Sanicro 25 steel were observed—investigations of creep carried out in the air. and low-cycle fatigue [5] and investigations of corrosion [6] The steel microstructure was analyzed using an Inspect and oxidation resistance in steam [7]. In both cases, one of F scanning electron microscope (SEM). The samples were the main focuses was the analysis of changes in the micro- prepared as metallographic cross-sections. The samples were structure of Sanicro 25. The low-cycle fatigue resistance ground on papers and polished on polishing wheels. Then it tests were conducted at both room temperature and 700 °C. was electrolytically etched in 50% HNO . At room temperature, Sanicro 25 is characterized by cyclic TEM samples for phase identification of precipitates in hardening at the initial stage, followed by cyclic softening, Sanicro 25 were prepared in the form of thin foils and lamel- which is related to the specificity of the deformation of aus- lae. Thin foils were made from the bulk samples by cutting tenitic steel and the supersaturation state of the Sanicro 25 into plate forms, mechanically polished and ion milled by structure. At 700 °C, Sanicro 25 shows the cyclic hardening Ar plasma at 5 keV to create a hole at the center. Lamellae effect associated with precipitation of the Z-phase inside the were made by the focused ion beam (FIB) technique using grains and M C carbides at the grain boundaries. Accord- an SEM/Ga-FIB Helios NanoLabTM 600i microscope (FEI 23 6 ing to the researchers, these carbides were mainly observed Company). TEM investigations were performed using an S/ in the test steel at the low-angle grain boundaries. They were TEM Titan 80–300 microscope (FEI Company) equipped the reason for the formation of Cr-depleted areas near the with a Cetcor Cs probe corrector (CEOS, Germany) and boundaries. Additionally, according to [5], the cyclic hard- EDS spectrometer for chemical composition analysis. Crys- ening of Sanicro 25 at 700 °C is related to the presence of tal Maker and Single Crystal software (version 10.4.1) were numerous dispersive ε_Cu and NbC precipitates. used to simulate the crystal structure and diffraction patterns. To this date, research carried out on Sanicro 25 mainly The qualitative and quantitative analysis of the chemi- covers issues related to low-cycle fatigue, weldability, and cal composition of the test steel was performed using the corrosion resistance. Tests were carried out on material aged EDS energy-dispersive X-ray spectrometer (from EDAX), for no more than 25,000 h. However, there is currently no which is attached to TITAN 80–300 high-resolution electron information on the stability of microstructure or the course microscope. Table 1 Chemical composition C Si Mn P S Cu Cr Ni W Co Nb B of the Sanicro 25 test steel, %wt 0.06 0.25 0.50 0.01 < 0.01 2.90 23.0 24.1 3.20 1.40 0.40 0.005 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 3 of 15 149 The mechanical properties were determined by hardness measurements and static tensile tests at room temperature. The hardness measurement was performed by the Vickers method: HV10, using the Swiss Max 300 hardness tester in accordance with EN ISO 6507-1. Mechanical property measurements were carried out on specimens aged 1,000, 10,000, 20,000, and 30,000 h. Investigations into the strength properties of Sanicro 25 were also carried out using a static tensile test according to EN 10002-1:2002 at room temperature and, according to EN ISO 6892-2:2011, at elevated temperature on flat test specimens. The tests at room temperature were performed using a Zwick 200 kN tensile testing machine, and tests at 700 °C were performed using an Amsler 200 kN tensile test- ing machine in the load range of 40 kN. The computer-based image analysis, including meas- uring the average diameter of M C carbide and σ-phase 23 6 precipitates, was performed using Image ProPlus software. The calibration was performed using a scale marker on the Fig. 1 The secondary electron (SE) image of the Sanicro 25 micro- microstructure images, assuming a calibration factor of 1 structure in the as-received condition pixel = 0.040 µm. All image analysis was carried out on pre- conditioned binary microstructure images. coarse-grained steels [3]. In addition, fine grain in these alloys also positively affects corrosion resistance [12]. 3 Results and discussion 3.2 Microstructure of Sanicro 25 after ageing 3.1 Initial microstructure of Sanicro 25 steel The primary microstructure degradation mechanism of The Sanicro 25 steel, as-received, had a fine-grained aus- creep-resistant austenitic steels is the precipitation processes occurring at the grain and twin boundaries and within the tenitic microstructure with numerous annealing twins and primary precipitates (Fig. 1). The grain size in the tested grains. The morphology and type of precipitates in the auste- nitic steel, both in the as-received condition and after ageing/ steel was determined to be 7 according to the ASTM stand- ard scale [8] (which corresponds to an average diameter of service, depending on the chemical composition of the steel itself, the heat (thermomechanical) treatment parameters, 31.2 μm). The precipitates observed in the microstructure of Sanicro 25 (Fig. 1) were identified as NbX carbonitrides and the holding temperature and time. The microstructure of Sanicro 25 steel after 1000 and 30,000 h ageing at 700 and NbCrN (Z-phase) precipitates (Fig. 2). These precipi- tates are considered primary particles that nucleate during and 750 °C is shown in Fig. 3. As shown in Fig. 3, many secondary phases precipitated the solidification process and grow during the production process. They are, therefore, mainly observed at or near the both within and at the grain boundaries, where, at the lat- ter, particles formed a so-called continuous grid of precipi- grain boundaries (Fig. 1). Primary precipitates of NbX and Z-phase in the microstructure of Sanicro 25 steel as deliv- tates. As high-energy surface defects and grain misalign- ment areas allow faster diffusion compared to that within ered were observed by Rutkowski [7] and Czempura [9]. On the other hand, Zurek [6] observed only particles of the the grains, grain boundaries are preferred sites for the pre- cipitation of secondary phases. At the grain boundaries Z-phase in this steel in the as-received state, while Zhou [10, 11] reported only particles of the MX type. The pur- in the tested steel, the presence of M C carbides was 23 6 revealed (Figs. 4, 5), and precipitation of the Laves phase pose of primary precipitates in austenitic steels is mainly to bind carbon and/or nitrogen atoms to limit the possibility was also shown (Fig. 5). Similar results, i.e., the presence of M C carbides and Laves phase precipitates at grain of precipitating particles rich in chromium, but also these 23 6 precipitates limit grain growth. boundaries, were reported by Zurek [6], and Rutkowski [7] who investigated the microstructure Sanicro 25 after Fine grain and numerous twins in the microstructure in austenitic steels have a positive effect on obtaining oxidation at 700 °C. However, Zhou [11], who examined the steel after ageing and creep at 700  °C, showed the high basic mechanical properties (yield strength, elonga- tion, impact energy) with creep resistance comparable to presence of only M C carbides at the grain boundaries. 23 6 1 3 149 Page 4 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 2 STEM-BF image of the Z-phase precipitate (a) and SAED diffraction pattern in [221] zone axis (b) Fig. 3 The secondary electron (SE) images of the Sanicro 25 microstructures after ageing for a 1,000 h at 700 °C, b 1,000 h at 750 °C, c 30,000 h at 700 °C, and d 30,000 h at 750 °C The preferential precipitation of M C carbides at grain the M C carbides (Fig. 5b). In austenitic steel, the Laves 23 6 23 6 boundaries was associated with the limited carbon sol- phase particles nucleated in the neighborhood of either ubility in the austenitic matrix and the short-range dif- Si-enriched spots present in the bulky M C carbides or 23 6 fusion of chromium atoms [3]. In turn, the Laves phase to the grain boundary areas rich in silicon. This made the in austenitic steels can be precipitated dependently and bulky M C carbides gradually divide into small pieces, 23 6 independently as neighborhoods of other phases within eventually forming a refined mixture of M C and Laves 23 6 the grains and at the grain boundaries. Precipitations of phase at the grain boundaries [13]. The M C carbides, 23 6 the Laves phase at grain boundaries were observed near in contrast, Laves phase precipitates, are characterized 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 5 of 15 149 Fig. 4 The M C precipitate in 23 6 Sanicro 25 steel after 10,000 h ageing at 750 °C. STEM- BF image (a). Selected area electron diffraction pattern from the area marked in Fig. a (b). Computer simulation of electron diffraction for M C in [121] 23 6 zone axis (c). EDS spectrum from the area marked in a (d) Fig. 5 The secondary electron (SE) images of the Sanicro 25 microstructure after 1000 h ageing at a 700 °C and b 750 °C by relatively low thermodynamical stability [14], which ageing time, which is reflected in the increase in the wid- results in them tending to grow relatively quickly (Fig. 6) ening of grain boundaries. and form a so-called continuous grid at the grain bounda-The M C carbides are also observed at the twin bounda- 23 6 ries (Figs.  3, 5). The size and relative amount of parti- ries. Still, their precipitation takes place at a later stage of cles precipitated at the grain boundaries increase with the ageing due to the lower energy of these boundaries (Fig. 3). The free surface energy of the incoherent twin boundary 1 3 149 Page 6 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 6 The average diameter of M C precipitates in Sanicro 23 6 25 as a function of ageing time represents approximately 0.7 of the large-angle boundary. from the high coagulability of the σ phase (Fig.  8). The In contrast, the coherent boundary represents approximately increase in the size of the σ phase is mainly achieved by the 0.2–0.3 [15]. consumption of M C carbides [15]. The disappearance of 23 6 For longer ageing times, both at 700 and 750  °C, the M C carbides ‘releases’ C and/or N atoms, and the lack of 23 6 occurrence of morphologically different precipitates are solubility of these elements in σ phase leads to their enrich- observed at the grain boundaries. These particles were iden- ment of them at the near-boundary micro-areas. This results tified as intermetallic σ phase (Fig.  7), which were initially in the precipitation of M C carbides within the grains and 23 6 observed at the intersection of three-grain boundaries (which at the ends of the twin boundaries [3]. However, according to gather in the σ phase) as preferred sites for nucleation and [17], the growth of the σ phase at the cost of M C carbides 23 6 growth [15]. Precipitation of σ phase particles at the inter- results in the diffusion of carbon at the grain boundaries and face of three-grain boundaries as the preferred places in aus- a further increase in the size of other carbides of this type. tenitic steels was also reported by Zieliński [3] and Sourmail In addition to M C carbides, Laves phase and σ phase 23 6 [15]. The nucleation rate and σ phase growth in austenitic precipitated at the grain boundaries at the longest age- steels depend mainly on the chromium content, where the ing times, and G phase precipitates were also revealed higher content, the faster the precipitation of this phase. The (Fig.  9). Precipitations of the G phase in the tested steel rate of nucleation and the increase in the size of the σ phase were observed in the neighborhood of M C carbides. Pre- 23 6 precipitates are influenced by the content of carbide-forming cipitates of the G phase in the tested steel were rich in iron, elements such as titanium or niobium, as well as the content nickel and chromium, but also in silicon (Fig. 9c). As in the of silicon [3, 16]. The amount and size of precipitates of this case of the σ phase, silicon also strongly affects the process phase increase with increasing ageing time, which results of G phase precipitation. An increase in the content of this Fig. 7 The secondary electron (SE) images of the Sanicro 25 microstructure after a 20,000 h ageing at 750 °C and b 30,000 h ageing at 700 °C 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 7 of 15 149 Fig. 8 Percentage share of σ phase precipitates in Sanicro 25 as a function of ageing time at 700 °C (blue) and 750 °C (red) Fig. 9 The G phase precipitate in Sanicro 25 after 30,000 h ageing at 750 °C. STEM-BF image (a). Selected area elec- tron diffraction pattern from the area marked in a (b). Computer simulation of electron diffrac- tion for Cr Ni Si in [011] zone 6 16 7 axis (c). EDS spectrum from the area marked in a (d) element in steel leads to a shortening of the incubation time were reported by Purzyńska [18]. According to the authors of G phase precipitation [17]. of this paper [18], the precipitation of the G phase was Precipitations of the G phase in austenitic 321H steel after related to the disappearance of M C carbides precipitated 23 6 a long-term operation, in the order of 200,000 h at 540 °C, at grain boundaries. Also, in [17], the authors indicate that 1 3 149 Page 8 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 the G phase precipitates in Ti and Nb-stabilized austenitic Cu to precipitate from the matrix at an ageing temperature. steels due to the in situ transformation of M C since the Also, ε_Cu /matrix low interphase energy lead to the quick 23 6 G phase and M C carbide have similar lattice constants. formation of many particles [21]. The interphase energy of 23 6 However, Vache [19] found the MC particles could trans- ε_Cu precipitates coherent with the matrix, which is approx- form into a G phase enriched with silicon and nickel atoms. imately 0.017 J/m [20]. Thus, according to [21], it depends In addition to precipitation processes at the grain/twin on manganese segregation to the interface of this phase and boundaries, the ageing of Sanicro 25 also contributed to the changes from 8.1 to 16.7 J/m after 10 and 10,000 h of age- precipitation of secondary phases that are different in terms ing of Sanicro 25 steel at 700 °C, respectively. of dispersion and morphology within the grains. These par- The ε_Cu precipitates revealed in the tested steel were ticles' relative amount and size increase with temperature rich in copper but contained iron, nickel and chromium and ageing time. The precipitates observed within the grains (Fig. 10c, d). The work [20] showed that the chemical com- were distributed both randomly and systematically. In auste- position of ε_Cu particles changes with ageing time. In the nitic steels containing copper, the first precipitates occurring initial ageing period, these particles may contain only up to within the grains are those rich in this element, referred to 20% of copper atoms, which increases to 90%. as ε_Cu (Fig. 10). The precipitation of numerous, excellent, In addition to the ε_Cu precipitates inside the grains, coherent Cu particles in austenitic steels takes place within a fine-dispersive secondary Z-phase was revealed in the the first hours of ageing [20]. The fast precipitation process microstructure of the tested steel (Fig. 11). Dispersive pre- 22 −3 and the high density of these precipitates (approx. 10  m cipitations of ε_Cu and Z-phase in aged Sanicro 25 steel [11]) in the steel microstructure are likely because of the were observed by both Zurek [6], Cempura [9] and Zhou low solubility of Cu in the steel. Heat treatment of austenitic [10]. Secondary Z-phase precipitates are mainly observed steels contributes to the supersaturation of the solid solu- within the grains on dislocations, which not only leads to tion with Cu, which will develop a high driving force for the dislocation pinning but also results in the formation of Fig. 10 The ε_Cu precipitate in Sanicro 25 steel after 1000 h ageing at 700 °C. STEM- HAADF image (a). Cu (red) and Fe (green) EDS distribu- tion map (b). STEM-HAADF image of a single precipitate (c). Analysis of changes in chemi- cal composition along the line marked in red in Fig. c (d) 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 9 of 15 149 Fig. 11 The Z-phase pre- cipitate in Sanicro 25 steel after 30,000 h ageing at 700 °C. STEM-BF image (a). Selected area electron diffraction pattern from the area marked in a (b). Computer simulation of electron diffraction for NbCrN in [110] zone axis (c). EDS spectrum from the area marked in a (d) the characteristic arrangement of these particles (Fig. 12). The high dispersion and high stability of these secondary particles contribute to their beneficial effect on both the strength properties and creep resistance. The precipitation of the Z-phase in the microstructure of austenitic steels is mainly related to the in situ transformation of metastable NbX particles into the higher stability of the Z-phase [22]. The Laves phase precipitates are observed not only at the grain boundaries, but also within the grains as particles nucleating independently (Fig. 13). The shape of these pre- cipitates—needle-like shape is related to the compromise between minimum distortion energy and the total surface energy, which translates into the precipitate/matrix inter- phase energy. In turn, the value of this energy affects the stability of the Laves phase precipitates coagulability [23]. Precipitations of the Laves phase inside grains in Sanicro 25 steel were observed by Zurek [6] and Rutkowski [7]. Zurek [6] reported particles nucleating independently inside grains. On the other hand, in Rutkowski [7], precipitations of the Laves phase nucleating heterogeneously on particles rich in niobium were reported. However, Suo et al. [13] showed Fig. 12 The TEM images of the Sanicro 25 substructure after these particles' presence in Sanicro 25 steel after 5665 h 20,000 h ageing at 700 °C 1 3 149 Page 10 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 13 The Laves phase in Sanicro 25 steel after 1000 h ageing at 750 °C. TEM-BF image (a), Selected Area Elec- tron Diffraction pattern from the area marked in a, characteristic for a stacking fault structure (b). EDS spectrum (c). Model of the Laves phase unit cell (Fe—yel- low, W – grey) (d) of creep at 700 °C. Precipitations of the Laves phase were In addition to the precipitates described above, compound observed on the grain boundaries near the M C carbides complexes of precipitates were revealed in the test steel, 23 6 and inside as particles nucleating independently and nucle- consisting of primary Z-phase precipitates, Laves phase, and ating on the primary precipitates of the Z-phase. Stress and M C carbides (Fig. 14), where the M C particles and 23 6 23 6 enrichment of the areas near the M C carbides in silicon the Laves phase nucleate heterogeneously on the primary 23 6 promoted phase formation [13]. Z-phase precipitates. Similar heterogeneous nucleation and Fig. 14 The STEM-BF image of the Sanicro 25 substructure after 10,000 h ageing at 700 °C (a). Enlargement of the middle sec- tion showing a single precipitate with a complex structure (b) 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 11 of 15 149 growth of M C carbides and Laves phase precipitates on complexes (Fig. 14) and the precipitation and growth of the 23 6 Z-phase primary particles were reported by Zurek [6] and σ phase (Fig. 7) and also Laves phase, resulting in the forma- Rutkowski [7] after oxidizing at 700 °C. Also, Suo [13] tion and growth of the PFZ. The width of the PFZ depends observed the formation of complex precipitates based on not only on the service/ ageing temperature, type and stabil- Z-phase primary particles in Sanicro 25 steel after creep. ity of precipitates at the grain boundaries or the diffusion The most promising nucleation sites are grain bounda- rate of the dominant component of the particles precipitated ries and precipitates (carbides and nitride). Heterogeneous at the boundaries [24], but also on the construction of its nucleation of the secondary phase particles on the primary grain boundary [28]. Z-phase precipitates may result from favorable crystalline relationships among the networks of precipitates, fluctua- 3.3 Properties of Sanicro 25 steel tions in chemical composition, and higher activation energy for nucleation at the inter-crystalline boundaries compared Austenitic steels, including creep-resistant austenitic steels, to that inside the grain. The presence of precipitate-free are delivered in a supersaturated condition, which is reflected zones (PFZs) near the grain boundaries was also revealed in the relatively low strength properties and good plasticity in the tested steel—Figs. 14a, 15. The zones are observed and toughness of these steels, which is due to the dominant mainly in precipitation-hardened materials, e.g., austenitic solid solution-strengthening mechanism. [24] and ferritic [25] stainless steel, and also titanium [26] In general, the increase in the strength properties is linked and aluminum [27] alloys. PFZs occur in the alloy because to the reduction of dislocation mobility. In austenitic steels, grain boundary precipitates coarsen more rapidly than those the important mechanism causing the boundary growth is in bulk, forcing the elements from these areas to diffuse to the solid solution-strengthening mechanism involving the growing precipitates. The formation of PFZs in Super 304H precipitation of secondary dispersive phases. The impact of steel was associated with the precipitation and growth of the this mechanism is mainly related to the type, size, shape and σ-phase particles at the grain boundaries [24]. The increase dispersion of these secondary particles. At equal volume in the size of the σ phase causes the consumption of M C fractions, particles of nanometric dimensions (but larger 23 6 carbides that precipitated at the grain boundaries near this than the critical average radius) and precipitates with a plate phase. At the same time, the aggregation of the Cu phase at shape rather than a round shape increase the hardening level the interface between the σ phase and the austenitic matrix [29]. From the point of view of both the interaction of the occurs. However, the work [25] demonstrated that the Laves precipitates with dislocations and their thermodynamic phase coagulating at the grain boundaries contributed to the stability, the expected presence of particles coherent or formation of PFZs. semi-coherent with the matrix is essential. There is a high- In the test steel, we believed there is most likely a combi- energy strain between the secondary particles in the grain nation of these mechanisms, i.e., the creation of compound that remain coherent and/or semi-coherent with the matrix, which can strongly affect the block dislocation slip during plastic deformation [29]. The ageing process of the tested steel increased in yield stress (YS) and tensile strength (TS), as well as hardness (Figs.  16, 17), regardless of the ageing temperature. The increase in the value of these properties should be mainly caused by the precipitation strengthening of the ε_Cu and Z-phase particles (Figs. 10, 11) precipitate within the grains, and also the M C carbides and Laves phase are observed 23 6 at the grain boundaries (Figs. 4, 6). The secondary dispersive precipitates within the grains constitute a considerable barrier to the free movement of dis- locations and, to a predominant extent, contribute to signifi- cant hardening of the alloy (as precipitates in creep-resistant austenitic steels are too hard to be cut by dislocations [16]). The precipitates, with a coherent (semi-coherent) boundary and nanometric dispersion dimensions, mainly the ε_Cu and Z-phase, contribute to significant hardening of the test alloy. The pinning force of the Z-phase and ε_Cu at the initial ageing stage is approximately 130 and 65 MPa, respectively Fig. 15 The PFZ area observed in Sanicro 25 steel after 10,000 h age- [11]. In turn, for M C carbide, this value is approximately ing at 700 °C; STEM-BF image 23 6 1 3 149 Page 12 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 16 Change in strength and plastic properties of Sanicro 25 steel after long-term ageing at 700 °C, determined at room temperature Fig. 17 Change in strength and plastic properties of Sanicro 25 steel after long-term ageing at 750 °C, determined at room temperature 30 MPa. This translates into 58, 28 and 9% fractions of the should be assumed that the increase in the strength proper- Z-phase, ε_Cu particles and M C carbides in the precipi- ties of the tested steel is mainly due to the ε_Cu particles and 23 6 tation hardening of the steel, respectively [11]. However, the precipitation of the Z-phase. according to [30], the Z-phase precipitates are responsible However, for longer holding times, the secondary dis- for approximately 75% of the hardening in HR3C steel. In persive Z-phase particles show a relatively more disper- work [21], he showed that the important precipitate in Sani- sive form and higher stability compared to ε_Cu particles. cro 25 steel is ε_Cu particles. These divisions constitute an It is also important that the Z-phase particles precipitate impact as a barrier to distribution movement depending on on dislocations (Fig. 12), resulting in their piling up. A the volume fraction and the average diameter. Ɛ_Cu parti- significant increase in the size of ε_Cu precipitates is cles, at the initial stage of ageing, act through the cutting observed after approximately 3000 h of ageing at 700 °C, mechanism and later through the bypassing mechanism due accompanied by a decrease in the density of precipitates to their size being smaller than the critical average radius by Oswald ripening phenomena [11]. The increase in the (which is 13 nm for such particles). This work [21] also size of ε_Cu particles in Sanicro 25 steel aged for 5000 h indicated the negligibly small role of M C carbides in the at 700 °C was also reported in [8]. The increase in the 23 6 hardening of this material. Compared to the Z-phase precipi- size of these particles results in a significant reduction tates and ε_Cu particles, the pinning force of M C carbides of the pinning effect of ε_Cu particles by approximately 23 6 is 10 and 27 times lower, respectively [30]. Therefore, it 50% while increasing the pinning force of the Z-phase 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 13 of 15 149 and M C by 14 and 5%, respectively [10]. The reduc- Furthermore, the softness of a PFZ compared to the adja- 23 6 tion of hardening due to the coagulation of ε_Cu parti- cent zones is much different, so cracks are likely to nucleate cles is also compensated by the Laves phase precipitating early at the boundaries. This leads to intergranular fracture, within the grains (Fig. 6). The precipitation hardening by weakens the ductility, and reduces the tensile strength [32]. the Laves phase particles depends on the Si content in However, according to [26], in the alloy with a PFZ, the the steel. It can range between 41.1 and 72.8 MPa [31]. ease with which dislocations are generated, and the resulting The increase in the volume fraction of M C carbides, dislocation pile-ups, lead to the reduction of the yield stress 23 6 and the precipitation of the σ phase, can also counterbal- in this area. In turn, [27] indicates that a PFZ in the material ance a small extent, the coagulation of dispersive particles does not affect yield strength. However, it does contribute [15]. A similar effect was seen in aged HR3C steel [3 ]. It to the reduction of ductility. should be assumed that the Laves phase precipitates at the grain boundaries also positively impact the strength [14]. Therefore, the continuous increase in the strength proper- ties and hardness is most likely observed for an ageing 4 Conclusions temperature of 700 °C (Fig. 16). In turn, for 750 °C, the increase in the strength properties and hardness was seen In conclusion, investigations into the changes in the micro- until the ageing time of 5000 h, whereas further holding structure and mechanical properties of Sanicro 25 austenitic leads to an over-ageing effect (Fig.  17). Raising the ageing steel after ageing at 700 and 750 °C at ageing times up to temperature increases the rate of the coagulation since the 30,000 h have been conducted. The tests performed allow activity of substitution solute atoms increases. This leads the following conclusions to be drawn: to an increase in the size of particles (Figs. 6 and 8) and, according to Ostwald's rule of ripening, takes place at the 1. In the as-received condition, the primary NbC and expense of the number of particles and increases the free Z-phase particles are observed. However, long-term age- path of particle motion, which translates into a reduction ing leads to the precipitation of secondary phase parti- of the pinning force of particles. The result of this is a cles in the steel microstructure, which were found to be progressive reduction in strength properties. M C , Laves phase, σ phase, and G phase at the grain 23 6 Regardless of the ageing temperature, changes in the boundaries, and ε_Cu, Z-phase, and Laves phase within strength properties and hardness are accompanied by a the grains. At the same time, compound complexes of plasticity drop, specifically elongation. The reduction of precipitates based on the primary Z-phase precipitates plasticity of the test steel is related to the changes in the were revealed. morphology of secondary precipitates. The precipitation of 2. In the steel microstructure, the presence of the PFZ was the M C carbides and Laves phase particles at the bounda- observed in the near-boundary areas of the grains for 23 6 ries contributes to a reduction in the strength cohesion of material ageing at both 700 and 750 °C. The formation the grain boundaries and are easier to nucleate and spread of the PFZ in the tested steel was related to the precipita- along the grain boundaries. In addition, M C carbides tion processes at the grain boundaries and in the border 23 6 and Laves phase particles at the grain boundaries promote areas. faster nucleation of cavities. Also, the precipitates within the 3. The increase in strength properties, the effect of over- grains themselves have a negative effect on plasticity. At the ageing and the decrease in the plastic properties of the interphase boundary of primary precipitates, easy nuclea- aged Sanicro 25 steel was associated with the precipita- tion of microcracks may occur, while the needle morphology tion and changes in the morphology of the secondary of Laves phase promotes a concentration of stress and the particles. nucleation of cavities [31]. Also, the recovery and recrystal- lization of the matrix that takes place during the ageing and Acknowledgements The results in this publication were obtained as manifests themselves, among other things, in an increase a part of research co-financed by the National Science Centre under in the grain size and decrease in the number of twins, have contract 2011/01/D/ST8/07219—Project: “Creep test application to model lifetime of materials for modern power generation industry”, and a negative effect not only on plasticity or ductility but also co-financed rector's grant in the area of scientific research and develop- reduce the strength. Also, PFZs (Figs. 14a, 15) observed in ment works, Silesian University of Technology, 10/010/RGJ23/1142. the steel microstructure impact the mechanical properties. The absence of precipitates in this area indicates that it most Author contribution MS: conceptualization, writing—original draft, investigation, resources. AZ: formal analysis, visualization, writ- likely has a lower hardness than inside the grain. Therefore, ing—review and editing. GG: formal analysis, investigation, writ- the preferential deformation occurs within PFZ in the initial ing—review. MP: investigation, data analysis. HP: investigation, data stage of deformation. This causes lower elongation regard- analysis. FN: investigation. less of the level of proof stress. 1 3 149 Page 14 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Funding Narodowe Centrum Nauki, 2011/01/D/ST8/07219, Adam 9. Czempura G, Gil A, Aguero A, Gutierrez M, Kruk A, Czyrska- Zieliński, Silesian University of Technology, 10/010/RGJ23/1142, Filemonowicz A. Microstructural studies of the scale on Sanicro Marek Sroka. 25 after 25 000 h of oxidation in steam using advanced electron microscopy techniques. Surf Coat Tech. 2019;377:124901. https:// Data availability The raw/processed data required to reproduce these doi. org/ 10. 1016/j. surfc oat. 2019. 124901. findings cannot be shared at this time as the data also form part of an 10. Zhou R, Zhu L, Liu Y, Lu Z, Chen L, Ma X. Microstructural ongoing study. evolution and the effect on hardness of Sanicro 25 welded joint base after creep at 973K. J Mater Sci. 2017;52:6161–72. https:// doi. org/ 10. 1007/ s10853- 017- 0758-6. Declarations 11. Zhou R, Zhu L, Yang Y, Lu Z, Chen L. Microstructural evolu- tion and the effect on hardness and impact toughness of Sani- Conflict of interest The authors declare that they have no known com- cro 25 welded joints after aging at 973K. Metall Mater Trans A. peting financial interests or personal relationships that could have ap- 2018;49:6290–307. https:// doi. org/ 10. 1007/ s11661- 018- 4906-7. peared to influence the work reported in this paper. 12. 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Evolution of the microstructure and mechanical properties of Sanicro 25 austenitic stainless steel after long-term ageing

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10.1007/s43452-023-00690-y
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Abstract

A newly developed heat-resistant austenitic steel, Sanicro 25 is currently considered the leading candidate material for an advanced ultra-supercritical installation. The test material was subjected to long-term ageing (up to 30,000 h) at 700 and 750 °C, after which investigations into the microstructure, identification of precipitates, and testing of mechanical proper - ties were conducted. Sanicro 25 had an austenitic microstructure with annealed twins and numerous large primary NbX and Z-phase precipitates in the as-received condition. It was found that the long-term ageing of the steel resulted in numerous precipitation processes. For example, M C carbides, Laves, σ and G phases occurred at the grain boundaries. However, 23 6 Z-phase precipitates, ε_Cu particles, and Laves phase were observed inside the grains. At the same time, compound com- plexes of precipitates based on the primary Z-phase precipitates were revealed in the microstructure. The ageing process increased the particle size of M C carbides and the σ phase. After longer ageing times, a precipitate-free zone (PFZ) near 23 6 the grain boundaries was observed. The precipitation processes initially lead to an increase in the strength properties of the steel. However, after 5000 h, an over-ageing effect was observed at 750 °C, which was not observed at 700 °C. Keywords Sanicro 25 steel · Microstructure · Precipitation · Ageing · Mechanical properties 1 Introduction in modern conventional power industries, including steels with an austenitic matrix [1]. The overall development of the power industry stimulates The scope of using modern materials for boiler com- the growth of new creep-resistant steels. In addition, eco- ponents, including new-generation ferritic and austenitic nomic and environmental considerations and continuous steels and nickel superalloys, depends on the temperature improvements in the thermal efficiency of power units have and stress conditions [2]. Bainitic and heat-resistant marten- forced the introduction of newer and newer materials for use sitic steels of 9–12% chromium, currently most commonly used in the power industry, are designed for operation at temperatures not exceeding 620–630 °C. However, due to the low corrosion resistance of 9% Cr steels and the low * M. Sroka microstructural stability of 12% Cr steels, the use of creep- marek.sroka@polsl.pl resistant austenitic steels is required at higher parameters. Austenitic steels show higher creep strength than ferritic Department of Engineering Materials and Biomaterials, Silesian University of Technology, Ul. Konarskiego 18a, steels and higher heat resistance and can be used at tempera- 44-100 Gliwice, Poland tures up to around 700 °C [3]. Compared to the currently Łukasiewicz Research Network - Institute for Ferrous available austenitic steels and nickel alloys (which can be Metallurgy, K. Miarki 12-14, 44-100 Gliwice, Poland used at above 700 °C but are considerably more expensive), Department of Materials Engineering, Czestochowa the best alternative is Sanicro 25 stainless steel, which can University of Technology, Al. Armii Krajowej 19, significantly reduce the investment cost of the plant [4 ]. 42-200 Częstochowa, Poland Sanicro 25 steel (X7NiCrWCuCoNbNB25) was devel- Materials Research Laboratory, Silesian University oped under the European Therme AD700 program by AB of Technology, Ul. Konarskiego 18a, 44-100 Gliwice, Poland Sandvik Material Technology in Sweden. Like other aus- Department of Materials Engineering, University of Zilina, tenitic steels, Sanicro 25 has high plastic properties and Univerzitná, 8215/1, 010 26 Žilina, Slovakia Vol.:(0123456789) 1 3 149 Page 2 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 ductility with relatively low strength properties. These prop- of precipitation processes in Sanicro 25 for ageing /service erties are mainly linked to the solid solution-strengthening times longer than 25,000 h. Studies on the impact of longer mechanism of steel, primarily provided by nitrogen, tung- ageing times are essential to obtain the complete picture of sten, and cobalt atoms. In addition, the balanced chemical changes in the microstructure and understand the proper- composition of Sanicro 25 allows significant resistance to ties of steels intended for long-term operation. This article high-temperature corrosion and oxidation. It gives a higher presents the results of investigations on the microstructure creep strength compared to other austenitic steels, e.g., and properties of Sanicro 25 steel after 30,000 h ageing at Super 304H and HR3C [4]. 700 and 750 °C. The creep strength of austenitic steels is associated with two strengthening mechanisms: solid solution strengthen- ing and precipitation hardening. The precipitation harden- 2 Material and research methodology ing mechanism is dominant during service under creep con- ditions. It is related to the presence of particles (carbides, Investigations were carried out on specimens of nitrides, intermetallic phases) interacting with dislocations ϕ38 × 8.8 mm taken from superheater coils in the form of a in the microstructure provided by the so-called carbide- tube along the axial direction and were composed of creep- forming elements (i.e., Cr, niobium, Nb, and tungsten, W, as resistant austenitic steel Sanicro 25 (X7NiCrWCuCoN- well as the addition of copper, Cu, which is released as ε_Cu bNB25-23-3-3-3-2). Details of the chemical composition nanoparticles). The value of creep strength determined for a of the test steel in the as-received condition are presented 100,000 h service at 700ºC is 95 MPa, comparable to that of in Table 1. the HR6W nickel-base alloy. The high functional properties The investigations of microstructure and mechanical of Sanicro 25 make it one of the primary materials eligible properties were carried out on steel in the as-received con- for implementation in supercritical and ultra-supercritical dition at both 700 and 750 °C. Various ageing times were power units with an efficiency of approximately 50% [4 ]. investigated at these temperatures and were chosen to be In previous literature, two main directions of the research 1,000, 5,000, 10,000, 20,000, and 30,000 h. All tests were on Sanicro 25 steel were observed—investigations of creep carried out in the air. and low-cycle fatigue [5] and investigations of corrosion [6] The steel microstructure was analyzed using an Inspect and oxidation resistance in steam [7]. In both cases, one of F scanning electron microscope (SEM). The samples were the main focuses was the analysis of changes in the micro- prepared as metallographic cross-sections. The samples were structure of Sanicro 25. The low-cycle fatigue resistance ground on papers and polished on polishing wheels. Then it tests were conducted at both room temperature and 700 °C. was electrolytically etched in 50% HNO . At room temperature, Sanicro 25 is characterized by cyclic TEM samples for phase identification of precipitates in hardening at the initial stage, followed by cyclic softening, Sanicro 25 were prepared in the form of thin foils and lamel- which is related to the specificity of the deformation of aus- lae. Thin foils were made from the bulk samples by cutting tenitic steel and the supersaturation state of the Sanicro 25 into plate forms, mechanically polished and ion milled by structure. At 700 °C, Sanicro 25 shows the cyclic hardening Ar plasma at 5 keV to create a hole at the center. Lamellae effect associated with precipitation of the Z-phase inside the were made by the focused ion beam (FIB) technique using grains and M C carbides at the grain boundaries. Accord- an SEM/Ga-FIB Helios NanoLabTM 600i microscope (FEI 23 6 ing to the researchers, these carbides were mainly observed Company). TEM investigations were performed using an S/ in the test steel at the low-angle grain boundaries. They were TEM Titan 80–300 microscope (FEI Company) equipped the reason for the formation of Cr-depleted areas near the with a Cetcor Cs probe corrector (CEOS, Germany) and boundaries. Additionally, according to [5], the cyclic hard- EDS spectrometer for chemical composition analysis. Crys- ening of Sanicro 25 at 700 °C is related to the presence of tal Maker and Single Crystal software (version 10.4.1) were numerous dispersive ε_Cu and NbC precipitates. used to simulate the crystal structure and diffraction patterns. To this date, research carried out on Sanicro 25 mainly The qualitative and quantitative analysis of the chemi- covers issues related to low-cycle fatigue, weldability, and cal composition of the test steel was performed using the corrosion resistance. Tests were carried out on material aged EDS energy-dispersive X-ray spectrometer (from EDAX), for no more than 25,000 h. However, there is currently no which is attached to TITAN 80–300 high-resolution electron information on the stability of microstructure or the course microscope. Table 1 Chemical composition C Si Mn P S Cu Cr Ni W Co Nb B of the Sanicro 25 test steel, %wt 0.06 0.25 0.50 0.01 < 0.01 2.90 23.0 24.1 3.20 1.40 0.40 0.005 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 3 of 15 149 The mechanical properties were determined by hardness measurements and static tensile tests at room temperature. The hardness measurement was performed by the Vickers method: HV10, using the Swiss Max 300 hardness tester in accordance with EN ISO 6507-1. Mechanical property measurements were carried out on specimens aged 1,000, 10,000, 20,000, and 30,000 h. Investigations into the strength properties of Sanicro 25 were also carried out using a static tensile test according to EN 10002-1:2002 at room temperature and, according to EN ISO 6892-2:2011, at elevated temperature on flat test specimens. The tests at room temperature were performed using a Zwick 200 kN tensile testing machine, and tests at 700 °C were performed using an Amsler 200 kN tensile test- ing machine in the load range of 40 kN. The computer-based image analysis, including meas- uring the average diameter of M C carbide and σ-phase 23 6 precipitates, was performed using Image ProPlus software. The calibration was performed using a scale marker on the Fig. 1 The secondary electron (SE) image of the Sanicro 25 micro- microstructure images, assuming a calibration factor of 1 structure in the as-received condition pixel = 0.040 µm. All image analysis was carried out on pre- conditioned binary microstructure images. coarse-grained steels [3]. In addition, fine grain in these alloys also positively affects corrosion resistance [12]. 3 Results and discussion 3.2 Microstructure of Sanicro 25 after ageing 3.1 Initial microstructure of Sanicro 25 steel The primary microstructure degradation mechanism of The Sanicro 25 steel, as-received, had a fine-grained aus- creep-resistant austenitic steels is the precipitation processes occurring at the grain and twin boundaries and within the tenitic microstructure with numerous annealing twins and primary precipitates (Fig. 1). The grain size in the tested grains. The morphology and type of precipitates in the auste- nitic steel, both in the as-received condition and after ageing/ steel was determined to be 7 according to the ASTM stand- ard scale [8] (which corresponds to an average diameter of service, depending on the chemical composition of the steel itself, the heat (thermomechanical) treatment parameters, 31.2 μm). The precipitates observed in the microstructure of Sanicro 25 (Fig. 1) were identified as NbX carbonitrides and the holding temperature and time. The microstructure of Sanicro 25 steel after 1000 and 30,000 h ageing at 700 and NbCrN (Z-phase) precipitates (Fig. 2). These precipi- tates are considered primary particles that nucleate during and 750 °C is shown in Fig. 3. As shown in Fig. 3, many secondary phases precipitated the solidification process and grow during the production process. They are, therefore, mainly observed at or near the both within and at the grain boundaries, where, at the lat- ter, particles formed a so-called continuous grid of precipi- grain boundaries (Fig. 1). Primary precipitates of NbX and Z-phase in the microstructure of Sanicro 25 steel as deliv- tates. As high-energy surface defects and grain misalign- ment areas allow faster diffusion compared to that within ered were observed by Rutkowski [7] and Czempura [9]. On the other hand, Zurek [6] observed only particles of the the grains, grain boundaries are preferred sites for the pre- cipitation of secondary phases. At the grain boundaries Z-phase in this steel in the as-received state, while Zhou [10, 11] reported only particles of the MX type. The pur- in the tested steel, the presence of M C carbides was 23 6 revealed (Figs. 4, 5), and precipitation of the Laves phase pose of primary precipitates in austenitic steels is mainly to bind carbon and/or nitrogen atoms to limit the possibility was also shown (Fig. 5). Similar results, i.e., the presence of M C carbides and Laves phase precipitates at grain of precipitating particles rich in chromium, but also these 23 6 precipitates limit grain growth. boundaries, were reported by Zurek [6], and Rutkowski [7] who investigated the microstructure Sanicro 25 after Fine grain and numerous twins in the microstructure in austenitic steels have a positive effect on obtaining oxidation at 700 °C. However, Zhou [11], who examined the steel after ageing and creep at 700  °C, showed the high basic mechanical properties (yield strength, elonga- tion, impact energy) with creep resistance comparable to presence of only M C carbides at the grain boundaries. 23 6 1 3 149 Page 4 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 2 STEM-BF image of the Z-phase precipitate (a) and SAED diffraction pattern in [221] zone axis (b) Fig. 3 The secondary electron (SE) images of the Sanicro 25 microstructures after ageing for a 1,000 h at 700 °C, b 1,000 h at 750 °C, c 30,000 h at 700 °C, and d 30,000 h at 750 °C The preferential precipitation of M C carbides at grain the M C carbides (Fig. 5b). In austenitic steel, the Laves 23 6 23 6 boundaries was associated with the limited carbon sol- phase particles nucleated in the neighborhood of either ubility in the austenitic matrix and the short-range dif- Si-enriched spots present in the bulky M C carbides or 23 6 fusion of chromium atoms [3]. In turn, the Laves phase to the grain boundary areas rich in silicon. This made the in austenitic steels can be precipitated dependently and bulky M C carbides gradually divide into small pieces, 23 6 independently as neighborhoods of other phases within eventually forming a refined mixture of M C and Laves 23 6 the grains and at the grain boundaries. Precipitations of phase at the grain boundaries [13]. The M C carbides, 23 6 the Laves phase at grain boundaries were observed near in contrast, Laves phase precipitates, are characterized 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 5 of 15 149 Fig. 4 The M C precipitate in 23 6 Sanicro 25 steel after 10,000 h ageing at 750 °C. STEM- BF image (a). Selected area electron diffraction pattern from the area marked in Fig. a (b). Computer simulation of electron diffraction for M C in [121] 23 6 zone axis (c). EDS spectrum from the area marked in a (d) Fig. 5 The secondary electron (SE) images of the Sanicro 25 microstructure after 1000 h ageing at a 700 °C and b 750 °C by relatively low thermodynamical stability [14], which ageing time, which is reflected in the increase in the wid- results in them tending to grow relatively quickly (Fig. 6) ening of grain boundaries. and form a so-called continuous grid at the grain bounda-The M C carbides are also observed at the twin bounda- 23 6 ries (Figs.  3, 5). The size and relative amount of parti- ries. Still, their precipitation takes place at a later stage of cles precipitated at the grain boundaries increase with the ageing due to the lower energy of these boundaries (Fig. 3). The free surface energy of the incoherent twin boundary 1 3 149 Page 6 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 6 The average diameter of M C precipitates in Sanicro 23 6 25 as a function of ageing time represents approximately 0.7 of the large-angle boundary. from the high coagulability of the σ phase (Fig.  8). The In contrast, the coherent boundary represents approximately increase in the size of the σ phase is mainly achieved by the 0.2–0.3 [15]. consumption of M C carbides [15]. The disappearance of 23 6 For longer ageing times, both at 700 and 750  °C, the M C carbides ‘releases’ C and/or N atoms, and the lack of 23 6 occurrence of morphologically different precipitates are solubility of these elements in σ phase leads to their enrich- observed at the grain boundaries. These particles were iden- ment of them at the near-boundary micro-areas. This results tified as intermetallic σ phase (Fig.  7), which were initially in the precipitation of M C carbides within the grains and 23 6 observed at the intersection of three-grain boundaries (which at the ends of the twin boundaries [3]. However, according to gather in the σ phase) as preferred sites for nucleation and [17], the growth of the σ phase at the cost of M C carbides 23 6 growth [15]. Precipitation of σ phase particles at the inter- results in the diffusion of carbon at the grain boundaries and face of three-grain boundaries as the preferred places in aus- a further increase in the size of other carbides of this type. tenitic steels was also reported by Zieliński [3] and Sourmail In addition to M C carbides, Laves phase and σ phase 23 6 [15]. The nucleation rate and σ phase growth in austenitic precipitated at the grain boundaries at the longest age- steels depend mainly on the chromium content, where the ing times, and G phase precipitates were also revealed higher content, the faster the precipitation of this phase. The (Fig.  9). Precipitations of the G phase in the tested steel rate of nucleation and the increase in the size of the σ phase were observed in the neighborhood of M C carbides. Pre- 23 6 precipitates are influenced by the content of carbide-forming cipitates of the G phase in the tested steel were rich in iron, elements such as titanium or niobium, as well as the content nickel and chromium, but also in silicon (Fig. 9c). As in the of silicon [3, 16]. The amount and size of precipitates of this case of the σ phase, silicon also strongly affects the process phase increase with increasing ageing time, which results of G phase precipitation. An increase in the content of this Fig. 7 The secondary electron (SE) images of the Sanicro 25 microstructure after a 20,000 h ageing at 750 °C and b 30,000 h ageing at 700 °C 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 7 of 15 149 Fig. 8 Percentage share of σ phase precipitates in Sanicro 25 as a function of ageing time at 700 °C (blue) and 750 °C (red) Fig. 9 The G phase precipitate in Sanicro 25 after 30,000 h ageing at 750 °C. STEM-BF image (a). Selected area elec- tron diffraction pattern from the area marked in a (b). Computer simulation of electron diffrac- tion for Cr Ni Si in [011] zone 6 16 7 axis (c). EDS spectrum from the area marked in a (d) element in steel leads to a shortening of the incubation time were reported by Purzyńska [18]. According to the authors of G phase precipitation [17]. of this paper [18], the precipitation of the G phase was Precipitations of the G phase in austenitic 321H steel after related to the disappearance of M C carbides precipitated 23 6 a long-term operation, in the order of 200,000 h at 540 °C, at grain boundaries. Also, in [17], the authors indicate that 1 3 149 Page 8 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 the G phase precipitates in Ti and Nb-stabilized austenitic Cu to precipitate from the matrix at an ageing temperature. steels due to the in situ transformation of M C since the Also, ε_Cu /matrix low interphase energy lead to the quick 23 6 G phase and M C carbide have similar lattice constants. formation of many particles [21]. The interphase energy of 23 6 However, Vache [19] found the MC particles could trans- ε_Cu precipitates coherent with the matrix, which is approx- form into a G phase enriched with silicon and nickel atoms. imately 0.017 J/m [20]. Thus, according to [21], it depends In addition to precipitation processes at the grain/twin on manganese segregation to the interface of this phase and boundaries, the ageing of Sanicro 25 also contributed to the changes from 8.1 to 16.7 J/m after 10 and 10,000 h of age- precipitation of secondary phases that are different in terms ing of Sanicro 25 steel at 700 °C, respectively. of dispersion and morphology within the grains. These par- The ε_Cu precipitates revealed in the tested steel were ticles' relative amount and size increase with temperature rich in copper but contained iron, nickel and chromium and ageing time. The precipitates observed within the grains (Fig. 10c, d). The work [20] showed that the chemical com- were distributed both randomly and systematically. In auste- position of ε_Cu particles changes with ageing time. In the nitic steels containing copper, the first precipitates occurring initial ageing period, these particles may contain only up to within the grains are those rich in this element, referred to 20% of copper atoms, which increases to 90%. as ε_Cu (Fig. 10). The precipitation of numerous, excellent, In addition to the ε_Cu precipitates inside the grains, coherent Cu particles in austenitic steels takes place within a fine-dispersive secondary Z-phase was revealed in the the first hours of ageing [20]. The fast precipitation process microstructure of the tested steel (Fig. 11). Dispersive pre- 22 −3 and the high density of these precipitates (approx. 10  m cipitations of ε_Cu and Z-phase in aged Sanicro 25 steel [11]) in the steel microstructure are likely because of the were observed by both Zurek [6], Cempura [9] and Zhou low solubility of Cu in the steel. Heat treatment of austenitic [10]. Secondary Z-phase precipitates are mainly observed steels contributes to the supersaturation of the solid solu- within the grains on dislocations, which not only leads to tion with Cu, which will develop a high driving force for the dislocation pinning but also results in the formation of Fig. 10 The ε_Cu precipitate in Sanicro 25 steel after 1000 h ageing at 700 °C. STEM- HAADF image (a). Cu (red) and Fe (green) EDS distribu- tion map (b). STEM-HAADF image of a single precipitate (c). Analysis of changes in chemi- cal composition along the line marked in red in Fig. c (d) 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 9 of 15 149 Fig. 11 The Z-phase pre- cipitate in Sanicro 25 steel after 30,000 h ageing at 700 °C. STEM-BF image (a). Selected area electron diffraction pattern from the area marked in a (b). Computer simulation of electron diffraction for NbCrN in [110] zone axis (c). EDS spectrum from the area marked in a (d) the characteristic arrangement of these particles (Fig. 12). The high dispersion and high stability of these secondary particles contribute to their beneficial effect on both the strength properties and creep resistance. The precipitation of the Z-phase in the microstructure of austenitic steels is mainly related to the in situ transformation of metastable NbX particles into the higher stability of the Z-phase [22]. The Laves phase precipitates are observed not only at the grain boundaries, but also within the grains as particles nucleating independently (Fig. 13). The shape of these pre- cipitates—needle-like shape is related to the compromise between minimum distortion energy and the total surface energy, which translates into the precipitate/matrix inter- phase energy. In turn, the value of this energy affects the stability of the Laves phase precipitates coagulability [23]. Precipitations of the Laves phase inside grains in Sanicro 25 steel were observed by Zurek [6] and Rutkowski [7]. Zurek [6] reported particles nucleating independently inside grains. On the other hand, in Rutkowski [7], precipitations of the Laves phase nucleating heterogeneously on particles rich in niobium were reported. However, Suo et al. [13] showed Fig. 12 The TEM images of the Sanicro 25 substructure after these particles' presence in Sanicro 25 steel after 5665 h 20,000 h ageing at 700 °C 1 3 149 Page 10 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 13 The Laves phase in Sanicro 25 steel after 1000 h ageing at 750 °C. TEM-BF image (a), Selected Area Elec- tron Diffraction pattern from the area marked in a, characteristic for a stacking fault structure (b). EDS spectrum (c). Model of the Laves phase unit cell (Fe—yel- low, W – grey) (d) of creep at 700 °C. Precipitations of the Laves phase were In addition to the precipitates described above, compound observed on the grain boundaries near the M C carbides complexes of precipitates were revealed in the test steel, 23 6 and inside as particles nucleating independently and nucle- consisting of primary Z-phase precipitates, Laves phase, and ating on the primary precipitates of the Z-phase. Stress and M C carbides (Fig. 14), where the M C particles and 23 6 23 6 enrichment of the areas near the M C carbides in silicon the Laves phase nucleate heterogeneously on the primary 23 6 promoted phase formation [13]. Z-phase precipitates. Similar heterogeneous nucleation and Fig. 14 The STEM-BF image of the Sanicro 25 substructure after 10,000 h ageing at 700 °C (a). Enlargement of the middle sec- tion showing a single precipitate with a complex structure (b) 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 11 of 15 149 growth of M C carbides and Laves phase precipitates on complexes (Fig. 14) and the precipitation and growth of the 23 6 Z-phase primary particles were reported by Zurek [6] and σ phase (Fig. 7) and also Laves phase, resulting in the forma- Rutkowski [7] after oxidizing at 700 °C. Also, Suo [13] tion and growth of the PFZ. The width of the PFZ depends observed the formation of complex precipitates based on not only on the service/ ageing temperature, type and stabil- Z-phase primary particles in Sanicro 25 steel after creep. ity of precipitates at the grain boundaries or the diffusion The most promising nucleation sites are grain bounda- rate of the dominant component of the particles precipitated ries and precipitates (carbides and nitride). Heterogeneous at the boundaries [24], but also on the construction of its nucleation of the secondary phase particles on the primary grain boundary [28]. Z-phase precipitates may result from favorable crystalline relationships among the networks of precipitates, fluctua- 3.3 Properties of Sanicro 25 steel tions in chemical composition, and higher activation energy for nucleation at the inter-crystalline boundaries compared Austenitic steels, including creep-resistant austenitic steels, to that inside the grain. The presence of precipitate-free are delivered in a supersaturated condition, which is reflected zones (PFZs) near the grain boundaries was also revealed in the relatively low strength properties and good plasticity in the tested steel—Figs. 14a, 15. The zones are observed and toughness of these steels, which is due to the dominant mainly in precipitation-hardened materials, e.g., austenitic solid solution-strengthening mechanism. [24] and ferritic [25] stainless steel, and also titanium [26] In general, the increase in the strength properties is linked and aluminum [27] alloys. PFZs occur in the alloy because to the reduction of dislocation mobility. In austenitic steels, grain boundary precipitates coarsen more rapidly than those the important mechanism causing the boundary growth is in bulk, forcing the elements from these areas to diffuse to the solid solution-strengthening mechanism involving the growing precipitates. The formation of PFZs in Super 304H precipitation of secondary dispersive phases. The impact of steel was associated with the precipitation and growth of the this mechanism is mainly related to the type, size, shape and σ-phase particles at the grain boundaries [24]. The increase dispersion of these secondary particles. At equal volume in the size of the σ phase causes the consumption of M C fractions, particles of nanometric dimensions (but larger 23 6 carbides that precipitated at the grain boundaries near this than the critical average radius) and precipitates with a plate phase. At the same time, the aggregation of the Cu phase at shape rather than a round shape increase the hardening level the interface between the σ phase and the austenitic matrix [29]. From the point of view of both the interaction of the occurs. However, the work [25] demonstrated that the Laves precipitates with dislocations and their thermodynamic phase coagulating at the grain boundaries contributed to the stability, the expected presence of particles coherent or formation of PFZs. semi-coherent with the matrix is essential. There is a high- In the test steel, we believed there is most likely a combi- energy strain between the secondary particles in the grain nation of these mechanisms, i.e., the creation of compound that remain coherent and/or semi-coherent with the matrix, which can strongly affect the block dislocation slip during plastic deformation [29]. The ageing process of the tested steel increased in yield stress (YS) and tensile strength (TS), as well as hardness (Figs.  16, 17), regardless of the ageing temperature. The increase in the value of these properties should be mainly caused by the precipitation strengthening of the ε_Cu and Z-phase particles (Figs. 10, 11) precipitate within the grains, and also the M C carbides and Laves phase are observed 23 6 at the grain boundaries (Figs. 4, 6). The secondary dispersive precipitates within the grains constitute a considerable barrier to the free movement of dis- locations and, to a predominant extent, contribute to signifi- cant hardening of the alloy (as precipitates in creep-resistant austenitic steels are too hard to be cut by dislocations [16]). The precipitates, with a coherent (semi-coherent) boundary and nanometric dispersion dimensions, mainly the ε_Cu and Z-phase, contribute to significant hardening of the test alloy. The pinning force of the Z-phase and ε_Cu at the initial ageing stage is approximately 130 and 65 MPa, respectively Fig. 15 The PFZ area observed in Sanicro 25 steel after 10,000 h age- [11]. In turn, for M C carbide, this value is approximately ing at 700 °C; STEM-BF image 23 6 1 3 149 Page 12 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Fig. 16 Change in strength and plastic properties of Sanicro 25 steel after long-term ageing at 700 °C, determined at room temperature Fig. 17 Change in strength and plastic properties of Sanicro 25 steel after long-term ageing at 750 °C, determined at room temperature 30 MPa. This translates into 58, 28 and 9% fractions of the should be assumed that the increase in the strength proper- Z-phase, ε_Cu particles and M C carbides in the precipi- ties of the tested steel is mainly due to the ε_Cu particles and 23 6 tation hardening of the steel, respectively [11]. However, the precipitation of the Z-phase. according to [30], the Z-phase precipitates are responsible However, for longer holding times, the secondary dis- for approximately 75% of the hardening in HR3C steel. In persive Z-phase particles show a relatively more disper- work [21], he showed that the important precipitate in Sani- sive form and higher stability compared to ε_Cu particles. cro 25 steel is ε_Cu particles. These divisions constitute an It is also important that the Z-phase particles precipitate impact as a barrier to distribution movement depending on on dislocations (Fig. 12), resulting in their piling up. A the volume fraction and the average diameter. Ɛ_Cu parti- significant increase in the size of ε_Cu precipitates is cles, at the initial stage of ageing, act through the cutting observed after approximately 3000 h of ageing at 700 °C, mechanism and later through the bypassing mechanism due accompanied by a decrease in the density of precipitates to their size being smaller than the critical average radius by Oswald ripening phenomena [11]. The increase in the (which is 13 nm for such particles). This work [21] also size of ε_Cu particles in Sanicro 25 steel aged for 5000 h indicated the negligibly small role of M C carbides in the at 700 °C was also reported in [8]. The increase in the 23 6 hardening of this material. Compared to the Z-phase precipi- size of these particles results in a significant reduction tates and ε_Cu particles, the pinning force of M C carbides of the pinning effect of ε_Cu particles by approximately 23 6 is 10 and 27 times lower, respectively [30]. Therefore, it 50% while increasing the pinning force of the Z-phase 1 3 Archives of Civil and Mechanical Engineering (2023) 23:149 Page 13 of 15 149 and M C by 14 and 5%, respectively [10]. The reduc- Furthermore, the softness of a PFZ compared to the adja- 23 6 tion of hardening due to the coagulation of ε_Cu parti- cent zones is much different, so cracks are likely to nucleate cles is also compensated by the Laves phase precipitating early at the boundaries. This leads to intergranular fracture, within the grains (Fig. 6). The precipitation hardening by weakens the ductility, and reduces the tensile strength [32]. the Laves phase particles depends on the Si content in However, according to [26], in the alloy with a PFZ, the the steel. It can range between 41.1 and 72.8 MPa [31]. ease with which dislocations are generated, and the resulting The increase in the volume fraction of M C carbides, dislocation pile-ups, lead to the reduction of the yield stress 23 6 and the precipitation of the σ phase, can also counterbal- in this area. In turn, [27] indicates that a PFZ in the material ance a small extent, the coagulation of dispersive particles does not affect yield strength. However, it does contribute [15]. A similar effect was seen in aged HR3C steel [3 ]. It to the reduction of ductility. should be assumed that the Laves phase precipitates at the grain boundaries also positively impact the strength [14]. Therefore, the continuous increase in the strength proper- ties and hardness is most likely observed for an ageing 4 Conclusions temperature of 700 °C (Fig. 16). In turn, for 750 °C, the increase in the strength properties and hardness was seen In conclusion, investigations into the changes in the micro- until the ageing time of 5000 h, whereas further holding structure and mechanical properties of Sanicro 25 austenitic leads to an over-ageing effect (Fig.  17). Raising the ageing steel after ageing at 700 and 750 °C at ageing times up to temperature increases the rate of the coagulation since the 30,000 h have been conducted. The tests performed allow activity of substitution solute atoms increases. This leads the following conclusions to be drawn: to an increase in the size of particles (Figs. 6 and 8) and, according to Ostwald's rule of ripening, takes place at the 1. In the as-received condition, the primary NbC and expense of the number of particles and increases the free Z-phase particles are observed. However, long-term age- path of particle motion, which translates into a reduction ing leads to the precipitation of secondary phase parti- of the pinning force of particles. The result of this is a cles in the steel microstructure, which were found to be progressive reduction in strength properties. M C , Laves phase, σ phase, and G phase at the grain 23 6 Regardless of the ageing temperature, changes in the boundaries, and ε_Cu, Z-phase, and Laves phase within strength properties and hardness are accompanied by a the grains. At the same time, compound complexes of plasticity drop, specifically elongation. The reduction of precipitates based on the primary Z-phase precipitates plasticity of the test steel is related to the changes in the were revealed. morphology of secondary precipitates. The precipitation of 2. In the steel microstructure, the presence of the PFZ was the M C carbides and Laves phase particles at the bounda- observed in the near-boundary areas of the grains for 23 6 ries contributes to a reduction in the strength cohesion of material ageing at both 700 and 750 °C. The formation the grain boundaries and are easier to nucleate and spread of the PFZ in the tested steel was related to the precipita- along the grain boundaries. In addition, M C carbides tion processes at the grain boundaries and in the border 23 6 and Laves phase particles at the grain boundaries promote areas. faster nucleation of cavities. Also, the precipitates within the 3. The increase in strength properties, the effect of over- grains themselves have a negative effect on plasticity. At the ageing and the decrease in the plastic properties of the interphase boundary of primary precipitates, easy nuclea- aged Sanicro 25 steel was associated with the precipita- tion of microcracks may occur, while the needle morphology tion and changes in the morphology of the secondary of Laves phase promotes a concentration of stress and the particles. nucleation of cavities [31]. Also, the recovery and recrystal- lization of the matrix that takes place during the ageing and Acknowledgements The results in this publication were obtained as manifests themselves, among other things, in an increase a part of research co-financed by the National Science Centre under in the grain size and decrease in the number of twins, have contract 2011/01/D/ST8/07219—Project: “Creep test application to model lifetime of materials for modern power generation industry”, and a negative effect not only on plasticity or ductility but also co-financed rector's grant in the area of scientific research and develop- reduce the strength. Also, PFZs (Figs. 14a, 15) observed in ment works, Silesian University of Technology, 10/010/RGJ23/1142. the steel microstructure impact the mechanical properties. The absence of precipitates in this area indicates that it most Author contribution MS: conceptualization, writing—original draft, investigation, resources. AZ: formal analysis, visualization, writ- likely has a lower hardness than inside the grain. Therefore, ing—review and editing. GG: formal analysis, investigation, writ- the preferential deformation occurs within PFZ in the initial ing—review. MP: investigation, data analysis. HP: investigation, data stage of deformation. This causes lower elongation regard- analysis. FN: investigation. less of the level of proof stress. 1 3 149 Page 14 of 15 Archives of Civil and Mechanical Engineering (2023) 23:149 Funding Narodowe Centrum Nauki, 2011/01/D/ST8/07219, Adam 9. Czempura G, Gil A, Aguero A, Gutierrez M, Kruk A, Czyrska- Zieliński, Silesian University of Technology, 10/010/RGJ23/1142, Filemonowicz A. Microstructural studies of the scale on Sanicro Marek Sroka. 25 after 25 000 h of oxidation in steam using advanced electron microscopy techniques. Surf Coat Tech. 2019;377:124901. https:// Data availability The raw/processed data required to reproduce these doi. org/ 10. 1016/j. surfc oat. 2019. 124901. findings cannot be shared at this time as the data also form part of an 10. Zhou R, Zhu L, Liu Y, Lu Z, Chen L, Ma X. Microstructural ongoing study. evolution and the effect on hardness of Sanicro 25 welded joint base after creep at 973K. J Mater Sci. 2017;52:6161–72. https:// doi. org/ 10. 1007/ s10853- 017- 0758-6. Declarations 11. Zhou R, Zhu L, Yang Y, Lu Z, Chen L. Microstructural evolu- tion and the effect on hardness and impact toughness of Sani- Conflict of interest The authors declare that they have no known com- cro 25 welded joints after aging at 973K. Metall Mater Trans A. peting financial interests or personal relationships that could have ap- 2018;49:6290–307. https:// doi. org/ 10. 1007/ s11661- 018- 4906-7. peared to influence the work reported in this paper. 12. Chen AY, Hu WF, Wang D, Zhu YK, Wang P, Yang H, Wang XY, Gu JF, Lu J. Improving the intergranular corrosion resist- Ethical approval This article does not contain any studies with human ance of austenitic stainless steel by high density twinned struc- participants or animals performed by any of the authors. ture. Scripta Mater. 2017;13:264–8. https:// doi. org/ 10. 1016/j. scrip tamat. 2016. 11. 032. Open Access This article is licensed under a Creative Commons Attri- 13. Suo J, Peng Z, Yang H, Chai G, Yu M. Formation of Laves bution 4.0 International License, which permits use, sharing, adapta- phase in Sanicro 25 austenitic steel during creep-rupture test at tion, distribution and reproduction in any medium or format, as long 700°C. Metallogr Microstruct Anal. 2019;8(2):281–6. https:// as you give appropriate credit to the original author(s) and the source, doi. org/ 10. 1007/ s13632- 019- 00529-0. provide a link to the Creative Commons licence, and indicate if changes 14. 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Journal

Archives of Civil and Mechanical EngineeringSpringer Journals

Published: May 19, 2023

Keywords: Sanicro 25 steel; Microstructure; Precipitation; Ageing; Mechanical properties

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