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Effect of solid-solution temperature on the microstructure and properties of ultra-high-strength ferrium S53® steel
Author’s Accepted Manuscript Effect of Solid-Solution Temperature on the Microstructure and Properties of Ultra-HighStrength Ferrium S53® Steel Yangpeng Zhang, Dongping Zhan, Xiwei Qi, Zhouhua Jiang www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(18)30767-6 https://doi.org/10.1016/j.msea.2018.05.099 MSA36537
To appear in: Materials Science & Engineering A Received date: 9 May 2018 Revised date: 24 May 2018 Accepted date: 25 May 2018 Cite this article as: Yangpeng Zhang, Dongping Zhan, Xiwei Qi and Zhouhua Jiang, Effect of Solid-Solution Temperature on the Microstructure and Properties of Ultra-High-Strength Ferrium S53® Steel, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.05.099 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Solid-Solution Temperature on the Microstructure and Properties of Ultra-High-Strength Ferrium S53® Steel a, b
a
b
Yangpeng Zhang , Dongping Zhan *, Xiwei Qi , Zhouhua Jiang a b
a
School of Metallurgy, Northeastern University, Shenyang 110819, China;
School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China;
*
Corresponding author. Tel.: +86 02483687723; [email protected]
Abstract Ferrium S53® is a new secondary hardening ultra-high-strength (UHS) stainless steel, with more Cr than other Co–Ni UHS steels (UHSSs). The solid-solution step is an important step in the heat treatment of Ferrium S53®. This study investigated the effect of solid-solution temperature on the microstructure and mechanical properties of Ferrium S53®. Steel samples were solid-solution-treated at six different temperatures between 935°C and 1185°C for one hour. The resulting microstructures were characterized by X-ray diffraction (XRD), optical microscopy, field emission scanning electron microscopy, and transmission electron microscopy. The tensile strength and impact toughness at different solid-solution temperatures were also evaluated, and their relationship with the microstructure was discussed. The secondary phase of Ferrium S53® steel precipitated during stress annealing was M23C6, mostly around 100 nm, having C, Cr, Mo, and W as main components. After one hour of solid-solution treatment at 1035°C, the steel was entirely austenitized, but up to 1085°C, the secondary phase was completely dissolved. The increase of the solid-solution temperature increased the retained austenite content and the toughness and led to the dissolution of the secondary phase. The last one is the main reason for the change of strength. The experimental results showed that the most appropriate solid-solution temperature for Ferrium S53® is 1085°C.
Keywords:
solid-solution,
ultra-high-strength.
microstructure,
tensile
strength,
impact
toughness,
1. Introduction Secondary hardening ultra-high-strength steel (UHSS) is mainly used in components with high strength and toughness requirements, such as submarine shells and aircraft landing gear. AF1410 and AerMet 100 are among the representative UHSSs [1, 2, 3]. In these steels, fine-scale M2C carbides are precipitated during the aging process, improving strength and hardness [4, 5]. These carbides have high coherency with bcc iron and can precipitate on a sufficiently fine scale; hence, secondary hardening UHSS shows good toughness and plasticity [6, 7], which are guaranteed also by the retained and the reverted austenite [8, 9, 10, 11]. Ferrium S53® is a new stainless UHSS that was developed by QuesTek Innovations in 2003 to solve the problem of toxic cadmium coating when using the steel [12, 13]. Compared with AerMet 100, Ferrium S53® contains more Cr and less Ni and presents higher corrosion resistance with no reduction of strength and toughness [13]. At present, research on the microstructure and properties of secondary hardening UHSSs is mainly concentrated on the aging process, while there are only few reports about solid-solution treatment [8, 14], although it is a crucial processing step. In addition to aging hardening, UHSS is also strengthened by a solid solution of alloying elements and martensitic transformation [15], which are two effects related to the solid-solution treatment. When the steel is kept at the solid-solution temperature, a single austenite phase can be formed, and all the secondary phases precipitated during the stress-relief annealing treatment can be dissolved into the matrix. Solid-solution treatment can improve the uniformity of the microstructure and reduce segregation [16, 17, 18]. Therefore, temperature can strongly affect the goals of solid-solution treatment to achieve a homogeneous microstructure, dissolution of secondary phases, solid-solution strengthening, and martensitic transformation strengthening. In this work, we studied the effect of solid-solution temperature on the tensile and impact properties of Ferrium S53® steel and analyzed the secondary phase and microstructure transformation at different solid-solution temperatures. The influence mechanism is also discussed on the basis of the analytical results.
2. Materials and Methods 2.1. Materials Preparation and Thermodynamic Calculation Ferrium S53® was produced by vacuum induction melting followed by vacuum arc
remelting. The VAR ingot was hot fired at 1150°C for 1 h and then forged as a 45 mm square bar, the finish-forging temperature was higher than 950°C. The composition (in wt. pct) of the samples was Fe–0.20C–14.07Co–5.56Ni–9.90Cr–1.97Mo–0.98W–0.3V. Figure 1 shows a thermodynamic diagram calculated by Thermo-Calc Software from the TCFE7 database; in the temperature range of 800–1200°C, once the thermodynamic equilibrium is reached, austenite and M23C6 carbide are the main phases in the steel. Most of the M23C6 is dissolved at about 1016°C, and the steel enters the single austenite region. With the increase in the solid-solution temperature, the Cr content in M23C6 decreases gradually, whereas the W content increases. According to these calculations, a good solid-solution effect can be achieved at a temperature higher than 1016°C. However, in practice, the equilibrium state is not reached in the heat treatment process, and if the solid-solution temperature is too high, the grain size will be too large and the properties will degrade. In this work, the Ferrium S53® samples were treated at different solid-solution temperatures; the effects of these temperatures on the microstructure and properties of the samples were studied, and the best solid-solution treatment temperature was determined.
Fig. 1. Relationship between precipitation and temperature of Ferrium S53® steel calculated by the Thermo-Calc Software. (a) Contents of each phase. (b) Contents of each element in the M23C6 phase.
2.2. Heat Treatment Process The size of the samples used in the heat treatment experiment was 13 × 20 × 65 mm. After the heat treatment, these samples were wire-electrode-cut into various sizes for different tests. All the forged specimens were homogenized at 1080°C for 1 h, cooled in air, immediately
transferred into a mixture of dry ice and alcohol (−73°C) for 1 h, heated in air to room temperature, and finally stress-relief-annealed at 680°C for 8 h. These annealed samples were used for the solid-solution experiment. The solid-solution and aging process and the samples’ numbers are shown in Fig. 2. Each sample was solid-solution-treated at a different temperature (935, 985, 1035, 1085, 1135, and 1185°C), but they were all water-quenched. After quenching, the samples were kept at −73°C for 1 h and then successively heated in air to room temperature to increase the martensitic transformation ratio. After the cryogenic treatment, the samples were named S935–S1185 in relation to the different solid-solution temperatures. To study the effect of solid-solution temperature on the properties of steel after heat treatment, the samples were then tempered and aged. They were first reheated to 501°C for 3 h and then water-quenched to room temperature. Then, they were cryogenically treated again (using the same process as the first time), aged at 482°C for 12 h, and finally cooled in air. According to the different solid-solution temperatures, these samples were named A935– A1185.
Fig. 2. Heat treatment process for two groups of samples.
2.3. Test Methods A solid solution of 1.5 g of CuCl2 in 33 mL of HCl and 33 mL of H2O was used as a metallographic etching agent. The etching time was 35 s and the soaking method was used.
The microstructures and secondary phases were observed using a Zeiss optical microscope (OM) and further analyzed by an ULTRA-X55 field emission scanning electron microscope (SEM) with an energy-dispersive X-ray spectrometer (EDS). The samples for the transmission electron microscope (TEM) analysis were subject to twin-jet electropolishing with a TenuPol-5 model at −20°C and −30°C using a solid solution of 12.5% perchloric acid and 87.5% alcohol. The treated samples were analyzed by a Tecnai G2 20 TEM with an EDS at a 200 kV operating voltage. X-ray diffraction (XRD) measurements, with Cu Kα radiation and an angular interval of 35–105°, were carried out to determine the volume fraction of austenite. The tensile properties of the samples were measured using a Shimadzu AG-Xplus electronic universal testing machine. The crosshead speed was 1 mm min−1 and the parallel segment of the tensile specimen was 30 × 3 × 2.5 mm. Impact tests were performed with a JBW-500 machine on V-notch Charpy impact specimens (notch depth = 2 mm) with dimensions of 5 × 10 × 55 mm.
3. Results 3.1. OM Observation Figure 3 shows the metallographic images of the S935–S1185 samples. The grain boundaries of austenite are not observed after solid-solution treatment at 935 and 985°C for 1 h. In the steel, there are a large number of secondary phases, which come from the stress-relief annealing at 680°C and are not completely dissolved into the matrix after solid-solution treatment at 935 and 985°C for 1 h. Figures 3(c)–3(f) demonstrate that the austenite grain boundaries appear when the solid-solution temperature is higher than 1035°C. With the increase of temperature, the austenite grains rapidly grow, becoming even larger than 300 μm in S1185, and the martensitic lath becomes wider after quenching. In the S1035 sample, secondary phases with smaller sizes disappear, but the larger ones can still be observed. However, no secondary phases can be observed in the S1085, S1135, and S1185 samples.
Fig. 3. OM images of samples that were solid-solution-treated at different temperatures.
3.2. SEM Analysis Figure 4 shows the SEM pictures for the S935, S985, and S1035 samples. As seen from Figs. 4(a) and 4(b), there are a large number of undissolved secondary phases in the steel, most of which are smaller than 150 nm, some even smaller than 50 nm, and part larger than 1 μm. The distribution of these secondary phases is inhomogeneous, and some of them aggregate in chains. Figure 4(c) shows that the secondary phase distribution in the S985 sample is similar to that in the S935 sample, but with an obviously lower number of secondary phases with small sizes. In addition, as shown in Fig. 4(d), because of the dissolution of some secondary phases, the distance between the remaining ones in the S985 sample is larger than in the S935 sample. The secondary phase is more dispersed, and their segregation is reduced. The S1035 sample is obviously different from the S935 and S985 samples. As shown in Fig. 4(e), secondary phases with small sizes substantially decrease and almost disappear in S1035. The remaining large-sized secondary phases become more spherical, as shown in Fig. 4(f). Therefore, the fast dissolution temperature range of secondary phases is 985–1035°C, which is consistent with the Thermo-Calc calculation results and metallographic observations. In addition, austenite grain boundaries can be clearly observed in the S1035 sample, which is also similar to the metallographic observation results.
Fig. 4. SEM images of the S935, S985, and S1035 samples.
The composition of secondary phases was characterized by analyzing the S935 sample with SEM mapping (Fig. 5). The composition of the large-sized secondary phases is similar to that of the smaller ones. The main elements are Cr, Mo, W, and C, whereas Fe is not included and other alloy elements such as Co and Ni are not increased in them.
Fig. 5. SEM image and elemental mapping of the S935 sample.
Figure 6 shows the SEM images of the S1085, S1135, and S1185 samples. No secondary phases were observed, which means that they are completely dissolved into the matrix when the solid-solution temperature is higher than 1085°C. As seen from Figs. 6(b), 6(d), and 6(f), the large-angle grain boundary between different austenite grains is very clear and the martensitic structure in austenite grains is also clearly visible, which indicates that the austenite is fully grown and the grain size is large enough.
Fig. 6. SEM images of the S1085, S1135, and S1185 samples.
3.3. TEM Analysis The S935, S1035, and S1085 samples were analyzed at 200 kV. The images of the matrix and secondary phases of the three samples are shown in Fig. 7. As seen from Figs. 7(a) and 7(b), there are a large number of undissolved secondary phases in the steel after solid-solution treatment at 935°C. These secondary phases have polygonal shapes larger than 150 nm, which indicates that their dissolution reaction at 935°C is still very weak, consistent with the results of OM and SEM observations. The sizes of the secondary phases in the S1035 sample, shown in Figs. 7(c) and 7(d), are significantly smaller compared with S935; most of them are smaller than 50 nm, and even the larger ones do not reach 100 nm in size. Another difference between the S935 and S1035 samples is the shape of the secondary phase, which is mostly spherical instead of polygonal in the second one. This is due to the preferential dissolution of the
particles’ edges and is consistent with the SEM results. Besides the differences in the secondary phase, a large number of martensite laths appeared in the S1035 sample. By comparison with the S1085 sample shown in Figs. 7(e) and 7(f), it results that the density of martensite laths and dislocations increases with the solid-solution temperature.
Fig. 7. TEM images of different samples. (a, b) sample S935; (c, d) sample S1035; (e, f) sample S1085.
In addition to the morphological observation, a diffraction analysis of the secondary phase shown in Fig. 7(a) was carried out. The selected-area electron diffraction (SAED) spot patterns and calibration results (Fig. 8) are in agreement with the diffraction spots of M23C6 carbides, which further confirms the calculation results shown in Fig. 1.
Fig. 8. SAED spot pattern of secondary phase in Fig. 7(a).
3.4. XRD Analysis Figure 9 shows the XRD analysis results of samples before (a) and after (b) aging. Figure 9(a) demonstrates that the main steel’s microstructure after quenching and cryogenic treatment consists of martensite and retained austenite. In the S935 sample, the retained austenite is even lower and the austenite peak (200)γ is not observed, but with increasing solid-solution temperature, the retained austenite content increases gradually and the (200)γ peak becomes visible. Figure 9(b) shows that the retained austenite content significantly decreases after aging. Moreover, after aging, the austenite content becomes very low in all the samples, reducing the differences between them.
Fig. 9. XRD spectra of martensite and austenite for the S935–S1185 (a) and A935– A1185 (b) samples.
3.5. Mechanical Properties Figure 10 shows the results of tensile and impact tests at room temperature for all the
samples, in particular, stress–strain curves before [Fig. 10(a)] and after [Fig. 10(b)] aging, tensile and yield strength values after treatment at different solid-solution temperatures [Fig. 10(c)], and the relationship between the impact energy and the solid-solution temperature [Fig. 10(c)]. According to Figs. 10(a) and 10(c), after the solid-solution treatment, the tensile strength increases with the solid-solution temperature, but the yield strength first increases and then decreases, and the decline rate becomes smaller and smaller. Elongation increases gradually. When the solid-solution temperature reaches 1085°C, the growth will no longer be observed once reaching 14%. As shown in Figs. 10(b) and 10(c), the tensile and yield strength of the samples after aging are 100–400 MPa higher than before aging, but the change trend with the solid-solution temperature is similar. The elongation is different from that before aging, as it increases gradually between 935 and 1085°C but decreases when the temperature exceeds 1085°C, meaning that when the sample is solid-solution-treated at 1085°C, the elongation reaches its maximum of about 13%. As seen in Fig. 10(d), whether before or after the aging treatment, the impact toughness increases with temperature, and it does that fastest between 985 and 1035°C. Once the solid-solution temperature reaches 1185°C, the toughness gets a little reduced.
Fig. 10. Results of tensile and impact tests on the samples. (a, b) Stress–strain curves before and after aging; (c) tensile and yield strength values; (d) impact energy.
4. Discussion The secondary phase has an important relationship with the strength and plasticity of UHSSs. First of all, secondary phase strengthening is the main means of improving strength properties [1, 12]. According to the Orowan strengthening mechanism, the secondary phase can impede dislocation movements and increase the yield strength of the steel [15, 19, 20]. The smaller the spacing and size of the secondary phase, the greater the contribution to the strength [15, 20]. Secondly, the interface of the secondary phase with the matrix and even its crack is an important source of material crack, and the size of the microcracks is positively related to the size of the secondary phase [21, 22]. Therefore, the maximum size of the secondary phase has a decisive influence on the tensile strength of the steel. Thirdly, the large-sized secondary phase usually has a bad effect on the plasticity and toughness of steel, but the smaller the size, the smaller the volume fraction, the closer the shape to spherical, and the lower the damage to the toughness of the steel [21, 23]. For the secondary hardening UHSS, M2C precipitates with a size below 20 nm are usually used as the strengthening phases [12]. Since the samples were stress-relief-annealed at 680°C for 8 h before solid-solution treatment, there was a large amount of precipitated secondary phases in the steel after annealing. The results of SEM [Fig. 4(a)] analysis of the S935 sample show that the size of some precipitates is larger than 1 μm, but most of them are about 100 nm. The results of mapping (Fig. 5) and TEM (Figs. 7 and 8) show that these secondary phases are M23C6 carbides rich in Cr, Mo, and W. As Ferrium S53® has higher Cr content compared with AerMet 100, it could promote the precipitation and coarsening rate of M23C6, which is rich in Cr. However, M23C6 is not the main strengthening phase of secondary hardening UHSSs. Therefore, all the M23C6 carbides need to be completely dissolved into the matrix during the solid-solution treatment stage, preparing for the precipitation of M2C carbides with a smaller size in the subsequent tempering and aging stages [4, 5]. When the solid-solution temperature was increased from 935 to 985°C, the secondary
phase with a small size was partially dissolved. The influence of this dissolution on the strength acts mainly through the following aspects. Firstly, according to the Orowan bypass strengthening mechanism of the secondary phase, the decrease of secondary phase content decreases the yield strength [15, 20]. Secondly, as shown in Fig. 4, when the secondary phase decreases, the segregation phenomenon gets enhanced and the secondary phase becomes more dispersed. The segregation of the secondary phase is one of the important reasons for the formation of microcracks in steel [21, 23]. Therefore, the tensile strength is improved if the secondary phase becomes more diffuse. The last point is that the alloy elements of the secondary phase, such as Cr and W, can be effectively dissolved into the matrix and can enhance the yield strength. After the solid-solution temperature was increased from 935 to 985°C, the improvement of segregation and solid-solution strengthening played a major role in increasing the tensile and yield strength. The retained austenite is one of the main factors affecting the toughness and ductility of secondary hardening UHSSs [9, 14]. According to the XRD analysis shown in Fig. 8, the austenite content is very low in the S935 and S985 samples, leading to poor toughness and plasticity as shown in Fig. 10. This is due to the fact that, on the one hand, solid-solution temperatures of 935 and 985°C are too low, austenitization is incomplete, and the austenite grains are very small, as shown in Fig. 4. On the other hand, a large number of secondary phases have not been dissolved in the steel, and hence the alloying elements in austenite are not too many. These two reasons can reduce the austenite stability and make martensitic transformation easier and more complete during quenching. When the solid-solution temperature was increased from 985 to 1035°C, the toughness of the steel obviously improved both before and after aging, but the tensile strength and yield strength were not the same because the tensile strength increased whereas the yield strength clearly decreased. As seen in Fig. 3, the most obvious change from 985 to 1035°C is in the appearance of austenite grain boundaries; that is, the austenitization of Ferrium S53® steel is more complete and the austenite grains grow fast with an increasing solid-solution temperature. According to the Hall–Petch formula, the increase of grain size reduces the yield strength of steel [24]. In addition, as seen from the SEM images (Fig. 4), most of the small-sized secondary phases completely dissolved into the matrix when the solid-solution temperature was increased to 1035°C. This means that the effect of secondary phases on the movement of dislocation glide gets weakened, which also leads to a decrease in the yield
strength [15, 20]. On the other hand, since the segregation phenomenon of the secondary phase disappears as it mostly dissolves and the shape of the remaining secondary phase becomes approximately spherical, when the samples are subjected to load, the stress concentration caused by segregation and irregular shape of the secondary phase is released, and the production and expansion of the microcracks in the steel are reduced [21, 23]. The capacity of steel to accommodate plastic deformation is enhanced, and the samples exert greater working hardening in the plastic deformation stage during the tensile test, which could cause the simultaneous enhancement of the steel plasticity and tensile strength [23, 25]. The toughness improvement is mainly due to the change of the retained austenite content. The XRD analysis indicated that the retained austenite content in the S1035 sample was clearly increased compared with the S985 sample, and the corresponding (200)γ and other peaks of austenite were obviously increased. This phenomenon has two main causes. First, the second-phase solid solution increases the alloy content in the austenite and improves the stability of the austenite at room temperature. Second, when the austenite grain size becomes larger, the martensite becomes larger after quenching and the corresponding internal stress increases, so that the transformed martensite in S1085 astrict more austenite [16, 26]. This austenite cannot be transformed into martensite because of the confined space, which eventually leads to an increase of the retained austenite. When the solid-solution temperature was increased from 1035 to 1085°C, the tensile strength, yield strength, impact toughness, and elongation of the steel showed the same trend as when the temperature was increased from 985 to 1035°C. This is due to the further dissolution of the precipitate and the further growth of the austenite grain, leading to a similar mechanism of action as observed in the previous analysis. When the solid-solution temperature was 1085°C, the secondary phase produced by stress-relief annealing was completely dissolved. Therefore, if the solid-solution temperature is further increased, the main change will be in the growth of austenite grains, as shown by the S1135 and S1185 samples in Fig. 3. However, when the grain size of austenite exceeded 100 μm at this stage, the rate of austenite grain growth slowed down, and the change of strength and toughness caused by the austenite became no longer obvious [16, 17], as confirmed by Fig. 10. Figure 9 also shows that, after aging treatment, the tensile properties were significantly improved, but the toughness was strongly reduced. A large number of M2C carbides of around
15 nm will precipitate from Ferrium S53® in the tempering and aging process, and their dispersion in the matrix will lead to the secondary hardening of the steel [4, 5]. The change of toughness before and after aging is mainly related to the content of the retained austenite. Because of the high alloy content of Ferrium S53® steel, the Ms and Mf temperatures of steel are lower [9, 14]. When the sample is quenched to room temperature, a large amount of austenite is retained and not completely transformed [14], even after a cryogenic treatment, as shown in Fig. 9(a). Austenite has a great influence on the steel’s toughness, and hence the impact toughness is higher in this case. However, the sample was cooled down in water after tempering, and a secondary cryogenic treatment followed. Therefore, the retained austenite was almost completely transformed into martensite, and its content in all the samples was strongly reduced, as shown in Fig. 9(b). Correspondingly, after tempering and aging, the impact toughness of the sample was significantly reduced. However, as seen in Fig. 10, the trend of change in strength and toughness with the solid-solution temperature after aging is the same as that observed before aging. Hence, the influence of solid-solution temperature on the steel’s microstructure will affect its performance after aging. According to the above discussion and analysis, it is reasonable to select 1085°C as the optimal solid-solution temperature for Ferrium S53® steel.
5. Conclusions (1) The secondary phase of Ferrium S53® steel precipitated during stress annealing is M23C6, in a large number and with serious segregation. The size of the precipitates can reach 1 μm but is mostly around 100 nm, and their main components are C, Cr, Mo, and W. (2) After one-hour solid-solution treatment at 1035°C, Ferrium S53® steel can be completely austenitized with clear grain boundaries. The secondary phase can be completely dissolved by heating at 1085°C for 1 h. (3) Increasing the solid-solution temperature increases the retained austenite content, the dissolution of the secondary phase, and the steel’s toughness. the steel’s toughness reaches the maximum at 1085°C. The changes in both the secondary phase and the grain size caused by the change in the solid-solution temperature influence the strength, and the dissolution of the secondary phase plays a major role.
(4) The influence of the solid-solution temperature on steel’s microstructure can affect its mechanical properties after aging. The comprehensive experimental results show that the appropriate solid-solution temperature for Ferrium S53® steel is 1085°C.
Acknowledgments The authors would like to thank National Natural Science Foundation of China (51574063) and Fundamental Research Funds for the Central Universities (N150204012 and N152306001). The authors would like to thank Enago (www.enago.cn) for the English language review.
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To appear in: Materials Science & Engineering A Received date: 9 May 2018 Revised date: 24 May 2018 Accepted date: 25 May 2018 Cite this article as: Yangpeng Zhang, Dongping Zhan, Xiwei Qi and Zhouhua Jiang, Effect of Solid-Solution Temperature on the Microstructure and Properties of Ultra-High-Strength Ferrium S53® Steel, Materials Science & Engineering A, https://doi.org/10.1016/j.msea.2018.05.099 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of Solid-Solution Temperature on the Microstructure and Properties of Ultra-High-Strength Ferrium S53® Steel a, b
a
b
Yangpeng Zhang , Dongping Zhan *, Xiwei Qi , Zhouhua Jiang a b
a
School of Metallurgy, Northeastern University, Shenyang 110819, China;
School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China;
*
Corresponding author. Tel.: +86 02483687723; [email protected]
Abstract Ferrium S53® is a new secondary hardening ultra-high-strength (UHS) stainless steel, with more Cr than other Co–Ni UHS steels (UHSSs). The solid-solution step is an important step in the heat treatment of Ferrium S53®. This study investigated the effect of solid-solution temperature on the microstructure and mechanical properties of Ferrium S53®. Steel samples were solid-solution-treated at six different temperatures between 935°C and 1185°C for one hour. The resulting microstructures were characterized by X-ray diffraction (XRD), optical microscopy, field emission scanning electron microscopy, and transmission electron microscopy. The tensile strength and impact toughness at different solid-solution temperatures were also evaluated, and their relationship with the microstructure was discussed. The secondary phase of Ferrium S53® steel precipitated during stress annealing was M23C6, mostly around 100 nm, having C, Cr, Mo, and W as main components. After one hour of solid-solution treatment at 1035°C, the steel was entirely austenitized, but up to 1085°C, the secondary phase was completely dissolved. The increase of the solid-solution temperature increased the retained austenite content and the toughness and led to the dissolution of the secondary phase. The last one is the main reason for the change of strength. The experimental results showed that the most appropriate solid-solution temperature for Ferrium S53® is 1085°C.
Keywords:
solid-solution,
ultra-high-strength.
microstructure,
tensile
strength,
impact
toughness,
1. Introduction Secondary hardening ultra-high-strength steel (UHSS) is mainly used in components with high strength and toughness requirements, such as submarine shells and aircraft landing gear. AF1410 and AerMet 100 are among the representative UHSSs [1, 2, 3]. In these steels, fine-scale M2C carbides are precipitated during the aging process, improving strength and hardness [4, 5]. These carbides have high coherency with bcc iron and can precipitate on a sufficiently fine scale; hence, secondary hardening UHSS shows good toughness and plasticity [6, 7], which are guaranteed also by the retained and the reverted austenite [8, 9, 10, 11]. Ferrium S53® is a new stainless UHSS that was developed by QuesTek Innovations in 2003 to solve the problem of toxic cadmium coating when using the steel [12, 13]. Compared with AerMet 100, Ferrium S53® contains more Cr and less Ni and presents higher corrosion resistance with no reduction of strength and toughness [13]. At present, research on the microstructure and properties of secondary hardening UHSSs is mainly concentrated on the aging process, while there are only few reports about solid-solution treatment [8, 14], although it is a crucial processing step. In addition to aging hardening, UHSS is also strengthened by a solid solution of alloying elements and martensitic transformation [15], which are two effects related to the solid-solution treatment. When the steel is kept at the solid-solution temperature, a single austenite phase can be formed, and all the secondary phases precipitated during the stress-relief annealing treatment can be dissolved into the matrix. Solid-solution treatment can improve the uniformity of the microstructure and reduce segregation [16, 17, 18]. Therefore, temperature can strongly affect the goals of solid-solution treatment to achieve a homogeneous microstructure, dissolution of secondary phases, solid-solution strengthening, and martensitic transformation strengthening. In this work, we studied the effect of solid-solution temperature on the tensile and impact properties of Ferrium S53® steel and analyzed the secondary phase and microstructure transformation at different solid-solution temperatures. The influence mechanism is also discussed on the basis of the analytical results.
2. Materials and Methods 2.1. Materials Preparation and Thermodynamic Calculation Ferrium S53® was produced by vacuum induction melting followed by vacuum arc
remelting. The VAR ingot was hot fired at 1150°C for 1 h and then forged as a 45 mm square bar, the finish-forging temperature was higher than 950°C. The composition (in wt. pct) of the samples was Fe–0.20C–14.07Co–5.56Ni–9.90Cr–1.97Mo–0.98W–0.3V. Figure 1 shows a thermodynamic diagram calculated by Thermo-Calc Software from the TCFE7 database; in the temperature range of 800–1200°C, once the thermodynamic equilibrium is reached, austenite and M23C6 carbide are the main phases in the steel. Most of the M23C6 is dissolved at about 1016°C, and the steel enters the single austenite region. With the increase in the solid-solution temperature, the Cr content in M23C6 decreases gradually, whereas the W content increases. According to these calculations, a good solid-solution effect can be achieved at a temperature higher than 1016°C. However, in practice, the equilibrium state is not reached in the heat treatment process, and if the solid-solution temperature is too high, the grain size will be too large and the properties will degrade. In this work, the Ferrium S53® samples were treated at different solid-solution temperatures; the effects of these temperatures on the microstructure and properties of the samples were studied, and the best solid-solution treatment temperature was determined.
Fig. 1. Relationship between precipitation and temperature of Ferrium S53® steel calculated by the Thermo-Calc Software. (a) Contents of each phase. (b) Contents of each element in the M23C6 phase.
2.2. Heat Treatment Process The size of the samples used in the heat treatment experiment was 13 × 20 × 65 mm. After the heat treatment, these samples were wire-electrode-cut into various sizes for different tests. All the forged specimens were homogenized at 1080°C for 1 h, cooled in air, immediately
transferred into a mixture of dry ice and alcohol (−73°C) for 1 h, heated in air to room temperature, and finally stress-relief-annealed at 680°C for 8 h. These annealed samples were used for the solid-solution experiment. The solid-solution and aging process and the samples’ numbers are shown in Fig. 2. Each sample was solid-solution-treated at a different temperature (935, 985, 1035, 1085, 1135, and 1185°C), but they were all water-quenched. After quenching, the samples were kept at −73°C for 1 h and then successively heated in air to room temperature to increase the martensitic transformation ratio. After the cryogenic treatment, the samples were named S935–S1185 in relation to the different solid-solution temperatures. To study the effect of solid-solution temperature on the properties of steel after heat treatment, the samples were then tempered and aged. They were first reheated to 501°C for 3 h and then water-quenched to room temperature. Then, they were cryogenically treated again (using the same process as the first time), aged at 482°C for 12 h, and finally cooled in air. According to the different solid-solution temperatures, these samples were named A935– A1185.
Fig. 2. Heat treatment process for two groups of samples.
2.3. Test Methods A solid solution of 1.5 g of CuCl2 in 33 mL of HCl and 33 mL of H2O was used as a metallographic etching agent. The etching time was 35 s and the soaking method was used.
The microstructures and secondary phases were observed using a Zeiss optical microscope (OM) and further analyzed by an ULTRA-X55 field emission scanning electron microscope (SEM) with an energy-dispersive X-ray spectrometer (EDS). The samples for the transmission electron microscope (TEM) analysis were subject to twin-jet electropolishing with a TenuPol-5 model at −20°C and −30°C using a solid solution of 12.5% perchloric acid and 87.5% alcohol. The treated samples were analyzed by a Tecnai G2 20 TEM with an EDS at a 200 kV operating voltage. X-ray diffraction (XRD) measurements, with Cu Kα radiation and an angular interval of 35–105°, were carried out to determine the volume fraction of austenite. The tensile properties of the samples were measured using a Shimadzu AG-Xplus electronic universal testing machine. The crosshead speed was 1 mm min−1 and the parallel segment of the tensile specimen was 30 × 3 × 2.5 mm. Impact tests were performed with a JBW-500 machine on V-notch Charpy impact specimens (notch depth = 2 mm) with dimensions of 5 × 10 × 55 mm.
3. Results 3.1. OM Observation Figure 3 shows the metallographic images of the S935–S1185 samples. The grain boundaries of austenite are not observed after solid-solution treatment at 935 and 985°C for 1 h. In the steel, there are a large number of secondary phases, which come from the stress-relief annealing at 680°C and are not completely dissolved into the matrix after solid-solution treatment at 935 and 985°C for 1 h. Figures 3(c)–3(f) demonstrate that the austenite grain boundaries appear when the solid-solution temperature is higher than 1035°C. With the increase of temperature, the austenite grains rapidly grow, becoming even larger than 300 μm in S1185, and the martensitic lath becomes wider after quenching. In the S1035 sample, secondary phases with smaller sizes disappear, but the larger ones can still be observed. However, no secondary phases can be observed in the S1085, S1135, and S1185 samples.
Fig. 3. OM images of samples that were solid-solution-treated at different temperatures.
3.2. SEM Analysis Figure 4 shows the SEM pictures for the S935, S985, and S1035 samples. As seen from Figs. 4(a) and 4(b), there are a large number of undissolved secondary phases in the steel, most of which are smaller than 150 nm, some even smaller than 50 nm, and part larger than 1 μm. The distribution of these secondary phases is inhomogeneous, and some of them aggregate in chains. Figure 4(c) shows that the secondary phase distribution in the S985 sample is similar to that in the S935 sample, but with an obviously lower number of secondary phases with small sizes. In addition, as shown in Fig. 4(d), because of the dissolution of some secondary phases, the distance between the remaining ones in the S985 sample is larger than in the S935 sample. The secondary phase is more dispersed, and their segregation is reduced. The S1035 sample is obviously different from the S935 and S985 samples. As shown in Fig. 4(e), secondary phases with small sizes substantially decrease and almost disappear in S1035. The remaining large-sized secondary phases become more spherical, as shown in Fig. 4(f). Therefore, the fast dissolution temperature range of secondary phases is 985–1035°C, which is consistent with the Thermo-Calc calculation results and metallographic observations. In addition, austenite grain boundaries can be clearly observed in the S1035 sample, which is also similar to the metallographic observation results.
Fig. 4. SEM images of the S935, S985, and S1035 samples.
The composition of secondary phases was characterized by analyzing the S935 sample with SEM mapping (Fig. 5). The composition of the large-sized secondary phases is similar to that of the smaller ones. The main elements are Cr, Mo, W, and C, whereas Fe is not included and other alloy elements such as Co and Ni are not increased in them.
Fig. 5. SEM image and elemental mapping of the S935 sample.
Figure 6 shows the SEM images of the S1085, S1135, and S1185 samples. No secondary phases were observed, which means that they are completely dissolved into the matrix when the solid-solution temperature is higher than 1085°C. As seen from Figs. 6(b), 6(d), and 6(f), the large-angle grain boundary between different austenite grains is very clear and the martensitic structure in austenite grains is also clearly visible, which indicates that the austenite is fully grown and the grain size is large enough.
Fig. 6. SEM images of the S1085, S1135, and S1185 samples.
3.3. TEM Analysis The S935, S1035, and S1085 samples were analyzed at 200 kV. The images of the matrix and secondary phases of the three samples are shown in Fig. 7. As seen from Figs. 7(a) and 7(b), there are a large number of undissolved secondary phases in the steel after solid-solution treatment at 935°C. These secondary phases have polygonal shapes larger than 150 nm, which indicates that their dissolution reaction at 935°C is still very weak, consistent with the results of OM and SEM observations. The sizes of the secondary phases in the S1035 sample, shown in Figs. 7(c) and 7(d), are significantly smaller compared with S935; most of them are smaller than 50 nm, and even the larger ones do not reach 100 nm in size. Another difference between the S935 and S1035 samples is the shape of the secondary phase, which is mostly spherical instead of polygonal in the second one. This is due to the preferential dissolution of the
particles’ edges and is consistent with the SEM results. Besides the differences in the secondary phase, a large number of martensite laths appeared in the S1035 sample. By comparison with the S1085 sample shown in Figs. 7(e) and 7(f), it results that the density of martensite laths and dislocations increases with the solid-solution temperature.
Fig. 7. TEM images of different samples. (a, b) sample S935; (c, d) sample S1035; (e, f) sample S1085.
In addition to the morphological observation, a diffraction analysis of the secondary phase shown in Fig. 7(a) was carried out. The selected-area electron diffraction (SAED) spot patterns and calibration results (Fig. 8) are in agreement with the diffraction spots of M23C6 carbides, which further confirms the calculation results shown in Fig. 1.
Fig. 8. SAED spot pattern of secondary phase in Fig. 7(a).
3.4. XRD Analysis Figure 9 shows the XRD analysis results of samples before (a) and after (b) aging. Figure 9(a) demonstrates that the main steel’s microstructure after quenching and cryogenic treatment consists of martensite and retained austenite. In the S935 sample, the retained austenite is even lower and the austenite peak (200)γ is not observed, but with increasing solid-solution temperature, the retained austenite content increases gradually and the (200)γ peak becomes visible. Figure 9(b) shows that the retained austenite content significantly decreases after aging. Moreover, after aging, the austenite content becomes very low in all the samples, reducing the differences between them.
Fig. 9. XRD spectra of martensite and austenite for the S935–S1185 (a) and A935– A1185 (b) samples.
3.5. Mechanical Properties Figure 10 shows the results of tensile and impact tests at room temperature for all the
samples, in particular, stress–strain curves before [Fig. 10(a)] and after [Fig. 10(b)] aging, tensile and yield strength values after treatment at different solid-solution temperatures [Fig. 10(c)], and the relationship between the impact energy and the solid-solution temperature [Fig. 10(c)]. According to Figs. 10(a) and 10(c), after the solid-solution treatment, the tensile strength increases with the solid-solution temperature, but the yield strength first increases and then decreases, and the decline rate becomes smaller and smaller. Elongation increases gradually. When the solid-solution temperature reaches 1085°C, the growth will no longer be observed once reaching 14%. As shown in Figs. 10(b) and 10(c), the tensile and yield strength of the samples after aging are 100–400 MPa higher than before aging, but the change trend with the solid-solution temperature is similar. The elongation is different from that before aging, as it increases gradually between 935 and 1085°C but decreases when the temperature exceeds 1085°C, meaning that when the sample is solid-solution-treated at 1085°C, the elongation reaches its maximum of about 13%. As seen in Fig. 10(d), whether before or after the aging treatment, the impact toughness increases with temperature, and it does that fastest between 985 and 1035°C. Once the solid-solution temperature reaches 1185°C, the toughness gets a little reduced.
Fig. 10. Results of tensile and impact tests on the samples. (a, b) Stress–strain curves before and after aging; (c) tensile and yield strength values; (d) impact energy.
4. Discussion The secondary phase has an important relationship with the strength and plasticity of UHSSs. First of all, secondary phase strengthening is the main means of improving strength properties [1, 12]. According to the Orowan strengthening mechanism, the secondary phase can impede dislocation movements and increase the yield strength of the steel [15, 19, 20]. The smaller the spacing and size of the secondary phase, the greater the contribution to the strength [15, 20]. Secondly, the interface of the secondary phase with the matrix and even its crack is an important source of material crack, and the size of the microcracks is positively related to the size of the secondary phase [21, 22]. Therefore, the maximum size of the secondary phase has a decisive influence on the tensile strength of the steel. Thirdly, the large-sized secondary phase usually has a bad effect on the plasticity and toughness of steel, but the smaller the size, the smaller the volume fraction, the closer the shape to spherical, and the lower the damage to the toughness of the steel [21, 23]. For the secondary hardening UHSS, M2C precipitates with a size below 20 nm are usually used as the strengthening phases [12]. Since the samples were stress-relief-annealed at 680°C for 8 h before solid-solution treatment, there was a large amount of precipitated secondary phases in the steel after annealing. The results of SEM [Fig. 4(a)] analysis of the S935 sample show that the size of some precipitates is larger than 1 μm, but most of them are about 100 nm. The results of mapping (Fig. 5) and TEM (Figs. 7 and 8) show that these secondary phases are M23C6 carbides rich in Cr, Mo, and W. As Ferrium S53® has higher Cr content compared with AerMet 100, it could promote the precipitation and coarsening rate of M23C6, which is rich in Cr. However, M23C6 is not the main strengthening phase of secondary hardening UHSSs. Therefore, all the M23C6 carbides need to be completely dissolved into the matrix during the solid-solution treatment stage, preparing for the precipitation of M2C carbides with a smaller size in the subsequent tempering and aging stages [4, 5]. When the solid-solution temperature was increased from 935 to 985°C, the secondary
phase with a small size was partially dissolved. The influence of this dissolution on the strength acts mainly through the following aspects. Firstly, according to the Orowan bypass strengthening mechanism of the secondary phase, the decrease of secondary phase content decreases the yield strength [15, 20]. Secondly, as shown in Fig. 4, when the secondary phase decreases, the segregation phenomenon gets enhanced and the secondary phase becomes more dispersed. The segregation of the secondary phase is one of the important reasons for the formation of microcracks in steel [21, 23]. Therefore, the tensile strength is improved if the secondary phase becomes more diffuse. The last point is that the alloy elements of the secondary phase, such as Cr and W, can be effectively dissolved into the matrix and can enhance the yield strength. After the solid-solution temperature was increased from 935 to 985°C, the improvement of segregation and solid-solution strengthening played a major role in increasing the tensile and yield strength. The retained austenite is one of the main factors affecting the toughness and ductility of secondary hardening UHSSs [9, 14]. According to the XRD analysis shown in Fig. 8, the austenite content is very low in the S935 and S985 samples, leading to poor toughness and plasticity as shown in Fig. 10. This is due to the fact that, on the one hand, solid-solution temperatures of 935 and 985°C are too low, austenitization is incomplete, and the austenite grains are very small, as shown in Fig. 4. On the other hand, a large number of secondary phases have not been dissolved in the steel, and hence the alloying elements in austenite are not too many. These two reasons can reduce the austenite stability and make martensitic transformation easier and more complete during quenching. When the solid-solution temperature was increased from 985 to 1035°C, the toughness of the steel obviously improved both before and after aging, but the tensile strength and yield strength were not the same because the tensile strength increased whereas the yield strength clearly decreased. As seen in Fig. 3, the most obvious change from 985 to 1035°C is in the appearance of austenite grain boundaries; that is, the austenitization of Ferrium S53® steel is more complete and the austenite grains grow fast with an increasing solid-solution temperature. According to the Hall–Petch formula, the increase of grain size reduces the yield strength of steel [24]. In addition, as seen from the SEM images (Fig. 4), most of the small-sized secondary phases completely dissolved into the matrix when the solid-solution temperature was increased to 1035°C. This means that the effect of secondary phases on the movement of dislocation glide gets weakened, which also leads to a decrease in the yield
strength [15, 20]. On the other hand, since the segregation phenomenon of the secondary phase disappears as it mostly dissolves and the shape of the remaining secondary phase becomes approximately spherical, when the samples are subjected to load, the stress concentration caused by segregation and irregular shape of the secondary phase is released, and the production and expansion of the microcracks in the steel are reduced [21, 23]. The capacity of steel to accommodate plastic deformation is enhanced, and the samples exert greater working hardening in the plastic deformation stage during the tensile test, which could cause the simultaneous enhancement of the steel plasticity and tensile strength [23, 25]. The toughness improvement is mainly due to the change of the retained austenite content. The XRD analysis indicated that the retained austenite content in the S1035 sample was clearly increased compared with the S985 sample, and the corresponding (200)γ and other peaks of austenite were obviously increased. This phenomenon has two main causes. First, the second-phase solid solution increases the alloy content in the austenite and improves the stability of the austenite at room temperature. Second, when the austenite grain size becomes larger, the martensite becomes larger after quenching and the corresponding internal stress increases, so that the transformed martensite in S1085 astrict more austenite [16, 26]. This austenite cannot be transformed into martensite because of the confined space, which eventually leads to an increase of the retained austenite. When the solid-solution temperature was increased from 1035 to 1085°C, the tensile strength, yield strength, impact toughness, and elongation of the steel showed the same trend as when the temperature was increased from 985 to 1035°C. This is due to the further dissolution of the precipitate and the further growth of the austenite grain, leading to a similar mechanism of action as observed in the previous analysis. When the solid-solution temperature was 1085°C, the secondary phase produced by stress-relief annealing was completely dissolved. Therefore, if the solid-solution temperature is further increased, the main change will be in the growth of austenite grains, as shown by the S1135 and S1185 samples in Fig. 3. However, when the grain size of austenite exceeded 100 μm at this stage, the rate of austenite grain growth slowed down, and the change of strength and toughness caused by the austenite became no longer obvious [16, 17], as confirmed by Fig. 10. Figure 9 also shows that, after aging treatment, the tensile properties were significantly improved, but the toughness was strongly reduced. A large number of M2C carbides of around
15 nm will precipitate from Ferrium S53® in the tempering and aging process, and their dispersion in the matrix will lead to the secondary hardening of the steel [4, 5]. The change of toughness before and after aging is mainly related to the content of the retained austenite. Because of the high alloy content of Ferrium S53® steel, the Ms and Mf temperatures of steel are lower [9, 14]. When the sample is quenched to room temperature, a large amount of austenite is retained and not completely transformed [14], even after a cryogenic treatment, as shown in Fig. 9(a). Austenite has a great influence on the steel’s toughness, and hence the impact toughness is higher in this case. However, the sample was cooled down in water after tempering, and a secondary cryogenic treatment followed. Therefore, the retained austenite was almost completely transformed into martensite, and its content in all the samples was strongly reduced, as shown in Fig. 9(b). Correspondingly, after tempering and aging, the impact toughness of the sample was significantly reduced. However, as seen in Fig. 10, the trend of change in strength and toughness with the solid-solution temperature after aging is the same as that observed before aging. Hence, the influence of solid-solution temperature on the steel’s microstructure will affect its performance after aging. According to the above discussion and analysis, it is reasonable to select 1085°C as the optimal solid-solution temperature for Ferrium S53® steel.
5. Conclusions (1) The secondary phase of Ferrium S53® steel precipitated during stress annealing is M23C6, in a large number and with serious segregation. The size of the precipitates can reach 1 μm but is mostly around 100 nm, and their main components are C, Cr, Mo, and W. (2) After one-hour solid-solution treatment at 1035°C, Ferrium S53® steel can be completely austenitized with clear grain boundaries. The secondary phase can be completely dissolved by heating at 1085°C for 1 h. (3) Increasing the solid-solution temperature increases the retained austenite content, the dissolution of the secondary phase, and the steel’s toughness. the steel’s toughness reaches the maximum at 1085°C. The changes in both the secondary phase and the grain size caused by the change in the solid-solution temperature influence the strength, and the dissolution of the secondary phase plays a major role.
(4) The influence of the solid-solution temperature on steel’s microstructure can affect its mechanical properties after aging. The comprehensive experimental results show that the appropriate solid-solution temperature for Ferrium S53® steel is 1085°C.
Acknowledgments The authors would like to thank National Natural Science Foundation of China (51574063) and Fundamental Research Funds for the Central Universities (N150204012 and N152306001). The authors would like to thank Enago (www.enago.cn) for the English language review.
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