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The influence of Ce micro-alloying on the precipitation of intermetallic sigma phase during solidification of super-austenitic stainless steels
Journal of Alloys and Compounds 815 (2020) 152418
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The influence of Ce micro-alloying on the precipitation of intermetallic sigma phase during solidification of super-austenitic stainless steels Qi Wang a, Lijun Wang a, *, Yanhui Sun a, Aimin Zhao a, Wei Zhang c, Jianmin Li c, Hongbiao Dong d, Kuochih Chou a, b a
Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing, 100083, PR China State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, PR China Research Center, Taiyuan Iron and Steel (Group) Co., Ltd, Shanxi Province, 030003, PR China d Department of Engineering, University of Leicester, Leicester, UK b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2019 Received in revised form 16 September 2019 Accepted 23 September 2019 Available online 24 September 2019
The influence of Ce addition on the precipitation of ferrite (d) and sigma (s) phase in super-austenitic stainless steels during solidification was investigated via a combined experimental and theoretical approach. Thermodynamic calculations reveal that the secondary phase of d and s are precipitated in sequence near the end of the solidification process due to the segregation of Mo and Cr. The Ce addition improves the thermal stability of the d, expands the range of Mo content for d phase precipitation and increases the d fraction in the solidified structure in the GullivereScheil model. Experimental observation shows that a small addition of Ce accelerates d nucleation in the interdendritic solute-rich liquid, refines the solidified structure and reduces Mo and Cr segregation, which suppresses precipitation of the s phase. The s phase precipitation is replaced by d phase precipitation after a Ce addition of 0.016%. The change in the secondary phase from s to d shortens the time required for homogenization and reduces the nano-hardness of the interdendritic secondary phase, which is significant for hot working. © 2019 Elsevier B.V. All rights reserved.
Keywords: Super-austenitic stainless steel Intermetallic compound Sigma phase Ce micro-alloying addition
1. Introduction Super-austenitic stainless steels (SASSs) have potential for use in extremely harsh service environments, such as for flue gas desulphurization and sea water desalination and also for applications in the petrochemical industry [1]. However, SASSs have a high content of alloying elements (>40%), especially Cr and Mo, which greatly reduces the microstructure stability and remarkably increases the precipitation sensitivity of secondary phase during solidification. The high levels of Cr and Mo promote the formation of secondary phase at the end of solidification due to strong segregation in the residual liquid. The secondary phase precipitates in an ingot of S31254 are mainly intermetallic sigma phase (s), which comprise (FeeCr) or (FeeCreMo) and negatively impacts the pitting resistance and mechanical properties [2,3]. Banovic S.W [4] noted that Mo is strongly segregated in the liquid during solidification of AL-6XN alloys and that the solidification terminates by a eutectic L / g þ s reaction. Generally, the size of s phases formed upon
* Corresponding author. E-mail address: [email protected] (L. Wang). https://doi.org/10.1016/j.jallcom.2019.152418 0925-8388/© 2019 Elsevier B.V. All rights reserved.
solidification is much larger than that formed upon solid-state transformation [5]. Due to its large size, it is difficult to redissolve the s phase during a post-solidification heat treatment, which deteriorates the hot working properties and leads to cracking of continuous-casting billets during hot rolling. Thus, it is very important to minimize the segregation and then control the precipitation of the s phase in the final solidified structure. Generally speaking, micro-structural refinement can minimize the segregation in steel ingots or castings, which is significant in SASSs, during solidification. The currently available grain refinement techniques used during steel casting can be categorized into three groups: electromagnetic stirring (EMS), fast cooling and chemical grain refinement [6]. However, EMS and fast cooling are limited by the actual casting equipment. REs (rare earths) are effective chemical grain refiners, and RE addition is an alternative approach to grain refinement in steel. Bartlett et al. [7] and Haakosen [8] both reported a reduction in the grain size in cast austenitic high-manganese steel after the addition of cerium. Many studies [9e14] indicate that grain refinement is due to the heterogeneous nucleation of austenite or ferrite on RE inclusions, including oxides, sulphides and oxysulfides, and solute effects. Thus, a small Ce addition is viable to refine the austenitic grain structure.
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The aim of this work is to control the precipitation of the intermetallic s phase in SASSs. A small Ce addition could refine the austenite grain size, which would minimize the segregation in interdendritic areas. In this study, the effect of Ce on the precipitation of the s phase in SASSs during solidification is investigated via a combined modelling and experimental approach using a Thermo-Calc simulation, a directional solidification test, and electron microscopy.
microstructure was determined by the scanning electron microscope that was equipped with electron back scatter diffraction (EBSD) and an HKL Channel 5 data acquisition and analysis system. The step size used for the EBSD analyses was 0.5 mm. The EBSD samples were mechanically polished, followed by electropolishing in a solution of 10% perchloric acid and 90% alcohol at approximately 25 C for approximately 30 s with an applied potential of 15 V.
2. Materials and experiments
2.4. Thermal analysis
2.1. Materials
Thermal analysis was conducted on a differential thermal analyzer (Netzsch STA 449C), and a dynamic high-purity argon atmosphere was used to minimize potential oxidizing effects. The specimen (400 mg) was subjected to the following heating/cooling cycles: room temperature / heating (5 C/min) to 1500 C / isothermal holding at 1500 C for 5 min / cooling (5 C/min) to room temperature.
In experiments with Ce addition, commercial S31254 superaustenite stainless steel (500 g) was melted at 1500 C in an MgO crucible, and the crucible was placed in a MoSi2 resistance furnace with the protection of high-purity Ar gas. To protect the Ce metal (99.9%, Aladdin) from oxidation, it was wrapped in a reduced iron powder and pressed into a metal cylinder, and then the metal cylinder was wrapped in pure iron chips and added to the molten steel. By stirring the molten steel with a quartz tube, good Ce homogeneity could be achieved. After homogenization, the molten steel was removed and quenched in water. In the present work, the chemical composition (wt%) of the S31254 stainless steel used in this work was measured by inductively coupled plasma mass spectrometry (ICP-MS), as shown in Table .1. These ingots were cut into samples with a size of F7.0 120 mm and then used for the directional solidification experiments. 2.2. Directional solidification Directional solidification experiments were carried out after evacuating the chamber and breaking vacuum with a flow of inert argon gas. During directional solidification, the as-machined rods were placed into alumina tubes with an inner diameter of 7.3 mm. The temperature gradient adjacent to the solid/liquid interface in the liquid was measured as approximately 20 C/mm, and the pulling rate (V) was set as 65 mm/s. During the experiments, the super-austenitic stainless steel rods were first heated to 1500 C, and then the alumina with rods were pulled down with the given velocity for steady growth after stabilizing at 1500 C for approximately 5 min. The alumina tubes with the solidified stainless steel were quenched into the Ga-In-Sn cooling liquid immediately after 60 mm of solidified stainless steel was obtained. 2.3. Microstructure analysis Microstructure analysis was carried out by scanning electron microscope (SEM, FEI Quanta) and optical microscope (OM). Phase identification was carried out by X-Ray diffraction (XRD, TTR3 instrument using Cu Ka at a voltage of 50 kV and current of 120 mA) and transmission electron microscope (TEM, Tecnai G2 F20). Electron probe micro analysis (EPMA, SHIMADZU-1720H, with an accelerating voltage of 30 kV and an emission current of 1 mA) was used to identify the element distribution in the solid-liquid interface and the secondary phase composition of the S31254 superaustenitic stainless steel. The phase distribution in the solidified
3. Results and discussion 3.1. Precipitation of the secondary phase during solidification of S31254 stainless steel As shown in Fig. 1 and Fig. 2, pseudo-binary phase diagrams and Gulliver-Scheil solidification of the S31254 stainless steel were calculated with Thermo-Calc software (TCFE9 database). Apparently, S31254 stainless steel solidifies as single-phase austenite from the liquid, i.e., the equilibrium solidification path can be described as L / L þ g. The effect of Mo and Cr segregation on the solidification structure is shown in Fig. 1a and b. As shown in Fig. 1a, the solidified phase change from g to g þ s with increasing Mo content in the liquid steel. However, with an increase in Cr content in liquid steel, the solidified phase changes from g to g þ d. The GullivereScheil solidification model prediction reveals that the real solidification path is L / L þ g / L þ g þ d / L þ g þ d þ s (Fig. 2). The primary austenite is formed from the liquid, then a small amount of high-temperature d phase is formed, and the s phase appears during the last stage of solidification. Ferrite is precipitated in the middle of the process during solidification; however, this phase is rarely mentioned in previous studies. Furthermore, the segregation degree of Mo mainly controls the d phase and s phase precipitation. As shown in Fig. 1, the d phase precipitates through a eutectic reaction when the content of Mo and Cr in the liquid steel is increased to 8.37% and 22.98%, respectively. This is a divorced eutectic reaction due to the high percentage of primary g phase. The nucleation of d phase must take place in each individual droplet. Thus, nucleation in a finite liquid volume is more difficult and requires a relatively large undercooling degree. The primary g phase continues to grow before the precipitation of d phase, which further limits the space for d nucleation. The enrichment of Mo and Cr in residual liquid continues to increase during the growth process of the primary g phase. The s phase instead of the d phase nucleates and grows due to the low element segregation at the end of the solidification process. Therefore, the secondary phase in the solidified S31254 stainless steel structure is mainly the s phase. 3.2. Effect of Ce on solidification phase structure
Table 1 Chemical composition of the S31254 super-austenitic stainless steels (wt %). Element
Cr
Ni
Mo
N
Cu
Si
Mn
C
Ce
Fe
S31254 S31254eCe
20 20
18 18
6.2 6.2
0.2 0.2
0.5 0.5
0.5 0.5
0.7 0.7
0.015 0.015
0 0.016%
bal bal
Pseudo-binary phase diagrams were calculated to evaluate the effect of Ce element segregation on the solidification structure. As shown in Fig. 3, the equilibrium phase structure changes from g phase to d phase with increasing Ce content during solidification. Furthermore, a high content of Ce in residual liquid steel could
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Fig. 1. Binary phase diagrams of S31254 with (a) Mo segregation and (b) Cr segregation.
Fig. 2. Solidification process calculation based on the GullivereScheil model.
easily be obtained due to the low solubility in austenite, which promotes d phase precipitation in residual liquid. The effect of Ce addition on the Mo and Cr pseudo-binary phase diagram is shown in Fig. 4. As shown in Fig. 4, the region of d phase in both pseudobinary phase diagrams is extended with Ce addition, which indicates that the Ce improves the thermodynamic stability of the
d phase during solidification. As shown in Fig. 4a, the initial precipitation of the Mo for the d phase decreases from 6.8% to 6.56% with the addition of 0.1% Ce. As shown in Fig. 4b, the initial precipitation of the Cr for the d phase decreases from 20.81% to 20.56% with the addition of 0.1% Ce. The phase proportion in the final solidified structure was calculated by the Gulliver-Scheil model, and the results are shown in Table .2. As listed in Table .2, the proportion of d phase increases from 6.722% to 7.168%, and the proportion of s phase decreases from 0.938% to 0.898% with increasing Ce content from 0 to 0.1%. The solid fraction for the initial d phase precipitation decreases from 0.645 to 0.626 with a Ce addition from 0 to 0.1%, respectively. Thus, Ce addition expands the range of Mo content for d phase precipitation. 3.3. Thermal analyses of secondary phase precipitation during solidification
Fig. 3. Binary phase diagram of S31254 with Ce addition.
Since significant micro-segregation occurs during solidification, freezing in super-austenite stainless steel and Ni-base super alloys does not follow equilibrium conditions, although local equilibrium is only maintained at the solid/liquid interface [15]. Thus differential thermal analysis (DTA) was used to study the actual
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Fig. 4. Effect of Ce addition on (a) Mo and (b) Cr pseudo-binary phase diagram.
Table 2 Phase ratios of the final solidified structure.
g
0 0.016% 0.05% 0.1%
d
s
Phase ratio
Phase ratio
Solid fraction
Phase ratio
Solid fraction
91.357% 91.326% 91.168% 90.821%
6.722% 6.728% 6.864% 7.168%
0.645 0.644 0.629 0.626
0.938% 0.935% 0.906% 0.898%
0.948 0.948 0.948 0.948
solidification path. Cooling curves of DTA two specimens from the current study with 0% and 0.016% Ce content are shown in Fig. 5. In the DTA curves, two stages of solid phase precipitation from the liquid melt occurred, which corresponded to (i) austenitic precipitation from liquid (Tn1) at 1388.7 C and 1385.9 C and (ii) the eutectic reaction of austenite and secondary phase precipitation from the liquid (Tn2) at 1373.3 C and 1368.7 C. In the case of S31254 SASS, the solidification sequence follows the pattern, L / g followed by L / g þ secondary phase, which is similar in austenite stainless steel [16,17], NieFe-based alloy [18] and Ni-based alloy [19]. The austenite nucleation and secondary phase nucleation temperatures of S31254 were determined to be 1395.6 C and 1369.8 C, respectively, via the tangent method. Compared to S31254, the nucleation temperatures of austenite and the
secondary phase in S31254eCe were slightly increased by 1.5 C and 4.7 C, respectively. Furthermore, the value of DT (the temperature difference between Tn1 and Tn2, DT ¼ Tn1-Tn2) decreases from 25.8 C to 22.6 C, which indicates that a small Ce addition initiates the secondary phase nucleation and growth. The secondary phase precipitated in S31254eCe is the middle phase d due to the large difference in DT between that and S31254. Ce atoms can be enriched in the interdendritic solute-rich liquid due to the low solubility in austenite, which substantially increases the constitutional undercooling degree for d nucleation.
3.4. Microstructure and element segregation evolution during solidification The distribution coefficient of ki (ki ¼ C0i , Ci is the measured Ci content of element i in the primary dendrites, C 0i is the initial content of element i) can be obtained by the measured content in the primary dendrites. The chemical composition at the centre primary dendrites was stable and measured by using EPMA, and the results are shown in Table .3. As shown in Table .3, 0.0160% Ce addition decreases the distribution coefficient of kMo from 0.645 to 0.641 and increases the distribution coefficient of kCr from 0.967 to 0.969. Thus, the change of the distribution coefficient after 0.016% Ce addition is limited, which has a poor effect on element segregation. Fig. 6a and Fig. 6b show OM images of the solid-liquid interface. A small Ce addition enriches in solid-liquid interface due to the low solubility in austenite, which increases the undercooling degree in front of the interface and makes the solid-liquid interface unstable. Thus, as shown in Fig. 6, the interface of S31254 stainless steel is a cellular dendrite and changes to a dendrite with 0.016% Ce addition. The average primary arm spacing and secondary arm spacing were measured using OM images, and the results are shown in Table .3. Table .3 shows that primary arm spacing decreases from 227 mm to 170 mm, and the secondary arm spacing decreases from 43.1 mm to 30.1 mm. Thus, the Ce element refines the grain structure. The interface element distribution was measured by EPMA, and the result is shown in Fig. 7. As shown in Fig. 7, strong Cr and Mo element segregation can be observed at the front of the solid-liquid interface and interdendritic areas in S31254 stainless steel. The Cmax interdendritic ; Cmax interdendritic , Cmin dendrite arm interdendritic, Cmin dendrite arm
maximum segregation ratio SR (SR ¼
Fig. 5. Cooling peak in the measured DTA curves.
is the
is the maximum element content in minimum element content in dendrite arm) is shown in Table .3, which indicates that element segregation, especially Mo, suppresses with a 0.016% Ce addition. Thus, small amount of Ce
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Table 3 Measured results at the solid-liquid interface. S31524 Wt Mo Cr primary arm spacing secondary arm spacing
3.867% ± 0.058% 19.330% ± 0.065%
S31254eCe ki
0.645 0.967 227 mm ± 12.63 mm 43.1 mm ± 5.47 mm
SR
Wt
4.11 1.35
3.848% ± 0.044% 19.391% ± 0.090%
ki 0.641 0.969 170 mm ± 9.24 mm 30.1 mm ± 4.92 mm
SR 2.71 1.21
which means d phase is precipitated in S31254eCe. H. Inoue and T. Koseki [20] investigated the solidification mechanism of austenitic stainless steel solidified with d phase and showed that the ferrite and the austenite grew independently so that the crystal orientation did not influence a change in the secondary phase. Fig. 10 a-d shows OM, BSED (back scattered electron detector) and EBSD maps of the phase distribution in the S31254 and S31254eCe specimens. As shown in Fig. 10a and b, the secondary phase is clearly observed in the BSED and OM images and of Fig. 6. The microstructure of solid-liquid interface (a) S31254 and (b) S31254eCe.
addition suppresses the element segregation in interdendritic region by grain refinement, which prevents the precipitation of the s phase. 3.5. Microstructure characterization Fig. 8 shows the XRD (X-ray diffraction) patterns of S31254 and S31254eCe specimen cross-sections. As expected, a set of characteristic diffraction peaks of the austenite are present in both specimens, but they have different textures, which are influenced by Ce addition. Moreover, characteristic diffraction peaks of the s phase and d phase are observed in specimens S31254 and S31254eCe, respectively. TEM images of the secondary phase and steel matrix in S31254 with/without Ce addition were shown in Fig. 9a and Fig. 9b. As shown in Fig. 9a, the selected area electron diffraction (SAED) pattern of secondary phase of S31254 without Ce presents a tetragonal structure, which indicates s phase precipitation in S31254. However, the corresponding SAED pattern of secondary phase in Fig. 9b gives a typical body-centred cubic (bcc) structure,
Fig. 8. XRD results of S31254 and S31254eCe.
Fig. 7. Mo and Cr distribution on the solid-liquid interface and interdendritic region of (a) S31254 and (b) S31254eCe.
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Fig. 9. TEM micrograph for (a) S31254 and (b) S31254eCe.
Fig. 10. OM, BSED and EBSD phase distribution images for (aec) S31254 and (def) S31254eCe.
S31254. As shown in Fig. 10d and e, the contrast of the secondary phase is not obvious in the BSED image and cannot be observed in the OM images with the same magnification. The secondary phase in both samples were measured by EBSD and identified as s phase and d phase in S31254 and S31254eCe, respectively. As shown in Fig. 10c and f, 0.016% Ce addition significantly promotes the main secondary phase change from s phase to d phase in the final solidified structure.
precipitation temperature, which could be regarded as a homogenization treatment due to the relatively high temperature and slower cooling rate. Thus, changing the secondary phase from s phase to d phase in the S31254 stainless steel by a small Ce addition
3.6. Effect of Ce addition on the performance of the secondary phase Fig. 11a and Fig. 11b show the OM images of the final microstructure of the DTA samples. Microstructure of the S31254 specimen under a 5 C/min cooling rate generally comprises primary dendritic phases with a small number of secondary phases. However, the solidified secondary phase mainly disappeared in the S3154eCe specimen with the same cooling process. There is a window between the solidus line and the secondary phase
Fig. 11. OM images of the final microstructure of DTA samples (a) S31254 and (b) S31254eCe.
Q. Wang et al. / Journal of Alloys and Compounds 815 (2020) 152418 Table 4 The nano-hardness of the secondary phase on interdendritic zone. Test No.
1 2 3 4 5 6 7 8 Average
Hardness/GPa S31254
S31254eCe
5.35 4.99 7.91 6.57 7.09 5.12 6.39 4.59 6.00 ± 1.16
3.24 4.13 3.76 4.90 4.63 6.52 4.40 4.10 4.46 ± 0.97
7
significant effect on suppressing the precipitation of the s phase. An addition of Ce improves the thermal stability of d phase and expands the range of Mo content for d phase precipitation. 3 A small amount of Ce addition changes the secondary phase from s to d, which shortens the time required for homogenization and reduces the nano-hardness of the interdendritic secondary phase. Acknowledgments The authors would like to express their appreciation to the National Key Research and Development Program of China (2016YFB0300204), Fundamental Research Funds for the Central Universities (FRF-TP-18-012B1) and the National Nature Science Foundation of China (U1810207, 51774027 and 51734002). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152418. References
Fig. 12. OM images of typical nano-indentation in nano-hardness test.
could shorten the time required for homogenization. Nanoindentation has been widely used for characterizing the nanomechanical properties of micro-zone, because of its high sensitivity and the excellent resolution for obtaining the nano-hardness, in a relatively easier fashion [21e23]. The nano-hardness in the micro-zone of the interdendritic secondary phase of S31254 and S31254eCe was measured by a nano-hardness tester (AgilenU9820A Nano Indenter G200) in continuous stiffness mode, and the results are shown in Table .4. As shown in Table .4, the average nano-hardness on the secondary phase of S31254 and S31254eCe is 6.00 GPa and 4.46 GPa, respectively. Typical nano-indentations in the two specimens are shown in Fig. 12. Thus, changing the secondary phase from s to d with 0.016% Ce addition can shorten the time for homogenization and reduce the nano-hardness of the interdendritic secondary phase. 4. Conclusion The effect of Ce on the precipitation of the s phase in a superaustenitic stainless steel during solidification has been investigated by a combined experimental and modelling approach. It is found that: 1 The GullivereScheil solidification model prediction reveals that solidification path is L / L þ g / L þ g þ d / L þ g þ d þ s. The d phase is precipitated through a divorced eutectic reaction of L / g þ d. The nucleation of d phase occurs in each individual droplet, which is more difficult and requires a relatively high undercooling degree. Therefore, the secondary phase in S31254 stainless steel solidification structure is s phase. 2 A small Ce addition transforms the secondary phase in S31254 stainless steel from s to d. First, Ce atoms can be enriched in the interdendritic solute-rich liquid and solid-liquid interface due to the low solubility in austenite, which not only accelerates d nucleation but also refines the solidified structure. Moreover, a small amount of Ce addition suppresses element segregation in interdendritic region by grain refinement, which has a
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The influence of Ce micro-alloying on the precipitation of intermetallic sigma phase during solidification of super-austenitic stainless steels Qi Wang a, Lijun Wang a, *, Yanhui Sun a, Aimin Zhao a, Wei Zhang c, Jianmin Li c, Hongbiao Dong d, Kuochih Chou a, b a
Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing, 100083, PR China State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, Beijing, 100083, PR China Research Center, Taiyuan Iron and Steel (Group) Co., Ltd, Shanxi Province, 030003, PR China d Department of Engineering, University of Leicester, Leicester, UK b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 May 2019 Received in revised form 16 September 2019 Accepted 23 September 2019 Available online 24 September 2019
The influence of Ce addition on the precipitation of ferrite (d) and sigma (s) phase in super-austenitic stainless steels during solidification was investigated via a combined experimental and theoretical approach. Thermodynamic calculations reveal that the secondary phase of d and s are precipitated in sequence near the end of the solidification process due to the segregation of Mo and Cr. The Ce addition improves the thermal stability of the d, expands the range of Mo content for d phase precipitation and increases the d fraction in the solidified structure in the GullivereScheil model. Experimental observation shows that a small addition of Ce accelerates d nucleation in the interdendritic solute-rich liquid, refines the solidified structure and reduces Mo and Cr segregation, which suppresses precipitation of the s phase. The s phase precipitation is replaced by d phase precipitation after a Ce addition of 0.016%. The change in the secondary phase from s to d shortens the time required for homogenization and reduces the nano-hardness of the interdendritic secondary phase, which is significant for hot working. © 2019 Elsevier B.V. All rights reserved.
Keywords: Super-austenitic stainless steel Intermetallic compound Sigma phase Ce micro-alloying addition
1. Introduction Super-austenitic stainless steels (SASSs) have potential for use in extremely harsh service environments, such as for flue gas desulphurization and sea water desalination and also for applications in the petrochemical industry [1]. However, SASSs have a high content of alloying elements (>40%), especially Cr and Mo, which greatly reduces the microstructure stability and remarkably increases the precipitation sensitivity of secondary phase during solidification. The high levels of Cr and Mo promote the formation of secondary phase at the end of solidification due to strong segregation in the residual liquid. The secondary phase precipitates in an ingot of S31254 are mainly intermetallic sigma phase (s), which comprise (FeeCr) or (FeeCreMo) and negatively impacts the pitting resistance and mechanical properties [2,3]. Banovic S.W [4] noted that Mo is strongly segregated in the liquid during solidification of AL-6XN alloys and that the solidification terminates by a eutectic L / g þ s reaction. Generally, the size of s phases formed upon
* Corresponding author. E-mail address: [email protected] (L. Wang). https://doi.org/10.1016/j.jallcom.2019.152418 0925-8388/© 2019 Elsevier B.V. All rights reserved.
solidification is much larger than that formed upon solid-state transformation [5]. Due to its large size, it is difficult to redissolve the s phase during a post-solidification heat treatment, which deteriorates the hot working properties and leads to cracking of continuous-casting billets during hot rolling. Thus, it is very important to minimize the segregation and then control the precipitation of the s phase in the final solidified structure. Generally speaking, micro-structural refinement can minimize the segregation in steel ingots or castings, which is significant in SASSs, during solidification. The currently available grain refinement techniques used during steel casting can be categorized into three groups: electromagnetic stirring (EMS), fast cooling and chemical grain refinement [6]. However, EMS and fast cooling are limited by the actual casting equipment. REs (rare earths) are effective chemical grain refiners, and RE addition is an alternative approach to grain refinement in steel. Bartlett et al. [7] and Haakosen [8] both reported a reduction in the grain size in cast austenitic high-manganese steel after the addition of cerium. Many studies [9e14] indicate that grain refinement is due to the heterogeneous nucleation of austenite or ferrite on RE inclusions, including oxides, sulphides and oxysulfides, and solute effects. Thus, a small Ce addition is viable to refine the austenitic grain structure.
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The aim of this work is to control the precipitation of the intermetallic s phase in SASSs. A small Ce addition could refine the austenite grain size, which would minimize the segregation in interdendritic areas. In this study, the effect of Ce on the precipitation of the s phase in SASSs during solidification is investigated via a combined modelling and experimental approach using a Thermo-Calc simulation, a directional solidification test, and electron microscopy.
microstructure was determined by the scanning electron microscope that was equipped with electron back scatter diffraction (EBSD) and an HKL Channel 5 data acquisition and analysis system. The step size used for the EBSD analyses was 0.5 mm. The EBSD samples were mechanically polished, followed by electropolishing in a solution of 10% perchloric acid and 90% alcohol at approximately 25 C for approximately 30 s with an applied potential of 15 V.
2. Materials and experiments
2.4. Thermal analysis
2.1. Materials
Thermal analysis was conducted on a differential thermal analyzer (Netzsch STA 449C), and a dynamic high-purity argon atmosphere was used to minimize potential oxidizing effects. The specimen (400 mg) was subjected to the following heating/cooling cycles: room temperature / heating (5 C/min) to 1500 C / isothermal holding at 1500 C for 5 min / cooling (5 C/min) to room temperature.
In experiments with Ce addition, commercial S31254 superaustenite stainless steel (500 g) was melted at 1500 C in an MgO crucible, and the crucible was placed in a MoSi2 resistance furnace with the protection of high-purity Ar gas. To protect the Ce metal (99.9%, Aladdin) from oxidation, it was wrapped in a reduced iron powder and pressed into a metal cylinder, and then the metal cylinder was wrapped in pure iron chips and added to the molten steel. By stirring the molten steel with a quartz tube, good Ce homogeneity could be achieved. After homogenization, the molten steel was removed and quenched in water. In the present work, the chemical composition (wt%) of the S31254 stainless steel used in this work was measured by inductively coupled plasma mass spectrometry (ICP-MS), as shown in Table .1. These ingots were cut into samples with a size of F7.0 120 mm and then used for the directional solidification experiments. 2.2. Directional solidification Directional solidification experiments were carried out after evacuating the chamber and breaking vacuum with a flow of inert argon gas. During directional solidification, the as-machined rods were placed into alumina tubes with an inner diameter of 7.3 mm. The temperature gradient adjacent to the solid/liquid interface in the liquid was measured as approximately 20 C/mm, and the pulling rate (V) was set as 65 mm/s. During the experiments, the super-austenitic stainless steel rods were first heated to 1500 C, and then the alumina with rods were pulled down with the given velocity for steady growth after stabilizing at 1500 C for approximately 5 min. The alumina tubes with the solidified stainless steel were quenched into the Ga-In-Sn cooling liquid immediately after 60 mm of solidified stainless steel was obtained. 2.3. Microstructure analysis Microstructure analysis was carried out by scanning electron microscope (SEM, FEI Quanta) and optical microscope (OM). Phase identification was carried out by X-Ray diffraction (XRD, TTR3 instrument using Cu Ka at a voltage of 50 kV and current of 120 mA) and transmission electron microscope (TEM, Tecnai G2 F20). Electron probe micro analysis (EPMA, SHIMADZU-1720H, with an accelerating voltage of 30 kV and an emission current of 1 mA) was used to identify the element distribution in the solid-liquid interface and the secondary phase composition of the S31254 superaustenitic stainless steel. The phase distribution in the solidified
3. Results and discussion 3.1. Precipitation of the secondary phase during solidification of S31254 stainless steel As shown in Fig. 1 and Fig. 2, pseudo-binary phase diagrams and Gulliver-Scheil solidification of the S31254 stainless steel were calculated with Thermo-Calc software (TCFE9 database). Apparently, S31254 stainless steel solidifies as single-phase austenite from the liquid, i.e., the equilibrium solidification path can be described as L / L þ g. The effect of Mo and Cr segregation on the solidification structure is shown in Fig. 1a and b. As shown in Fig. 1a, the solidified phase change from g to g þ s with increasing Mo content in the liquid steel. However, with an increase in Cr content in liquid steel, the solidified phase changes from g to g þ d. The GullivereScheil solidification model prediction reveals that the real solidification path is L / L þ g / L þ g þ d / L þ g þ d þ s (Fig. 2). The primary austenite is formed from the liquid, then a small amount of high-temperature d phase is formed, and the s phase appears during the last stage of solidification. Ferrite is precipitated in the middle of the process during solidification; however, this phase is rarely mentioned in previous studies. Furthermore, the segregation degree of Mo mainly controls the d phase and s phase precipitation. As shown in Fig. 1, the d phase precipitates through a eutectic reaction when the content of Mo and Cr in the liquid steel is increased to 8.37% and 22.98%, respectively. This is a divorced eutectic reaction due to the high percentage of primary g phase. The nucleation of d phase must take place in each individual droplet. Thus, nucleation in a finite liquid volume is more difficult and requires a relatively large undercooling degree. The primary g phase continues to grow before the precipitation of d phase, which further limits the space for d nucleation. The enrichment of Mo and Cr in residual liquid continues to increase during the growth process of the primary g phase. The s phase instead of the d phase nucleates and grows due to the low element segregation at the end of the solidification process. Therefore, the secondary phase in the solidified S31254 stainless steel structure is mainly the s phase. 3.2. Effect of Ce on solidification phase structure
Table 1 Chemical composition of the S31254 super-austenitic stainless steels (wt %). Element
Cr
Ni
Mo
N
Cu
Si
Mn
C
Ce
Fe
S31254 S31254eCe
20 20
18 18
6.2 6.2
0.2 0.2
0.5 0.5
0.5 0.5
0.7 0.7
0.015 0.015
0 0.016%
bal bal
Pseudo-binary phase diagrams were calculated to evaluate the effect of Ce element segregation on the solidification structure. As shown in Fig. 3, the equilibrium phase structure changes from g phase to d phase with increasing Ce content during solidification. Furthermore, a high content of Ce in residual liquid steel could
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Fig. 1. Binary phase diagrams of S31254 with (a) Mo segregation and (b) Cr segregation.
Fig. 2. Solidification process calculation based on the GullivereScheil model.
easily be obtained due to the low solubility in austenite, which promotes d phase precipitation in residual liquid. The effect of Ce addition on the Mo and Cr pseudo-binary phase diagram is shown in Fig. 4. As shown in Fig. 4, the region of d phase in both pseudobinary phase diagrams is extended with Ce addition, which indicates that the Ce improves the thermodynamic stability of the
d phase during solidification. As shown in Fig. 4a, the initial precipitation of the Mo for the d phase decreases from 6.8% to 6.56% with the addition of 0.1% Ce. As shown in Fig. 4b, the initial precipitation of the Cr for the d phase decreases from 20.81% to 20.56% with the addition of 0.1% Ce. The phase proportion in the final solidified structure was calculated by the Gulliver-Scheil model, and the results are shown in Table .2. As listed in Table .2, the proportion of d phase increases from 6.722% to 7.168%, and the proportion of s phase decreases from 0.938% to 0.898% with increasing Ce content from 0 to 0.1%. The solid fraction for the initial d phase precipitation decreases from 0.645 to 0.626 with a Ce addition from 0 to 0.1%, respectively. Thus, Ce addition expands the range of Mo content for d phase precipitation. 3.3. Thermal analyses of secondary phase precipitation during solidification
Fig. 3. Binary phase diagram of S31254 with Ce addition.
Since significant micro-segregation occurs during solidification, freezing in super-austenite stainless steel and Ni-base super alloys does not follow equilibrium conditions, although local equilibrium is only maintained at the solid/liquid interface [15]. Thus differential thermal analysis (DTA) was used to study the actual
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Fig. 4. Effect of Ce addition on (a) Mo and (b) Cr pseudo-binary phase diagram.
Table 2 Phase ratios of the final solidified structure.
g
0 0.016% 0.05% 0.1%
d
s
Phase ratio
Phase ratio
Solid fraction
Phase ratio
Solid fraction
91.357% 91.326% 91.168% 90.821%
6.722% 6.728% 6.864% 7.168%
0.645 0.644 0.629 0.626
0.938% 0.935% 0.906% 0.898%
0.948 0.948 0.948 0.948
solidification path. Cooling curves of DTA two specimens from the current study with 0% and 0.016% Ce content are shown in Fig. 5. In the DTA curves, two stages of solid phase precipitation from the liquid melt occurred, which corresponded to (i) austenitic precipitation from liquid (Tn1) at 1388.7 C and 1385.9 C and (ii) the eutectic reaction of austenite and secondary phase precipitation from the liquid (Tn2) at 1373.3 C and 1368.7 C. In the case of S31254 SASS, the solidification sequence follows the pattern, L / g followed by L / g þ secondary phase, which is similar in austenite stainless steel [16,17], NieFe-based alloy [18] and Ni-based alloy [19]. The austenite nucleation and secondary phase nucleation temperatures of S31254 were determined to be 1395.6 C and 1369.8 C, respectively, via the tangent method. Compared to S31254, the nucleation temperatures of austenite and the
secondary phase in S31254eCe were slightly increased by 1.5 C and 4.7 C, respectively. Furthermore, the value of DT (the temperature difference between Tn1 and Tn2, DT ¼ Tn1-Tn2) decreases from 25.8 C to 22.6 C, which indicates that a small Ce addition initiates the secondary phase nucleation and growth. The secondary phase precipitated in S31254eCe is the middle phase d due to the large difference in DT between that and S31254. Ce atoms can be enriched in the interdendritic solute-rich liquid due to the low solubility in austenite, which substantially increases the constitutional undercooling degree for d nucleation.
3.4. Microstructure and element segregation evolution during solidification The distribution coefficient of ki (ki ¼ C0i , Ci is the measured Ci content of element i in the primary dendrites, C 0i is the initial content of element i) can be obtained by the measured content in the primary dendrites. The chemical composition at the centre primary dendrites was stable and measured by using EPMA, and the results are shown in Table .3. As shown in Table .3, 0.0160% Ce addition decreases the distribution coefficient of kMo from 0.645 to 0.641 and increases the distribution coefficient of kCr from 0.967 to 0.969. Thus, the change of the distribution coefficient after 0.016% Ce addition is limited, which has a poor effect on element segregation. Fig. 6a and Fig. 6b show OM images of the solid-liquid interface. A small Ce addition enriches in solid-liquid interface due to the low solubility in austenite, which increases the undercooling degree in front of the interface and makes the solid-liquid interface unstable. Thus, as shown in Fig. 6, the interface of S31254 stainless steel is a cellular dendrite and changes to a dendrite with 0.016% Ce addition. The average primary arm spacing and secondary arm spacing were measured using OM images, and the results are shown in Table .3. Table .3 shows that primary arm spacing decreases from 227 mm to 170 mm, and the secondary arm spacing decreases from 43.1 mm to 30.1 mm. Thus, the Ce element refines the grain structure. The interface element distribution was measured by EPMA, and the result is shown in Fig. 7. As shown in Fig. 7, strong Cr and Mo element segregation can be observed at the front of the solid-liquid interface and interdendritic areas in S31254 stainless steel. The Cmax interdendritic ; Cmax interdendritic , Cmin dendrite arm interdendritic, Cmin dendrite arm
maximum segregation ratio SR (SR ¼
Fig. 5. Cooling peak in the measured DTA curves.
is the
is the maximum element content in minimum element content in dendrite arm) is shown in Table .3, which indicates that element segregation, especially Mo, suppresses with a 0.016% Ce addition. Thus, small amount of Ce
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Table 3 Measured results at the solid-liquid interface. S31524 Wt Mo Cr primary arm spacing secondary arm spacing
3.867% ± 0.058% 19.330% ± 0.065%
S31254eCe ki
0.645 0.967 227 mm ± 12.63 mm 43.1 mm ± 5.47 mm
SR
Wt
4.11 1.35
3.848% ± 0.044% 19.391% ± 0.090%
ki 0.641 0.969 170 mm ± 9.24 mm 30.1 mm ± 4.92 mm
SR 2.71 1.21
which means d phase is precipitated in S31254eCe. H. Inoue and T. Koseki [20] investigated the solidification mechanism of austenitic stainless steel solidified with d phase and showed that the ferrite and the austenite grew independently so that the crystal orientation did not influence a change in the secondary phase. Fig. 10 a-d shows OM, BSED (back scattered electron detector) and EBSD maps of the phase distribution in the S31254 and S31254eCe specimens. As shown in Fig. 10a and b, the secondary phase is clearly observed in the BSED and OM images and of Fig. 6. The microstructure of solid-liquid interface (a) S31254 and (b) S31254eCe.
addition suppresses the element segregation in interdendritic region by grain refinement, which prevents the precipitation of the s phase. 3.5. Microstructure characterization Fig. 8 shows the XRD (X-ray diffraction) patterns of S31254 and S31254eCe specimen cross-sections. As expected, a set of characteristic diffraction peaks of the austenite are present in both specimens, but they have different textures, which are influenced by Ce addition. Moreover, characteristic diffraction peaks of the s phase and d phase are observed in specimens S31254 and S31254eCe, respectively. TEM images of the secondary phase and steel matrix in S31254 with/without Ce addition were shown in Fig. 9a and Fig. 9b. As shown in Fig. 9a, the selected area electron diffraction (SAED) pattern of secondary phase of S31254 without Ce presents a tetragonal structure, which indicates s phase precipitation in S31254. However, the corresponding SAED pattern of secondary phase in Fig. 9b gives a typical body-centred cubic (bcc) structure,
Fig. 8. XRD results of S31254 and S31254eCe.
Fig. 7. Mo and Cr distribution on the solid-liquid interface and interdendritic region of (a) S31254 and (b) S31254eCe.
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Fig. 9. TEM micrograph for (a) S31254 and (b) S31254eCe.
Fig. 10. OM, BSED and EBSD phase distribution images for (aec) S31254 and (def) S31254eCe.
S31254. As shown in Fig. 10d and e, the contrast of the secondary phase is not obvious in the BSED image and cannot be observed in the OM images with the same magnification. The secondary phase in both samples were measured by EBSD and identified as s phase and d phase in S31254 and S31254eCe, respectively. As shown in Fig. 10c and f, 0.016% Ce addition significantly promotes the main secondary phase change from s phase to d phase in the final solidified structure.
precipitation temperature, which could be regarded as a homogenization treatment due to the relatively high temperature and slower cooling rate. Thus, changing the secondary phase from s phase to d phase in the S31254 stainless steel by a small Ce addition
3.6. Effect of Ce addition on the performance of the secondary phase Fig. 11a and Fig. 11b show the OM images of the final microstructure of the DTA samples. Microstructure of the S31254 specimen under a 5 C/min cooling rate generally comprises primary dendritic phases with a small number of secondary phases. However, the solidified secondary phase mainly disappeared in the S3154eCe specimen with the same cooling process. There is a window between the solidus line and the secondary phase
Fig. 11. OM images of the final microstructure of DTA samples (a) S31254 and (b) S31254eCe.
Q. Wang et al. / Journal of Alloys and Compounds 815 (2020) 152418 Table 4 The nano-hardness of the secondary phase on interdendritic zone. Test No.
1 2 3 4 5 6 7 8 Average
Hardness/GPa S31254
S31254eCe
5.35 4.99 7.91 6.57 7.09 5.12 6.39 4.59 6.00 ± 1.16
3.24 4.13 3.76 4.90 4.63 6.52 4.40 4.10 4.46 ± 0.97
7
significant effect on suppressing the precipitation of the s phase. An addition of Ce improves the thermal stability of d phase and expands the range of Mo content for d phase precipitation. 3 A small amount of Ce addition changes the secondary phase from s to d, which shortens the time required for homogenization and reduces the nano-hardness of the interdendritic secondary phase. Acknowledgments The authors would like to express their appreciation to the National Key Research and Development Program of China (2016YFB0300204), Fundamental Research Funds for the Central Universities (FRF-TP-18-012B1) and the National Nature Science Foundation of China (U1810207, 51774027 and 51734002). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152418. References
Fig. 12. OM images of typical nano-indentation in nano-hardness test.
could shorten the time required for homogenization. Nanoindentation has been widely used for characterizing the nanomechanical properties of micro-zone, because of its high sensitivity and the excellent resolution for obtaining the nano-hardness, in a relatively easier fashion [21e23]. The nano-hardness in the micro-zone of the interdendritic secondary phase of S31254 and S31254eCe was measured by a nano-hardness tester (AgilenU9820A Nano Indenter G200) in continuous stiffness mode, and the results are shown in Table .4. As shown in Table .4, the average nano-hardness on the secondary phase of S31254 and S31254eCe is 6.00 GPa and 4.46 GPa, respectively. Typical nano-indentations in the two specimens are shown in Fig. 12. Thus, changing the secondary phase from s to d with 0.016% Ce addition can shorten the time for homogenization and reduce the nano-hardness of the interdendritic secondary phase. 4. Conclusion The effect of Ce on the precipitation of the s phase in a superaustenitic stainless steel during solidification has been investigated by a combined experimental and modelling approach. It is found that: 1 The GullivereScheil solidification model prediction reveals that solidification path is L / L þ g / L þ g þ d / L þ g þ d þ s. The d phase is precipitated through a divorced eutectic reaction of L / g þ d. The nucleation of d phase occurs in each individual droplet, which is more difficult and requires a relatively high undercooling degree. Therefore, the secondary phase in S31254 stainless steel solidification structure is s phase. 2 A small Ce addition transforms the secondary phase in S31254 stainless steel from s to d. First, Ce atoms can be enriched in the interdendritic solute-rich liquid and solid-liquid interface due to the low solubility in austenite, which not only accelerates d nucleation but also refines the solidified structure. Moreover, a small amount of Ce addition suppresses element segregation in interdendritic region by grain refinement, which has a
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