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Microstructure and high-temperature strength of high Cr ODS tempered martensitic steels
Journal of Nuclear Materials 442 (2013) S89–S94
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Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat
Microstructure and high-temperature strength of high Cr ODS tempered martensitic steels S. Ohtsuka ⇑, T. Kaito, T. Tanno, Y. Yano, S. Koyama, K. Tanaka Japan Atomic Energy Agency, 4002, Narita, Oarai, Ibaraki 311-1393, Japan
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Article history: Available online 19 June 2013
a b s t r a c t 11-12Cr oxide dispersion strengthened (ODS) tempered martensitic steels underwent manufacturing tests and their ferritic–martensitic duplex structures were quantitatively evaluated by three methods: high-temperature X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and metallography. It was demonstrated that excessive formation of residual-a ferrite, due to increasing Cr content, could be suppressed by appropriately controlling the concentration of the ferrite-forming and austenite-forming elements on the basis of the parameter ‘‘chemical driving force of a to c reverse transformation. 11CrODS steel containing a small portion of residual-a ferrite was successfully manufactured. In the asreceived condition, this 11Cr-ODS steel was shown to have satisfactory creep strength and ductility, both as high as those of the 9Cr-ODS steel, while its 0.2% proof strength at 973 K was lower than in the 9CrODS steel. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxide dispersion strengthened (ODS) steels have been developed as high-temperature and radiation-resistant core materials used in advanced type reactors (e.g., fusion and advanced fast breeder reactors) [1–7]. ODS steels having a ferritic matrix are classified into two types: ODS ferritic and ODS tempered martensitic steels. The alloy design in the former permits a high concentration of Cr, exceeding 12 wt%, which offers an advantage in terms of corrosion resistance over the ODS tempered martensitic steels for which Cr concentration is limited below 12 wt%. ODS tempered martensitic steels are expected to have better radiation resistance due to a high concentration of sink sites for irradiation defects in the matrix. In addition, they have better workability, which is provided by the high-temperature heat treatment for softening using the ferrite–austenite phase transformation [1]. In the development of ODS tempered martensitic steels for nuclear applications, a Cr concentration of 8–9 wt% is preferable to suppress the ductility loss by irradiation hardening and improve the microstructure stability and creep strength at high temperature. Conversely, from the viewpoint of corrosion resistance, Cr concentration as high as 11– 12 wt%Cr is preferable. A recent study revealed that 12CrODS ferritic steel with a recrystallized microstructure showed suitable ductility after neutron irradiation to 17 dpa at 693 K in JOYO [5]. This result suggests that high Cr ODS tempered martensitic steels containing 11–12 wt% Cr could be candidates for a new radiation-resistant material with better corrosion resistance than ⇑ Corresponding author. Tel.: +81 029 267 4141; fax: +81 029 267 7130 E-mail address: [email protected] (S. Ohtsuka). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.06.010
9Cr-ODS steel. In this study, manufacturing tests and characterizations of the matrix duplex structure were carried out as a first step in the development of high Cr ODS tempered martensitic steels. Thermo-dynamic calculations were used to find a methodology for control of the matrix duplex structure. In addition, mechanical properties were evaluated in the as-received condition and compared with those of 9Cr-ODS steel. 2. Procedure 2.1. Manufacturing of high Cr ODS tempered martensitic steels Chemical compositions of high Cr ODS tempered martensitic steels were selected based on the phase diagram constructed by the Thermo-calc code and the SSOL4/SSUB3 database [8]. The chemical compositions of the high Cr ODS tempered martensitic steels manufactured are shown in Table 1. The high Cr ODS tempered martensitic steels were manufactured using a series of technologies, developed by Japan Atomic Energy Agency (JAEA) for fast breeder reactor fuel cladding tubes [1], consisting of mechanical alloying (MA), canning, and consolidating by hot-extrusion. The mixture of raw material powders were mechanically alloyed in an attrition-type ball mill with a rotating speed of 220 rpm for 48 h in high purity argon atmosphere (99.9999 wt% Ar). The MA conditions were judged to be well optimized on the basis of electron probe micro analyzer (EPMA) mapping views in 1000 times magnification, which demonstrated the uniform distribution of Cr and W in MA powder produced with elementary powders. The reference 9Cr-ODS steels (9CR, 9CRTI) was made from Ar-gas atomized alloy powder and oxide (Y2O3) powder; the rest of steels
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Table 1 Chemical analysis results of high-Cr ODS tempered martensitic steels. ID
9CR 9CRTI 12CRTI-NiO 12CRTI15 W-Ni 12CRTI15 W-O 11CRTI15 W-lowNi
Chemical analysis result (wt%)
Calculated (wt%)
C
Si
Mn
P
S
Ni
Cr
W
Ti
Y
O
N
Ar
Y2O3
ExO
0.14 0.13 0.15 0.16 0.16 0.16
0.06 0.08 <0.01 0.01 <0.01 0.01
0.04 0.12 <0.01 <0.01 <0.01 <0.01
<0.005 <0.005 <0.005 <0.005 <0.005 0.018
0.003 0.006 0.003 0.002 0.002 0.002
0.02 0.04 0.78 0.73 0.01 0.38
8.9 9.0 12.1 11.8 11.7 11.0
2.0 2.0 1.9 1.5 1.5 1.4
0.20 0.28 0.29 0.28 0.29 0.28
0.27 0.28 0.28 0.27 0.28 0.27
0.14 0.17 0.24 0.18 0.23 0.16
0.020 0.014 0.015 0.010 0.007 0.009
0 0.01 0.01 0.01 0.01 0.01
0.34 0.36 0.36 0.34 0.36 0.34
0.07 0.09 0.16 0.11 0.15 0.09
were made from elemental powders (Fe, Cr, C, W, Ti) and Y2O3 powder. The hot-extrusion for consolidation of MA powder was conducted at 1423 K for all the steels manufactured in this study. The extruded round bars were normalized and tempered (1323 K 1 h, Ar-gas cooling => 1073 K 1 h, Ar-gas cooling). The cooling rates were roughly 2000–3000 K/h. 2.2. Duplex microstructure characterization The ferrite phase in tempered martensite matrix of ODS steels has been called both d-ferrite and residual-a ferrite. In general, the ferrite phase in the austenite matrix at the austenitization heat treatment is called d-ferrite in the production process of heat-resistant steels by a melting method. However, in the manufacturing of ODS steels, alloying is not performed by high-temperature melting but by mechanical alloying (MA). MA does not use a higher temperature than the c loop temperature. Ukai et al. [1] designated this ferrite phase as residual-a ferrite. In this paper, the same designation is adopted. Cr is a ferrite-forming element, so its addition in excess will result in excessive residual-a ferrite formation. Incorporating the residual-a ferrite in tempered martensite matrix improves the high-temperature strength of ODS tempered martensitic steel [1,6], however, it produces anisotropy in the mechanical properties through working processes such as cold-rolling. This is because the ferrite phase in ODS steels is anisotropically elongated by the working process and not easily recrystallized, even by high-temperature annealing, due to the presence of oxide particles [7]. Moreover, thermal aging embrittlement by Laves phase precipitation may occur in excessive residual-a ferrite, because Tungsten (W) concentration in residual-a ferrite is higher than in tempered martensite. To identify the basic chemical composition with the smallest amount of residual-a ferrite while increasing Cr, several high Cr ODS tempered martensitic steels were manufactured and their residual-a ferrite proportions quantitatively measured. Three methods were applied for measurements: high-temperature X-ray diffraction (XRD), electron probe microanalyzer (EPMA) mapping, and metallography. 2.2.1. High-temperature XRD High-temperature XRD measurements were carried out using X’ Pert-PRO (Spectris Co., Ltd.) with Mo K-a as the X-ray source. A powder sample was filed off the ODS bulk material and used for the measurement without considering the effect of preferred orientation. The XRD measurement was carried out in vacuum (1 Pa) at 1323 K to suppress sample oxidation. The volume fraction of residual a-ferrite was evaluated from the peak intensity of several crystal planes in austenite (c) and ferrite (a) matrices measured by XRD. High intensity peaks were used for the analysis (i.e., (1 1 0), (2 0 0), (2 1 1) planes in the a phase and (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes in the c phase). The volume fraction of residual a-ferrite (Va) was calculated using the following equations:
Va ¼
Aa ; Aa þ Ac
ð1Þ
Aa ¼
/ IODS ð1 1 0Þ I/ODS ð2 0 0Þ I/ODS ð2 1 1Þ =3; þ þ I/O ð1 1 0Þ I/O ð2 0 0Þ I/O ð2 1 1Þ
ð2Þ
Ac ¼
c IODS ð1 1 1Þ IcODS ð2 0 0Þ IcODS ð2 2 0Þ IcODS ð3 1 1Þ þ c þ c þ c =4: c IO ð1 1 1Þ IO ð2 0 0Þ IO ð2 2 0Þ IO ð3 1 1Þ
ð3Þ
The symbol IxODS ðijkÞ (X = a, c) is the peak intensity of the crystal plane in phase X of the ODS steel powder sample measured by high-temperature XRD. The symbol Ix0 ðijkÞ (X = a, c) is the peak intensity of the crystal plane in the single phase X of pure Fe, which is derived from the Joint Committee Power Diffraction Standards (JCPDS) database. The volume fraction of residual a-ferrite (Va) calculated using Eqs. (1)–(3) was corrected by the standard curve. The standard curve was prepared as follows. 12Cr-ODS steel (Fe– 12 wt%Cr–0.04C-2W–0.2Ti–0.25Y2O3) and 9Cr–1Mo steel were used as the a-ferrite single phase sample and austenite single phase sample, respectively, at 1323 K. These two samples were each ground into powder. The two powders were mixed at eight ratios: a-ferrite volume fractions of 1%, 2%, 5%, 10%, 25%, 50%, 80%, and 90%. These standard powder samples were subjected to the high-temperature XRD measurement and the residual-a ferrite volume fraction (Va) was calculated from the measured peak intensities using Eqs. (1)–(3). The standard curve was derived by plotting the mixture fractions of a-ferrite in standard powder samples (i.e. the actual volume fraction of residual a phase) as a function of the calculated volume fraction (Va) from the measured peak intensities and Eqs. (1)–(3). 2.2.2. EPMA mapping W is a ferrite-forming element with a higher partitioning ratio into ferrite than into austenite. It follows that W preferentially partitions into ferrite in the ferrite–austenite duplex matrix at austinization heat treatment [9]. W distribution in ODS steels was evaluated using EPMA of a longitudinal section after 1323 K 1 h annealing followed by rapid cooling. The area fraction of high W content, which is equivalent to the area of residual a-ferrite, was evaluated by image analysis. Four images with a size of 80 80 lm (for a total size of 160 160 lm) were used for the analysis of each sample. 2.2.3. Metallography Metallographic observations were carried out in the longitudinal section after the 1323 K 1 h annealing followed by rapid cooling, in which the Cr and W concentration gap between ferrite and austenite was considerably high. Chemical etching was carried out with picral for an appropriate period to ensure the residual-a ferrite clearly appeared in the metallographs. For each sample, one image (200 250 lm) was used for the analysis.
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2.3. Thermodynamic analysis The chemical driving force of a to c reverse transformation at 1323 K was calculated using the Thermo-calc code and TCFE3/ SSUB3 database [8]. Concentrations of major constituents such as Fe, C, Ni, Cr, W, Ti, Y, and O were applied to the calculation. In the first step, the Gibbs energy at 1323 K was calculated for the various compositions of manufactured ODS steels in which, according to the calculation, the main matrix was austenite. In the next step, the austenite phase was set to be dormant to get the Gibbs energy of the ferrite matrix system at 1323 K. The chemical driving force for a to c reverse transformation at 1323 K was calculated by subtracting the calculated Gibbs energy of the austenite matrix system from that of the ferrite matrix system. 2.4. Mechanical properties evaluation The uni-axial creep rupture test at 973 K and uni-axial tensile tests at RT and 973 K were carried out for one of the high Cr ODS tempered martensitic steels, in which the residual-a ferrite proportion was controlled to be lower than in 9Cr-ODS steel. The direction of stress loading in the creep and tensile tests was parallel to the hot extrusion direction. The test results were compared with those of the 9Cr-ODS steel. 3. Results 3.1. Selection of chemical composition The basic chemical compositions of high Cr ODS tempered martensitic steels were examined on the basis of the information for 9Cr-ODS steel. A few researchers have reported data for 9Cr-ODS steel showing good high-temperature strength and radiation resistance [1,4,5]. Thus, modification of the chemical composition other than Cr content was examined to minimize the trade-off effect produced by increasing the Cr concentration (i.e., degradations in the resistance to irradiation embrittlement, the microstructure stability, and the mechanical properties at high temperature). Increasing Cr concentration from 9 wt% to 11–12 wt% is known to be effective for improvement of corrosion resistance. The 12CrODS steel, with a recrystallized microstructure and the conventional heat-resistant steel HT9 and PNC-FMS, contains 11–12 wt% Cr and was reported to have adequate ductility after neutron irradiation from 15 to
Fig. 1. Multi-component phase diagram of Fe-9–12 wt%Cr-0.15C-0.3Ti-0.1Ex.OxW-yNi system (x,y: variables) at 1323 K constructed using the Thermo-calc code and TCFE3/SSUB3 database [8].
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33 dpa at 693 K [5,10]. Thus, the Cr concentration of the high Cr ODS tempered martensitic steels was set to 11–12 wt% in this study. Fig. 1 shows the phase boundary of the Fe-9–12 wt%Cr-0.15C– 0.3Ti–0.1O–xW–yNi system (x, y: variables) at 1323 K calculated by the Thermo-calc code and TCFE3/SSUB3 database [8]. In the composition containing 9 wt%Cr–2 W–0Ni, the heat treatment at 1323 K annealing followed by rapid cooling produces the martensite single phase matrix (i.e. no residual-a ferrite) as shown in Fig. 1. However, in contrast to Fig. 1, residual-a ferrite is actually formed in 9Cr–2 W–0Ni-ODS steel [1,6]. It is believed that pinning of the ac interface by oxide particles suppresses the a to c reverse transformation and produces the residual-a ferrite in the nonequilibrium state [11]. It can certainly be said that the formation of duplex matrix in ODS tempered martensitic steels is easier than in the conventional tempered martensitic steels produced by the melting method. Thus, it follows that the minor adjustment of chemical composition (e.g., decreasing the ferrite-forming element and increasing the austenite-forming element) is required to prevent excessive residual-a ferrite formation in 11-12Cr ODS tempered martensitic steels. Among the constituent elements of 9CrODS steels (Fe, C, Cr, W, Ti), the ferrite-forming elements besides Cr are W and Ti, and the austenite-forming element is C. The main strengthening factor of ODS steels is the oxide dispersion. Solute W certainly strengthens the steel by a solid-solution strengthening; however, that strength decreases under long-term thermal aging at 773–943 K due to Laves phase precipitation. Decreasing the W concentration has been reported to be effective for improving toughness and ductility in ferritic steels [12]. In this study, W content was decreased to suppress excessive residual-a formation. The addition of Ti is known to lead to fine oxide particles, which are densely dispersed in the matrix, notably increasing ODS [7,13]. To make up for the strength degradation caused by decreasing the W content, the Ti concentration was increased to 0.3 wt%, which is reported to be the optimum concentration in terms of creep strength improvement [6]. Increasing C concentration was excluded as an option, because C causes Cr depletion of the matrix through Cr carbide formation. Ni and Mn are often used in manufacturing conventional heat-resistant ferritic steel to decrease dferrite formation and produce the martensite single phase matrix. However, Ni addition was not desirable here from the viewpoint of irradiation hardening because it is reported to enhance irradiation hardening and ductility loss in low temperature neutron irradiation below 673 K [14]. For the same level of Ni equivalent, the decline in the Ac1 point by Mn addition is larger than in Ni addition [15]. This study attached greater importance to high-temperature performance (i.e. suppressing the drop in the Ac1 point) than the ductility after irradiation below 673 K, and Ni was chosen as a minor constitutive element suppressing formation of excessive residual-a ferrite. Based on the idea described above, high Cr ODS tempered martensitic steels with different concentrations of W and Ni were manufactured for this study. The impurity element O was also chosen as a parameter, because it considerably decreases the residual-a ferrite proportion by causing matrix Ti depletion through compound formation with Ti. The chemical compositions of the manufactured ODS steels are shown in Table 1. 9CR is the standard 9Cr-ODS steel and 9CRTI is a high Ti-containing 9Cr-ODS steel. 12CRTI-NiO is the 12Cr-ODS steel containing Ni and a high amount of excess oxygen (Ex.O), where Ex.O is defined as the value obtained by subtracting oxygen content in Y2O3 powder from the total oxygen concentration in the steel. 12CRTI15 W-Ni is the 12Cr-ODS steel containing Ni and 1.5 wt%W that is lower than the W concentration of 9CR. 12CRTI15 W-O is the 12Cr-ODS steel containing 1.5 wt%W and a high amount of Ex.O. 11CRTI15 W-lowNi is the 11Cr-ODS steel containing 1.5 wt%W and a small amount of Ni.
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Fig. 2. High-temperature XRD spectrum of 9CR steel powder measured at 1323 K in vacuum (1 Pa).
3.2. Microstructure: quantitative characterization of matrix duplex structure Fig. 2 shows an example of the high-temperature X-ray diffraction spectrum at 1323 K. This spectrum indicates that 9Cr-ODS steel surely has the duplex matrix consisting of ferrite and martensite. Fig. 3 shows the W mapping results and metallographic views of 9CRTI, 9CR, and 12CRTI15 W-Ni steels. These images indisputably display the duplex microstructures of the ODS steels. Fig. 4 indicates the residual-a ferrite proportion measured by three methods as a function of chemical driving force for a to c reverse transformation at 1323 K. The residual-a ferrite proportion declines with increasing driving force. The absolute values of residual-a ferrite proportion evaluated by the three methods do not agree with each other. High-temperature XRD should provide the
Fig. 4. Evaluation results of residual-a ferrite proportion in the ODS steels by three procedures: high-temperature XRD, EPMA mapping, and metallography.
most reliable result, considering that the measured values were corrected by the standard curve. The two-dimensional image analyses of EPMA and metallographic views may not correctly provide absolute values, because these analyses were carried out only in a one-directional view. In addition, the lack of clear phase boundaries in the EPMA mapping images may result in overestimation of the residual-a ferrite proportion. The image analysis of metallographic views provides underestimated values because only the brightest fields were summed up and the tiny-sized residual-a ferrite particles might not be detected. Fig. 4 shows that the chemical driving force is a useful indication for control of the residual-a ferrite proportion. In the case of the ODS steels, the ferritic-martensitic duplex structure does not coincide with the phase diagram as discussed in Section 3.1. This inconsistency made it necessary to
Fig. 3. W mapping results of (a) 9CRTI, (b) 9CR and (c) 12CRTI15 W-Ni steels. Metallographic observation results of (d) 9CRTI, (e) 9CR and (f) 12CRTI15 W-Ni steels. All results were obtained after the 1323 K annealing for 1 h followed by Ar gas cooling.
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carry out many manufacturing tests and microstructure characterizations for optimization of the residual-a ferrite proportion. This study successfully demonstrated that the parameter ‘‘chemical driving force for a to c reverse transformation’’ is a useful indicator for control of residual-a ferrite proportion in ODS tempered martensitic steel. It should be noted that, based on the model[11] accounting for the retention of residual-a ferrite by pinning of ac interface motion by nano-sized oxide particles, not only the chemical driving force but also oxide particle dispersion condition have an effect on the proportion of residual-a ferrite. Concentrations of Y2O3, Ti and Ex.O in ODS tempered martensitic steels affect the dispersion condition of nano-sized oxide particles [13], so that they would affect the proportion of residual-a ferrite in the steels. Taking into account the definite correlation between residual-a ferrite fraction and chemical driving force as seen in Fig. 4, the effect of chemical driving force prevails over that of oxide particle dispersion condition in the steels used in this study. If the ODS tempered martensitic steels containing the wide range of Y2O3, Ti, Ex.O concentrations will be dealt with, the careful choice of chemical compositions will be needed from the viewpoint of not only chemical driving force but also pinning force of ac interface motion by nano-sized oxide particles for the control of residual-a ferrite proportion. 3.3. Mechanical properties evaluation The uni-axial creep rupture test results of 11CRTI15 W-lowNi steel are shown in Fig. 5. 9Cr-ODS steel [6] and PNC-FMS (conventional 11Cr ferritic steel)[16] data are also plotted for comparison. The data of 11CRTI15 W-lowNi indicate adequate creep strength, although W concentration is lower than in the standard 9Cr-ODS steel. The uni-axial tensile test results at RT and 973 K are shown in Table 2. 11CRTI15 W-lowNi steel displayed satisfactory total elongation (i.e. 15.9% at RT and 12.9% at 973 K). The 0.2% proof strength in 11CRTI15 W-lowNi at 973 K is 231 MPa, which is much lower than that of 9CR (316 MPa). Ultimate tensile strength of 11CRTI15 W-lowNi at 973 K is 328 MPa which is as high as that of 9CR (357 MPa). Focusing on the chemical compositions of both steels, concentrations of Ti, Cr and Ni in 11CRTI15 W-lowNi are higher than those in 9CR while W concentration in 11CRTI15 WlowNi is lower than that in 9CR. Increasing Ti concentration
Table 2 Tensile test results of high Cr ODS tempered martensitic steels. ID
11CRTI15 W-LowNi
Test temp.(K) 0.2% proof strength (MPa) Ultimate tensile strength (MPa) Total elongation (%)
RT 932 1080 15.9
9CR 973 231 328 12.9
973 316 357 14.6
produces fine oxide particles which are densely dispersed in the matrix [13], thus improving tensile strength at high-temperature. Addition of Cr into ferritic steel is reported to provide little solid solution strengthening [17,18]. Nickel is a solid solution strengthening element [17,18], however, its addition is reported to provide little improvement of tensile strength of ferritic steel at high-temperature [19]. Additionally taking into account the low Ni concentration in 11CRTI15 W-lowNi, Ni addition is thought to have little effect on 0.2% proof strength in 11CRTI15 W-lowNi at 973 K. Tungsten is a solid solution element [17,18] improving tensile and creep strength of ferritic steels at high temperature. Considering the knowledge described above, a low 0.2% proof strength in 11CRTI15 W-lowNi steel is thought to be caused by the lower W concentration in 11CRTI15 W-lowNi than in 9CR. Regarding the capability evaluation of 11Cr ODS tempered martensitic steel as a heat-resistant and radiation-resistant material, further investigations will be carried out on the long-term creep property, the microstructure stability, and the mechanical properties after neutron irradiation and high-temperature thermal aging. 4. Summary The manufacturing tests and characterizations of 11–12Cr ODS tempered martensitic steels were carried out. Thermodynamic calculations were also carried out to find a methodology to control the residual-a ferrite proportion. (1) Several 11–12Cr ODS tempered martensitic steels were manufactured and their ferritic–martensitic duplex structures were quantitatively evaluated by three methods: high-temperature XRD, EPMA, and metallography. (2) Excessive formation of residual-a ferrite caused by increasing Cr content was demonstrated to be suppressed by appropriately controlling the concentration of the ferrite-forming element and austenite-forming element on the basis of the parameter ‘‘chemical driving force of a to c reverse transformation’’ as a useful indication. (3) 11Cr-ODS steel containing a small portion of residual-a ferrite was successfully manufactured. As-received 11Cr-ODS steel had satisfactory creep strength and ductility (i.e., both as high as those of the 9Cr-ODS steel) and 0.2% proof strength at 973 K (lower than in the 9Cr-ODS steel).
Acknowledgements The authors would like to thank Dr. M. Fujiwara, Dr. T. Okuda and Mr. T. Nakai, KOBELCO Research Institute, for fruitful discussions concerning this study. Special thanks also go to Dr. M. Yamanaka, Nippon Steel Technoresearch Corporation, for carrying out the precise, high temperature XRD measurements. References
Fig. 5. Uni-axial creep rupture test results of 11CRTI15 W-lowNi and 9CR steels tested at 973 K.
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Microstructure and high-temperature strength of high Cr ODS tempered martensitic steels S. Ohtsuka ⇑, T. Kaito, T. Tanno, Y. Yano, S. Koyama, K. Tanaka Japan Atomic Energy Agency, 4002, Narita, Oarai, Ibaraki 311-1393, Japan
a r t i c l e
i n f o
Article history: Available online 19 June 2013
a b s t r a c t 11-12Cr oxide dispersion strengthened (ODS) tempered martensitic steels underwent manufacturing tests and their ferritic–martensitic duplex structures were quantitatively evaluated by three methods: high-temperature X-ray diffraction (XRD), electron probe microanalyzer (EPMA), and metallography. It was demonstrated that excessive formation of residual-a ferrite, due to increasing Cr content, could be suppressed by appropriately controlling the concentration of the ferrite-forming and austenite-forming elements on the basis of the parameter ‘‘chemical driving force of a to c reverse transformation. 11CrODS steel containing a small portion of residual-a ferrite was successfully manufactured. In the asreceived condition, this 11Cr-ODS steel was shown to have satisfactory creep strength and ductility, both as high as those of the 9Cr-ODS steel, while its 0.2% proof strength at 973 K was lower than in the 9CrODS steel. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Oxide dispersion strengthened (ODS) steels have been developed as high-temperature and radiation-resistant core materials used in advanced type reactors (e.g., fusion and advanced fast breeder reactors) [1–7]. ODS steels having a ferritic matrix are classified into two types: ODS ferritic and ODS tempered martensitic steels. The alloy design in the former permits a high concentration of Cr, exceeding 12 wt%, which offers an advantage in terms of corrosion resistance over the ODS tempered martensitic steels for which Cr concentration is limited below 12 wt%. ODS tempered martensitic steels are expected to have better radiation resistance due to a high concentration of sink sites for irradiation defects in the matrix. In addition, they have better workability, which is provided by the high-temperature heat treatment for softening using the ferrite–austenite phase transformation [1]. In the development of ODS tempered martensitic steels for nuclear applications, a Cr concentration of 8–9 wt% is preferable to suppress the ductility loss by irradiation hardening and improve the microstructure stability and creep strength at high temperature. Conversely, from the viewpoint of corrosion resistance, Cr concentration as high as 11– 12 wt%Cr is preferable. A recent study revealed that 12CrODS ferritic steel with a recrystallized microstructure showed suitable ductility after neutron irradiation to 17 dpa at 693 K in JOYO [5]. This result suggests that high Cr ODS tempered martensitic steels containing 11–12 wt% Cr could be candidates for a new radiation-resistant material with better corrosion resistance than ⇑ Corresponding author. Tel.: +81 029 267 4141; fax: +81 029 267 7130 E-mail address: [email protected] (S. Ohtsuka). 0022-3115/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2013.06.010
9Cr-ODS steel. In this study, manufacturing tests and characterizations of the matrix duplex structure were carried out as a first step in the development of high Cr ODS tempered martensitic steels. Thermo-dynamic calculations were used to find a methodology for control of the matrix duplex structure. In addition, mechanical properties were evaluated in the as-received condition and compared with those of 9Cr-ODS steel. 2. Procedure 2.1. Manufacturing of high Cr ODS tempered martensitic steels Chemical compositions of high Cr ODS tempered martensitic steels were selected based on the phase diagram constructed by the Thermo-calc code and the SSOL4/SSUB3 database [8]. The chemical compositions of the high Cr ODS tempered martensitic steels manufactured are shown in Table 1. The high Cr ODS tempered martensitic steels were manufactured using a series of technologies, developed by Japan Atomic Energy Agency (JAEA) for fast breeder reactor fuel cladding tubes [1], consisting of mechanical alloying (MA), canning, and consolidating by hot-extrusion. The mixture of raw material powders were mechanically alloyed in an attrition-type ball mill with a rotating speed of 220 rpm for 48 h in high purity argon atmosphere (99.9999 wt% Ar). The MA conditions were judged to be well optimized on the basis of electron probe micro analyzer (EPMA) mapping views in 1000 times magnification, which demonstrated the uniform distribution of Cr and W in MA powder produced with elementary powders. The reference 9Cr-ODS steels (9CR, 9CRTI) was made from Ar-gas atomized alloy powder and oxide (Y2O3) powder; the rest of steels
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Table 1 Chemical analysis results of high-Cr ODS tempered martensitic steels. ID
9CR 9CRTI 12CRTI-NiO 12CRTI15 W-Ni 12CRTI15 W-O 11CRTI15 W-lowNi
Chemical analysis result (wt%)
Calculated (wt%)
C
Si
Mn
P
S
Ni
Cr
W
Ti
Y
O
N
Ar
Y2O3
ExO
0.14 0.13 0.15 0.16 0.16 0.16
0.06 0.08 <0.01 0.01 <0.01 0.01
0.04 0.12 <0.01 <0.01 <0.01 <0.01
<0.005 <0.005 <0.005 <0.005 <0.005 0.018
0.003 0.006 0.003 0.002 0.002 0.002
0.02 0.04 0.78 0.73 0.01 0.38
8.9 9.0 12.1 11.8 11.7 11.0
2.0 2.0 1.9 1.5 1.5 1.4
0.20 0.28 0.29 0.28 0.29 0.28
0.27 0.28 0.28 0.27 0.28 0.27
0.14 0.17 0.24 0.18 0.23 0.16
0.020 0.014 0.015 0.010 0.007 0.009
0 0.01 0.01 0.01 0.01 0.01
0.34 0.36 0.36 0.34 0.36 0.34
0.07 0.09 0.16 0.11 0.15 0.09
were made from elemental powders (Fe, Cr, C, W, Ti) and Y2O3 powder. The hot-extrusion for consolidation of MA powder was conducted at 1423 K for all the steels manufactured in this study. The extruded round bars were normalized and tempered (1323 K 1 h, Ar-gas cooling => 1073 K 1 h, Ar-gas cooling). The cooling rates were roughly 2000–3000 K/h. 2.2. Duplex microstructure characterization The ferrite phase in tempered martensite matrix of ODS steels has been called both d-ferrite and residual-a ferrite. In general, the ferrite phase in the austenite matrix at the austenitization heat treatment is called d-ferrite in the production process of heat-resistant steels by a melting method. However, in the manufacturing of ODS steels, alloying is not performed by high-temperature melting but by mechanical alloying (MA). MA does not use a higher temperature than the c loop temperature. Ukai et al. [1] designated this ferrite phase as residual-a ferrite. In this paper, the same designation is adopted. Cr is a ferrite-forming element, so its addition in excess will result in excessive residual-a ferrite formation. Incorporating the residual-a ferrite in tempered martensite matrix improves the high-temperature strength of ODS tempered martensitic steel [1,6], however, it produces anisotropy in the mechanical properties through working processes such as cold-rolling. This is because the ferrite phase in ODS steels is anisotropically elongated by the working process and not easily recrystallized, even by high-temperature annealing, due to the presence of oxide particles [7]. Moreover, thermal aging embrittlement by Laves phase precipitation may occur in excessive residual-a ferrite, because Tungsten (W) concentration in residual-a ferrite is higher than in tempered martensite. To identify the basic chemical composition with the smallest amount of residual-a ferrite while increasing Cr, several high Cr ODS tempered martensitic steels were manufactured and their residual-a ferrite proportions quantitatively measured. Three methods were applied for measurements: high-temperature X-ray diffraction (XRD), electron probe microanalyzer (EPMA) mapping, and metallography. 2.2.1. High-temperature XRD High-temperature XRD measurements were carried out using X’ Pert-PRO (Spectris Co., Ltd.) with Mo K-a as the X-ray source. A powder sample was filed off the ODS bulk material and used for the measurement without considering the effect of preferred orientation. The XRD measurement was carried out in vacuum (1 Pa) at 1323 K to suppress sample oxidation. The volume fraction of residual a-ferrite was evaluated from the peak intensity of several crystal planes in austenite (c) and ferrite (a) matrices measured by XRD. High intensity peaks were used for the analysis (i.e., (1 1 0), (2 0 0), (2 1 1) planes in the a phase and (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes in the c phase). The volume fraction of residual a-ferrite (Va) was calculated using the following equations:
Va ¼
Aa ; Aa þ Ac
ð1Þ
Aa ¼
/ IODS ð1 1 0Þ I/ODS ð2 0 0Þ I/ODS ð2 1 1Þ =3; þ þ I/O ð1 1 0Þ I/O ð2 0 0Þ I/O ð2 1 1Þ
ð2Þ
Ac ¼
c IODS ð1 1 1Þ IcODS ð2 0 0Þ IcODS ð2 2 0Þ IcODS ð3 1 1Þ þ c þ c þ c =4: c IO ð1 1 1Þ IO ð2 0 0Þ IO ð2 2 0Þ IO ð3 1 1Þ
ð3Þ
The symbol IxODS ðijkÞ (X = a, c) is the peak intensity of the crystal plane in phase X of the ODS steel powder sample measured by high-temperature XRD. The symbol Ix0 ðijkÞ (X = a, c) is the peak intensity of the crystal plane in the single phase X of pure Fe, which is derived from the Joint Committee Power Diffraction Standards (JCPDS) database. The volume fraction of residual a-ferrite (Va) calculated using Eqs. (1)–(3) was corrected by the standard curve. The standard curve was prepared as follows. 12Cr-ODS steel (Fe– 12 wt%Cr–0.04C-2W–0.2Ti–0.25Y2O3) and 9Cr–1Mo steel were used as the a-ferrite single phase sample and austenite single phase sample, respectively, at 1323 K. These two samples were each ground into powder. The two powders were mixed at eight ratios: a-ferrite volume fractions of 1%, 2%, 5%, 10%, 25%, 50%, 80%, and 90%. These standard powder samples were subjected to the high-temperature XRD measurement and the residual-a ferrite volume fraction (Va) was calculated from the measured peak intensities using Eqs. (1)–(3). The standard curve was derived by plotting the mixture fractions of a-ferrite in standard powder samples (i.e. the actual volume fraction of residual a phase) as a function of the calculated volume fraction (Va) from the measured peak intensities and Eqs. (1)–(3). 2.2.2. EPMA mapping W is a ferrite-forming element with a higher partitioning ratio into ferrite than into austenite. It follows that W preferentially partitions into ferrite in the ferrite–austenite duplex matrix at austinization heat treatment [9]. W distribution in ODS steels was evaluated using EPMA of a longitudinal section after 1323 K 1 h annealing followed by rapid cooling. The area fraction of high W content, which is equivalent to the area of residual a-ferrite, was evaluated by image analysis. Four images with a size of 80 80 lm (for a total size of 160 160 lm) were used for the analysis of each sample. 2.2.3. Metallography Metallographic observations were carried out in the longitudinal section after the 1323 K 1 h annealing followed by rapid cooling, in which the Cr and W concentration gap between ferrite and austenite was considerably high. Chemical etching was carried out with picral for an appropriate period to ensure the residual-a ferrite clearly appeared in the metallographs. For each sample, one image (200 250 lm) was used for the analysis.
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2.3. Thermodynamic analysis The chemical driving force of a to c reverse transformation at 1323 K was calculated using the Thermo-calc code and TCFE3/ SSUB3 database [8]. Concentrations of major constituents such as Fe, C, Ni, Cr, W, Ti, Y, and O were applied to the calculation. In the first step, the Gibbs energy at 1323 K was calculated for the various compositions of manufactured ODS steels in which, according to the calculation, the main matrix was austenite. In the next step, the austenite phase was set to be dormant to get the Gibbs energy of the ferrite matrix system at 1323 K. The chemical driving force for a to c reverse transformation at 1323 K was calculated by subtracting the calculated Gibbs energy of the austenite matrix system from that of the ferrite matrix system. 2.4. Mechanical properties evaluation The uni-axial creep rupture test at 973 K and uni-axial tensile tests at RT and 973 K were carried out for one of the high Cr ODS tempered martensitic steels, in which the residual-a ferrite proportion was controlled to be lower than in 9Cr-ODS steel. The direction of stress loading in the creep and tensile tests was parallel to the hot extrusion direction. The test results were compared with those of the 9Cr-ODS steel. 3. Results 3.1. Selection of chemical composition The basic chemical compositions of high Cr ODS tempered martensitic steels were examined on the basis of the information for 9Cr-ODS steel. A few researchers have reported data for 9Cr-ODS steel showing good high-temperature strength and radiation resistance [1,4,5]. Thus, modification of the chemical composition other than Cr content was examined to minimize the trade-off effect produced by increasing the Cr concentration (i.e., degradations in the resistance to irradiation embrittlement, the microstructure stability, and the mechanical properties at high temperature). Increasing Cr concentration from 9 wt% to 11–12 wt% is known to be effective for improvement of corrosion resistance. The 12CrODS steel, with a recrystallized microstructure and the conventional heat-resistant steel HT9 and PNC-FMS, contains 11–12 wt% Cr and was reported to have adequate ductility after neutron irradiation from 15 to
Fig. 1. Multi-component phase diagram of Fe-9–12 wt%Cr-0.15C-0.3Ti-0.1Ex.OxW-yNi system (x,y: variables) at 1323 K constructed using the Thermo-calc code and TCFE3/SSUB3 database [8].
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33 dpa at 693 K [5,10]. Thus, the Cr concentration of the high Cr ODS tempered martensitic steels was set to 11–12 wt% in this study. Fig. 1 shows the phase boundary of the Fe-9–12 wt%Cr-0.15C– 0.3Ti–0.1O–xW–yNi system (x, y: variables) at 1323 K calculated by the Thermo-calc code and TCFE3/SSUB3 database [8]. In the composition containing 9 wt%Cr–2 W–0Ni, the heat treatment at 1323 K annealing followed by rapid cooling produces the martensite single phase matrix (i.e. no residual-a ferrite) as shown in Fig. 1. However, in contrast to Fig. 1, residual-a ferrite is actually formed in 9Cr–2 W–0Ni-ODS steel [1,6]. It is believed that pinning of the ac interface by oxide particles suppresses the a to c reverse transformation and produces the residual-a ferrite in the nonequilibrium state [11]. It can certainly be said that the formation of duplex matrix in ODS tempered martensitic steels is easier than in the conventional tempered martensitic steels produced by the melting method. Thus, it follows that the minor adjustment of chemical composition (e.g., decreasing the ferrite-forming element and increasing the austenite-forming element) is required to prevent excessive residual-a ferrite formation in 11-12Cr ODS tempered martensitic steels. Among the constituent elements of 9CrODS steels (Fe, C, Cr, W, Ti), the ferrite-forming elements besides Cr are W and Ti, and the austenite-forming element is C. The main strengthening factor of ODS steels is the oxide dispersion. Solute W certainly strengthens the steel by a solid-solution strengthening; however, that strength decreases under long-term thermal aging at 773–943 K due to Laves phase precipitation. Decreasing the W concentration has been reported to be effective for improving toughness and ductility in ferritic steels [12]. In this study, W content was decreased to suppress excessive residual-a formation. The addition of Ti is known to lead to fine oxide particles, which are densely dispersed in the matrix, notably increasing ODS [7,13]. To make up for the strength degradation caused by decreasing the W content, the Ti concentration was increased to 0.3 wt%, which is reported to be the optimum concentration in terms of creep strength improvement [6]. Increasing C concentration was excluded as an option, because C causes Cr depletion of the matrix through Cr carbide formation. Ni and Mn are often used in manufacturing conventional heat-resistant ferritic steel to decrease dferrite formation and produce the martensite single phase matrix. However, Ni addition was not desirable here from the viewpoint of irradiation hardening because it is reported to enhance irradiation hardening and ductility loss in low temperature neutron irradiation below 673 K [14]. For the same level of Ni equivalent, the decline in the Ac1 point by Mn addition is larger than in Ni addition [15]. This study attached greater importance to high-temperature performance (i.e. suppressing the drop in the Ac1 point) than the ductility after irradiation below 673 K, and Ni was chosen as a minor constitutive element suppressing formation of excessive residual-a ferrite. Based on the idea described above, high Cr ODS tempered martensitic steels with different concentrations of W and Ni were manufactured for this study. The impurity element O was also chosen as a parameter, because it considerably decreases the residual-a ferrite proportion by causing matrix Ti depletion through compound formation with Ti. The chemical compositions of the manufactured ODS steels are shown in Table 1. 9CR is the standard 9Cr-ODS steel and 9CRTI is a high Ti-containing 9Cr-ODS steel. 12CRTI-NiO is the 12Cr-ODS steel containing Ni and a high amount of excess oxygen (Ex.O), where Ex.O is defined as the value obtained by subtracting oxygen content in Y2O3 powder from the total oxygen concentration in the steel. 12CRTI15 W-Ni is the 12Cr-ODS steel containing Ni and 1.5 wt%W that is lower than the W concentration of 9CR. 12CRTI15 W-O is the 12Cr-ODS steel containing 1.5 wt%W and a high amount of Ex.O. 11CRTI15 W-lowNi is the 11Cr-ODS steel containing 1.5 wt%W and a small amount of Ni.
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Fig. 2. High-temperature XRD spectrum of 9CR steel powder measured at 1323 K in vacuum (1 Pa).
3.2. Microstructure: quantitative characterization of matrix duplex structure Fig. 2 shows an example of the high-temperature X-ray diffraction spectrum at 1323 K. This spectrum indicates that 9Cr-ODS steel surely has the duplex matrix consisting of ferrite and martensite. Fig. 3 shows the W mapping results and metallographic views of 9CRTI, 9CR, and 12CRTI15 W-Ni steels. These images indisputably display the duplex microstructures of the ODS steels. Fig. 4 indicates the residual-a ferrite proportion measured by three methods as a function of chemical driving force for a to c reverse transformation at 1323 K. The residual-a ferrite proportion declines with increasing driving force. The absolute values of residual-a ferrite proportion evaluated by the three methods do not agree with each other. High-temperature XRD should provide the
Fig. 4. Evaluation results of residual-a ferrite proportion in the ODS steels by three procedures: high-temperature XRD, EPMA mapping, and metallography.
most reliable result, considering that the measured values were corrected by the standard curve. The two-dimensional image analyses of EPMA and metallographic views may not correctly provide absolute values, because these analyses were carried out only in a one-directional view. In addition, the lack of clear phase boundaries in the EPMA mapping images may result in overestimation of the residual-a ferrite proportion. The image analysis of metallographic views provides underestimated values because only the brightest fields were summed up and the tiny-sized residual-a ferrite particles might not be detected. Fig. 4 shows that the chemical driving force is a useful indication for control of the residual-a ferrite proportion. In the case of the ODS steels, the ferritic-martensitic duplex structure does not coincide with the phase diagram as discussed in Section 3.1. This inconsistency made it necessary to
Fig. 3. W mapping results of (a) 9CRTI, (b) 9CR and (c) 12CRTI15 W-Ni steels. Metallographic observation results of (d) 9CRTI, (e) 9CR and (f) 12CRTI15 W-Ni steels. All results were obtained after the 1323 K annealing for 1 h followed by Ar gas cooling.
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carry out many manufacturing tests and microstructure characterizations for optimization of the residual-a ferrite proportion. This study successfully demonstrated that the parameter ‘‘chemical driving force for a to c reverse transformation’’ is a useful indicator for control of residual-a ferrite proportion in ODS tempered martensitic steel. It should be noted that, based on the model[11] accounting for the retention of residual-a ferrite by pinning of ac interface motion by nano-sized oxide particles, not only the chemical driving force but also oxide particle dispersion condition have an effect on the proportion of residual-a ferrite. Concentrations of Y2O3, Ti and Ex.O in ODS tempered martensitic steels affect the dispersion condition of nano-sized oxide particles [13], so that they would affect the proportion of residual-a ferrite in the steels. Taking into account the definite correlation between residual-a ferrite fraction and chemical driving force as seen in Fig. 4, the effect of chemical driving force prevails over that of oxide particle dispersion condition in the steels used in this study. If the ODS tempered martensitic steels containing the wide range of Y2O3, Ti, Ex.O concentrations will be dealt with, the careful choice of chemical compositions will be needed from the viewpoint of not only chemical driving force but also pinning force of ac interface motion by nano-sized oxide particles for the control of residual-a ferrite proportion. 3.3. Mechanical properties evaluation The uni-axial creep rupture test results of 11CRTI15 W-lowNi steel are shown in Fig. 5. 9Cr-ODS steel [6] and PNC-FMS (conventional 11Cr ferritic steel)[16] data are also plotted for comparison. The data of 11CRTI15 W-lowNi indicate adequate creep strength, although W concentration is lower than in the standard 9Cr-ODS steel. The uni-axial tensile test results at RT and 973 K are shown in Table 2. 11CRTI15 W-lowNi steel displayed satisfactory total elongation (i.e. 15.9% at RT and 12.9% at 973 K). The 0.2% proof strength in 11CRTI15 W-lowNi at 973 K is 231 MPa, which is much lower than that of 9CR (316 MPa). Ultimate tensile strength of 11CRTI15 W-lowNi at 973 K is 328 MPa which is as high as that of 9CR (357 MPa). Focusing on the chemical compositions of both steels, concentrations of Ti, Cr and Ni in 11CRTI15 W-lowNi are higher than those in 9CR while W concentration in 11CRTI15 WlowNi is lower than that in 9CR. Increasing Ti concentration
Table 2 Tensile test results of high Cr ODS tempered martensitic steels. ID
11CRTI15 W-LowNi
Test temp.(K) 0.2% proof strength (MPa) Ultimate tensile strength (MPa) Total elongation (%)
RT 932 1080 15.9
9CR 973 231 328 12.9
973 316 357 14.6
produces fine oxide particles which are densely dispersed in the matrix [13], thus improving tensile strength at high-temperature. Addition of Cr into ferritic steel is reported to provide little solid solution strengthening [17,18]. Nickel is a solid solution strengthening element [17,18], however, its addition is reported to provide little improvement of tensile strength of ferritic steel at high-temperature [19]. Additionally taking into account the low Ni concentration in 11CRTI15 W-lowNi, Ni addition is thought to have little effect on 0.2% proof strength in 11CRTI15 W-lowNi at 973 K. Tungsten is a solid solution element [17,18] improving tensile and creep strength of ferritic steels at high temperature. Considering the knowledge described above, a low 0.2% proof strength in 11CRTI15 W-lowNi steel is thought to be caused by the lower W concentration in 11CRTI15 W-lowNi than in 9CR. Regarding the capability evaluation of 11Cr ODS tempered martensitic steel as a heat-resistant and radiation-resistant material, further investigations will be carried out on the long-term creep property, the microstructure stability, and the mechanical properties after neutron irradiation and high-temperature thermal aging. 4. Summary The manufacturing tests and characterizations of 11–12Cr ODS tempered martensitic steels were carried out. Thermodynamic calculations were also carried out to find a methodology to control the residual-a ferrite proportion. (1) Several 11–12Cr ODS tempered martensitic steels were manufactured and their ferritic–martensitic duplex structures were quantitatively evaluated by three methods: high-temperature XRD, EPMA, and metallography. (2) Excessive formation of residual-a ferrite caused by increasing Cr content was demonstrated to be suppressed by appropriately controlling the concentration of the ferrite-forming element and austenite-forming element on the basis of the parameter ‘‘chemical driving force of a to c reverse transformation’’ as a useful indication. (3) 11Cr-ODS steel containing a small portion of residual-a ferrite was successfully manufactured. As-received 11Cr-ODS steel had satisfactory creep strength and ductility (i.e., both as high as those of the 9Cr-ODS steel) and 0.2% proof strength at 973 K (lower than in the 9Cr-ODS steel).
Acknowledgements The authors would like to thank Dr. M. Fujiwara, Dr. T. Okuda and Mr. T. Nakai, KOBELCO Research Institute, for fruitful discussions concerning this study. Special thanks also go to Dr. M. Yamanaka, Nippon Steel Technoresearch Corporation, for carrying out the precise, high temperature XRD measurements. References
Fig. 5. Uni-axial creep rupture test results of 11CRTI15 W-lowNi and 9CR steels tested at 973 K.
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