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Influence of austenitization temperature on phase transformation features of modified high Cr ferritic heat-resistant steel
Nuclear Engineering and Design 256 (2013) 148–152
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Influence of austenitization temperature on phase transformation features of modified high Cr ferritic heat-resistant steel Qiuzhi Gao a , Yongchang Liu a,∗ , Xinjie Di a , Liming Yu a , Zesheng Yan a , Zhixia Qiao b a b
State Key Lab of Hydaulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300072, PR China School of Mechanical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China
h i g h l i g h t s Ms firstly increases then decreases with increasing austenitization temperature. ␦-Ferrite abnormally forms austenitized at 950 ◦ C. Ms as a function of austenite grain size based on kinetics model.
a r t i c l e
i n f o
Article history: Received 27 March 2012 Received in revised form 8 December 2012 Accepted 19 December 2012
a b s t r a c t The phase transformation features of high Cr ferritic heat-resistant steel have been investigated using Differential Thermal Analysis (DTA) in accordance with different austenitization temperatures as 950 ◦ C, 1050 ◦ C, 1150 ◦ C, 1200 ◦ C and predicted by Thermo-calc calculation. The results based on DTA curves on heating showed that the magnetic transition and the Ac1 temperature occurred at 744.9 ◦ C and 850.9 ◦ C, respectively. The measured Ac3 temperature was around 919.5 ◦ C, the obtained austenitization temperature range was about 69 ◦ C. Two new phenomena were found: firstly, the onset temperature of martensite transformation increases strongly and then not obviously with the increase of austenitization temperature; secondly, the ␦-ferrite abnormally forms in the specimen austenitized at 950 ◦ C due to the existence of un-dissolved M23 C6 particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The high Cr ferritic heat-resistant steels are widely used in critical components of power plant with its high temperature stability, high creep strength, low thermal expansion coefficient and outstanding corrosion resistance (Mythili et al., 2003; Klueh and Nelson, 2007). It has been proven that martensite is the predominant structure for the modified high Cr ferritic heat-resistant steel (Bhadeshia et al., 1998; Ning et al., 2009). The effect of the austenitization temperature on the austenite transformation kinetics, for the high Cr ferrtic heat-resistant steel (i.e., modified 9Cr–1Mo steel), was studied by DSC (Differential Scanning Calorimeter) or DTA (Differential Thermal Analysis) under continuous cooling (Cota et al., 2004; Tokunaga et al., 2009). The beginning of ␦-ferrite formation and of austinitization transformation time is strongly dependent on the austinitization temperature which will effect on the austenite grain size.
∗ Corresponding author. Tel.: +86 22 87401873; fax: +86 22 87401873. E-mail address: [email protected] (Y. Liu). 0029-5493/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2012.12.009
In the paper, the phase transformation features of the modified high Cr ferritic heat-resistant steel upon different austenitization temperatures were studied, and the phase transition process was also computed by Thermo-calc software. A Differential Thermal Analysis (DTA) employing inductive heating was used to obtain the information of ␦-ferrite formation, martensite transformation and carbides precipitation as a function of austenitization temperature. The kinetic model between the Ms point and the austenite grain size is fitted. 2. Experimental details The material employed in present work is the modified high Cr ferritic heat-resistant steel upon hot rolling and followed by air cooling. The chemical compositions are displayed in Table 1. The samples used the Differential Thermal Analysis were the cylinderal samples, 5 mm × 3 mm, which were machined from the rolled pipe with a thickness of 9 mm. The experiment was carried out in high purity argon atmosphere to avoid oxidation, and the thermal treatment cycle during DTA for the modified high Cr ferritic heat-resistant steel as follows: the samples were firstly heated from room temperature up to 950 ◦ C,
Q. Gao et al. / Nuclear Engineering and Design 256 (2013) 148–152
149
Table 1 Chemical compositions of high Cr ferritic heat-resistant steel (in wt.%). Element
C
Si
Mn
Cr
Mo
W
V
Nb
N
Co
B
Ti
Al
wt.%
0.05
0.21
0.44
9.81
0.43
1.73
0.22
0.07
0.03
1.21
0.0045
<0.01
0.014
Fig. 1. DTA curves of heat flow as a function of austenitization temperature during heating.
1050 ◦ C, 1150 ◦ C and 1200 ◦ C at a rate of 20 K/min, respectively, and held at the temperature for 20 min, then cooled to room temperature with the same rate. 3. Results and discussion The obtained DTA curves with a heating rate of 20 K/min to differential austenitization temperature are displayed in Fig. 1. During heating, two distinct endothermic peaks can be found: The first one around 745 ◦ C indicated the transition from a ferromagnetic state to a paramagnetic state, i.e., so-called the Curie Point or the magnetic transition. The measured Curie Point in our work was about 744.9 ◦ C. The other was a new phase formation peak with transformation from martensite to austenite, so the observed average Ac1 temperature was about 850.9 ◦ C, and the Ac3 was around 919.5 ◦ C, respectively. Austenitizing from 1050 ◦ C to 1200 ◦ C, a gentle endothermic peak was observed after 950 ◦ C, respectively. Maehara (Maehara
and Ohmori, 1987) had shown that a lamellar/cellular structure of M23 C6 and austenite initiated nucleating at the interface of ␦ferrite/matrix austenite at around 1000 ◦ C, in other words, M23 C6 particles are certainly complete dissolution at 1050 ◦ C. The gentle peak after 950 ◦ C should be result from the dissolution of M23 C6 particles. The last gentle peak after 1050 ◦ C for austenitization at 1150 ◦ C and 1200 ◦ C implied austenite homogenization and the formation of ␦-ferrite. According to the early report, the computed constant carbon section of (Fe–xCr–0.1C) system suggested that for a 9Cr steel, the ␦-ferrite phase formed at 1100 ◦ C (Jeya Ganesh et al., 2011); and for the standard 9Cr–1Mo steel, the inflection at 1050 ◦ C can be taken to mark the entry of the system into the ␥ + ␦-ferrite two phase field on continuous heating by estimated using Thermocalc software (Helis et al., 2009). Combining the two possibilities, the inflection which is seen after 1050 ◦ C was associated with both austenite homogenization and the possible emergence of ␦-ferrite from homogeneous austenite. In short, M23 C6 particles have dissolved partly at 950 ◦ C and completely before 1050 ◦ C, whereas, new ␦-ferrite appears until heating to 1150 ◦ C. The result of phase mole fraction as a function of the temperature for the high Cr ferritic heat-resistant steel in our work was computed by Thermo-calc software is shown in Fig. 2. The liquid temperature is about 1490 ◦ C, and the phase is all ␦-ferrite range exits above 1320 ◦ C. The ␦-ferrite starts to transform austenite below 1320 ◦ C and finishes approximately at around 1020 ◦ C. The room temperature phases were composed by martensite (formed by ␣-ferrite) and 0.06–0.08 volume fraction ␦-ferrite based on the result of computation. The detail of precipitated phase was represented in Fig. 2b. MX phase and M23 C6 carbides dissolve completely at about 1200 ◦ C and 950 ◦ C, respectively. Hence the gentle endothermic peak at around 950 ◦ C is M23 C6 carbides dissolution and that at 1050 ◦ C is the formation of ␦-ferrite for the DTA heating curves (seen as Fig. 2), and the same as the phases formation during continuous cooling. Fig. 3 indicates the martensite fractions as a function of austenitization temperature during continuous cooling. The most obvious one is the shape of martensite transformation curve, at 950 ◦ C austenitization temperature which M23 C6 starts to dissolve in the austenite, was different with others, that is to say, the
Fig. 2. Mole fraction of phase transition of high Cr ferritic heat resistant steel as a function of temperature based upon computation by Thermo-calc software. (a) Ferrite and austenite transformation, and (b) details of precipitates.
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Q. Gao et al. / Nuclear Engineering and Design 256 (2013) 148–152
Fig. 3. Martensite fraction based on integrated the data of martensite transformation as a function of temperature.
transformation kinetic mechanism of martensite is not same before M23 C6 dissolving complete and after that. It also can be found that the beginning reaction rate of martensite was lower at 950 ◦ C than that at other temperatures. The obtained onset points of martensite transformation (Ms point) as a function of austenitization temperatures are shown in Fig. 4. Several researches (Hsu and Linfah, 1983; Harris and Cohen, 1949) have reported that the vacancies in the austenite would increase with the increase of austenitization temperature due to the strong mobility of atoms. The nucleation of martensite may be boosted because of freezing the vacancies in the austenite grain due to higher quenching temperatures, and it means that the increase in vacancies makes austenite less stable by increasing martensite nucleation sites. Besides, with increasing austenitization temperature, the chemical compositions in the austenite are more uniform which will promote martensite transformation. Therefore, in general, the Ms should increase as austenitization temperature increases, and the same results can be found based on our experimental data, whereas it is increase strongly at the beginning and not obviously with the temperature raising. These results can be attributed to the followed four major points: Firstly, before 1050 ◦ C, new ␦-ferrite does not form, and the alloying elements dissolving in austenite are more homogenous with the increase of
austenitization temperature, which will lead to the increase of onset point of martensite transformation. Secondly, the undissolved precipitates, such as M23 C6 precipitating in austenite grain boundary and MX particles locating in the austenite grain, lead to form carbon depletion zone which restrains martensite transformation and decreases the onset point. So the more completely precipitates dissolved, the higher martensite transformation onset point. Thirdly, the effect of ␦-ferrite formation. After 1050 ◦ C, the content of ␦-ferrite is sufficiently high to affect martensite transformation, and becomes the major factor comparing with the effects of compositions of austenites and undissolved particles (mainly MX). Lastly, the austenite will be stable with the precipitates complete dissolving and the chemical composition distributing uniformly in the austenite, which lead to the Ms point does not change obviously. Combined with the above factors, the change of Ms point presents the characteristic as shown in Fig. 4. Generally, Ms point is just relevant to the chemical compositions of materials, but some researchers recently reported that it also has certain relations with the austenite grain size (Krauss et al., 1989; Garcia-Junceda et al., 2008; Gil et al., 1995). Based on Fisher’s nucleation model about the new phase, it is given as (Fisher et al., 1949): df m(1 − f ) = mV¯ C (1 − f ) ≡ dNV NVC
(1)
where f is the formed martensite fraction, NV is the number of martensite laths per unit volume, m is the aspect ratio of martensite lath (set as 0.06 (Zanardi, 1952)), V¯ C is the mean volume of austenite that divided by martensite lath, and NVC is the number of austenite compartments per unit volume. Each new martensite lath formed will produce one additional compartment, so that: NVC =
1 + NV V
(2)
where V is the mean austenite grain volume. Substituting Eq. (2) into Eq. (1) and integrating, the number of martensite laths per unit volume can be obtained as: NV =
1 V
exp
ln(1 − f ) −
m
−1
(3)
the formed martensite fraction, f, can be obtained by measuring the square of exothermic peak in DTA curves which in detail as follows: f =
ST S
(4)
where ST is the square of exothermic peak that the temperature reduced from Ms point to T, and S is the total square of exothermic peak. It should be noted that Eq. (3) has not described the relationship between Ms point and austenite grain size, Yang and Bhadeshia (2009) showed an new equation about the number of martensite laths per unit volume as: NV = a[exp{b(T ∗ − T )} − 1]
(5)
where a and b are the fitted constants, T* is the martensite start temperature at which martensite formed in infinite austenite grain. Combining Eqs. (3) and (5), it can be obtained as:
1 1 ln T −T = aV b ∗
Fig. 4. Martensite transformation start temperature Ms as a function of austenitization temperature.
ln(1 − f )
exp −
m
−1
+1
(6)
It is assumed that the measured Ms point is the temperature when martensite fraction is 0.01, so the parameter T in Eq. (6) can be substituted by Ms . It should be noted that the added experiments that austenitized at 1000 ◦ C and 1100 ◦ C are finished, then only the Ms and the relative austenite grain size are measured to obtain more reliable fitted results. The fitted results based on experimental data are displayed in Fig. 5. So, the fitted related constants can
Q. Gao et al. / Nuclear Engineering and Design 256 (2013) 148–152
Fig. 5. Ms point as a function of austenite grain size at various austenitization temperatures in the new high Cr ferritic heat-resistant steel.
be obtained as a = 3.84, b = 2.23 × 10−5 , and T* = 455.08 ◦ C, respectively. It can be seen that Eq. (6) can be appropriately described the relationship between Ms and austenite grain size for the high Cr ferritic heat resistant steel. Fig. 6 exhibits optical morphologies of microstructures austenitized at various temperatures. It is clear that the microstructures are consisted of martensite and ␦-ferrite, and the content of ␦-ferrite austenitized at 950 ◦ C is obviously higher than that at other temperatures. By observing the microstructures carefully, it also should be noted that the formed martensite existed with lath style, and the corresponding martensite laths become coarser with the increase of austenitization temperature. The primary ␦-ferrite which formed in hot rolling and followed by air cooling, cannot completely transform to austenite/martensite during the subsequent cooling, the ␦-ferrite + austenite duplex phase coexisting in room temperature have been researched in our previous work (Gao et al., 2011).
151
Fig. 7. ␦-Ferrite fraction as a function of austenitization temperature.
The measured ␦-ferrite fraction based upon optical morphologies of microstructures using the relative Computer-aided Software, as a function of austenitization temperature as revealed in Fig. 7. The highest ␦-ferrite fraction abnormally occurs at 950 ◦ C as can be seen, and sharply decreases at 1050 ◦ C, then increases with the increase of austenitization temperature. For the experimental temperature, it has been pointed out that the new ␦-ferrite appears at about 1150 ◦ C in the DTA curves and the calculated results by Thermo-calc that is consistent with the results as shown in Fig. 7. The abnormally formed ␦-ferrite at 950 ◦ C can be considered the effect of un-dissolved M23 C6 and MX particles, and there are also some particles can be found in the optical morphologies for austenitizing at 950 ◦ C (as seen Fig. 6a). As is known to all, the alloying elements uniformly distribute in the austenite grain, and more uniform with the increase of temperature (Cota et al., 2004; Campos et al., 2006). However, if there is the occurrence of particles precipitation, some alloying elements will transfer into these particles which may lead to form so-called carbon depletion zone.
Fig. 6. Optical microstructures of the samples after DTA thermal cycle with different austenitization temperature: (a) 950 ◦ C, (b) 1050 ◦ C, (c) 1150 ◦ C, and (d) 1200 ◦ C.
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Fig. 8. TEM images showing the microstructures of (a) austenitized at 950 ◦ C and (b) austenitzed at 1200 ◦ C.
The so-called carbon depletion zone provides advantaged nucleation sites for ␦-ferrite transformation while the temperature being high enough. So austenited at 950 ◦ C, the un-dissolved M23 C6 and MX particles promote ␦-ferrite transformation on the contrary, and ␦-ferrite transformation abnormally occurs. Fig. 8 demonstrates the typical TEM images of the modified high Cr ferritic heat resistant steel upon different austenitization temperatures. It can be found that the un-dissolved M23 C6 particles precipitated in ␦-ferrite grain while austenitizing at 950 ◦ C, and distributed parallelly with the direction of ␦-ferrite. Nevertheless, there is just one M23 C6 particle can be found in ␦-ferrite grain for the sample austenitized at 1200 ◦ C, which manifests that the M23 C6 particles should have dissolved completely while heating to 1200 ◦ C. The M23 C6 particles promote the transformation from austenite to ␦-ferrite around themselves, and so new ␦-ferrite forms (Hamada et al., 1995). With holding at M23 C6 induced ␦ferrite formation temperature (950 ◦ C in the paper), ␦-ferrite grows up and links together, which results in a morphology like Fig. 6a. The ␦-ferrite formed at 1200 ◦ C due to the temperature is high enough to induce the transformation from austenite phase to ␦ferrite phase, and part ␦-ferrite will remain to room temperature during continuous cooling. 4. Conclusions The effect of austenitization temperature on phase transition features of high Cr ferritic heat-resistant steel was studied by Differential Thermal Analysis experiment. The M23 C6 particles dissolved peak and the ␦-ferrite formed peak were observed during heating, respectively. The onset of martensite transformation firstly increases and then decreases with the increase of austenitization temperature. The ␦-ferrite abnormally forms austenitized at 950 ◦ C due to be induced by the un-dissolved M23 C6 particles. The austenite grain size and martensite laths become larger and coarser with increasing austenitization temperature, respectively. Acknowledgements The authors are grateful to the National Natural Science Foundation of China and Shanghai Baosteel Group Company (No. 50834011), and National Natural Science Foundation of China (Nos. 51204121 and 50901049) for grant and financial support.
References Bhadeshia, H., Strang, A., Gooch, D.J., 1998. Ferritic power plant steels: remanent life assessment and approach to equilibrium. Int. Mater. Rev. 43 (2), 45–69. Campos, M., Blanco, L., Torralba, J.M., 2006. Thermal analysis of prealloyed Fe–3Cr–0.5 Mo sintered steel. J. Therm. Anal. Calorim. 84 (2), 483–487. Cota, A., Lacerda, C., Oliveira, F., Machado, F., 2004. Effect of the austenitizing temperature on the kinetics of ferritic grain growth under continuous cooling of a Nb microalloyed steel. Scr. Mater. 51 (7), 721–725. Fisher, J.C., Hollomon, J.H., Turnbull, D., 1949. Kinetics of the austenite–martensite transformation. Trans. AIME 185, 691–700. Gao, Q.Z., Liu, Y.C., Di, X.J., Dong, Z.Z., Yan, Z.S., 2011. The isochronal ␦ → ␥ transformation of high Cr ferritic heat-resistant steel during cooling. J. Mater. Sci. 46 (21), 6910–6915. Garcia-Junceda, A., Capdevila, C., Caballero, F.G., de Andrés, C.G., 2008. Dependence of martensite start temperature on fine austenite grain size. Scr. Mater. 58 (2), 134–137. Gil, F., Manero, J., Planell, J., 1995. Effect of grain size on the martensitic transformation in NiTi alloy. J. Mater. Sci. 30 (10), 2526–2530. Hamada, K., Tokuno, K., Tomita, Y., Mabuchi, H., Okamoto, K., 1995. Effects of precipitate shape on high temperature strength of modified 9Cr–1Mo Steels. ISIJ Int. 35 (1), 86–91. Harris, W., Cohen, M., 1949. Stabilization of the austenite–martensite transformation. Trans. AIME 180, 447–470. Helis, L., Toda, Y., Hara, T., Miyazaki, H., Abe, F., 2009. Effect of cobalt on the microstructure of tempered martensitic 9Cr steel for ultra-supercritical power plants. Mater. Sci. Eng. A 510, 88–94. Hsu, T.Y., Linfah, Y., 1983. The effect of quenched-in vacancies on the martensitic transformation. J. Mater. Sci. 18 (11), 3213–3218. Jeya Ganesh, B., Raju, S., Kumar Rai, A., Mohandas, E., Vijayalakshmi, M., Rao, K., Raj, B., 2011. Differential scanning calorimetry study of diffusional and martensitic phase transformations in some 9wt-% Cr low carbon ferritic steels. Mater. Sci. Technol. 27 (2), 500–512. Klueh, R.L., Nelson, A.T., 2007. Ferritic/martensitic steels for next-generation reactors. J. Nucl. Mater. 371 (1–3), 37–52. Krauss, W., Pabi, S.K., Gleiter, H., 1989. On the mechanism of martensite nucleation. Acta Metall. 37 (1), 25–30. Maehara, Y., Ohmori, Y., 1987. Microstructural change during superplastic deformation of ␦-ferrite/austenite duplex stainless steel. Metall. Mater. Trans. A 18 (5), 663–672. Mythili, R., Thomas Paul, V., Saroja, S., Vijayalakshmi, M., Raghunathan, V., 2003. Microstructural modification due to reheating in multipass manual metal arc welds of 9Cr–1Mo steel. J. Nucl. Mater. 312 (2–3), 199–206. Ning, B.Q., Shi, Q.Z., Yan, Z.S., Fu, J.C., Liu, Y.C., Bie, L.J., 2009. Variation of martensite phase transformation mechanism in minor-stressed T91 ferritic steel. J. Nucl. Mater. 393 (1), 54–60. Tokunaga, T., Hasegawa, K., Masuyama, F., 2009. Phase transformation behavior of Grade 91 ferritic steel. Mater. Sci. Eng. A 510, 158–161. Yang, H.S., Bhadeshia, H., 2009. Austenite grain size and the martensite-start temperature. Scr. Mater. 60 (7), 493–495. Zanardi, T.J., 1952. The Martensitic Transformation in High Chromium Steels, Thesis (M.S.). Massachusetts Institute of Technology.
Contents lists available at SciVerse ScienceDirect
Nuclear Engineering and Design journal homepage: www.elsevier.com/locate/nucengdes
Influence of austenitization temperature on phase transformation features of modified high Cr ferritic heat-resistant steel Qiuzhi Gao a , Yongchang Liu a,∗ , Xinjie Di a , Liming Yu a , Zesheng Yan a , Zhixia Qiao b a b
State Key Lab of Hydaulic Engineering Simulation and Safety, School of Materials Science & Engineering, Tianjin University, Tianjin 300072, PR China School of Mechanical Engineering, Tianjin University of Commerce, Tianjin 300134, PR China
h i g h l i g h t s Ms firstly increases then decreases with increasing austenitization temperature. ␦-Ferrite abnormally forms austenitized at 950 ◦ C. Ms as a function of austenite grain size based on kinetics model.
a r t i c l e
i n f o
Article history: Received 27 March 2012 Received in revised form 8 December 2012 Accepted 19 December 2012
a b s t r a c t The phase transformation features of high Cr ferritic heat-resistant steel have been investigated using Differential Thermal Analysis (DTA) in accordance with different austenitization temperatures as 950 ◦ C, 1050 ◦ C, 1150 ◦ C, 1200 ◦ C and predicted by Thermo-calc calculation. The results based on DTA curves on heating showed that the magnetic transition and the Ac1 temperature occurred at 744.9 ◦ C and 850.9 ◦ C, respectively. The measured Ac3 temperature was around 919.5 ◦ C, the obtained austenitization temperature range was about 69 ◦ C. Two new phenomena were found: firstly, the onset temperature of martensite transformation increases strongly and then not obviously with the increase of austenitization temperature; secondly, the ␦-ferrite abnormally forms in the specimen austenitized at 950 ◦ C due to the existence of un-dissolved M23 C6 particles. © 2012 Elsevier B.V. All rights reserved.
1. Introduction The high Cr ferritic heat-resistant steels are widely used in critical components of power plant with its high temperature stability, high creep strength, low thermal expansion coefficient and outstanding corrosion resistance (Mythili et al., 2003; Klueh and Nelson, 2007). It has been proven that martensite is the predominant structure for the modified high Cr ferritic heat-resistant steel (Bhadeshia et al., 1998; Ning et al., 2009). The effect of the austenitization temperature on the austenite transformation kinetics, for the high Cr ferrtic heat-resistant steel (i.e., modified 9Cr–1Mo steel), was studied by DSC (Differential Scanning Calorimeter) or DTA (Differential Thermal Analysis) under continuous cooling (Cota et al., 2004; Tokunaga et al., 2009). The beginning of ␦-ferrite formation and of austinitization transformation time is strongly dependent on the austinitization temperature which will effect on the austenite grain size.
∗ Corresponding author. Tel.: +86 22 87401873; fax: +86 22 87401873. E-mail address: [email protected] (Y. Liu). 0029-5493/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.nucengdes.2012.12.009
In the paper, the phase transformation features of the modified high Cr ferritic heat-resistant steel upon different austenitization temperatures were studied, and the phase transition process was also computed by Thermo-calc software. A Differential Thermal Analysis (DTA) employing inductive heating was used to obtain the information of ␦-ferrite formation, martensite transformation and carbides precipitation as a function of austenitization temperature. The kinetic model between the Ms point and the austenite grain size is fitted. 2. Experimental details The material employed in present work is the modified high Cr ferritic heat-resistant steel upon hot rolling and followed by air cooling. The chemical compositions are displayed in Table 1. The samples used the Differential Thermal Analysis were the cylinderal samples, 5 mm × 3 mm, which were machined from the rolled pipe with a thickness of 9 mm. The experiment was carried out in high purity argon atmosphere to avoid oxidation, and the thermal treatment cycle during DTA for the modified high Cr ferritic heat-resistant steel as follows: the samples were firstly heated from room temperature up to 950 ◦ C,
Q. Gao et al. / Nuclear Engineering and Design 256 (2013) 148–152
149
Table 1 Chemical compositions of high Cr ferritic heat-resistant steel (in wt.%). Element
C
Si
Mn
Cr
Mo
W
V
Nb
N
Co
B
Ti
Al
wt.%
0.05
0.21
0.44
9.81
0.43
1.73
0.22
0.07
0.03
1.21
0.0045
<0.01
0.014
Fig. 1. DTA curves of heat flow as a function of austenitization temperature during heating.
1050 ◦ C, 1150 ◦ C and 1200 ◦ C at a rate of 20 K/min, respectively, and held at the temperature for 20 min, then cooled to room temperature with the same rate. 3. Results and discussion The obtained DTA curves with a heating rate of 20 K/min to differential austenitization temperature are displayed in Fig. 1. During heating, two distinct endothermic peaks can be found: The first one around 745 ◦ C indicated the transition from a ferromagnetic state to a paramagnetic state, i.e., so-called the Curie Point or the magnetic transition. The measured Curie Point in our work was about 744.9 ◦ C. The other was a new phase formation peak with transformation from martensite to austenite, so the observed average Ac1 temperature was about 850.9 ◦ C, and the Ac3 was around 919.5 ◦ C, respectively. Austenitizing from 1050 ◦ C to 1200 ◦ C, a gentle endothermic peak was observed after 950 ◦ C, respectively. Maehara (Maehara
and Ohmori, 1987) had shown that a lamellar/cellular structure of M23 C6 and austenite initiated nucleating at the interface of ␦ferrite/matrix austenite at around 1000 ◦ C, in other words, M23 C6 particles are certainly complete dissolution at 1050 ◦ C. The gentle peak after 950 ◦ C should be result from the dissolution of M23 C6 particles. The last gentle peak after 1050 ◦ C for austenitization at 1150 ◦ C and 1200 ◦ C implied austenite homogenization and the formation of ␦-ferrite. According to the early report, the computed constant carbon section of (Fe–xCr–0.1C) system suggested that for a 9Cr steel, the ␦-ferrite phase formed at 1100 ◦ C (Jeya Ganesh et al., 2011); and for the standard 9Cr–1Mo steel, the inflection at 1050 ◦ C can be taken to mark the entry of the system into the ␥ + ␦-ferrite two phase field on continuous heating by estimated using Thermocalc software (Helis et al., 2009). Combining the two possibilities, the inflection which is seen after 1050 ◦ C was associated with both austenite homogenization and the possible emergence of ␦-ferrite from homogeneous austenite. In short, M23 C6 particles have dissolved partly at 950 ◦ C and completely before 1050 ◦ C, whereas, new ␦-ferrite appears until heating to 1150 ◦ C. The result of phase mole fraction as a function of the temperature for the high Cr ferritic heat-resistant steel in our work was computed by Thermo-calc software is shown in Fig. 2. The liquid temperature is about 1490 ◦ C, and the phase is all ␦-ferrite range exits above 1320 ◦ C. The ␦-ferrite starts to transform austenite below 1320 ◦ C and finishes approximately at around 1020 ◦ C. The room temperature phases were composed by martensite (formed by ␣-ferrite) and 0.06–0.08 volume fraction ␦-ferrite based on the result of computation. The detail of precipitated phase was represented in Fig. 2b. MX phase and M23 C6 carbides dissolve completely at about 1200 ◦ C and 950 ◦ C, respectively. Hence the gentle endothermic peak at around 950 ◦ C is M23 C6 carbides dissolution and that at 1050 ◦ C is the formation of ␦-ferrite for the DTA heating curves (seen as Fig. 2), and the same as the phases formation during continuous cooling. Fig. 3 indicates the martensite fractions as a function of austenitization temperature during continuous cooling. The most obvious one is the shape of martensite transformation curve, at 950 ◦ C austenitization temperature which M23 C6 starts to dissolve in the austenite, was different with others, that is to say, the
Fig. 2. Mole fraction of phase transition of high Cr ferritic heat resistant steel as a function of temperature based upon computation by Thermo-calc software. (a) Ferrite and austenite transformation, and (b) details of precipitates.
150
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Fig. 3. Martensite fraction based on integrated the data of martensite transformation as a function of temperature.
transformation kinetic mechanism of martensite is not same before M23 C6 dissolving complete and after that. It also can be found that the beginning reaction rate of martensite was lower at 950 ◦ C than that at other temperatures. The obtained onset points of martensite transformation (Ms point) as a function of austenitization temperatures are shown in Fig. 4. Several researches (Hsu and Linfah, 1983; Harris and Cohen, 1949) have reported that the vacancies in the austenite would increase with the increase of austenitization temperature due to the strong mobility of atoms. The nucleation of martensite may be boosted because of freezing the vacancies in the austenite grain due to higher quenching temperatures, and it means that the increase in vacancies makes austenite less stable by increasing martensite nucleation sites. Besides, with increasing austenitization temperature, the chemical compositions in the austenite are more uniform which will promote martensite transformation. Therefore, in general, the Ms should increase as austenitization temperature increases, and the same results can be found based on our experimental data, whereas it is increase strongly at the beginning and not obviously with the temperature raising. These results can be attributed to the followed four major points: Firstly, before 1050 ◦ C, new ␦-ferrite does not form, and the alloying elements dissolving in austenite are more homogenous with the increase of
austenitization temperature, which will lead to the increase of onset point of martensite transformation. Secondly, the undissolved precipitates, such as M23 C6 precipitating in austenite grain boundary and MX particles locating in the austenite grain, lead to form carbon depletion zone which restrains martensite transformation and decreases the onset point. So the more completely precipitates dissolved, the higher martensite transformation onset point. Thirdly, the effect of ␦-ferrite formation. After 1050 ◦ C, the content of ␦-ferrite is sufficiently high to affect martensite transformation, and becomes the major factor comparing with the effects of compositions of austenites and undissolved particles (mainly MX). Lastly, the austenite will be stable with the precipitates complete dissolving and the chemical composition distributing uniformly in the austenite, which lead to the Ms point does not change obviously. Combined with the above factors, the change of Ms point presents the characteristic as shown in Fig. 4. Generally, Ms point is just relevant to the chemical compositions of materials, but some researchers recently reported that it also has certain relations with the austenite grain size (Krauss et al., 1989; Garcia-Junceda et al., 2008; Gil et al., 1995). Based on Fisher’s nucleation model about the new phase, it is given as (Fisher et al., 1949): df m(1 − f ) = mV¯ C (1 − f ) ≡ dNV NVC
(1)
where f is the formed martensite fraction, NV is the number of martensite laths per unit volume, m is the aspect ratio of martensite lath (set as 0.06 (Zanardi, 1952)), V¯ C is the mean volume of austenite that divided by martensite lath, and NVC is the number of austenite compartments per unit volume. Each new martensite lath formed will produce one additional compartment, so that: NVC =
1 + NV V
(2)
where V is the mean austenite grain volume. Substituting Eq. (2) into Eq. (1) and integrating, the number of martensite laths per unit volume can be obtained as: NV =
1 V
exp
ln(1 − f ) −
m
−1
(3)
the formed martensite fraction, f, can be obtained by measuring the square of exothermic peak in DTA curves which in detail as follows: f =
ST S
(4)
where ST is the square of exothermic peak that the temperature reduced from Ms point to T, and S is the total square of exothermic peak. It should be noted that Eq. (3) has not described the relationship between Ms point and austenite grain size, Yang and Bhadeshia (2009) showed an new equation about the number of martensite laths per unit volume as: NV = a[exp{b(T ∗ − T )} − 1]
(5)
where a and b are the fitted constants, T* is the martensite start temperature at which martensite formed in infinite austenite grain. Combining Eqs. (3) and (5), it can be obtained as:
1 1 ln T −T = aV b ∗
Fig. 4. Martensite transformation start temperature Ms as a function of austenitization temperature.
ln(1 − f )
exp −
m
−1
+1
(6)
It is assumed that the measured Ms point is the temperature when martensite fraction is 0.01, so the parameter T in Eq. (6) can be substituted by Ms . It should be noted that the added experiments that austenitized at 1000 ◦ C and 1100 ◦ C are finished, then only the Ms and the relative austenite grain size are measured to obtain more reliable fitted results. The fitted results based on experimental data are displayed in Fig. 5. So, the fitted related constants can
Q. Gao et al. / Nuclear Engineering and Design 256 (2013) 148–152
Fig. 5. Ms point as a function of austenite grain size at various austenitization temperatures in the new high Cr ferritic heat-resistant steel.
be obtained as a = 3.84, b = 2.23 × 10−5 , and T* = 455.08 ◦ C, respectively. It can be seen that Eq. (6) can be appropriately described the relationship between Ms and austenite grain size for the high Cr ferritic heat resistant steel. Fig. 6 exhibits optical morphologies of microstructures austenitized at various temperatures. It is clear that the microstructures are consisted of martensite and ␦-ferrite, and the content of ␦-ferrite austenitized at 950 ◦ C is obviously higher than that at other temperatures. By observing the microstructures carefully, it also should be noted that the formed martensite existed with lath style, and the corresponding martensite laths become coarser with the increase of austenitization temperature. The primary ␦-ferrite which formed in hot rolling and followed by air cooling, cannot completely transform to austenite/martensite during the subsequent cooling, the ␦-ferrite + austenite duplex phase coexisting in room temperature have been researched in our previous work (Gao et al., 2011).
151
Fig. 7. ␦-Ferrite fraction as a function of austenitization temperature.
The measured ␦-ferrite fraction based upon optical morphologies of microstructures using the relative Computer-aided Software, as a function of austenitization temperature as revealed in Fig. 7. The highest ␦-ferrite fraction abnormally occurs at 950 ◦ C as can be seen, and sharply decreases at 1050 ◦ C, then increases with the increase of austenitization temperature. For the experimental temperature, it has been pointed out that the new ␦-ferrite appears at about 1150 ◦ C in the DTA curves and the calculated results by Thermo-calc that is consistent with the results as shown in Fig. 7. The abnormally formed ␦-ferrite at 950 ◦ C can be considered the effect of un-dissolved M23 C6 and MX particles, and there are also some particles can be found in the optical morphologies for austenitizing at 950 ◦ C (as seen Fig. 6a). As is known to all, the alloying elements uniformly distribute in the austenite grain, and more uniform with the increase of temperature (Cota et al., 2004; Campos et al., 2006). However, if there is the occurrence of particles precipitation, some alloying elements will transfer into these particles which may lead to form so-called carbon depletion zone.
Fig. 6. Optical microstructures of the samples after DTA thermal cycle with different austenitization temperature: (a) 950 ◦ C, (b) 1050 ◦ C, (c) 1150 ◦ C, and (d) 1200 ◦ C.
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Fig. 8. TEM images showing the microstructures of (a) austenitized at 950 ◦ C and (b) austenitzed at 1200 ◦ C.
The so-called carbon depletion zone provides advantaged nucleation sites for ␦-ferrite transformation while the temperature being high enough. So austenited at 950 ◦ C, the un-dissolved M23 C6 and MX particles promote ␦-ferrite transformation on the contrary, and ␦-ferrite transformation abnormally occurs. Fig. 8 demonstrates the typical TEM images of the modified high Cr ferritic heat resistant steel upon different austenitization temperatures. It can be found that the un-dissolved M23 C6 particles precipitated in ␦-ferrite grain while austenitizing at 950 ◦ C, and distributed parallelly with the direction of ␦-ferrite. Nevertheless, there is just one M23 C6 particle can be found in ␦-ferrite grain for the sample austenitized at 1200 ◦ C, which manifests that the M23 C6 particles should have dissolved completely while heating to 1200 ◦ C. The M23 C6 particles promote the transformation from austenite to ␦-ferrite around themselves, and so new ␦-ferrite forms (Hamada et al., 1995). With holding at M23 C6 induced ␦ferrite formation temperature (950 ◦ C in the paper), ␦-ferrite grows up and links together, which results in a morphology like Fig. 6a. The ␦-ferrite formed at 1200 ◦ C due to the temperature is high enough to induce the transformation from austenite phase to ␦ferrite phase, and part ␦-ferrite will remain to room temperature during continuous cooling. 4. Conclusions The effect of austenitization temperature on phase transition features of high Cr ferritic heat-resistant steel was studied by Differential Thermal Analysis experiment. The M23 C6 particles dissolved peak and the ␦-ferrite formed peak were observed during heating, respectively. The onset of martensite transformation firstly increases and then decreases with the increase of austenitization temperature. The ␦-ferrite abnormally forms austenitized at 950 ◦ C due to be induced by the un-dissolved M23 C6 particles. The austenite grain size and martensite laths become larger and coarser with increasing austenitization temperature, respectively. Acknowledgements The authors are grateful to the National Natural Science Foundation of China and Shanghai Baosteel Group Company (No. 50834011), and National Natural Science Foundation of China (Nos. 51204121 and 50901049) for grant and financial support.
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