J. Mater. Sci. Technol., 2011, 27(11), 1059-1064. Improvement of Stress-rupture Life for Modified-HR6W Austenitic Stainless Steel Shiyun Cui 1), Zixing Zhang 1), Yulai Xu 1), Jun Li 1), Xueshan Xiao 1) and Changchun Zhu 2) 1) Institute of Materials, Shanghai University, Shanghai 200072, China 2) Baoshan Iron & Steel Co., Ltd., Shanghai 200940, China [Manuscript received December 21, 2010, in revised form March 16, 2011] Stress-rupture life of HR6W austenitic stainless modified with B and Mg additions was measured, and the microstructures were analyzed by optical microscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy equipped with energy dispersive spectroscopy. The results indicated that the enhancement of the stress-rupture life was mainly due to the precipitation with B in the elemental form at the grain boundaries, and the improvement of the form of carbides at grain boundaries and the removal of O and S elements by addition of Mg. The micro-alloying elements have a beneficial effect on stress-rupture life of the modified-hr6w austenitic stainless at high temperature. KEY WORDS: HR6W; Micro-alloying; Stress-rupture life; Microstructure 1. Introduction With the rapid development of economy and the sustaining improvement of people living standards, the requirement for electric power is becoming more urgent than ever, which promotes the electric power construction and rapid development of manufacturing industry for power equipments. In this new century, government pays more attention to environmental protection and reduction of CO 2 emission. Increased efficiency and decreased emissions of pulverized coalfired boilers can be simultaneously realized through increasing the steam temperatures and pressures [1]. The major efforts are devoted to develop supercritical and ultra-supercritical coal-fired power plants with higher thermal efficiency and cleanness. Due to the excellent creep properties, HR6W austenitic stainless (ASS) can be selected for high temperature services like super heater tubes in steam power plants, boiling water reactors and chemical plants [2]. But the applications of these components at high temperature and high stress are strongly influenced by the Corresponding author. Prof., Ph.D.; Tel.: +86 21 56331484; Fax: +86 21 56331484; E-mail address: xsxiao@mail.shu.edu.cn (X.S. Xiao). performance of HR6W ASS. As a result, significant efforts have been taken to improve the high temperature properties. In most instances it is assumed that element B is concentrated at the grain boundaries where it enters into the precipitates to suppress the microcavity formation. So it is considered to be a beneficial additive which can improve mechanical properties at high temperature [3 5]. The segregation of elements like O, S etc. at grain boundaries greatly lower the interfacial energy of the boundaries and enhance the creep cavitations that can directly lead to the Stage III creep or the eventual failure of metals [6]. Minute addition of Mg was found to be highly effective in removing O and S through formations of MgO and MgS [7,8]. Dong [9] found that the addition of appropriate content of Mg changed the shape of carbides at grain boundaries. However, there was no related literature on the effect of micro-alloying elements B and Mg on the high temperature creep properties in HR6W. The purpose of this paper is to improve the stressrupture life of HR6W by investigating the effect of micro-alloying elements B and Mg on the microstructures, tensile properties and stress-rupture properties at.
1060 S.Y. Cui et al.: J. Mater. Sci. Technol., 2011, 27(11), 1059 1064 Table 1 Chemical compositions of the test s (wt%) Steels C Cr Ni W Nb Ti N O S B Mg Fe Base 0.030 22.98 45.02 7.01 0.20 0.10 0.0115 0.0317 0.0090 Bal. +B 0.030 22.97 45.07 6.98 0.20 0.10 0.0109 0.0376 0.0086 0.008 Bal. + (B, Mg) 0.030 23.02 45.10 7.02 0.20 0.10 0.0104 0.0096 0.0036 0.008 0.005 Bal. Fig. 1 Schematic diagram of the stress-rupture test specimen at (all dimensions are in millimeters) 2. Experimental The experimental HR6W ASS was modified with additions of 80 10 6 B and 80 10 6 B + 50 10 6 Mg, respectively, and melted in a ZG-50 vacuum furnace, using pure iron, titanium, nickel, FeCr, FeNb, FeW and FeCrN alloys. The chemical compositions of all the castings designated as tested s are shown in Table 1. The ingots were hot forged into rods. Steels were solution heat treated at 1200 C for 30 min followed by water quenching. Room temperature tensile tests were performed on round bar samples with a gauge length of 25 mm and a diameter of 5 mm according to a National Standard of China, GB/T228-2002, with a strain rate of 0.4 10 3 s 1 to obtain the mechanical properties including ultimate tensile strength (UTS), yield strength (YS) and total elongation (Ψ). X-ray diffraction (XRD) analyses conducted on a D\max-2550 diffractometer with a scan rate of 8 /min were employed to analyze the phase structure. Stress-rupture tests were carried out at. Details of the specimen are shown in Fig. 1. The microstructure after stress-rupture tests was observed by using a KEYENCE VH-Z100 optical microscope (OM) and a JSM 6700F scanning electron microscope (SEM). The metallographic samples were prepared by mounting the specimens in an epoxy mold, grinding using SiC papers to 1000 grit, polishing using 1 µm diamond abrasive, then etching in a solution of 10 ml pure nitric acid, 10 ml pure hydrochloric acid and 15 ml pure acetic acid for about 60 s. Transmission electron microscopy (TEM) equipped with energy dispersive spectroscopy (EDS) was carried out on JEOL JEM 2101F to examine the effect of micro-alloying elements. TEM samples were prepared by using a conventional twin-jet electro-polishing machine at voltage 45 V for about 45 s in a solution of 20% perchloric acid and 80% ethanol at a liquid nitrogen temperature. Fig. 2 Optical micrographs of the test s solution heat treated at 1200 C for 30 min: (a) base ; (b) + B ; (c) + (B, Mg) 3. Results and Discussion The solution treated tested s possessed an average ASTM grain size of 4 5 µm. The microstructure of the as-supplied was fully austenitic, and some twins could be observed, as shown in Fig. 2. The addition of micro-alloying elements had no obvious influence on average grain size. Figure 3 is the XRD patterns of the three tested s, and only austenite phase was found with no precipitation. The variations of UTS, YS and Ψ of the Base, + B and
S.Y. Cui et al.: J. Mater. Sci. Technol., 2011, 27(11), 1059 1064 1061 Intensity / a.u. (c) (b) (a) (111) (200) (220) 30 40 50 60 70 80 90 2 / deg. Fig. 3 XRD patterns of the s solution heat treated at 1200 C for 30 min: (a) base ; (b) + B ; (c) + (B, Mg) UTS and YS / MPa 1000 800 600 400 200 0 Base UTS YS + B + (B, Mg) 100 90 80 70 60 50 40 30 / % Fig. 4 Variation of UTS, YS and Ψ of the s solution heat treated at 1200 C for 30 min + (B, Mg) s at room temperature are shown in Fig. 4. By comparing those results, it can be seen that although there was slight influence on strength of the s with additions of B and Mg, the Ψ was much higher for the micro-alloying s than that of the base at room temperature. The variations of the stress-rupture life of the Base and micro-alloying s are shown in Fig. 5. It can be seen that the stress-rupture life of + (B, Mg) was about 433 h, which is nearly 2.7 times longer than that of the Base (160 h) and more than 1.3 times of that of the + B (332 h). The elongation shown in Fig. 6 after stress-rupture for the Base, + B and + (B, Mg) s were 27%, 35% and 60%, respectively. The above results indicate that the addition of small amount of B and Mg can increase the stress-rupture life remarkably, and that the modified s also displayed excellent creep ductility. Optical micrographs along the tensile direction of the Base, + B and + (B, Mg) s after stressrupture tests at are shown in Fig. 7, showing that the microstructure has changed in the + (B, Mg) (Fig. 7(c)) compared to those of the Creep rupture life / h 450 400 350 300 250 200 150 100 50 0 Base + B + (B, Mg) Fig. 5 Variation of stress-rupture life of the s tested at Elongation / % 60 50 40 30 20 10 0 Base + B + (B, Mg) Fig. 6 Variation of elongation of the s tested at base and +B (Fig. 7(a) and Fig. 7(b)). Figure 7(c) shows that the austenite grains have been transformed into the rafted structure significantly along the direction of the stress axis, which appears to be in agreement with the result of superior ductility of + (B, Mg) shown in Fig. 6. Figure 8 shows SEM morphologies of the precipitates at grain boundaries of the tested s after stress-rupture tests at. The precipitates were characterized as carbides through EDS analyses, mainly composed of C and Cr with small amounts of Fe, Ni and W. TEM observations and corresponding selected area electron diffractions (SADP) were carried out for further characterizing the microstructure of the carbides. Figure 9 presents a set of typical TEM micrographs, and the corresponding SADP displays that the grain boundaries are decorated with M 23 C 6. The morphologies of M 23 C 6 at the grain boundary are strip and block, as shown in Fig. 8(a). The morphology of M 23 C 6 had no significant change with addition of B (Fig. 8(b)). Mg made the M 23 C 6 distribute uniformly at the grain boundary, which greatly changed the existed form of M 23 C 6 particles, as shown in Fig. 8(c). This is because the segregation of active Mg element with appropriate concentration at the grain boundary could
1062 S.Y. Cui et al.: J. Mater. Sci. Technol., 2011, 27(11), 1059 1064 Fig. 7 Optical micrographs along the tensile direction of the s after stress-rupture test at : (a) base, (b) + B, (c) + (B, Mg) make the lattice distortion for austenite phase and M 23 C 6, which caused the mismatch of the coherent lattice interface on both sides, thereby increasing the elastic energy between the parent phase and carbide. The elastic energy of M 23 C 6 increases with increasing the thickness of M 23 C 6. When the thickness of M 23 C 6 reaches a critical value, the coherent boundary between the matrix and M 23 C 6 will be destroyed. Thus morphology of the carbide can be changed and granulated in order to reduce its surface energy. So it is not easy for the glide of dislocations to pass through the grain boundary. Moreover, M 23 C 6 plays a significant role in the hardening in the process of creep. For the dislocation motion between two particles, the pinning force by the particles, τ, can be estimated by the following Eq. (1) [10] : τ = αgb λ (1) Fig. 8 SEM morphologies of precipitates on grain boundaries of the s after stress-rupture test at 700 C/180MPa: (a) base, (b) + B, (c) + (B, Mg) where α is a constant, G is the shear modulus of matrix, b is the Burgers vector of dislocation, G and b also are constant, and λ is the mean free path among dispersed particles. λ can be given by Eq. (2) as a function of the volume fraction f and diameter of dispersed particles d [11] λ = d(1 f 1/3 ) f 1/3 (2) From Eqs. (1) and (2), τ depends on λ, while λ depends on d and f. The volume fraction of precipitates f is assumed to be constant. Therefore, the smaller the particles, the stronger the pinning force τ. And the M 23 C 6 precipitates at the grain boundaries formed in strip and block particles were the major reason to improve the short-term stress-rupture performance of the. Some fine precipitates appeared in the +B, as
S.Y. Cui et al.: J. Mater. Sci. Technol., 2011, 27(11), 1059 1064 1063 Fig. 9 TEM micrographs of the M 23C 6 at the grain boundaries after stress-rupture test at Fig. 10 SEM micrographs and compositions of precipitates in the +B after stress-rupture test at Fig. 12 Morphologies and compositions of precipitates of the s after stress-rupture test at bides in the containing B is expected to decrease the creep rate. But the beneficial effect of B addition on the creep strength and ductility was not mainly due to the enhanced carbonitride precipitation [13]. B element was found to stay at the grain boundaries in the +B by TEM selected area diffraction pattern in the inset of Fig. 11. So it is reasonable to say that the growth of stress-rupture life at for the + B is primarily due to the presence of B element. The segregation of B element at grain boundaries is expected to suppress the grain boundaries sliding because of its relatively high melting point of around 2303 K [14]. So the main high resisting stress may come from the higher strength of grain boundaries. Mg has a strong affinity to O and S, and thermodynamics of the reactions can be dramatically expressed as follows [15,16] : Mg(g) + O = MgO(s), G = 597665 + 203.56T (J/mol) (3) Mg(g) + S = MgS(s), G = 410934 + 180.45T (J/mol) (4) Fig. 11 TEM micrographs of the B element at the grain boundaries after stress-rupture test at shown in Fig. 10. Being an interstitial element, B has been known to decrease the solubility of carbon and nitrogen in ASS [12] and accelerates the carbonitride precipitation. Possible enhanced precipitation of car- The above thermodynamic standard free energies of O, S and Mg display that adding Mg can effectively remove the O and S in the s. The contents of O and S in the s shown in Table 1 also confirm this conclusion, in which the S and O contents were significantly decreased, especially O. The removal of O and S in s can eliminate the harmful effects of low melting point oxides and sulfides, which can reduce the reaction possibility of O with Ti, Nb, etc. in the melting process. Nb and Ti are often taken as the strengthening elements, because Nb and Ti have strong affinities for combining with C and N to form carbides and nitrides. Shinya et al. [17 19] found that the formation of fine NbC and TiC during the hot-deformation improved the high temperature creep properties. And the removal of O and S enhanced the precipitation of NbC and TiC in the s, which might partially contribute to the increase of the
1064 S.Y. Cui et al.: J. Mater. Sci. Technol., 2011, 27(11), 1059 1064 creep resistance properties. Precipitation of NbC and TiC was observed in the present tested s by SEM and EDS, as shown in Fig. 12. 4. Conclusion The effects of micro-alloying elements, B and Mg, on the mechanical properties of the modified-hr6w ASS have been investigated, showing a small effect on improving the room temperature tensile strength, but significant increase in the stress-rupture life at high temperature. Due to the precipitation with B element at the grain boundaries, the diffusivity along the grain boundaries was decreased. Mg was found effective in improving the existing form of carbide particles and removing the O and S, which provided the with longer stress-rupture life. Precipitates of NbC and TiC were identified in the matrix and at the grain boundaries, which improve the creep life by precipitation strengthening. Acknowledgement The authors would like to thank Mr. Y.L. Chu and Dr. Q. Li of the Instrumental Analysis & Research Center for their assistance on SEM and TEM observations. REFERENCES [1 ] R. Viswanathan, K. Coleman and U. Rao: Int. J. Pres. Ves. Pip., 2006, 83(11-12), 778. [2 ] J.P. Shingledecker and N.D. Evans: Int. J. Pres. Ves. Pip., 2010, 87(6), 345. [3 ] M. Fujiwara, H. Uchida and S. Ohta: J. Mater. Sci. Lett., 1994, 13, 557. [4 ] X.X. Yao: Mater. Sci. Eng. A, 1999, 271, 353. [5 ] N. Li, W.R. Sun, Y. Xu, S.R. Guo, D.Z. Lu and Z.Q. Hu: Mater. Lett., 2006, 60, 2232. [6 ] M.E. Kassner and T.A. Hayes: Int. J. Plast., 2003, 19, 1715. [7 ] N.Y. Deng, Z.L. Yang, M.Y. Zhang, G.J. Zhu and Z.D. Ren: J. Chongqing Polytechnic College, 2001, 16(2), 40. (in Chinese) [8 ] K. Wang, X.H. Hu, Y.J. Bi, M. Liu and D.Q. Zhao: Shandong Metallurgy, 2006, 28(3), 47. (in Chinese) [9 ] J. Dong: Heilongjiang Metallurgy, 2009, 29(4), 8. (in Chinese) [10] B.C. Peng, H.X. Zhang, J. Hong, J.Q. Gao, H.Q. Zhang, J.F. Li and Q.J. Wang: Mater. Sci. Eng. A, 2010, 527, 4424 [11] H. Kimura: Tetsu To Hagane-J. Iron Steel Inst. Jpn.,, 2000, 86, 343. (in Japanese) [12] M. Fujiwara, H. Uchida and S. Ohta: J. Mater. Sci. Lett., 1994, 13, 557. [13] K. Laha, J. Kyono, S. Kishimoto and N. Shinya: Scripta Mater., 2005, 52(7), 675. [14] Y.L. Xu, H. Nie, J. Li, X.S. Xiao, C.C. Zhu and J.L. Zhao: Mater. Sci. Eng. A, 2010, 528, 643. [15] H.X. Huang: in Metallurgical Principles, eds, Metallurgy Industry Press, Beijing, 1993, 141. (in Chinese) [16] J.X. Chen: in Metallurgy of Iron and Steel, Metallurgy Industry Press, Beijing, 2005, 115. (in Chinese) [17] N. Shinya, J. Kyono and K. Laha: Mater. Sci. Forum, 2003, 426-432, 1107 [18] A.Y. Kina, V.M. Souza, S.S.M. Tavares, J.A. Souza and H.F.G. de Abreu: J. Mater. Process. Technol., 2008, 199, 391. [19] J.P. Shingledecker, P.J. Maziasz, N.D. Evans and M.J. Pollard: Int. J. Pres. Ves. Pip., 2007, 84, 21.