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Development of new 11%Cr heat resistant ferritic steels with enhanced creep resistance for steam power plants with operating steam temperatures up to 650 °C
Materials Science and Engineering A 510–511 (2009) 180–184
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Development of new 11%Cr heat resistant ferritic steels with enhanced creep resistance for steam power plants with operating steam temperatures up to 650 ◦ C Y. Wang a,∗ , K.-H. Mayer b , A. Scholz a , C. Berger a , H. Chilukuru c , K. Durst c , W. Blum c a
Institut für Werkstoffkunde (IfW) Technische Universität Darmstadt, Germany ALSTOM Energie GmbH, Nürnberg, Germany c University of Erlangen Nürnberg, Germany b
a r t i c l e
i n f o
Article history: Received 13 December 2007 Received in revised form 3 April 2008 Accepted 30 April 2008 Keywords: Creep behavior Oxidation resistance Microstructure Precipitation hardening 11%CrWCoCuVB(Ta Nb) steels
a b s t r a c t The goal of developing new heat resistant 11%Cr ferritic–martensitic steels with sufficient creep and oxidation resistance up to 650 ◦ C was pursued within a joint project following an alloying concept based on physical metallurgy principles. The highest creep strength combined with good oxidation resistance was achieved for a Ta-alloyed test melt (11 wt.% Cr, W, Co, Mo, V, 0.09 wt.% Ta, relatively high contents of C and B). The microstructural evolution during creep was investigated by transmission electron microscopy for the Ta-alloyed melt in comparison to a sister melt where Ta was exchanged for 0.04%Nb. It is proposed that fine particles of types MX and M23 C6 (M: metallic element, X: interstitial elements) are the cause of the outstanding creep resistance of the Ta-alloyed melt. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The creep properties of construction steels play an important role in the process of developing new steam power plants with enhanced thermal efficiency and reduced environmental impact. Significant improvements of the creep resistance of the 9–12%Cr steels were achieved in the last decades through slight changes in chemical composition of well proven steels like X20CrMoV12-1 [1–6] comprising • balancing the contents of C, V, Nb and/or Ta and N to generate stable MX precipitates (M: metal, X: C, N); • increasing the contents of Mo and W for solution hardening and precipitation hardening by M23 C6 and Laves phase; • adding Co, Cu, Mn and C to suppress delta-ferrite; • adding Cu to nucleate Laves phase at Cu precipitates; • alloying with B to stabilize M23 C6 -precipitates. B-alloying appears to be the most effective way to improve the creep strength of the 9–12%Cr steels. Already four decades ago,
∗ Corresponding author. E-mail address: [email protected] (Y. Wang). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.04.116
Fujita found a substantial improvement of creep strength by adding 0.027–0.040 wt.% of B [2]. Meanwhile Fujita confirmed the positive effect of 0.040 wt.% B in creep of TAF steel up to 130,000 h at 650 ◦ C [3], where the target of 105 h creep life at 650 ◦ C and 100 MPa was nearly achieved. However, a disadvantage of high B contents lies in the impaired forgeability of large components. Fig. 1 exhibits the temperature dependence of the 105 h creep strengths of TAF steel in comparison to the steel X20CrMoV12-1 and to the two rotor steels X12CrMo(W)VNbN10-1(1) and X18CrMoV-NbB9-1 developed in the COST 501 Programme of the European Cooperation in the field of Scientific and Technical Research [6]. Apart from B, Cr is a key element influencing both the oxidation resistance and the creep strength. Experience shows that 12 wt.% Cr are needed to raise the oxidation resistance to an acceptable level [6]. The highest creep strength, on the other hand, is obtained for a Cr-content of only about 9.5 wt.%. This is substantiated by the 100 MPa creep rupture strengths at 650 ◦ C of ferritic–martensitic steels which were newly developed and investigated during the last decades in international programs [6]; from Fig. 2 an overall trend of increase of creep strength with decrease of Cr-content becomes clearly apparent. An outstanding position is held by test melts of TAF steel with relatively high Cr-contents of 10.5 to 10.7 wt.% and B-contents of 0.027– 0.040 wt.%.
Y. Wang et al. / Materials Science and Engineering A 510–511 (2009) 180–184
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Fig. 3. Creep rupture strengths of 59As and 62As at 650 ◦ C as function of time to fracture in comparison to P92 and TAF [9]. Fig. 1. 105 h creep rupture strength of heat resistant turbine steels [6].
2. Alloying concept The aim of the present cooperative Deutsche Forschungsgemeinschaft DFG; German Research Foundation project was to develop new heat resistant 11–12%Cr ferritic–martensitic steels for applications at 650 ◦ C. A systematic approach based on physical metallurgy was attempted to accelerate the commonly used trialand-error procedure [1]. The tasks of alloy development, testing and analysis of microstructure were distributed among the partners of a working group (Max-Planck-Institut fuer Eisenforschung, Duesseldorf; Institut fuer Werkstoffwissenschaften, Erlangen; Institut fuer Werkstoffe der Energietechnik, Juelich and Institut fuer Werkstoffkunde, Darmstadt) [1]. The ultimate goal was to combine a 105 h creep rupture strength of 100 MPa at 650 ◦ C with sufficient corrosion resistance. Tables 1 and 2 give the chemical compositions, the mechanical properties and the grain sizes for two key melts called 59As and 62As and selected from a total of about 80 melts of the DFG project. The Cr-content was fixed at about 11% to ensure an oxidation resistance of sufficient level. In order to generate a ferritic–martensitic microstructure with precipitates of M23 (C, B)6 , MX and Laves phase, the following choices of alloying elements were made (see Table 1):
• Cu to promote a fine distribution of Laves phase by nucleation at Cu-precipitates; • V and Ta or Nb for strengthening by MX precipitates; and • Si in addition to Cr for improved oxidation resistance. The test melts were prepared in a high frequency furnace and vacuum arc remelted by the forgemaster Saarschmiede, Völklingen, Germany. The heat treatment of the forged bars of 40 mm × 20 mm cross-section was been carried out by IfW Darmstadt. On the basis of calculations with the software ThermoCalc the heat treatment was specified as 0.5 h at 1150 ◦ C + 2 h at 765 ◦ C + air cooling. For comparison, the advanced 9%Cr piping steel P92, manufactured by Nippon Steel Corporation and limited to applications at 620 ◦ C, was investigated in the state after heat treatment for 2 h at 1065 ◦ C + air cooling + 2 h at 770 ◦ C [7]. 3. Results of creep tests
• balanced amounts of Co, Mn, C and Cu to obtain a ferritic–martensitic matrix; • Cr, W, B and C for strengthening by M23 C6 precipitates; • W for strengthening by Laves phase;
The creep rupture strengths of the key melts 59As and 62As at 650 ◦ C are plotted in Fig. 3 together with those of TAF steel [3] and P92 [8]. The differences in chemical composition of the two melts appear to have substantial influence on their creep strengths. While the long-term rupture strength of the Nb-alloyed 62As is similar to that the P92, the strength of the Ta-alloyed 59As is substantial higher till 25,000 h and, according to current tests, probably beyond. This is also seen from the fact that for three creep stresses 59As shows the lowest creep rates at a given time (Figs. 4 and 5) and even at a given strain (Fig. 6). The low creep rates of 59As retard
Fig. 2. Creep rupture time of newly developed ferritic–martensitic steels versus Cr content for a test stress of 100 MPa at 650 ◦ C [6].
Fig. 4. Creep rate ε˙ p at 95 MPa and 650 ◦ C as function of time t for 59As and 62As in comparison to P92.
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Table 1 Chemical composition of test melts and of steel P92 (wt.%). Steel
C
Si
Mn
Cr
Mo
W
Co
59As 62As P92*
.19 .20 .106
.37 .28 .04
.30 .32 .46
11.0 10.8 9.0
.50 .51 .47
2.10 2.00 1.84
1.20 1.17 –
*
Cu .71 .76 –
V
Nb
Ta
Ti
N
B
.27 .28 .20
– .04 .07
.09 – –
– – –
.030 .030 .050
.020 .022 .001
Data from [7].
Table 2 Mechanical properties and grain size at RT of test steels and comparison test melt P92. Steel
0.2-Limit MPa
Tensile strength MPa
Elongation %
Reduction of area %
Hardness HV30
ASTM grain size
59As 62As P92a
675 636 528
880 844 688
17.4 18.4 25.3
51 55 71
270 262 226
2–3 2–3 7–10
a
Data from [7].
Fig. 5. As Fig. 4 for 130 and 83 MPa.
the onset of tertiary creep compared to P92 in accordance with the observed creep rupture behavior. 4. Discussion of creep results Our results show that the goal of times beyond 105 h for creep rupture at 100 MPa and 650 ◦ C has not been achieved with the present alloying concept. However, the Ta-alloyed melt 59As exhibits a distinctly higher creep strength than the advanced 9%Cr piping steel P92. The comparison of Ta-alloyed 59As and Nb-alloyed
Fig. 6. Creep rate as function of strain for tests of Fig. 5.
62As demonstrates that the substitution a single alloying element may significantly change the creep behavior. The maximum rate at which log ε˙ increases with creep strain in the tertiary stage equals 30 for 62As and 130 MPa (Fig. 6). These values lie in the range of those values determined for similar alloys under conditions, where hardening precipitates coarsen, but do not dissolve in favor of a stabler phase [10]. This indicates that the hardening particles in 59As are equally stable. 59As not only has excellent creep resistance, but also excellent oxidation resistance although the general requirement of a minimum Cr-content of 12 wt.% is not fulfilled. This was found within the DFG project by the partner IWV 2 Jülich who investigated specimens of the melts 59As and 62As (with 11 wt.% Cr) as well as the melt 71As (12 wt.%Cr, 4%W and 4%Co) and the piping steel P92 in an environment of Ar–50%H2 O at 600 and 650 ◦ C [11]. It turned out that the oxidation resistance of 59As and 62As is even somewhat higher than that of 71As (Fig. 7), and distinctly better than that of the piping steel P92, consistent with its low Cr-content of 9 wt.%. 5. Microstructure Investigations of the initial states by light optical and transmission electron microscopy (TEM) show that there are no significant differences between 59As and 62As in the prior austenite grain structure and the initial subgranular dislocation structure within the austenite grains (Fig. 8). The average initial subgrain sizes between 300 and 400 nm lie the range normally found for tempered martensite Cr-steels. The particle structure was investigated by TEM of extraction replicas. Each particle was characterized by its size dp (average
Fig. 7. Weight change due to oxidation of the test melts 59As, 62As, 71As and steel P92 at 600 and 650 ◦ C in Ar–50H2 O [11].
Y. Wang et al. / Materials Science and Engineering A 510–511 (2009) 180–184
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Fig. 8. Representative TEM images of particle-stabilized subgrain structure in 59As and 62As after heat treatment.
Fig. 9. Particle sizes dp and volume fractions fp of precipitated phases in 59As and 62As in initial state and after creep at 650 ◦ C and 110 and 95 MPa, respectively, for (a) M23 C6 , (b) VX and TaX, (c) Laves phase. Lines: Ostwald ripening according to d3 p − d3 p,o = kp t (kp : ripening constant).
equivalent diameter) and composition (from energy-dispersive Xray spectrometry). The volume fraction fp of the particles was roughly estimated using the formula given in [13]. The results are displayed in Fig. 9. Both test melts contain particles of M23 C6 , MX and Laves phase. The experimental data for dp and fp lie in the range commonly found for 9-12%Cr steels (see [10]). For a detailed discussion including the systematic errors related with extraction replica technique we refer to [13].
The fact that the fraction of Laves phase is higher in crept 62As than in 59As in combination with the observation that Cu-particles are missing after creep in 62As indicates that Cu precipitates acts as nucleation sites of Laves phase. The point to be emphasized here is that the relatively small MX particles found after creep in 59As consist of TaX particles in addition to the well known VX particles. A similar effect of Nb in 62As does not exist, suggesting that, in contrast to Ta, Nb is precipitated
Fig. 10. Initial state of 59As in TEM; left: bright field view of dislocation structure with several coarse precipitates at subgrain boundaries; center: dark field view of rectangular area (from left) in subgrain interior; bright spots probably due to MX precipitates; right: diffraction pattern.
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in the form of coarse primary particles. It is probable that the volume fraction of MX has increased in consequence of the additional supply of MX-forming atoms at temper temperature. Thus the Ta content is expected to enhance the strengthening of the subgrain interior by MX in 59As; however, the fp data are not exact enough for an unequivocal proof. It is important to note that the resolution limit of the replica technique is near 20 nm so that smaller particles which may have a strong hardening effect may have been overlooked. Inspection of a replica area, which was seemingly free from extracted particles, revealed the presence of TaX, indicating the existence of small TaX precipitates with sizes below the limit of resolution. Fig. 10 shows TEM images of 59As in the initial state where the extraction technique did not work for some unknown reason. The marked region of the bright-field image, which lies in the interior of a large subgrain, was investigated under weak-beam conditions. The weak beam image shows numerous small particles (bright spots), which according to the diffraction pattern were coherent with the matrix and may be of type MX. Observations in high-resolution TEM of steel foils at ARC Seibersdorf [12] on 59As in the virgin condition revealed the existence of fine M23 C6 precipitates, around 10 nm in size, probably lying in the interiors of subgrains. The fact that fine M23 C6 particles were found not only in the initial state but also after creep suggests a high stability against coarsening. It is therefore proposed that the specific advantage of 59As is due to enhanced strengthening by fine precipitates, in particular Ta-containing MX precipitates, which have sizes below the limit of resolution of the extraction replica technique and are sufficiently resistant against coarsening and replacement by more stable phases. 6. Summary While the goal of 105 h creep rupture strength of 100 MPa at 650 ◦ C could not be achieved with the present alloying concept, the test melt 59As with 11 wt.% Cr, relatively high contents of C and B, low contents of W and Co, and 0.09 wt.% Ta yields promising results. The results confirm the finding derived from the literature that the effectivity of strengthening by fine precipitates is enhanced, when the Cr content is somewhat reduced relative to the value of 12 wt.% recommended for oxidation stability. The creep strength of 59As markedly exceeds that of the piping steel P92 and the
Nb-alloyed test melt 62As in tests extending over 25,000 h. TEM examinations indicates that the extraordinary creep resistance of 59As is due to fine Ta-containing precipitates of the types MX and M23 C6 . Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. We thank the members of the working group for providing the test materials and performing ThermoCalc calculations. We also acknowledge the financial support for continuation of creep tests of 59As and 62As samples beyond 25,000 h within framework of the COST 536 Action from German Industry and the Bundesministerium für Wirtschaft und Technologie. References [1] R. Agamennone, C. Berger, W. Blum, J. Ehlers, P.J. Ennis, J. Granacher, G. Inden, V. Knezevic, J.W. Quadakkers, G. Sauthoff, A. Scholz, L. Singheiser, J. Vilk, Y. Wang, in: J. Lecomte-Beckers, M. Carton, F. Schubert und, P.J. Ennis (Eds.), Materials for Advanced Power Engineering 2002, Proc. of 7th Liége Conference, vol. III, Forschungszentrum Jülich, Germany, 2002, pp. 1279–1288. [2] T. Fujita, Trans. JIM 9 (1968) 167–169. [3] T. Fujita, Proc. of International Workshop on Development of Advanced Heat Resisting Steels, Yokohama, Japan, 8th November 1999, NIMS Tsukuba, Japan, 1999. [4] F. Abe, Proc. of 4th EPRI Conference on Advances in Materials Technogy for Fossil Power Plants, Hilton Oceanfront Resort, Hilton Head Island, SC, USA 25–28 October 2004, pp. 202–216, EPRI Palo Alto, USA, 2004. [5] M. Hättestrand, Doctoral thesis for the degree of Doctor of Philosopy, Chalmers University of Technology and Göteborg University, Sweden, 2000. [6] K.H. S Mayer, Proc. of 29. MPA Seminar Materials & Component Behaviour in Energy & Plant Technology Stuttgart 9–10, October 2003, MPA Stuttgart, Germany, 2003. [7] Nippon Steel Corporation: Data Package for NF 616 Ferritic Steel (9Cr–0.5Mo–1.8W–Nb–N), January 1993. [8] J. Hald, ECCC E911-P92 Assessment, ECCC Data Sheet, presented in September 2005 at the ECCC conference Creep & Fracture in High Temperature Components, September 12–14, 2005, London, UK, ERA Technology Ltd., Leatherhead, Surrey, KT22 7SAS, UK 2003. [9] C. Berger, A. Scholz, Y. Wang, K.H. Mayer, Z. Metallkd. 96 (2005) 668–674. [10] R. Agamennone, W. Blum, C. Gupta, J.K. Chakravartty, Acta Mater. 54 (2006) 3003–3014. [11] J. Pirón Abellán, P.J. Ennis, L. Singheiser, W.J. Quadakkers, in: J. Lecomte-Beckers, M. Carton, F. Schubert und, P.J. Ennis (Eds.), Materials for Advanced Power Engineering 2006, Proc. of 8th Liège Conference, vol III, Forschungszentrum Jülich, Germany, 2006, pp. 1285–1295. [12] S. Höfinger, ARC Seibersdorf, Austria, Personal communication. [13] H. Chilukuru, K. Durst, S. Wadekar, M. Schwienheer, A. Scholz, C. Berger, K.H. Mayer, W. Blum, Mater. Sci. Eng. A. in press.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Development of new 11%Cr heat resistant ferritic steels with enhanced creep resistance for steam power plants with operating steam temperatures up to 650 ◦ C Y. Wang a,∗ , K.-H. Mayer b , A. Scholz a , C. Berger a , H. Chilukuru c , K. Durst c , W. Blum c a
Institut für Werkstoffkunde (IfW) Technische Universität Darmstadt, Germany ALSTOM Energie GmbH, Nürnberg, Germany c University of Erlangen Nürnberg, Germany b
a r t i c l e
i n f o
Article history: Received 13 December 2007 Received in revised form 3 April 2008 Accepted 30 April 2008 Keywords: Creep behavior Oxidation resistance Microstructure Precipitation hardening 11%CrWCoCuVB(Ta Nb) steels
a b s t r a c t The goal of developing new heat resistant 11%Cr ferritic–martensitic steels with sufficient creep and oxidation resistance up to 650 ◦ C was pursued within a joint project following an alloying concept based on physical metallurgy principles. The highest creep strength combined with good oxidation resistance was achieved for a Ta-alloyed test melt (11 wt.% Cr, W, Co, Mo, V, 0.09 wt.% Ta, relatively high contents of C and B). The microstructural evolution during creep was investigated by transmission electron microscopy for the Ta-alloyed melt in comparison to a sister melt where Ta was exchanged for 0.04%Nb. It is proposed that fine particles of types MX and M23 C6 (M: metallic element, X: interstitial elements) are the cause of the outstanding creep resistance of the Ta-alloyed melt. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The creep properties of construction steels play an important role in the process of developing new steam power plants with enhanced thermal efficiency and reduced environmental impact. Significant improvements of the creep resistance of the 9–12%Cr steels were achieved in the last decades through slight changes in chemical composition of well proven steels like X20CrMoV12-1 [1–6] comprising • balancing the contents of C, V, Nb and/or Ta and N to generate stable MX precipitates (M: metal, X: C, N); • increasing the contents of Mo and W for solution hardening and precipitation hardening by M23 C6 and Laves phase; • adding Co, Cu, Mn and C to suppress delta-ferrite; • adding Cu to nucleate Laves phase at Cu precipitates; • alloying with B to stabilize M23 C6 -precipitates. B-alloying appears to be the most effective way to improve the creep strength of the 9–12%Cr steels. Already four decades ago,
∗ Corresponding author. E-mail address: [email protected] (Y. Wang). 0921-5093/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.04.116
Fujita found a substantial improvement of creep strength by adding 0.027–0.040 wt.% of B [2]. Meanwhile Fujita confirmed the positive effect of 0.040 wt.% B in creep of TAF steel up to 130,000 h at 650 ◦ C [3], where the target of 105 h creep life at 650 ◦ C and 100 MPa was nearly achieved. However, a disadvantage of high B contents lies in the impaired forgeability of large components. Fig. 1 exhibits the temperature dependence of the 105 h creep strengths of TAF steel in comparison to the steel X20CrMoV12-1 and to the two rotor steels X12CrMo(W)VNbN10-1(1) and X18CrMoV-NbB9-1 developed in the COST 501 Programme of the European Cooperation in the field of Scientific and Technical Research [6]. Apart from B, Cr is a key element influencing both the oxidation resistance and the creep strength. Experience shows that 12 wt.% Cr are needed to raise the oxidation resistance to an acceptable level [6]. The highest creep strength, on the other hand, is obtained for a Cr-content of only about 9.5 wt.%. This is substantiated by the 100 MPa creep rupture strengths at 650 ◦ C of ferritic–martensitic steels which were newly developed and investigated during the last decades in international programs [6]; from Fig. 2 an overall trend of increase of creep strength with decrease of Cr-content becomes clearly apparent. An outstanding position is held by test melts of TAF steel with relatively high Cr-contents of 10.5 to 10.7 wt.% and B-contents of 0.027– 0.040 wt.%.
Y. Wang et al. / Materials Science and Engineering A 510–511 (2009) 180–184
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Fig. 3. Creep rupture strengths of 59As and 62As at 650 ◦ C as function of time to fracture in comparison to P92 and TAF [9]. Fig. 1. 105 h creep rupture strength of heat resistant turbine steels [6].
2. Alloying concept The aim of the present cooperative Deutsche Forschungsgemeinschaft DFG; German Research Foundation project was to develop new heat resistant 11–12%Cr ferritic–martensitic steels for applications at 650 ◦ C. A systematic approach based on physical metallurgy was attempted to accelerate the commonly used trialand-error procedure [1]. The tasks of alloy development, testing and analysis of microstructure were distributed among the partners of a working group (Max-Planck-Institut fuer Eisenforschung, Duesseldorf; Institut fuer Werkstoffwissenschaften, Erlangen; Institut fuer Werkstoffe der Energietechnik, Juelich and Institut fuer Werkstoffkunde, Darmstadt) [1]. The ultimate goal was to combine a 105 h creep rupture strength of 100 MPa at 650 ◦ C with sufficient corrosion resistance. Tables 1 and 2 give the chemical compositions, the mechanical properties and the grain sizes for two key melts called 59As and 62As and selected from a total of about 80 melts of the DFG project. The Cr-content was fixed at about 11% to ensure an oxidation resistance of sufficient level. In order to generate a ferritic–martensitic microstructure with precipitates of M23 (C, B)6 , MX and Laves phase, the following choices of alloying elements were made (see Table 1):
• Cu to promote a fine distribution of Laves phase by nucleation at Cu-precipitates; • V and Ta or Nb for strengthening by MX precipitates; and • Si in addition to Cr for improved oxidation resistance. The test melts were prepared in a high frequency furnace and vacuum arc remelted by the forgemaster Saarschmiede, Völklingen, Germany. The heat treatment of the forged bars of 40 mm × 20 mm cross-section was been carried out by IfW Darmstadt. On the basis of calculations with the software ThermoCalc the heat treatment was specified as 0.5 h at 1150 ◦ C + 2 h at 765 ◦ C + air cooling. For comparison, the advanced 9%Cr piping steel P92, manufactured by Nippon Steel Corporation and limited to applications at 620 ◦ C, was investigated in the state after heat treatment for 2 h at 1065 ◦ C + air cooling + 2 h at 770 ◦ C [7]. 3. Results of creep tests
• balanced amounts of Co, Mn, C and Cu to obtain a ferritic–martensitic matrix; • Cr, W, B and C for strengthening by M23 C6 precipitates; • W for strengthening by Laves phase;
The creep rupture strengths of the key melts 59As and 62As at 650 ◦ C are plotted in Fig. 3 together with those of TAF steel [3] and P92 [8]. The differences in chemical composition of the two melts appear to have substantial influence on their creep strengths. While the long-term rupture strength of the Nb-alloyed 62As is similar to that the P92, the strength of the Ta-alloyed 59As is substantial higher till 25,000 h and, according to current tests, probably beyond. This is also seen from the fact that for three creep stresses 59As shows the lowest creep rates at a given time (Figs. 4 and 5) and even at a given strain (Fig. 6). The low creep rates of 59As retard
Fig. 2. Creep rupture time of newly developed ferritic–martensitic steels versus Cr content for a test stress of 100 MPa at 650 ◦ C [6].
Fig. 4. Creep rate ε˙ p at 95 MPa and 650 ◦ C as function of time t for 59As and 62As in comparison to P92.
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Table 1 Chemical composition of test melts and of steel P92 (wt.%). Steel
C
Si
Mn
Cr
Mo
W
Co
59As 62As P92*
.19 .20 .106
.37 .28 .04
.30 .32 .46
11.0 10.8 9.0
.50 .51 .47
2.10 2.00 1.84
1.20 1.17 –
*
Cu .71 .76 –
V
Nb
Ta
Ti
N
B
.27 .28 .20
– .04 .07
.09 – –
– – –
.030 .030 .050
.020 .022 .001
Data from [7].
Table 2 Mechanical properties and grain size at RT of test steels and comparison test melt P92. Steel
0.2-Limit MPa
Tensile strength MPa
Elongation %
Reduction of area %
Hardness HV30
ASTM grain size
59As 62As P92a
675 636 528
880 844 688
17.4 18.4 25.3
51 55 71
270 262 226
2–3 2–3 7–10
a
Data from [7].
Fig. 5. As Fig. 4 for 130 and 83 MPa.
the onset of tertiary creep compared to P92 in accordance with the observed creep rupture behavior. 4. Discussion of creep results Our results show that the goal of times beyond 105 h for creep rupture at 100 MPa and 650 ◦ C has not been achieved with the present alloying concept. However, the Ta-alloyed melt 59As exhibits a distinctly higher creep strength than the advanced 9%Cr piping steel P92. The comparison of Ta-alloyed 59As and Nb-alloyed
Fig. 6. Creep rate as function of strain for tests of Fig. 5.
62As demonstrates that the substitution a single alloying element may significantly change the creep behavior. The maximum rate at which log ε˙ increases with creep strain in the tertiary stage equals 30 for 62As and 130 MPa (Fig. 6). These values lie in the range of those values determined for similar alloys under conditions, where hardening precipitates coarsen, but do not dissolve in favor of a stabler phase [10]. This indicates that the hardening particles in 59As are equally stable. 59As not only has excellent creep resistance, but also excellent oxidation resistance although the general requirement of a minimum Cr-content of 12 wt.% is not fulfilled. This was found within the DFG project by the partner IWV 2 Jülich who investigated specimens of the melts 59As and 62As (with 11 wt.% Cr) as well as the melt 71As (12 wt.%Cr, 4%W and 4%Co) and the piping steel P92 in an environment of Ar–50%H2 O at 600 and 650 ◦ C [11]. It turned out that the oxidation resistance of 59As and 62As is even somewhat higher than that of 71As (Fig. 7), and distinctly better than that of the piping steel P92, consistent with its low Cr-content of 9 wt.%. 5. Microstructure Investigations of the initial states by light optical and transmission electron microscopy (TEM) show that there are no significant differences between 59As and 62As in the prior austenite grain structure and the initial subgranular dislocation structure within the austenite grains (Fig. 8). The average initial subgrain sizes between 300 and 400 nm lie the range normally found for tempered martensite Cr-steels. The particle structure was investigated by TEM of extraction replicas. Each particle was characterized by its size dp (average
Fig. 7. Weight change due to oxidation of the test melts 59As, 62As, 71As and steel P92 at 600 and 650 ◦ C in Ar–50H2 O [11].
Y. Wang et al. / Materials Science and Engineering A 510–511 (2009) 180–184
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Fig. 8. Representative TEM images of particle-stabilized subgrain structure in 59As and 62As after heat treatment.
Fig. 9. Particle sizes dp and volume fractions fp of precipitated phases in 59As and 62As in initial state and after creep at 650 ◦ C and 110 and 95 MPa, respectively, for (a) M23 C6 , (b) VX and TaX, (c) Laves phase. Lines: Ostwald ripening according to d3 p − d3 p,o = kp t (kp : ripening constant).
equivalent diameter) and composition (from energy-dispersive Xray spectrometry). The volume fraction fp of the particles was roughly estimated using the formula given in [13]. The results are displayed in Fig. 9. Both test melts contain particles of M23 C6 , MX and Laves phase. The experimental data for dp and fp lie in the range commonly found for 9-12%Cr steels (see [10]). For a detailed discussion including the systematic errors related with extraction replica technique we refer to [13].
The fact that the fraction of Laves phase is higher in crept 62As than in 59As in combination with the observation that Cu-particles are missing after creep in 62As indicates that Cu precipitates acts as nucleation sites of Laves phase. The point to be emphasized here is that the relatively small MX particles found after creep in 59As consist of TaX particles in addition to the well known VX particles. A similar effect of Nb in 62As does not exist, suggesting that, in contrast to Ta, Nb is precipitated
Fig. 10. Initial state of 59As in TEM; left: bright field view of dislocation structure with several coarse precipitates at subgrain boundaries; center: dark field view of rectangular area (from left) in subgrain interior; bright spots probably due to MX precipitates; right: diffraction pattern.
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in the form of coarse primary particles. It is probable that the volume fraction of MX has increased in consequence of the additional supply of MX-forming atoms at temper temperature. Thus the Ta content is expected to enhance the strengthening of the subgrain interior by MX in 59As; however, the fp data are not exact enough for an unequivocal proof. It is important to note that the resolution limit of the replica technique is near 20 nm so that smaller particles which may have a strong hardening effect may have been overlooked. Inspection of a replica area, which was seemingly free from extracted particles, revealed the presence of TaX, indicating the existence of small TaX precipitates with sizes below the limit of resolution. Fig. 10 shows TEM images of 59As in the initial state where the extraction technique did not work for some unknown reason. The marked region of the bright-field image, which lies in the interior of a large subgrain, was investigated under weak-beam conditions. The weak beam image shows numerous small particles (bright spots), which according to the diffraction pattern were coherent with the matrix and may be of type MX. Observations in high-resolution TEM of steel foils at ARC Seibersdorf [12] on 59As in the virgin condition revealed the existence of fine M23 C6 precipitates, around 10 nm in size, probably lying in the interiors of subgrains. The fact that fine M23 C6 particles were found not only in the initial state but also after creep suggests a high stability against coarsening. It is therefore proposed that the specific advantage of 59As is due to enhanced strengthening by fine precipitates, in particular Ta-containing MX precipitates, which have sizes below the limit of resolution of the extraction replica technique and are sufficiently resistant against coarsening and replacement by more stable phases. 6. Summary While the goal of 105 h creep rupture strength of 100 MPa at 650 ◦ C could not be achieved with the present alloying concept, the test melt 59As with 11 wt.% Cr, relatively high contents of C and B, low contents of W and Co, and 0.09 wt.% Ta yields promising results. The results confirm the finding derived from the literature that the effectivity of strengthening by fine precipitates is enhanced, when the Cr content is somewhat reduced relative to the value of 12 wt.% recommended for oxidation stability. The creep strength of 59As markedly exceeds that of the piping steel P92 and the
Nb-alloyed test melt 62As in tests extending over 25,000 h. TEM examinations indicates that the extraordinary creep resistance of 59As is due to fine Ta-containing precipitates of the types MX and M23 C6 . Acknowledgements Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. We thank the members of the working group for providing the test materials and performing ThermoCalc calculations. We also acknowledge the financial support for continuation of creep tests of 59As and 62As samples beyond 25,000 h within framework of the COST 536 Action from German Industry and the Bundesministerium für Wirtschaft und Technologie. References [1] R. Agamennone, C. Berger, W. Blum, J. Ehlers, P.J. Ennis, J. Granacher, G. Inden, V. Knezevic, J.W. Quadakkers, G. Sauthoff, A. Scholz, L. Singheiser, J. Vilk, Y. Wang, in: J. Lecomte-Beckers, M. Carton, F. Schubert und, P.J. Ennis (Eds.), Materials for Advanced Power Engineering 2002, Proc. of 7th Liége Conference, vol. III, Forschungszentrum Jülich, Germany, 2002, pp. 1279–1288. [2] T. Fujita, Trans. JIM 9 (1968) 167–169. [3] T. Fujita, Proc. of International Workshop on Development of Advanced Heat Resisting Steels, Yokohama, Japan, 8th November 1999, NIMS Tsukuba, Japan, 1999. [4] F. Abe, Proc. of 4th EPRI Conference on Advances in Materials Technogy for Fossil Power Plants, Hilton Oceanfront Resort, Hilton Head Island, SC, USA 25–28 October 2004, pp. 202–216, EPRI Palo Alto, USA, 2004. [5] M. Hättestrand, Doctoral thesis for the degree of Doctor of Philosopy, Chalmers University of Technology and Göteborg University, Sweden, 2000. [6] K.H. S Mayer, Proc. of 29. MPA Seminar Materials & Component Behaviour in Energy & Plant Technology Stuttgart 9–10, October 2003, MPA Stuttgart, Germany, 2003. [7] Nippon Steel Corporation: Data Package for NF 616 Ferritic Steel (9Cr–0.5Mo–1.8W–Nb–N), January 1993. [8] J. Hald, ECCC E911-P92 Assessment, ECCC Data Sheet, presented in September 2005 at the ECCC conference Creep & Fracture in High Temperature Components, September 12–14, 2005, London, UK, ERA Technology Ltd., Leatherhead, Surrey, KT22 7SAS, UK 2003. [9] C. Berger, A. Scholz, Y. Wang, K.H. Mayer, Z. Metallkd. 96 (2005) 668–674. [10] R. Agamennone, W. Blum, C. Gupta, J.K. Chakravartty, Acta Mater. 54 (2006) 3003–3014. [11] J. Pirón Abellán, P.J. Ennis, L. Singheiser, W.J. Quadakkers, in: J. Lecomte-Beckers, M. Carton, F. Schubert und, P.J. Ennis (Eds.), Materials for Advanced Power Engineering 2006, Proc. of 8th Liège Conference, vol III, Forschungszentrum Jülich, Germany, 2006, pp. 1285–1295. [12] S. Höfinger, ARC Seibersdorf, Austria, Personal communication. [13] H. Chilukuru, K. Durst, S. Wadekar, M. Schwienheer, A. Scholz, C. Berger, K.H. Mayer, W. Blum, Mater. Sci. Eng. A. in press.