Development of creep- and corrosion-resistant steels for future steam power plants Dr.-Ing. B. Kuhn, M. Talik, C. Li, Dr. J. Zurek, Prof. Dr.-Ing. W. J. Quadakkers, Prof. Dr.-Ing. T. Beck, Prof. Dr.-Ing. L. Singheiser Forschungszentrum Jülich GmbH Institut für Energie und Klimaforschung Werkstoffstruktur und Eigenschaften (IEK-2) 1. Motivation Reliable energy supply is - among many others - one of the most important requirements modern industrial societies are based on. The scheduled transition in energy policy towards renewable sources postulates a reduction of Germany's energy needs by about 10 % [UBA] until 2050, but future scenarios nevertheless predict an increase in global energy demand for the upcoming decades. Moderate economic growth in the OECD countries, but especially the rapidly growing economies of the non-OECD countries, may cause an almost tripled global energy demand by 2050 [Shell]. Having in mind the environmental impact of energy supply and the need for valuable primary resource conservation efficient and sustainable technologies for energy conversion are required. Although renewable energy technologies are of great interest fossil fuels will continue to play a major role in future energy supply security. Due to decreasing availability of fossil fuels, its increasingly cost intensive exploitation, the fact that large amounts of CO2 are emitted in combustion whose separation from power plant exhaust gases may cause a loss of overall plant efficiency of about 10 – 25 % [Metz] system efficiency will play the leading role in resource conservation, cost saving and environmental protection. Improved plant efficiency, however, requires increased process temperatures and pressures and can therefore only be reached by improved materials. 2. Material requirements Over the past decades material development has enabled a gradual increase of steam parameters from 530 °C / 150 bar [Kather] to appr. 600 °C / 280 bar [Alstom] thus increasing the efficiency of steam power plants from appr. 31 % in 1960 [RWE ] to appr. 43 % today (Niederaussem, D: 43.2% [RWE], Neurath BoA 2 +3 D [Alstom], Skaerbaek, DK: 45% [Hald]). In this course a reduction in CO2 emitted per megawatt hour of generated electricity by approximately 26 % [RWE] was reached. Present state of the art power plants predominantly employ ferritic-martensitic (9-12% Cr) steels, im some cases also (for superheaters) austenitic steels. A further improvement in efficiency to about 50 % requires a further rise in steam parameters to about 650 °C / 300 bar [Mayer1, Visvan1]. A material suitable for these operating conditions must - among many others - meet the following main requirements [Kern, Bendick, Gabrel, Visvan1]:  100.000 hour creep rupture strength of about 100 MPa at 650 °C  creep rupture elongation greater than 10%  long-term oxidation resistance in steam and combustion atmosphere (future so called "oxyfuel" processes included: 70% CO2 / 29% H 2 O / 1% O2) at 650 ° C  high thermal conductivity  high thermo-mechanical fatigue resistance  little/no notch sensitivity  short-term yield strength / tensile strength of about 450 MPa / 600 MPa (at ambient temperature) Referring to this profile the materials used in today's power plants all have one or another kind of disadvantage and it seems rather impossible or only possible at exceptional expense (e.g. by application of corrosion protective coatings) to make further improvement towards a 650 °C steam power plant. 3. State of the art materials 105 h creep rupture strength [MPa] During the past decades the 100.000 h creep rupture strength of 9-12 (wt. -%) chromium steels was nearly doubled from alloy P9 (around 1940) to steel P92 (appr. 113 MPa / 58 MPa at 600 °C / 650 °C, Fig. 1). 200 150 ~ 60 Years 100 +W - Mo, + W ? + N, V, Nb 50 0 P9 P91 E911 P92 Target Fig. 1: Evolution of creep strength reached by selected ferritic-martensitic steel grades To achieve the goal of increased steam parameters outlined in the previous section a further doubling of the 105 h creep rupture strength to 100 MPa at 650 °C is required (Fig. 1). Due to its limited creep resistance and especially because of the enhanced steam oxidation resistance required conventional ferritic-martensitic 9-12 wt.-% chromium steels seem to reach technical limitations. The creep resistance of these materials is based on microstructural stabilization by finely precipitated carbides and nitrides [Mayer2]. Stress [MPa] 1000 P92 T122 TAF NF12 MarBN 100 10 10 100 1000 10000 100000 1000000 Time to rupture [h] Fig. 2: Creep rupture strength: 9 Cr (P92, MarBN), 10.5 Cr (TAF) and 12 Cr (NF12, T122). Data from [Abe, NIMS DataSheet51, Naoi, Hald / Danielsen]. The limited thermodynamic stability of these strengthening precipitates as well as the constricted steam oxidation resistance, caused by the comparatively low chromium content of about 9 wt.-%, restricts the application of alloy P92 to maximum operation temperatures of about 620 °C [Visvan1]. Adequate steam oxidation resistance up to 650 °C requires significantly higher chromium contents. Newly developed materials employing higher chromium contents in the range from 11 to 12 wt.-% such as VM12 [Gabrel], NF12 [Danielsen1] and T122 [Danielsen1] however display an accelerated drop in creep rupture strength in continuous high-temperature operation (Fig. 2). Because of the elevated chromium content these steels are believed to be prone to increased precipitation of the coarse Z-phase (Cr (V, Nb) N) [Danielsen1], that occurs at the expense of the strengthening small MN ((V, Nb) N) nitrides and causes the strength reduction in long-term operation [Danielsen2] (Fig. 2). For the outlined reasons the application limit of currently available ferritic-martensitic materials is fixed at temperatures of about 610 °C - 630 °C [Kern1, Scarlin, Visvan1]. 4. State of international research - Evolution or revolution? 4.1. Evolution in martensitic steels Global research in the field of ferritic-martensitic power plant materials currently focuses on further improving creep resistance. By alloying of relatively large amounts of tungsten (> 2 wt.-%) improved by solid solution strengthening and increased precipitation of the intermetallic Laves phase can be reached. From 1995 - 2005 attempts were made to apply "principles and concepts of physical metallurgy (...)” to overcome the traditional 'trial and error' alloy development philosophy [Mayer3, Knezevic1, 2] in the framework of a German project called "Superwarmfeste ferritische Werkstoffe". In the course of this project some 80 [Kern] alloy variants incorporating Cr / W contents of 8.4 - 14.4 / 1 - 6 wt.-% were produced and tested regarding the effects of various other alloying elements such as Co, Cu and Ta. Result of this development effort was a 12Cr steel having a creep rupture strength comparable to P92 (9Cr), but improved oxidation resistance due to enhanced chromium content [Wang]. A 9Cr experimental material alloyed with 120 ppm of boron was developed by the Technical University of Graz. After 25.000 hours of testing this material shows promising creep resistance – increased by about 20% in direct comparison to P92 - and reduced steam oxidation [Mayr]. The positive effect of adding small amounts of boron (> 0.01 wt.-%) on creep properties was discovered by the Japanese National Institute of Materials Science (NIMS) [ABE1, Semba]. [Fountain] and [Sakuraya] demonstrated that the precipitation of coarse BN particles, which are believed to compensate the advantageous effect of dissolved boron and nitrogen, can be avoided by fixing the boron / nitrogen ratio to a value greater than 1.1. Consequently, many of today’s commercially available and also some semi-commercial trial alloys feature corresponding boron / nitrogen ratios. Examples are P92, the Japanese high chromium (10.5 wt.-% Cr) TAF steels [Uehara, Sawada] (see Fig. 2), novel 9 wt.-% Cr trial rotor alloys (FB3 [Kern]) evolving from the European COST536 program and the Japanese MAR steels [aBE2]. An innovative approach is being pursued by Danielsen and Hald. According to the motto "If you can not beat them, join them!" [Danielsen2] they try to exploit the adverse effect of Zphase transformation in 12Cr-steels for strengthening. By accelerating the formation of nitrides and its subsequent transformation finely distributed small Z-phase particles are generated, which - according to thermodynamic modelling - are less prone to coarsening [Hald2, 3] than the original nitrides and therefore promise improved creep strength in the long-term [Danielsen1]. The feasibility of such a material was demonstrated by model alloys [Danielsen2] that do not form other precipitates except the desired nitrides. Incorporating this new idea into the concept of heat resistant ferritic-martensitic steels promises to be a challenging optimization task. However, all the alloys described in this brief review do have one aspect in common: According to the Schaeffler diagram [Schaeffler] the possible chromium content is limited to approximately 13.5 wt.-% if a martensitic microstructure shall be retained. In practice this value has to be considered to be even lower because of undesired -ferrite formation [NIMS1]. Detailed steam oxidation studies on the behavior of 9-12 Cr steels show that today's 9 Cr steels have to be regarded as so-called "borderline" materials [Zurek1, Wright], that are at the border to the formation of protective chromium oxide layers under exposition to water vapor at temperatures between 550 °C and 650 °C. For this reason the steam oxidation resistance of 9-12 Cr steels strongly depends on a multitude of parameters and exhibits relatively high fluctuation [Zurek1-3, Quad1, Ess, Shreirs], that urges caution in the selection of materials and operating parameters [Zurek1]. Utilizing binary FeCr model alloys of different compositions Nieto-Hierro et al. [Shreirs] demonstrated that chromium contents higher than 15 wt.-% are necessary for a reliable reduction of steam oxidation rates at 650 °C. Assuming that the above mentioned innovative development approaches help to solve or alleviate the problem of microstructural instability of >11.5 wt.-% Cr steels these materials are likely to be limited in steam oxidation resistance because of their relatively low absolute chromium content. 4.2 Revolution! - Fully ferritic steels Having in mind the outlined drawbacks a paradigm shift in alloy development - away from improving creep strength with steam oxidation resistance treated as a subordinate goal towards simultaneous improvement of both crucial properties seems to be mandatory. Fully ferritic steels with chromium contents higher than 15 wt.-% - without martensitic transformation - are considered to provide a suitable basis for such development as their resistance to steam oxidation up to 650 °C [Shreirs] is considered to be sufficiently good. At such high chromium contents adequate strengthening can not be reached based on precipitation of carbides and nitrides, since the solubility of C and N is too low in the ferritic alloy matrix. Apart from finely dispersed oxide particles employed in so called ODS(oxide dispersion strengthened)-alloys, that are produced by relatively expensive powdermetallurgical processes, therefore only precipitates of intermetallic phases come into question for strengthening. Back in the year 2000 the Japanese National Institute of Materials Science (NIMS) published its research activities on ferritic 15 Cr steels that are strengthened by precipitation of intermetallic phases [NIMSPat]. 260 NIMS 15 Cr, two-phase, water quenched Stress [MPa] 220 180 140 NIMS 15 Cr, single-phase solution annealed 100 P92 60 10 100 1000 Time to rupture [h] 10000 100000 Fig. 3: Creep rupture strength comparison (650 °C) of the ferritic-martensitic steel P92 and two variants of the ferritic 15 Cr NIMS steels. Data from [Toda1-4, NIMS Data Sheet No. 51, Shibuya]. Several variants of this material do exhibit promising creep properties at 650 °C [Toda1, 2]. Within this alloying philosophy a basic distinction into single-phase (i.e., ferritic matrix with intermetallic precipitates only) and two-phase (ferrite matrix with intermetallic plus (rest)martensite phase with carbide and nitride precipitates) materials can be made [Toda1, 2]. The single-phase variant is roughly equal (Fig. 3) to common materials such as ferriticmartensitic P92 with respect to creep resistance. Some of the two-phase variants in contrast even outplay [Toda2, 3, Shibuya] common materials. In [Toda4] a two-phase 6 wt.-% W, 3 wt.-% Co alloy, which might show realistic potential to achieve the targeted 100.000 hour creep rupture strength of 100 MPa at 650 °C (Fig. 3) is described. For the two-phase materials also the effects of heat treatment on microstructure and creep properties are described in relative detail [Toda4]. High amounts tungsten decrease formability, due to increased high temperature strength [Schatt] and thus potentially complicate the production of large components. Literature data on the steam oxidation properties of these materials is not yet available. Depending on the volume fraction of precipitates (that consume chromium from the matrix) the chromium content that remains in the matrix is reported to drop down to appr. 12.5 wt.-% [Toda3]. Following the findings of Nieto-Hierro et al. [Shreirs] chromium contents as low as that do not ensure sufficient resistance to steam oxidation at 650 °C. At the same time, the Institute for Energy and Climate Research (IEK), Microstructure and Properties of Materials (IEK-2) at Forschungszentrum Jülich, Germany developed fully ferritic high chromium (22 wt.-%) steels for application in high temperature fuel cell stacks [Froitz] in cooperation with ThyssenKrupp VDM. Result of this collaboration are today commercially available interconnector steels with the trade names Crofer® 22 APU [APU] and Crofer® 22 H [H]. Its use in high temperature fuel cell technology requires the formation of electrically conductive surface oxides at operating temperature. Therefore standard deoxidants like aluminum and silicon must be omitted in the production of Crofer® 22 APU because these elements would contaminate the steel and form electrically non-conductive oxides on stack components during fuel cell operation. For this reason Crofer ® 22 APU is produced utilizing relatively expensive vacuum processes (VIM). The variant Crofer ® 22 H is a logical further development of the original material Crofer ® 22 APU. The development targets were the improvement of creep resistance in the temperature range from 700 to 800 °C and the reduction of manufacturing costs with unchanged or even improved fuel-cell-related properties (thermal expansion, corrosion resistance, electrical conductivity of oxide scales). All the objectives were reached by combined alloying with comparatively small amounts of tungsten (2 wt.-%), niobium (0.5 wt.-%) and silicon (0.25 wt.-%). In the relevant temperature range a reduction of the minimum creep rates by at least one order of magnitude compared to Crofer ® 22 APU was achieved [Kuhn1], by the precipitation of small, finely dispersed, mixed intermetallic Laves-phase particles (Fe,Cr)2(Nb,W). Since the Laves phase has substantial solubility for silicon [Hosoi, Kuhn1] the alloy composition was optimized in a way that the silicon added during production for deoxidation of the melt is tied up by the intermetallic precipitates formed during high temperature operation and thus cannot form undesired electrically non-conductive oxide on component surfaces. Crofer ® 22 H can therefore be produced employing conventional arc-melting technique, which allows a significant reduction in manufacturing cost. As a part of the described material development, the composition of the experimental alloys was systematically varied step by step to guarantee a property profile ideal for fuel cell operation between 700 °C and 800 °C. The starting point were steels solely alloyed with Nb (up to 1 wt.-%) or W (2 - 7 wt.-%) to provoke the formation of intermetallic phases. In the following the proportions of both elements were successively altered in mixed alloying and the supplemental addition of silicon (up to 0.4 wt %) was studied. Some of the experimental alloys created during the development of Crofer ® 22 H show exceedingly good steam oxidation resistance at 600 °C due to their relatively high chromium contents [Quad2] of 18 wt.-% (Fig. 4: LJO) to 22 wt.-% (Fig. 4. KSX). Additionally they also display promising creep properties in the relevant temperature range without any further optimization done yet (Fig. 5) for 600 – 650 °C application. 100 2 Weight gain [mg/cm ] P92 P91 600 °C, Ar50Vol.-%H2O KSX (22Cr) LJO (18Cr) 10 1 0,1 0,01 0 500 1000 1500 2000 2500 3000 3500 Time [h] Fig. 4: Weight gain curves of a 22 Cr and an 18 Cr fully ferritic model alloy in direct comparison to ferritic-martensitic state of the art 9 Cr alloys P91 and P92 (600 °C, Ar50Vol.-%H2O). The high chromium content of these model alloys does not only yield improved steam oxidation properties. Highly chromium alloyed materials in turn, tend to form an embrittling and therefore undesired FeCr-phase () in relevant temperature range. In a first optimization step to meet this challenge the chromium content was lowered from 22 wt.-% in alloy batch KSX to 18 percent by weight in variant "LJO". Like it is demonstrated in the stress rupture diagram displayed in Fig. 5 the reduction in chromium content did not yield adverse effects on the creep behavior when compared to batch KSX (tR: KSX(22Cr), 600 °C, 120 MPa = 31.134 h, tR: LJO(18Cr), 600 °C, 120 MPa = 29.395 h). Additionally Fig. 5 shows a comparison of selected model alloys with the commercial ferritic-martensitic alloys P92 (9 Cr) and VM12 (12 Cr). 180 170 P92 - 600 °C Stress [MPa] 160 VM12 - 600 °C 150 140 KSX (22Cr) 130 LJM (22Cr) 120 LFO (22Cr) 110 LJO (18Cr) P92 - 650°C VM12 650°C LFM (22Cr) 100 10 100 1000 10000 100000 Time [h] Fig. 5: Stress-rupture diagram of the materials P92 [ECCC Data sheet 2005], VM12 [Gabrel] and some assorted fully ferritic model alloys (filled symbols: ruptured, open symbols: experiments running) All the model alloys prove to be superior to the most oxidation resistant commercial material VM12 at 600 and 650 °C. Some of the trial materials (KSX, LJM, LFO: 22 Cr, 2 – 2.4 W, 0.5 Nb, 0.25 Si) showed to be nearly equal to P92 at 600 / 650 °C at high stress and up to appr. 15.000 hours of testing duration. The high tungsten (LFM: 7 W, 0.25 Si) variants even outplay P92 in the high stress (145 MPa) range. The true potential of this alloying approach as a base for further improvement is subject of ongoing work. 5. Conclusion All previous attempts to combined high creep resistance with adequate steam oxidation properties in the field of ferritic-martensitic 9-12 Cr steels failed due to the reasons described in section three of this paper, but current development efforts however suggest still some potential in this class of materials [Kern]. However, the limited chromium content poses an element of uncertainty regarding the achievable steam oxidation resistance. The described paradigm shift towards fully ferritic alloys seems to have sufficient potential in store for further improvement towards future application in high-efficiency steam power processes. Literature [UBA] T. Klaus, C. Vollmer, K. Werner, H. Lehmann, K. Müschen: Energy Target 2050: 100 % Renewable Electricity Supply. 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Kimura: Effect of Nickel and Cobalt Addition on the Precipitation-Strength of 15Cr Ferritic Steels. In: Materials Science and Engineering A, 528, 2011, S. 5387 – 5393 Y. Toda, H. Kushima, K. Kimura, F. Abe: Improvement in Creep Strength of Heat-Resistant Ferritic Steel Precipitation-Strengthened by Intermetallic Compound. In: Materials Science Forum, Vols. 539-543, 2007, S. 2994 – 2999 J. Froitzheim, G.H. Meier, L. Niewolak, P.J. Ennis, H. Hattendorf, L. Singheiser, W.J. Quadakkers: Development of High Strength Ferritic Steel for Interconnect Application in SOFCs“, Journal of Power Sources 178, 2008, S. 163 – 173 http://www.thyssenkruppCrofer®22APU – Materialdatenblatt. vdm.com/downloads/materialdatenblaetter.html?L=1 [H] [Kuhn1] [Quad2] Crofer®22H – Material Data Sheet. http://www.thyssenkruppvdm.com/downloads/materialdatenblaetter.html?L=1 B. Kuhn, L. Niewolak, T. Hüttel, T. Beck, W. J. Quadakkers, L. Singheiser, H. Hattendorf: Effect of Laves Phase Strengthening on the Mechanical Properties of high Cr Ferritic Steels for Solid Oxide Fuel Cell Interconnect Application. In: Material Science and Engineering A, 528 (2011) S. 5888 – 5899 Unpublished experimental steam oxidation results on fully ferritic model alloys. Personal communication with Prof. Dr.-Ing. W. J. Quadakkers, FZ Juelich GmbH, IEK-2, High temperature corrosion department