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Anomalous steam oxidation behavior of a creep resistant martensitic 9 wt. % Cr steel
Materials Chemistry and Physics 141 (2013) 432e439
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Anomalous steam oxidation behavior of a creep resistant martensitic 9 wt. % Cr steel Alina Agüero a, *, Vanessa González a, Peter Mayr b, Krystina Spiradek-Hahn c a
Instituto Nacional de Técnica Aeroespacial, Ctra. de Ajalvir Km 4, 28850 Torrejón de Ardoz, Spain Chair of Welding Engineering, Chemnitz University of Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany c Alloy Development Group, Montanuniversität Leoben, 8700 Leoben, Austria b
h i g h l i g h t s High steam oxidation resistant 9 wt. % Cr martensitic steel at 650 C. Multilayer thin protective CreFe oxide. Nano-grain sub-oxide metal zone.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 October 2012 Received in revised form 26 April 2013 Accepted 14 May 2013
The efficiency of thermal power plants is currently limited by the long-term creep strength and the steam oxidation resistance of the commercially available ferritic/martensitic steel grades. Higher operating pressures and temperatures are essential to increase efficiency but impose important requirements on the materials, from both the mechanical and chemical stability perspective. It has been shown that in general, a Cr wt. % higher than 9 is required for acceptable oxidation rates at 650 C, but on the other hand such high Cr content is detrimental to the creep strength. Surprisingly, preliminary studies of an experimental 9 wt. % Cr martensitic steel, exhibited very low oxidation rates under flowing steam at 650 C for exposure times exceeding 20,000 h. A metallographic investigation at different time intervals has been carried out. Moreover, scanning transmission electron microscopy (STEM) analysis of a ground sample exposed to steam for 10,000 h at 650 C revealed the formation of a complex tri-layered protective oxide comprising a top and bottom Fe and Cr rich spinel layer with a magnetite intermediate layer on top of a very fine grained zone. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Alloys Oxidation Surface properties Multilayers
1. Introduction Higher operating pressures and temperatures are essential to increase efficiency of steam power plants. These harsher conditions impose important requirements on the materials from both the mechanical and chemical stability perspective [1]. For instance, a creep rupture strength of 100 MPa after 100,000 h at 650 C has been defined as the target for new steel development. Moreover, steam oxidation resistance is required as otherwise, at temperatures higher than 600 C, the resulting thick oxide scales will spall, causing blockage of tube bends as well as overheating of heat exchangers due to a thermal insulation effect, erosion of downstream components and loss of cross-section in critical
* Corresponding author. Tel.: þ34 915201561; fax: þ34 915201381. E-mail address: [email protected] (A. Agüero). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.041
components such as blades [2]. Substantial efforts are being carried out in Europe, North America and Japan attempting to design and produce steels with such improved properties. The steam oxidation as well as the creep and welding behavior of high creep resistant new steels developed in Europe, as well as of a NIMS developed martensitic steel, have been studied within the framework of European COST Actions 522 and 536 [3e5]. It has been shown that in general, a Cr content higher than 9 wt. % is required for acceptable oxidation rates at 650 C [6], but on the other hand such high Cr content results in a reduction of the creep strength due to precipitation of modified Z-phase on expense of MX carbides. However, other minor alloying elements such as Si, Co, Mn, Mo and W are also known to have important effects on the steam oxidation resistance [7e13]. In fact, Quaddakers et al. [10] have proposed that the critical Cr content of the alloy required to form a protective oxide depends on the presence and amount of these minor elements, as well as the alloy microstructure and the
A. Agüero et al. / Materials Chemistry and Physics 141 (2013) 432e439
Mass change (mg/cm2)
60
433
P92 P91
50
NPM
40 30 20 10 0
0
5000
10000
15000
20000
Time (h) Fig. 2. Mass change of ground P91, P92 and NPM as a function of time when exposed to flowing steam at 650 C.
Fig. 1. Creep strength of NPM a 650 C (ECCC: European Creep Collaborative Committee, VdTUV: Materials Datasheet from Technischer Ueberwachungs-Verein).
service temperature. As an exception, several 9 wt. % steels developed by Abe [14] also containing Si and Mn, exhibit good resistance to steam oxidation but only after having been subjected to a preoxidation heat treatment in argon containing 0.3 ppm of oxygen at 700 C for 50 h Si and Mn are known to enhance the steam oxidation resistance of steels. For instance Quadakkers and Ennis [15] found significantly lower mass gain when 9e12 wt. % Cr steels containing these elements were oxidized at 600 and 650 C. Osgerby and Fry [16] also found that the oxide growth kinetics of this type of steels can be correlated not only with the Cr, content but also with that of Si, Mn, Mo and W, with W appearing to have a detrimental effect whereas all of the other mentioned elements tend to increase the oxidation resistance. Quadakkers and collaborators [10] have also found than Co has a positive effect in increasing the oxidation resistance of 11 wt. % Cr steels and they attributed this effect to a higher activity and diffusivity of Cr as there was no Co found in the scale. Besides chemical composition, surface preparation and/or mechanical treatments can affect the oxidation behavior of steels. It has been shown that shot peening increases the steam oxidation resistance of austenitic stainless steels [17], but it does not significantly affect the oxidation behavior of FeCr model alloys containing less than 19.9 wt. % in Cr [18] or commercial ferritic steels [19]. Matsuo [20] has also shown a significant reduction of the steam oxidation rate when using fine grained (w50 mm) austenitic alloys at temperatures up to 700 C. This effect is increased by shot peening and it is attributed to accelerated diffusion of Cr through the higher density of grain boundaries along with a higher density of cold work induced dislocations present in the surface area. At higher temperatures, the dislocation density decreases as confirmed by Galerie and coworkers [21]. They showed that neither cold working nor surface roughness influences the oxidation rate of a ferritic steel at 800 C, and that cold working induced by grinding rather reduced scale adhesion. In contrast, Grabke and coworkers
[22] showed that a high degree of surface deformation as for instance induced by grinding or sand blasting, results in enhanced Cr diffusion in ferritic steels, the higher the degree of surface deformation the higher the apparent Cr diffusion coefficient. Although the oxide scales produced in the more deformed surfaces where thicker, they were also richer in Cr. Within a family of steels developed by Abe [14] and coworkers, B containing 9Cr3W3CoVNb steels exhibit a significantly higher creep strength reaching 21,000 h at 100 MPa, exceeding by far the creep strength of other 9 wt. % ferritic/martensitic steels such as commercial grade P92 according to the ECCC values (see Fig. 1) and also unexpectedly showing high steam oxidation resistance on preliminary experiments as shown by Mayr and coworkers [5]. The goal of this work was to study the steam oxidation resistance of this experimental alloy at 650 C by exposing it to pure flowing steam in the laboratory for up to 20,000 h. Specimens of standard steel grades P91 and 92 have also been included in the test as reference materials. To characterize the oxidation behavior metallographic investigations at different time intervals have been carried out including scanning transmission electron microscopy (STEM) investigation of a ground sample, exposed to steam for 10,000 h at 650 C. 2. Experimental 2.1. Materials A 20 kg heat of an experimental 9Cr3W3CoVNb steel (NPM) with controlled additions of 120 ppm boron and 130 ppm nitrogen was produced by vacuum induction melting. The addition of boron and nitrogen was set to avoid the formation of large boron nitrides but still allow the precipitation of strengthening MX particles. For homogenization, the ingot (110 mm square) was forged to final dimensions of 50 mm square. The final quality heat treatment consisted of normalizing at 1150 C for 1 h followed by tempering at 770 C for 4 h. The chemical composition in weight (wt.) % is given in Table 1. P92 material was obtained from Vallourec Mannesmann (France) whereas P91 material was received from Monitor (UK).
Table 1 Alloy compositions. Analysis
C
Si
Mn
Cr
W
Mo
Co
V
Al
Ni
Nb
B
N
NPM P91 P92
0.074 0.12 0.12
0.29 0.44 0.10
0.44 0.38 0.61
9.26 9.96 9.37
2.84 e 1.98
e 0.89 0.45
2.95 e e
0.21 0.22 0.23
e 0.01 e
e e e
0.056 0.070 e
0.012 e e
0.013 0.069 0.04
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A. Agüero et al. / Materials Chemistry and Physics 141 (2013) 432e439
Fig. 3. Cross-section images of a) P91 and b) P92 specimens exposed to 10,000 h of flowing steam at 650 C.
The chemistry of all steels investigated in this study is given in Table 1. The specimens were ground with 120 grit prior to testing. 2.2. Steam oxidation laboratory testing The schematics of the closed loop laboratory rig employed at INTA are shown elsewhere [23]. Prior to testing, laboratory air is displaced from the specimen chamber by means of N2 which is kept flowing while heating up to the test temperature (approximately at a rate of 600 C h1). Once the test temperature has been reached, the N2 flow is stopped and pure steam is introduced at a linear velocity of 8 cm s1. To carry out weight measurements or to remove samples, the furnace is cooled to about 300 C under N2 atmosphere and the specimens are subsequently removed. The reheat cycle is also carried out under N2 atmosphere. 2.3. Characterization The oxidized specimens were characterized by light optical microscopy (Leica MEF 4) and field emission scanning electron microscopy (FESEM) employing a JEOL JSM 840 system equipped with an energy dispersive X-ray spectrometer (EDS) KEVEX MICROANALYST 8000 with a RÖNTEC signal processor. Scanning transmission electron microscopy (STEM) was carried out by CM20STEM (Philips) operating at 200 kV accelerating voltage and equipped with an SE detector and an EDS unit.
3. Results and discussion The oxidation rate was determined as a function of the mass variation of the specimens with time. The mass variation of NPM specimens as a function of exposure time at 650 C under flowing steam is shown in Fig. 2. The oxidation rates of reference grades P91 and 92 were also included in the test for comparison purposes. Specimen preparation consisted of degreasing and grinding (120 grit) prior to exposure. None of the specimens was subjected to a pre-oxidation heat treatment. Cross-section images of the P91 and 92 specimens exposed for 10,000 h are shown in Fig. 3a and b respectively. As expected, the formation of a thick dual layered oxide scale is observed on both substrates with that on P92 significantly thicker than that on P91. These complex layers are characteristic for 9 wt. % ferritic steels [24] and consist of a top outwards growing Fe3O4 layer and an inwards growing mixed oxide layer containing (Fe, Cr)3O4, FeO and Cr2O3. The average Cr content in this inner oxide layer is 12 and 10 wt. % for P91 and P92 respectively. However, Cr is not uniformly distributed as shown in the EDS composition map of the P92 specimen (Fig. 4). Cr rich layers are revealed within the inner oxide and the original specimen surface can be clearly observed. According to Quaddakers [24] and coworkers, this pattern results from the repetition of cycles in which Cr rich spinels form due to rapid Cr diffusion from the bulk alloy until the spinel cannot be sustained due to continuous depletion of Cr, resulting in the formation of Fe rich oxides. The main difference between these two
Fig. 4. SEM-EDS mapping of P92 exposed to 10,000 h of flowing steam at 650 C.
A. Agüero et al. / Materials Chemistry and Physics 141 (2013) 432e439
Fig. 5. Low magnification cross-section of an NPM sample exposed to steam for 1260 h at 650 C.
substrates is the higher oxidation resistance exhibited by P91 which has been attributed to the higher content of Si and Mo as well as by the absence of W in this alloy. As already mentioned, it has been shown by other authors that both Mo and Si decrease oxidation rates whereas W has a negative effect on the oxidation resistance. Unexpectedly, ground specimens of the experimental 9 wt. % Cr NPM alloy displayed very high oxidation resistance. Most of the surface of the specimen exposed for 1260 h was covered with a very thin protective oxide layer with some thicker oxide nodules (Fig. 5). This explains the significantly lower weight gain compared to grades P91 and 92. The nodules are composed of a dual magnetite/ Cr, Fe spinel layer similar in morphology to that observed as a continuous layer in P91 and 92 (Fig. 6). Indeed the same oscillating content profile as in P92 is observed on the NPM spinel inner layer, regarding the Cr content. Rather than by inhomogeneities in the chemical composition, the nodules seem to have their origin in mechanical defects of the protective oxide as it forms initially, through which steam can get in contact with the underlying steel substrate. As observed in Fig. 7, all edges of the specimens were totally covered with oxide, supporting this hypothesis, and
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moreover, no newly formed nodules appeared after longer exposure times. Those nodules observed at short exposures slowly grew “sideways” with increasing exposure time. The thickness of the thin protective oxide layer which covers most of the specimen surface, increases very slowly, as illustrated in Fig. 8a and b and has a very high Cr content (30 wt. %) as well as some Mn (8e9 wt. %). In Fig. 8b a tiny growth in the oxide can be observed. As no inner oxide growth can be observed below this feature, rather than a nodule it is though to be an outwards growing flake as also seen in Fig. 8b. This type of feature has already been observed by Quaddakers and collaborators, and are considered as evidence of fast Cr transport through oxides formed under water vapor containing atmospheres [25]. This protective oxide is similar to that observed by Abe and collaborators [14] on similar alloys, requiring a 700 C pre-oxidizing heat treatment (for 50 h) in argon containing 0.3 ppm of oxygen to be carried out before testing. The formation of a protective oxide on a 9 wt. % Cr steel is difficult to explain on the basis of the composition, as the alloy has the same content of Cr than grades P91 and 92, with a Si content in between those of P91 and P92 and a significantly higher W content in both alloys. Moreover, there is no Mo in NPM and its Mn content is also between that of the other two alloys. As mentioned in the introduction, Mo and Mn appear to be beneficial for oxidation resistance but W seems to be detrimental. On the other hand, NPM contains 3 wt. % Co which according to Quadakkers [10], has a positive effect on the oxidation resistance of 11 wt. % Cr steels. In order to understand this rather puzzling behavior as well as to study the protective oxide morphology and obtain its composition, an STEM investigation of a specimen exposed to steam for 10,000 h at 650 C was carried out. As it can be observed in Fig. 9, the thickness of the protective oxide scale measures approximately 500 nm and surprisingly a dislocation depleted substrate zone is located immediately beneath the scale. This zone exhibits a very fine grained morphology with a thickness between 1.5 and 5 mm.
Fig. 6. Cross-section micrograph and EDS mapping of an NPM specimen exposed to steam for 10,000 h at 650 C at a specimen edge.
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Fig. 9. STEM image of the protective oxide formed on NPM on exposure to flowing steam at 650 C after 10,000 h.
Fig. 7. SEM image of an NPM specimen exposed to steam at 650 C for: a) 5998 h and b) 12,500 and c) 20,000 h.
The grains in this zone, with diameters between 0.5 and 2.5 mm, are at least one order of magnitude smaller than those present in the bulk material, as can be seen in Fig. 10. Another interesting feature that can be observed is the presence of a high density of Laves
phase precipitates (Fe2W), mostly at grain boundaries, as well as the absence of chromium carbide precipitates within this fine grained zone. These precipitates are always present in 9e12 wt. % Cr ferritic/martensitic steels and although their absence beneath the scale may indicate a Cr depleted zone, according to EDS measurements there is no difference with respect to the Cr content within the bulk material [26,27]. Quadakkers and collaborators [28] have found the same situation in a FeeCreNb model alloy. The presence of this fine grained zone allowed faster diffusion of Cr and is likely the main reason for the formation of the protective Cr rich oxide. A higher magnification image showed a 1.5 mm thick protective scale exhibiting three distinct layers of different thicknesses: the top and bottom have a similar morphology composed of nanosized grains (10e50 nm) whereas the middle layer has larger grains ranging from approximately 50e200 nm (Fig. 11). Moreover, some porosity between the sub-layers is visible. According to the electron diffraction patterns, the top and bottom layers are composed of (Cr, Mn, Fe)3O4 with little Fe (w3 wt. %) and Cr2O3 whereas the middle layer has very little Cr and no Mn and can be identified as Fe3O4. The voids can have their origin in Fe diffusion from the Fe3O4 into the spinel. There were also small amounts of Si in all layers. Moreover, the EDS elemental concentration profile revealed two other even thinner layers (<50 nm) at the interface with the substrate as seen in Fig. 12, the most inner being enriched in Cr and Mn while the
Fig. 8. Protective oxide formed on NPM on exposure to flowing steam at 650 C after a) 5000 h and b) 10,000 h.
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Fig. 10. NPM grains: a) beneath de surface and b) materials bulk.
Fig. 11. STEM image and electron diffraction of the protective oxide scale formed on NPM after 10,000 h of exposure to flowing steam at 650 C.
outer layer is rich in Fe. This two layers may be identified as (Cr, Mn, Fe)3O4 and Fe3O4 respectively. Therefore, the protective scale present after 10,000 h of exposure to steam is composed of five layers from the surface to the substrate: a) 200e300 nm (Cr, Mn, Fe)3O4,
Fig. 12. EDS element concentration profiling if the protective scale obtained on NPM after 10,000 of exposure to steam at 650 C.
b) w 200 nm Fe3O4, c) 600e700 nm (Cr, Mn, Fe)3O4, d) < 50 nm Fe3O4 and e) < 50 nm (Cr, Mn, Fe)3O4. This multilayered thin scale evokes the inner layer present in the non protective oxides found in P91 and 92 as well as in the oxide nodules found on NPM, in which the Cr concentrations oscillate from high to low as mentioned earlier. The proposed interpretation, regarding the repetition of cycles in which Cr rich spinels form due to rapid Cr diffusion from the bulk alloy, until the spinel cannot be sustained due to Cr depletion, may also apply for this protective scale. A second STEM analysis was carried out at a different spot, near an oxide nodule. Fig. 13 shows how oxidation has taken place via grain boundaries or cracks surrounding grains of the alloy. This oxide is rich in Cr and Mn as it can be observed in the EDS mapping shown in Fig. 14. If oxidation continues advancing inwards through grain boundaries or cracks until reaching the interface between the fine grained zone and the bulk, the advantages provided by the fine grained zone would be lost and locally, the material would no longer form and maintain a protective oxide. As these localized nodules appear at the initial stages of oxidation, it is suggested that the observed intergranular oxidation mechanism originates from intergranular micro-cracks originally present in the material. Once an oxide nodule starts forming, its high rate of growth can cause tension, and promote further micro-crack formation in adjacent zones, facilitating the observed “sideways” growth provoked by the above mentioned process.
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A. Agüero et al. / Materials Chemistry and Physics 141 (2013) 432e439
Fig. 13. STEM image of the protective oxide formed on NPM on exposure to flowing steam at 650 C after 10,000 h at a zone near an oxide nodule.
Another protective layer formation mechanism could be derived from the observed grain surrounded by oxides growing through cracks or grain boundaries. The metal surface grains would oxidize and end up surrounded by Cr rich oxides, depleting the grains’ bulk in Cr and therefore generating Fe3O4. As the grains are very small in size, soon they would fully oxidize generating Fe3O4 under the Cr rich spinel. However, such a mechanism would not result in continuous layers as those observed in the present study but rather in concentric nano-layered oxide grains. Abe and collaborators [14] observed a different protective scale when pre-oxidizing 9Cr3W3Co0.2V0.05Nb (C: 0.082, Si: 0.73, Mn 0.49, Cr: 9.16, W: 2.47, Co: 3.3, V: 0.2, N: 0.002, B: 0.019, wt. %) at 700 C for 50 h in argon. After exposure to steam for 9000 h at 650 C the scale observed in this case is composed of a 100 nm film of (Cr, Mn, Fe)3O4 on top, a thicker layer of (Fe, Cr)2O3 (aprox. 500 nm) in the middle and a very thin Si enriched oxide-substrate interface. The substrate layer directly beneath the scale is also very
fine grained but the authors did not comment this feature. Similarly to this investigation, it is enriched in Laves phase precipitates. Although very alike in composition, the NPM substrate used in this work had about half the Si content of the steels analyzed by Abe and coworkers. The unexpected high oxidation resistance of NPM is therefore related to the very fine grained zone found beneath the scale and likely to enhance Cr diffusion within this layer. It is however, difficult to explain how it was formed. As described in the experimental section, samples were taken from a forged ingot, which was heat treated (normalized and tempered) and cut from the materials bulk and later ground with 120 grit paper. Cutting was carried out with a water cooled wheel and it is perhaps during this process where the grain refinement may have taken place. In this case, the grinding process would not have removed all of the finely grained. Another alternative could be that grinding caused the formation of this fine grained zone and as mentioned in the introduction Grabke
Fig. 14. EDS element concentration mapping of the protective scale obtained on NPM after 10,000 of exposure to steam at 650 C at a spot near an oxide nodule.
A. Agüero et al. / Materials Chemistry and Physics 141 (2013) 432e439
[22] found that what he called “deformation” by grinding increased the oxidation resistance of ferritic steels. Further work is certainly required to clarify why and how this layer was formed in this steel and not in others such as P91 and P92 which received exactly the same surface treatment prior to exposure. 4. Conclusions As already known, chemical composition plays a very important role on the oxidation behavior of ferritic/martensitic steels. However, as a result of the present study it can be concluded that the material morphology near the surface may also be of high importance in ferritic steels. A 9 wt % Cr steel ferritic/martensitic steel containing Mn, Co and W, developed a Cr and Mn rich protective scale which was stable up to at least 20,000 h of exposure to flowing steam at 650 C. Some oxidation nodules were present from the beginning of the test and slowly grew, but no new ones appeared to form during the exposure. Surprisingly, a multilayer thin protective oxide develops without requiring a previous preoxidizing heat treatment on top of a very fine grained zone rich in Laves precipitates within the substrate. The multilayer oxide structure is composed of alternating layers of (Cr, Mn, Fe)3O4 and Fe3O4 which may form as a result a Cr depletionerepletion cycles. Thicker oxide nodules also form likely due to mechanical defects of the protective oxide rather than to inhomogeneities in local composition. It is not clear yet how and why this finely grained zone forms and further investigations are on-going. Acknowledgments The authors would like to acknowledge the support by the European Cooperation in Science and Technology (COST) through Action 536 “Alloy development for Critical Components of Environmental friendly Power planT (ACCEPT)”. AA and VG wish to acknowledge the Spanish Ministry of Science and Innovation for the financial support (ENE2008-06755-C02-01) as well as all of the members of the Metallic Materials Area at INTA for technical support in particular Javier García de Blas for his useful input. KSH would like to acknowledge the Austrian Research Promotion Agency FFG for the financial support.
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References [1] T.U. Kern, M. Staubli, K.H. Meyer, B. Donth, G. Zeiler, A. DiGianfrancesco, Materials for Advanced Power Engineering II (2006) 846. [2] M.G. Angell, S.K. Lister, A.Rudge, http://www.icpws15.de/papers/06_Electro10_angell.pdf. [3] T.U. Kern, K.H. Mayer, B. Donth, G. Zeiler, A. DiGianfrancesco, Materials for Advanced Power Engineering (2010) 7. [4] T. Fry, A. Agüero, Materials for Advanced Power Engineering (2010) 935. [5] P. Mayr, I. Holzer, F. Mendez-Martin, M. Albu, S. Mitsche, V. González, A. Agüero, Materials for Advanced Power Engineering (2010) 351. [6] R. Viswanathan, J. Server, J.M. Tanzosh, Journal of Materials Engineering and Performance 15 (2006) 255. [7] J.C. Nava Paz, R. Knödler, Materials for Advanced Power Engineering (1998) 451. [8] R. Knödler, B. Scarlin, Materials for Advanced Power Engineering (2002) 1601. [9] V. Leplingle, G. Louis, D. Petelot, B. Lefevre, J.C. Vaillant, Material Science Forum 69e372 (2001) 239. [10] J. Zurek, L. Nieto-Hierro, J. Piron-Abellan, L. Niewolak, L. Singheiser, W.J. Quadakkers, Materials Science Forum 461e464 (2004) 791. [11] Y. Wouters, G. Bamba, A. Galerie, M. Mermoux, J.P. Pettit, Materials Science Forum 461e464 (2004) 835. [12] Y. Fukuda, K. Tamura, in: Proc. Int. Symp. Plant Aging and Life Prediction of Corrodible Structures, Sapporo, Japan (May 1995), p. 835. [13] T. Ishitsuka, Y. Inoue, H. Ogawa, Oxidation of Metals 61 (2004) 125. [14] H. Okubo, S. Muneki, T. Hara, H. Kutsumi, H. Haruyama, F. Abe, in: Proceedings of the 3rd Symposium on Heat Resistant Steels and Alloys for High Efficiency USC Power Plants 2009, Tsukuba, Japan (June 2e4, 2009). [15] P.J. Ennis, W.J. Quadakkers, Materials for Advanced Power Engineering I (2002) 1131. [16] S. Osgerby, A. Fry, Materials Science Forum 522e523 (2006) 129. [17] A. Miname, M. Tookayaam, K. Seki, K. Nonaka, K. Koga, NKK Technical Review 75 (1996) 1. [18] D. Caplan, Corrosion Science 6 (1966) 509. [19] H. Haruyama, H. Kutsumi, S. Kuroda, F. Abe, in: Proceedings of the 4th International Conference on Advances in Materials Technology for Fossil Fuel Plants, Hilton Head Island South Carolina (2004), p. 412. [20] H. Matsuo, Y. Nishiyama, T. Yamadera, in: Proceedings of the 4th International Conference on Advances in Materials Technology for Fossil Fuel Plants, Hilton Head Island, South Carolina (2004), p. 441. [21] Y. Madi, A. Salhi, F. Charlot, A. Galerie, Y. Wouters, Oxidation of Metals 75 (2011) 167. [22] C. Piehl, Z. Tokei, H.J. Grabke, Materials Science Forum 369e372 (2001) 319. [23] A. Agüero, R. Muelas, B. Scarlin, R. Knödler, Materials for Advanced Power Engineering (2002) 1143. [24] W.J. Quadakkers, P.J. Ennis, Materials for Advanced Power Engineering I (1998) 123. [25] M. Hänsel, W.J. Quadakkers, D.J. Young, Oxidation of Metals 59 (2003) 285. [26] J. Hald, Steel Research 67 (1996) 369. [27] C. Panait, A.F. Gourguez Lorenzon, J. Besson, A. Fuchsmann, W. Bendick, J. Gabrel, M. Piette, Materials for Advanced Power Engineering (2010). [28] C. Asencio, A. Chyrkin, V. Konoval, H. Hattendorf, B. Kuhn, L. Singheiser, W.J. Quadakkers, Electrochemical Society Letters 14 (2011) 17.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Anomalous steam oxidation behavior of a creep resistant martensitic 9 wt. % Cr steel Alina Agüero a, *, Vanessa González a, Peter Mayr b, Krystina Spiradek-Hahn c a
Instituto Nacional de Técnica Aeroespacial, Ctra. de Ajalvir Km 4, 28850 Torrejón de Ardoz, Spain Chair of Welding Engineering, Chemnitz University of Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany c Alloy Development Group, Montanuniversität Leoben, 8700 Leoben, Austria b
h i g h l i g h t s High steam oxidation resistant 9 wt. % Cr martensitic steel at 650 C. Multilayer thin protective CreFe oxide. Nano-grain sub-oxide metal zone.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 October 2012 Received in revised form 26 April 2013 Accepted 14 May 2013
The efficiency of thermal power plants is currently limited by the long-term creep strength and the steam oxidation resistance of the commercially available ferritic/martensitic steel grades. Higher operating pressures and temperatures are essential to increase efficiency but impose important requirements on the materials, from both the mechanical and chemical stability perspective. It has been shown that in general, a Cr wt. % higher than 9 is required for acceptable oxidation rates at 650 C, but on the other hand such high Cr content is detrimental to the creep strength. Surprisingly, preliminary studies of an experimental 9 wt. % Cr martensitic steel, exhibited very low oxidation rates under flowing steam at 650 C for exposure times exceeding 20,000 h. A metallographic investigation at different time intervals has been carried out. Moreover, scanning transmission electron microscopy (STEM) analysis of a ground sample exposed to steam for 10,000 h at 650 C revealed the formation of a complex tri-layered protective oxide comprising a top and bottom Fe and Cr rich spinel layer with a magnetite intermediate layer on top of a very fine grained zone. Ó 2013 Elsevier B.V. All rights reserved.
Keywords: Alloys Oxidation Surface properties Multilayers
1. Introduction Higher operating pressures and temperatures are essential to increase efficiency of steam power plants. These harsher conditions impose important requirements on the materials from both the mechanical and chemical stability perspective [1]. For instance, a creep rupture strength of 100 MPa after 100,000 h at 650 C has been defined as the target for new steel development. Moreover, steam oxidation resistance is required as otherwise, at temperatures higher than 600 C, the resulting thick oxide scales will spall, causing blockage of tube bends as well as overheating of heat exchangers due to a thermal insulation effect, erosion of downstream components and loss of cross-section in critical
* Corresponding author. Tel.: þ34 915201561; fax: þ34 915201381. E-mail address: [email protected] (A. Agüero). 0254-0584/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.matchemphys.2013.05.041
components such as blades [2]. Substantial efforts are being carried out in Europe, North America and Japan attempting to design and produce steels with such improved properties. The steam oxidation as well as the creep and welding behavior of high creep resistant new steels developed in Europe, as well as of a NIMS developed martensitic steel, have been studied within the framework of European COST Actions 522 and 536 [3e5]. It has been shown that in general, a Cr content higher than 9 wt. % is required for acceptable oxidation rates at 650 C [6], but on the other hand such high Cr content results in a reduction of the creep strength due to precipitation of modified Z-phase on expense of MX carbides. However, other minor alloying elements such as Si, Co, Mn, Mo and W are also known to have important effects on the steam oxidation resistance [7e13]. In fact, Quaddakers et al. [10] have proposed that the critical Cr content of the alloy required to form a protective oxide depends on the presence and amount of these minor elements, as well as the alloy microstructure and the
A. Agüero et al. / Materials Chemistry and Physics 141 (2013) 432e439
Mass change (mg/cm2)
60
433
P92 P91
50
NPM
40 30 20 10 0
0
5000
10000
15000
20000
Time (h) Fig. 2. Mass change of ground P91, P92 and NPM as a function of time when exposed to flowing steam at 650 C.
Fig. 1. Creep strength of NPM a 650 C (ECCC: European Creep Collaborative Committee, VdTUV: Materials Datasheet from Technischer Ueberwachungs-Verein).
service temperature. As an exception, several 9 wt. % steels developed by Abe [14] also containing Si and Mn, exhibit good resistance to steam oxidation but only after having been subjected to a preoxidation heat treatment in argon containing 0.3 ppm of oxygen at 700 C for 50 h Si and Mn are known to enhance the steam oxidation resistance of steels. For instance Quadakkers and Ennis [15] found significantly lower mass gain when 9e12 wt. % Cr steels containing these elements were oxidized at 600 and 650 C. Osgerby and Fry [16] also found that the oxide growth kinetics of this type of steels can be correlated not only with the Cr, content but also with that of Si, Mn, Mo and W, with W appearing to have a detrimental effect whereas all of the other mentioned elements tend to increase the oxidation resistance. Quadakkers and collaborators [10] have also found than Co has a positive effect in increasing the oxidation resistance of 11 wt. % Cr steels and they attributed this effect to a higher activity and diffusivity of Cr as there was no Co found in the scale. Besides chemical composition, surface preparation and/or mechanical treatments can affect the oxidation behavior of steels. It has been shown that shot peening increases the steam oxidation resistance of austenitic stainless steels [17], but it does not significantly affect the oxidation behavior of FeCr model alloys containing less than 19.9 wt. % in Cr [18] or commercial ferritic steels [19]. Matsuo [20] has also shown a significant reduction of the steam oxidation rate when using fine grained (w50 mm) austenitic alloys at temperatures up to 700 C. This effect is increased by shot peening and it is attributed to accelerated diffusion of Cr through the higher density of grain boundaries along with a higher density of cold work induced dislocations present in the surface area. At higher temperatures, the dislocation density decreases as confirmed by Galerie and coworkers [21]. They showed that neither cold working nor surface roughness influences the oxidation rate of a ferritic steel at 800 C, and that cold working induced by grinding rather reduced scale adhesion. In contrast, Grabke and coworkers
[22] showed that a high degree of surface deformation as for instance induced by grinding or sand blasting, results in enhanced Cr diffusion in ferritic steels, the higher the degree of surface deformation the higher the apparent Cr diffusion coefficient. Although the oxide scales produced in the more deformed surfaces where thicker, they were also richer in Cr. Within a family of steels developed by Abe [14] and coworkers, B containing 9Cr3W3CoVNb steels exhibit a significantly higher creep strength reaching 21,000 h at 100 MPa, exceeding by far the creep strength of other 9 wt. % ferritic/martensitic steels such as commercial grade P92 according to the ECCC values (see Fig. 1) and also unexpectedly showing high steam oxidation resistance on preliminary experiments as shown by Mayr and coworkers [5]. The goal of this work was to study the steam oxidation resistance of this experimental alloy at 650 C by exposing it to pure flowing steam in the laboratory for up to 20,000 h. Specimens of standard steel grades P91 and 92 have also been included in the test as reference materials. To characterize the oxidation behavior metallographic investigations at different time intervals have been carried out including scanning transmission electron microscopy (STEM) investigation of a ground sample, exposed to steam for 10,000 h at 650 C. 2. Experimental 2.1. Materials A 20 kg heat of an experimental 9Cr3W3CoVNb steel (NPM) with controlled additions of 120 ppm boron and 130 ppm nitrogen was produced by vacuum induction melting. The addition of boron and nitrogen was set to avoid the formation of large boron nitrides but still allow the precipitation of strengthening MX particles. For homogenization, the ingot (110 mm square) was forged to final dimensions of 50 mm square. The final quality heat treatment consisted of normalizing at 1150 C for 1 h followed by tempering at 770 C for 4 h. The chemical composition in weight (wt.) % is given in Table 1. P92 material was obtained from Vallourec Mannesmann (France) whereas P91 material was received from Monitor (UK).
Table 1 Alloy compositions. Analysis
C
Si
Mn
Cr
W
Mo
Co
V
Al
Ni
Nb
B
N
NPM P91 P92
0.074 0.12 0.12
0.29 0.44 0.10
0.44 0.38 0.61
9.26 9.96 9.37
2.84 e 1.98
e 0.89 0.45
2.95 e e
0.21 0.22 0.23
e 0.01 e
e e e
0.056 0.070 e
0.012 e e
0.013 0.069 0.04
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Fig. 3. Cross-section images of a) P91 and b) P92 specimens exposed to 10,000 h of flowing steam at 650 C.
The chemistry of all steels investigated in this study is given in Table 1. The specimens were ground with 120 grit prior to testing. 2.2. Steam oxidation laboratory testing The schematics of the closed loop laboratory rig employed at INTA are shown elsewhere [23]. Prior to testing, laboratory air is displaced from the specimen chamber by means of N2 which is kept flowing while heating up to the test temperature (approximately at a rate of 600 C h1). Once the test temperature has been reached, the N2 flow is stopped and pure steam is introduced at a linear velocity of 8 cm s1. To carry out weight measurements or to remove samples, the furnace is cooled to about 300 C under N2 atmosphere and the specimens are subsequently removed. The reheat cycle is also carried out under N2 atmosphere. 2.3. Characterization The oxidized specimens were characterized by light optical microscopy (Leica MEF 4) and field emission scanning electron microscopy (FESEM) employing a JEOL JSM 840 system equipped with an energy dispersive X-ray spectrometer (EDS) KEVEX MICROANALYST 8000 with a RÖNTEC signal processor. Scanning transmission electron microscopy (STEM) was carried out by CM20STEM (Philips) operating at 200 kV accelerating voltage and equipped with an SE detector and an EDS unit.
3. Results and discussion The oxidation rate was determined as a function of the mass variation of the specimens with time. The mass variation of NPM specimens as a function of exposure time at 650 C under flowing steam is shown in Fig. 2. The oxidation rates of reference grades P91 and 92 were also included in the test for comparison purposes. Specimen preparation consisted of degreasing and grinding (120 grit) prior to exposure. None of the specimens was subjected to a pre-oxidation heat treatment. Cross-section images of the P91 and 92 specimens exposed for 10,000 h are shown in Fig. 3a and b respectively. As expected, the formation of a thick dual layered oxide scale is observed on both substrates with that on P92 significantly thicker than that on P91. These complex layers are characteristic for 9 wt. % ferritic steels [24] and consist of a top outwards growing Fe3O4 layer and an inwards growing mixed oxide layer containing (Fe, Cr)3O4, FeO and Cr2O3. The average Cr content in this inner oxide layer is 12 and 10 wt. % for P91 and P92 respectively. However, Cr is not uniformly distributed as shown in the EDS composition map of the P92 specimen (Fig. 4). Cr rich layers are revealed within the inner oxide and the original specimen surface can be clearly observed. According to Quaddakers [24] and coworkers, this pattern results from the repetition of cycles in which Cr rich spinels form due to rapid Cr diffusion from the bulk alloy until the spinel cannot be sustained due to continuous depletion of Cr, resulting in the formation of Fe rich oxides. The main difference between these two
Fig. 4. SEM-EDS mapping of P92 exposed to 10,000 h of flowing steam at 650 C.
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Fig. 5. Low magnification cross-section of an NPM sample exposed to steam for 1260 h at 650 C.
substrates is the higher oxidation resistance exhibited by P91 which has been attributed to the higher content of Si and Mo as well as by the absence of W in this alloy. As already mentioned, it has been shown by other authors that both Mo and Si decrease oxidation rates whereas W has a negative effect on the oxidation resistance. Unexpectedly, ground specimens of the experimental 9 wt. % Cr NPM alloy displayed very high oxidation resistance. Most of the surface of the specimen exposed for 1260 h was covered with a very thin protective oxide layer with some thicker oxide nodules (Fig. 5). This explains the significantly lower weight gain compared to grades P91 and 92. The nodules are composed of a dual magnetite/ Cr, Fe spinel layer similar in morphology to that observed as a continuous layer in P91 and 92 (Fig. 6). Indeed the same oscillating content profile as in P92 is observed on the NPM spinel inner layer, regarding the Cr content. Rather than by inhomogeneities in the chemical composition, the nodules seem to have their origin in mechanical defects of the protective oxide as it forms initially, through which steam can get in contact with the underlying steel substrate. As observed in Fig. 7, all edges of the specimens were totally covered with oxide, supporting this hypothesis, and
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moreover, no newly formed nodules appeared after longer exposure times. Those nodules observed at short exposures slowly grew “sideways” with increasing exposure time. The thickness of the thin protective oxide layer which covers most of the specimen surface, increases very slowly, as illustrated in Fig. 8a and b and has a very high Cr content (30 wt. %) as well as some Mn (8e9 wt. %). In Fig. 8b a tiny growth in the oxide can be observed. As no inner oxide growth can be observed below this feature, rather than a nodule it is though to be an outwards growing flake as also seen in Fig. 8b. This type of feature has already been observed by Quaddakers and collaborators, and are considered as evidence of fast Cr transport through oxides formed under water vapor containing atmospheres [25]. This protective oxide is similar to that observed by Abe and collaborators [14] on similar alloys, requiring a 700 C pre-oxidizing heat treatment (for 50 h) in argon containing 0.3 ppm of oxygen to be carried out before testing. The formation of a protective oxide on a 9 wt. % Cr steel is difficult to explain on the basis of the composition, as the alloy has the same content of Cr than grades P91 and 92, with a Si content in between those of P91 and P92 and a significantly higher W content in both alloys. Moreover, there is no Mo in NPM and its Mn content is also between that of the other two alloys. As mentioned in the introduction, Mo and Mn appear to be beneficial for oxidation resistance but W seems to be detrimental. On the other hand, NPM contains 3 wt. % Co which according to Quadakkers [10], has a positive effect on the oxidation resistance of 11 wt. % Cr steels. In order to understand this rather puzzling behavior as well as to study the protective oxide morphology and obtain its composition, an STEM investigation of a specimen exposed to steam for 10,000 h at 650 C was carried out. As it can be observed in Fig. 9, the thickness of the protective oxide scale measures approximately 500 nm and surprisingly a dislocation depleted substrate zone is located immediately beneath the scale. This zone exhibits a very fine grained morphology with a thickness between 1.5 and 5 mm.
Fig. 6. Cross-section micrograph and EDS mapping of an NPM specimen exposed to steam for 10,000 h at 650 C at a specimen edge.
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Fig. 9. STEM image of the protective oxide formed on NPM on exposure to flowing steam at 650 C after 10,000 h.
Fig. 7. SEM image of an NPM specimen exposed to steam at 650 C for: a) 5998 h and b) 12,500 and c) 20,000 h.
The grains in this zone, with diameters between 0.5 and 2.5 mm, are at least one order of magnitude smaller than those present in the bulk material, as can be seen in Fig. 10. Another interesting feature that can be observed is the presence of a high density of Laves
phase precipitates (Fe2W), mostly at grain boundaries, as well as the absence of chromium carbide precipitates within this fine grained zone. These precipitates are always present in 9e12 wt. % Cr ferritic/martensitic steels and although their absence beneath the scale may indicate a Cr depleted zone, according to EDS measurements there is no difference with respect to the Cr content within the bulk material [26,27]. Quadakkers and collaborators [28] have found the same situation in a FeeCreNb model alloy. The presence of this fine grained zone allowed faster diffusion of Cr and is likely the main reason for the formation of the protective Cr rich oxide. A higher magnification image showed a 1.5 mm thick protective scale exhibiting three distinct layers of different thicknesses: the top and bottom have a similar morphology composed of nanosized grains (10e50 nm) whereas the middle layer has larger grains ranging from approximately 50e200 nm (Fig. 11). Moreover, some porosity between the sub-layers is visible. According to the electron diffraction patterns, the top and bottom layers are composed of (Cr, Mn, Fe)3O4 with little Fe (w3 wt. %) and Cr2O3 whereas the middle layer has very little Cr and no Mn and can be identified as Fe3O4. The voids can have their origin in Fe diffusion from the Fe3O4 into the spinel. There were also small amounts of Si in all layers. Moreover, the EDS elemental concentration profile revealed two other even thinner layers (<50 nm) at the interface with the substrate as seen in Fig. 12, the most inner being enriched in Cr and Mn while the
Fig. 8. Protective oxide formed on NPM on exposure to flowing steam at 650 C after a) 5000 h and b) 10,000 h.
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Fig. 10. NPM grains: a) beneath de surface and b) materials bulk.
Fig. 11. STEM image and electron diffraction of the protective oxide scale formed on NPM after 10,000 h of exposure to flowing steam at 650 C.
outer layer is rich in Fe. This two layers may be identified as (Cr, Mn, Fe)3O4 and Fe3O4 respectively. Therefore, the protective scale present after 10,000 h of exposure to steam is composed of five layers from the surface to the substrate: a) 200e300 nm (Cr, Mn, Fe)3O4,
Fig. 12. EDS element concentration profiling if the protective scale obtained on NPM after 10,000 of exposure to steam at 650 C.
b) w 200 nm Fe3O4, c) 600e700 nm (Cr, Mn, Fe)3O4, d) < 50 nm Fe3O4 and e) < 50 nm (Cr, Mn, Fe)3O4. This multilayered thin scale evokes the inner layer present in the non protective oxides found in P91 and 92 as well as in the oxide nodules found on NPM, in which the Cr concentrations oscillate from high to low as mentioned earlier. The proposed interpretation, regarding the repetition of cycles in which Cr rich spinels form due to rapid Cr diffusion from the bulk alloy, until the spinel cannot be sustained due to Cr depletion, may also apply for this protective scale. A second STEM analysis was carried out at a different spot, near an oxide nodule. Fig. 13 shows how oxidation has taken place via grain boundaries or cracks surrounding grains of the alloy. This oxide is rich in Cr and Mn as it can be observed in the EDS mapping shown in Fig. 14. If oxidation continues advancing inwards through grain boundaries or cracks until reaching the interface between the fine grained zone and the bulk, the advantages provided by the fine grained zone would be lost and locally, the material would no longer form and maintain a protective oxide. As these localized nodules appear at the initial stages of oxidation, it is suggested that the observed intergranular oxidation mechanism originates from intergranular micro-cracks originally present in the material. Once an oxide nodule starts forming, its high rate of growth can cause tension, and promote further micro-crack formation in adjacent zones, facilitating the observed “sideways” growth provoked by the above mentioned process.
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Fig. 13. STEM image of the protective oxide formed on NPM on exposure to flowing steam at 650 C after 10,000 h at a zone near an oxide nodule.
Another protective layer formation mechanism could be derived from the observed grain surrounded by oxides growing through cracks or grain boundaries. The metal surface grains would oxidize and end up surrounded by Cr rich oxides, depleting the grains’ bulk in Cr and therefore generating Fe3O4. As the grains are very small in size, soon they would fully oxidize generating Fe3O4 under the Cr rich spinel. However, such a mechanism would not result in continuous layers as those observed in the present study but rather in concentric nano-layered oxide grains. Abe and collaborators [14] observed a different protective scale when pre-oxidizing 9Cr3W3Co0.2V0.05Nb (C: 0.082, Si: 0.73, Mn 0.49, Cr: 9.16, W: 2.47, Co: 3.3, V: 0.2, N: 0.002, B: 0.019, wt. %) at 700 C for 50 h in argon. After exposure to steam for 9000 h at 650 C the scale observed in this case is composed of a 100 nm film of (Cr, Mn, Fe)3O4 on top, a thicker layer of (Fe, Cr)2O3 (aprox. 500 nm) in the middle and a very thin Si enriched oxide-substrate interface. The substrate layer directly beneath the scale is also very
fine grained but the authors did not comment this feature. Similarly to this investigation, it is enriched in Laves phase precipitates. Although very alike in composition, the NPM substrate used in this work had about half the Si content of the steels analyzed by Abe and coworkers. The unexpected high oxidation resistance of NPM is therefore related to the very fine grained zone found beneath the scale and likely to enhance Cr diffusion within this layer. It is however, difficult to explain how it was formed. As described in the experimental section, samples were taken from a forged ingot, which was heat treated (normalized and tempered) and cut from the materials bulk and later ground with 120 grit paper. Cutting was carried out with a water cooled wheel and it is perhaps during this process where the grain refinement may have taken place. In this case, the grinding process would not have removed all of the finely grained. Another alternative could be that grinding caused the formation of this fine grained zone and as mentioned in the introduction Grabke
Fig. 14. EDS element concentration mapping of the protective scale obtained on NPM after 10,000 of exposure to steam at 650 C at a spot near an oxide nodule.
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[22] found that what he called “deformation” by grinding increased the oxidation resistance of ferritic steels. Further work is certainly required to clarify why and how this layer was formed in this steel and not in others such as P91 and P92 which received exactly the same surface treatment prior to exposure. 4. Conclusions As already known, chemical composition plays a very important role on the oxidation behavior of ferritic/martensitic steels. However, as a result of the present study it can be concluded that the material morphology near the surface may also be of high importance in ferritic steels. A 9 wt % Cr steel ferritic/martensitic steel containing Mn, Co and W, developed a Cr and Mn rich protective scale which was stable up to at least 20,000 h of exposure to flowing steam at 650 C. Some oxidation nodules were present from the beginning of the test and slowly grew, but no new ones appeared to form during the exposure. Surprisingly, a multilayer thin protective oxide develops without requiring a previous preoxidizing heat treatment on top of a very fine grained zone rich in Laves precipitates within the substrate. The multilayer oxide structure is composed of alternating layers of (Cr, Mn, Fe)3O4 and Fe3O4 which may form as a result a Cr depletionerepletion cycles. Thicker oxide nodules also form likely due to mechanical defects of the protective oxide rather than to inhomogeneities in local composition. It is not clear yet how and why this finely grained zone forms and further investigations are on-going. Acknowledgments The authors would like to acknowledge the support by the European Cooperation in Science and Technology (COST) through Action 536 “Alloy development for Critical Components of Environmental friendly Power planT (ACCEPT)”. AA and VG wish to acknowledge the Spanish Ministry of Science and Innovation for the financial support (ENE2008-06755-C02-01) as well as all of the members of the Metallic Materials Area at INTA for technical support in particular Javier García de Blas for his useful input. KSH would like to acknowledge the Austrian Research Promotion Agency FFG for the financial support.
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