Manufacturing of Fe-Cr-Al weld overlay coatings for high temperature applications

Transcription

1 Manufacturing of Fe-Cr-Al weld overlay coatings for high temperature applications Authors: Teemu Sarikka, Risto Ilola, Hannu Hänninen, Aalto University School of Engineering, Department of Engineering Design and Production, Laoratory of Engineering Materials, Finland Rami Pohja, Satu Tuurna, VTT Technical Research Centre of Finland

2 Astract Boiler and tuing materials used in iomass-fired energy production are susceptile to corrosion in severe corrosive environments caused y inorganic constituents such as alkali metals and chlorine at the operating temperatures. Alumina forming alloys have een considered as possile candidates in these applications. In this study, overlay welds with FeCrAl type consumales having a nominal Al content of 5.8% (Kanthal A1) and 10% (prototype consumale) were developed and produced. The overlay welds were manufactured on T22 steel (10CrMo9-10). The overlay welds were made with a single-wire or two-wire GTAW method. Performance of the welds was tested in 168 h potassium chloride (KCl) exposure tests at 600 C in as-welded condition and after 24 h preoxidation at 950 C. Aluminium distriution in the welds and the formed oxide layers in the KCl exposure were characterized with SEM. After the exposure tests the oxide scales were mainly iron and chromium oxides in samples of as-welded condition and aluminium oxide in samples of preoxidized condition. Introduction Conventional heat-resistant stainless steels used in the critical components of a iomass-fired power plant rely on chromia (chromium oxide, Cr 2 O 3 ) scales for protection against high-temperature corrosion attack. However, since the higher efficiency demands the operating temperatures of the power plants to increase, the chromia scales may no longer provide sufficient protection against high-temperature corrosion attack and, thus, new approaches are needed to replace the chromia as the provider of the high-temperature corrosion resistance of the construction materials. One possile replacement to chromia in iron-ased high-temperature alloys is alumina (aluminium oxide, Al 2 O 3 ). Aluminium is a very effective alloying element in improving the alloy s resistance to oxidation and other types of high-temperature corrosion attack 1. As an alloying element, the purpose of Al is to form a healing layer of alumina on the surface of the oxide layer at the gas-scale interface to protect the structural material against high-temperature corrosion attack. The alumina scale protects the material against high-temperature oxidation y acting as a diffusion arrier to metal and oxygen ions trying to penetrate the oxide scale 2. Growth rate of an alumina scale is 1 to 2 orders of magnitude lower than that of Cr 2 O 3. Alumina is also significantly more thermodynamically stale than chromia. Alumina scales have een proven to e specifically eneficial in the presence of aggressive caron- or sulphur-species encountered in comustion and chemical process industry applications 3. The most critical factors in the formation of the protecting alumina scale on Fe-Cr-Al alloys are the temperature in which the scale forms and the aluminium content which has to e high enough to develop and maintain an alumina scale and prevent the following reakaway oxidation of the alloy 2. There are several different types of alumina scales formed on the surface of Fe-Cr-Al alloy and the type of the scale depends on its formation temperature. When the oxide scale formation occurs in temperatures higher than 1000 C, the forming oxide scale will e thermodynamically stale α- Al 2 O 3 scale and it is continuous, has very low defect concentration, and will provide the material with outstanding oxidation resistance 4. α-al 2 O 3 is a stale high temperature form of alumina, which has a slow growth rate and a large and gap which makes the electronic conduction difficult 2. The Al 2 O 3 scale formed in temperatures higher than 1000 C consists mainly of α-al 2 O 3. The α-al 2 O 3 scale provides effective protection against high-temperature corrosion ecause it is chemically inert and grows relatively slowly. The oxidation rate of the material slows down rapidly after a continuous layer of α-al 2 O 3 has formed 5. The aluminium oxide scales, which are formed in temperatures < 1000 C, are so called transition aluminas, e.g. γ- and θ-al 2 O 3. They have different crystal structures compared to the stale α- 2

3 Al 2 O 3. When oxidation takes place in temperatures aove 1000 C, the transient aluminas can e oserved during the initial stages of oxidation 2. These transition aluminas are metastale and they will convert into the thermodynamically stale α-al 2 O 3 over time, however, the transformation is temperature dependent and it is relatively slow at temperatures elow 1000 C. Transition aluminas are not desired due to their larger defect concentration and higher growth rate 4. Experimental methods The samples used in this study were manufactured using GTAW (gas-tungsten arc welding) procedure. The cladding materials chosen for the study were ferritic alumina-forming Kanthal A1 alloy and manually composed ferritic alumina-forming alloy with a nominal composition of Fe- 17Cr-10Al. Standard 10CrMo9-10 steel was chosen as ase metal for the cladding. Nominal chemical compositions of the chosen materials are presented in Tale 1. Kanthal A1 coating layers were welded over the structural layer using a single-wire GTAW method. Manually composed Fe- 17Cr-10Al samples were manufactured using GTAW with dual wires, one eing commercially pure 1070 Al and the other one eing ferritic RW 430. Exact nominal composition of the cladding material was sought to e as close as possile of eing 10 wt. % of Al y controlling the composition y adjusting the wire feed speeds of the additive wires. The cladding layer was welded on the top of the ase metal either as a single layer or, to minimize the amount of susequent dilution of Al and Cr from the cladding to the ase material, as two pass-layers. Tale 1. Nominal chemical compositions of the studied materials. Sample Fe (%) Cr (%) Si (%) Mo (%) Cu (%) Ni (%) Kanthal A1 Balance RW 430 Balance CrMo9-10 Balance Sample Mn (%) V (%) S (%) P (%) Al (%) C (%) Kanthal A , RW CrMo KCl-tests were performed in order to study the material ehaviour in high chlorine environments. Four 20x20x12 mm samples of each cladding material were cut from the overlay welds, two from single-layer weld and two from two-layer weld. The cutting surfaces of the samples were polished using 320 grit paper, one sample of each type was preoxidized in air atmosphere at 950 C for 24 h, and all the samples were then exposed to potassium chloride containing environment for 168 h at 600 C. The exposed samples were examined in order to determine the effects KCl environment had on the oxide scales of the samples. Results Tale 2 shows the measured chemical compositions of the studied samples. The measurements were done using a standard SEM-EDS point analysis method with 15 different points from each sample making the total of 30 measurements per cladding. The Al was evenly distriuted throughout the Kanthal A1 claddings and the variation in Al composition in Kanthal A1 samples was quite low. In Fe-17Cr-10Al samples, the Al composition distriution was much larger. The Al composition 3

4 measured from on pass of the sample was totally different than the composition measured from the other pass of the sample. Also, the chemical composition of Al was much lower in Fe-17Cr-Al samples than in Kanthal A1 samples. Tale 2. Measured chemical compositions of the studied materials. Acquisition Fe (wt. %) Cr (wt. %) Al (wt. %) Kanthal A1 1-layer Kanthal A1 2-layer FeCrAl 1-layer FeCrAl 2-layer SEM (scanning electron microscope) pictures of the oxide scale on the surface of the KCl-tested Kanthal A1 two-layer sample of as-welded (a) and preoxidized () states are presented in Figure 1. The oxide scale covered the surfaces of the claddings thoroughly and was similar all over the surface. As can e seen, the oxide scale on the cladding of the as-welded sample consists of larger amount of individual oxide layers and the scale is clearly thicker on as-welded sample than on the preoxidized sample. The oxide layers were optically similar on one-layer Kanthal A1 samples. a Fig.1. SEM pictures of Kanthal A1 oxide scale. Figure 2 shows SEM pictures of the oxide scales on the surface of the cladding material on KCltested Fe-17Cr-10Al two-layer samples of as-welded (a) and preoxidized () states. The surfaces of the claddings were completely covered with oxide scale; however, the oxide scale was not similar throughout the preoxidized sample. The oxide scale on the preoxidized sample was partly identical to the oxide scale on the as-welded sample and partly it was noticealy thinner, as can e seen from the Figure 2. The oxide scales on the surfaces of the Fe-17Cr-10Al samples were overall thicker compared to the Kanthal A1 samples. The oxide layers were optically similar also on one-layer Fe- 17Cr-10Al samples; the oxide layer of the preoxidized sample was not similar throughout the surface of the cladding and the oxide scales were thicker than the oxide scales on Kanthal A1 samples. 4

5 a Fig.2. SEM pictures of Fe-17Cr-10Al oxide scale. Figure 3 shows the SEM-EDS elemental maps of the oxide scales on the surface of the Kanthal A1 two-layer samples of as-welded (a) and preoxidized () states. As can e seen, the oxide scale on the surface of the as-welded sample consists of the top layer of iron oxides, the middle layer of chromium and aluminium oxides, and eneath the chromium oxide layer is a very thin internal layer of aluminium oxide. The oxide scale on the surface of the preoxidized sample, on the other hand, consists only of an external layer of aluminium oxide. a Fig.3. SEM-EDS elemental maps of Kanthal A1 samples (the difference in the colours of the elements noticed). The SEM-EDS elemental maps of the oxide scales on the surface of the Fe-17Cr-10Al two-layer samples of as-welded (a) and preoxidized () states are show in Figure 4. The oxide scale on the surface of the as-welded sample consists of layers of iron and chromium oxides and aluminium is nearly undetectale in the specimen. The oxide scale on the surface of the preoxidized sample has external aluminium oxide scale; however, it is not even nearly continuous and does not cover the whole oxide scale. Underneath the aluminium oxide particles, there is a layer of iron oxides. 5

6 a Fig.4. SEM-EDS elemental maps of Fe-17Cr-10Al samples. Conclusions The manufacturing of Kanthal A1 cladding was successful; the aluminium in the cladding was evenly distriuted and the composition was sufficient to provide the material with the aility to form an external aluminium oxide scale. The manufacturing of the cladding with 10 wt. % nominal aluminium composition was not successful; the aluminium in the cladding was unevenly distriuted and the composition of aluminium in the cladding was clearly lower than desired, even clearly lower than in Kanthal A1 cladding. The low aluminium composition was likely due to the uneven distriution of aluminium in the cladding and vaporisation of aluminium during the welding process. The Kanthal A1 sample did not form an external aluminium oxide layer in the KCl environment; instead it formed an oxide scale with multiple different oxide layers with non-continuous internal aluminium oxide layer. The preoxidation of Kanthal A1 cladding formed a thin, continuous, external aluminium oxide layer on the surface of the cladding which resisted the KCl environment and, thus, provided the material with good resistance against chlorine containing environment. The cladding with 10 wt. % nominal aluminium composition was not understandaly ale to resist the KCl environment neither in as-welded nor in preoxidized state since the aluminium composition was not high enough to provide the material with the aility to form any kind of external aluminium oxide scale. Acknowledgements This study was made as a part of a collaorative project, called Matexon, etween Aalto University and VTT (Technical Research Centre of Finland). The authors wish to express their gratitude for all the participants of the project. References 1. Lai, G. (2007) High Temperature Corrosion and Materials Applications. ASM International, USA. 461 p. ISBN-10: Prescott, R. and Graham, M. (1992A) The Formation of Aluminum Oxide Scales on High- Temperature Alloys. Oxidation of Metals, Vol. 38, No. 3, pp Brady, M.P., Yamamoto, Y., Santella, M.L., Maziasz, P.J., Pint, B.A., Liu, C.T., Lu, Z.P., and Bei, H. (2008) The Development of Alumina-Forming Austenitic Stainless Steels for 6

7 High-Temperature Structural Use. The Minerals, Metals & Materials Society, Vol. 60, pp Liu, F., Götlind, H., Svensson, J.-E., Johansson, L.-G., and Halvarsson, M. (2008) Early Stages of the Oxidation of a FeCrAlRE Alloy (Kanthal AF) at 900 C: A Detailed Microstructural Investigation. Corrosion Science, Vol. 50, No. 8, pp Engkvist, J., Canovic, S., Hellström, K., Järdnäs, A., Svensson, J.-E., Johansson, L.-G., Olsson, M., and Halvarsson, M. (2010) Alumina Scale Formation on a Powder Metallurgical FeCrAl Alloy (Kanthal APMT) at 900-1,100 C in Dry O 2 and in O 2 + H 2 O. Oxidation of Metals, Vol. 73, No. 2, pp