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9th International DAAAM Baltic Conference "INDUSTRIAL ENGINEERING"- 24-26 April 2014, Tallinn, Estonia

INVESTIGATION OF BOILER MATERIALS WITH RELATION TO CORROSION AND WEAR: REVIEW

Priss, J.; Klevtsov, I. & Dedov, A., Antonov, M.

Abstract: Elevated temperatures, oxidizing atmospheres and impacts by fluidized ash particles may cause significant degradation of some components in Circulating Fluidized Bed boilers [4]. The objective of presented research has been to investigate degradation process for some boiler steels in severe operation conditions and to rank investigated steel grades in these conditions. High temperature corrosion tests under impact of oil shale ash and tribotests have been used for research. Key words: boiler component, corrosion, wear, tribotests, analysis. 1. INTRODUCTION Oil shale is the main energy source in Estonia. Around 90% of the electricity consumed in the country is produced from oil shale at the Narva Power Plants (NPP). The lower heating value (LHV) of oil shale used for electricity production is 8.3-8.4 MJ/kg [1]. Oil shale is a fuel with a high content of carbonate minerals. Burning that kind low-grade fuel is associated with several problems: dissociation of carbonate minerals, high CO2 emission levels, ash handling and storing. For minimizing the environmental impact of power production, the application of modern Circulating Fluidized Bed (CFB) technology at the NPP provides advantages in utilization of upgraded oil shale with the LHV at up to 11.0 MJ/kg [1]. Fluidized-bed combustion addresses to these challenges, like burning a wider range of fuels and generating power with improved thermal efficiency with low emissions compared to other existing combustion techniques.

However, conditions in fluidized-bed boilers (e.g., elevated temperatures, oxidizing atmospheres and impacts by fluidized sand and ash particles) may cause significant degradation of some boiler components, such as tubes of heating surfaces, by a combination of oxidative attack and erosive wear [4]. Scheme of CFB boiler in NPP is illustrated in Fig. 1.

Fig. 1. Scheme of oil shale-fired CFB boiler (sample parts). 1-Raw fuel silo, 2- Fuel feeder, 3- Grate, 4- Secondary fuel crusher, 5- Furnace chamber, 6- Separating chamber, 7- Fluidized bed internal heat exchange (Intrex), 8- Separator of solids, 9- Convective superheater and reheater, 10- Economizer, 11- Air preheater, 12- Electrical precipitator [5]. 2. MODES OF WEAR IN BOILER According to previous studies it is well known that combustion products of Estonian oil shale highly accelerate the corrosion of the superheater tubes [17-19]. The main corroding ions are chlorides of alkaline metals, which cause the high corrosivity of combustion products of oil shale. Erosion-corrosion process is a result of material wastage rates in waste incineration

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systems and power generation industry. According to [2] convection pass tubes, such as superheater, reheater, and economizer tubes in fluidized bed boilers are suffering from fly-ash erosion (Fig.2). Major constitutes of the fly-ash are SiO2, Al2O3, and Fe2O3. There is imposed limits for the flue gas velocities (12-13 ms-

1 for erosive ash having high silica content and 18-20 ms-1 for less-erosive ash) to reduce the wear rates, but this also results in the drop of boiler efficiency.

STEAM

~700-1000 C0COMBUSTION

FLY ASH

OXIDATIONOUTSIDE

STEAM

STEAM STEAM

BOILER HEAT EXCHANGE AREA

CORROSIONEROSION

~450-600 C0T METAL

T GAS

Fig. 2. Fireside corrosion and wear of boiler steels. 3. CORROSION AND TRIBOTESTS In present research work high temperature corrosion (HTC) test and two tribotests: high temperature erosion (HTE) test, high temperature cyclic impact-abrasion (HT-CIA) test have been involved in process. According to [3] tribotests could be classified by rising of the degree of realism (how closely they imitate the conditions of real application) as model test. The model test usually involves standard test specimens and could be executed in short time (several minutes, hours). The test samples could be smaller than the real component and of simple geometry that makes higher the level of the conditions control. Model test enables to raise the level of tracking the corrosion- or tribosystem behaviour [3].

4. MATERIALS TESTED In the present study, four different materials have been investigated in the HTE, HTC and HT-CIA tests. List of investigated steels is present in Table 1.

Grade

(A) X12CrNiTi18-9 austenitic

(B) X10CrMoVNb9-1 martensitic (C) 13CrMoV42 pearlitic (D) TP347HFG austenitic Table 1. List of investigated steels. Material A is an austenitic, heat-resistant steel 12X18H10T, equivalent grade is X12CrNiTi18-9 (DIN 17440). This steel has a high temperature resistant microstructure with a chromium content of 18% and additionally 10% of Ni, which improves the resistance against corrosion in reducing environments and stress corrosion cracking [9]. Additionally steel is alloyed by titanium (0.52%) to stabilize the carbon and the chromium in the matrix and increase the fracture toughness by precipitating intergranular TiC [10]. Material B is a chromium alloyed high strength martensitic steel 18X12BMБФР, equivalent grade is X10CrMoVNb9-1 (EN 10216-2) with increased creep resistance. At the chromium content of 9% and the added molybdenum (1.08%) the pitting and crevice corrosion resistance is increased [11-12]. Material C is a pearlitic steel 12Х1МФ, equivalent grade is 13CrMoV42 (DIN 17155) with a chromium content of 1.44%. Material D is austenitic steel TP347HFG (ASTM 213). It has homogeneous structure in bimodal distribution of the grain size. In this material Nb is added to increase the strength and the resistance against different corrosive media [13]. The hardness have been measured at room temperature (RT), 100, 300, 500, 600, 700 and 800°C, respectively. Steels have an initial HV10 Vickers hardness of ~221(A);

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235(B); 146(C); 180(D) at RT which is slowly declining with increasing temperature due to softening mechanisms, as seen in Fig. 3 [7].

0

50

100

150

200

250

300

0 100 200 300 400 500 600 700 800

Temperature [°C]

Har

dnes

s [H

V10]

(A) X12CrNiTi18-9(B) X10CrMoVNb9-1(C) 13CrMoV42(D) TP347HFG

a)

Fig. 3. Summary of hardness data [7]. 5. EXPERIMENTALS AND RESULTS 5.1 High temperature corrosion test Experiments have been performed at 500 °C and 600 °C with an exposure time of 24 h. The test conditions are presented in Fig.4. The samples were coated by manual brushing with a mixture of oil shale ash and ethanol for the optimal simulation of oil shale ash on-tube deposits for the corrosion tests. The tests have been carried out in the temperature-controlled horizontal tube furnace of the HTC test. For the experiments HCl was passed through the chamber at a velocity of 1.9 cm/s. The samples were positioned vertically and parallel to the flow direction. Before and after the test procedure the samples were weighed, determining the mass change for the calculation of the corrosion rates [6].

HCl 0.019m/s

HT CORROSION TEST CONDITIONS

CRUCIBLE

ASHSAMPLE

500,600 C24hGAS FLOW:

0

Fig. 4. The high temperature corrosion (HTC) test conditions.

Based on the test results following conclusions can be drawn, that the high temperature corrosion of different boiler steels is strongly dependent on the content of oxidation layer forming alloying elements, such as chromium and nickel. The corrosion rate in presence of HCl in high temperature applications can be influenced by different other corroding anions, e.g. sulphate and potassium ions, presented in oil shale. Also oxidation cannot be neglected in corrosive environment at elevated temperatures [6]. Austenitic steel grades (materials A and D) and martensitic steel grade (material B) show better oxidation resistance against oxidation than the pearlitic steel grade (material C) [6]. Under HTC test conditions in HCl environment in presence of sulphur and potassium containing oil shale ash the best corrosion resistance shows austenitic steel grade X12CrNiTi18-9, then austenitic steel TP347HFG, pearlitic steel 13CrMoV42 and martensitic X10CrMoVNb9-1. That sequence represents the corrosion ranking of the tested steel grades for their application as high-temperature heating surfaces in boilers firing oil shale which contain chlorine, sulphur and potassium in outer deposits [6]. 5.2 High temperature erosion test Solid particle erosion tests have been performed in a conventional centrifugal four-channel accelerator (HTE) test. The abrasive particles used in this work were rounded silica particles. “Investigation of steady state erosion rate was made as a function of the impact angle at the abrasive particles velocity of 80 m/s” [4]. Erosion tests were conducted at impact angles of 30° and 90°, respectively. The duration of each erosive test was approximately 1 hour. In each test, 10 specimens of the size of 20x15x4.5mm were treated simultaneously. Parameters of the tests are presented in Fig. 5 [8].

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HT EROSION TEST CONDITIONS

=30, 90

1hERODENT:

0

500,600 C0

FLOW=

O

SILICA SAND6 kg80 m/s

Fig. 5. The high temperature erosion test conditions. The investigated materials show a change of the wear in the temperature range from 500 °C to 600 °C. Considering materials B, C and D volumetric wear rate for both tests increases with increasing temperature (Fig. 6, 7). Opposite behavior can be detected for austenitic material A which exhibits better wear performance at higher testing temperature of 600 °C in test with impact angle of abrasive 30°. That may be explained by particle sticking [8]. Analyzing materials wear rate in respect to impact angle of abrasive particle influence, it should be mentioned that wear rate is lower with impact angle of 90° for materials investigated [8].

Fig. 6. Volumetric wear rate of materials at elevated temperatures under the impact angles of 30°, 80 m/s [8].

Fig. 7. Volumetric wear rate of materials at elevated temperatures under the impact angles of 90°, 80 m/s [8]. 5.3 High temperature cyclic impact-abrasion test Test principle is based on potential energy (0.8 J) which is cyclically turned into kinetic energy by free fall. Samples are fixed at 45° and are periodically hit by the plunger with 1.8-2.0 Hz frequency with a constant flow of abrasive (3 g/s, silica sand with particle size of 0.4-0.9 mm) between plunger and sample (Fig. 8) [14]. Tests were done at 500°C and 600°C. Wear was determined by mass loss and wear volume was calculated via materials density. The plunger material used for these tests was a Co-rich high-speed steel. Test duration was set constant to 1 h.

HT-CIA TEST CONDITIONS500,600 C1h

0PLUNGERSILICASANDFLOW=3 g/s

=45 0O

Fig. 8. The high temperature cyclic-impact abrasion test conditions.

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Fig. 9. SEM micrographs of typical worn surfaces of material B (martensitic X10CrMoVNb9-1) after cyclic impact/abrasion testing at 600 °C. Formation of mechanically mixed layer (MML) [7]. Wear under high temperature cyclic impact/abrasion strongly depends on the materials microstructure. Generally, the wear resistance decreases with increasing temperature due to different softening mechanisms for the investigated steels A, B and D. Softer materials tend to incorporate abrasive particles in the surface and near-surface region by forming a mechanically mixed layer (MML) (Fig.9) [15-16] which can decrease the volumetric wear. Materials with higher initial hardness such as martensitic steels do not show this mentioned effect [7]. 6. CONCLUDING REMARKS Boiler steels ranking according to their resistance to high temperature corrosion does not agree with the ranking on the basis of tribotests. Under HTC test conditions the best corrosion resistance shows austenitic steel grade (A) X12CrNiTi18-9, than are going austenitic steel (D) TP347HFG, pearlitic steel grade (C) 13CrMoV42 and martensitic (B) X10CrMoVNb9-1. In order to compare the behaviour of steels the wear rate map was created (Fig.10, 11).

Fig. 10. Wear rate map of studied steels under at 500 °C.

Fig. 11. Wear rate map of studied steels under at 600 °C. 7. REFERENCES 1. Plamus K., Ots, A., Pihu, T., Neshumayev, D. Firing Estonian oil shale in CFB boilers- ash balance and behavior of carbonate minerals. Oil Shale, 2011, vol. 28, No.1, pp.58-67. 2. M.Antonov, M., Veinthal, R., Huttunen-Saarivirta, E., Hussainova, I., Vallikivi, A., Lelis, M., Priss, J. Effect of oxidation on erosive wear behaviour of boiler steels. Tribo-Corrosion 2013, 68, 35-44. 3. Antonov, M., Michalczewki, D., Pasaribu, R., W.Piekoszewski, W. Comparison of tribological model and real component test methods for lubricated contacts. Estonian Journal of Engineering, 2009, 15, 4-349-358. 4. Huttunen-Saarivirta, E., Antonov, M., Venthal, R., Tuiremo, J., Mäkelä, K., Siitonen, P. Influence of particle impact conditions and temperature on erosion-oxidation of steels at elevated temperatures. Wear, 2011, vol. 272, pp.159-175. 5. Neshumayev, D., Ots, A., Parve, T., Pihu, T., Plamus, K., Prikk, A. Combustion

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of Baltic oil shales in boilers with fluidized bed combustion. Power technology and Engineering, 2011, vol. 44, No.5. 6. Priss, J., Rojacz, H., Klevtsov, I., Dedov, A., Winkelmann, H., Badisch, E. High temperature corrosion of boiler steels in hydrochloric atmosphere under oil shale ashes. Corrosion Science, 2014, vol. 82, 36-44. 7. Priss, J., Klevtsov, I., Dedov, A., Antonov, M., Rojacz, H., Badisch, E. High temperature cyclic impact/abrasion testing of boiler steels. Key Engineering, Materials, 2014, vol. 604, 289-292. 8. Priss, J., Klevtsov, I., Dedov, A., Antonov, M. Erosion wear testing of boiler steels at elevated temperatures. Proceeding of the 14th International Symposium „Topical Problems in the Field of Electrical and Power Engineering“ Pärnu, Estonia, January 13-17, 2014, 248-250. 9. Bose, S. High Temperature Coatings, Butterworth-Heinemann, Oxford, 2007. 10. Kim, J.K., Kim, Y.H., Lee, J.S., Kim, K.Y. Corros. Sci., 2010, 52, 1847-1852. 11. Ilevbare, G.O., Burstein, G.T. Corros. Sci., 2001, 43, 485-513. 12. Wanklyn, J.N. Corros. Sci., 1981, 21, 211-225. 13. Nam, N.D., Kim, J.G. Corros. Sci., 2010, 52, 3377–3384. 14. Badisch, E., Katsich, C., Winkelmann, H., Franek, F., Manish, R. Wear behaviour of hardfaced Fe-Cr-C alloy and austenitic steel under 2-body and 3-body conditions at elevated temperature. Tribology International, 2010, 43, 1234-1244. 15. Varga, M., Rojacz, H., Winkelmann, H., Mayer, H., Badisch, E. Wear reducing

effects and temperature dependence of tribolayer formation in harsh environment. Tribology International, 2013, 65, 190-199. 16. Antonov, M., Hussainova, I., Pirso, J., Volobueva, O. Assessment of mechanically mixed layer developed during high temperature erosion of cermets. Wear, 2007, 263, 878–886. 17. Tallermo, H., Lausmaa, T., Klevtsov, I., Nuutre, M. The influence of chlorine in ash deposits on 12Cr1MoV alloy high temperature corrosion. - Oil Shale, 1997, vol. 14, No. 3, pp. 307-316. 18. Klevtsov I., Tallermo H., Bojarinova T., Lausmaa T., High Temperature Corrosion Resistance of the Austenitic Steels in the Presence of Chlorine-Containing On-Tube Deposits. Oil Shale, 2003, vol. 20, No. 4, pp. 501-509. 19. Tallermo H., Klevtsov I., Bojarinova T., Dedov A. Laboratory tests of high-temperature corrosion of steels B-407, X8CrNiNb1613 and X8CrNiMoNb1616 under impact of ash formed at pulverized firing of oil shale. Oil Shale, 2005, vol. 22, No. 4, pp. 467-474. 6. ADDITIONAL DATA ABOUT AUTHORS 1) MSc. J. Priss, Prof. I.Klevtsov, Ph.D A. Dedov. Ph.D M.Antonov. 2) Compeering analysis of boiler materials according to tribotests. 3) Professor / Ivan Klevtsov /klevtsov@staff.ttu.ee/ Ph.D / Andrei Dedov / dedov@staff.ttu.ee Ph.D / Maksim Antonov / maksim.antonov@ttu.ee MS.C / Jelena Priss/ jelenapriss@gmail.com 4) Corresponding Author: (Jelena Priss, jelenapriss@gmail.com)