Sandvik Sanicro®25, A high performance austenitic material for superheaters and reheaters in

advanced coal fired power boilers

N.Lintzén U.Forsberg

Sandvik Materials Technology SE-811 81 Sandviken

Sweden

Abstract The worldwide electricity demand is growing at the same time as much of today’s generating fleet is getting closer to the end of its service life. This means there is a big demand for new electric power generating plants. The today’s predominantly used feedstock, oil and gas, are resources with limited availability and thus the costs for these fuels has increased dramatically. Coal is found on every continent and the World Coal Institute forecasts it to last for over 150 years consumption. Coal conversion in different forms will therefore continue to contribute with a substantial part of the electric power generation industry in the future. Limiting environmental impact is at the same time of great importance. The new generation of advanced ultra super critical pulverized coal fired boilers will operate at very high steam temperatures and pressures (700°C, 350 bar) targeting net efficiencies above 50% with reduced fuel consumption and CO2 emissions. To meet the material properties required for some of the critical components in these boilers, Sandvik Materials Technology has developed the austenitic alloy Sandvik Sanicro®25. Main applications are for superheater and reheater tubes for use up to about 700°C material design temperature. The alloy is designed to have excellent strength at elevated temperatures, high oxidation resistance combined with high structural stability and good fabricability. The so far obtained test results indicate that Sandvik Sanicro 25 has higher mechanical (creep) properties than any other known commercial austenitic material. It is also a substantially more economical alternative to nickel base alloys. This paper presents material properties and results from material tests generated during the development and the further verification of data for this material.

1. Introduction Sandvik Sanicro®25 is an austenitic stainless steel intended to be used as superheater and reheater tubes in high efficiency coal fired power boilers in areas in which material temperatures are up to around 700°C. Use in high temperature applications in other types of steam boilers and with other fuels might also be of interest. The alloy is designed to have high creep- and oxidation resistance, high microstructural stability and good fabricability. The very high elevated temperature strength is reached by precipitate strengthening through precipitations of Nb(C,N), fine NbCrN, M23C6 and Cu-rich precipitates [1]. Addition of W also improves the high temperature strength. The chemical composition is shown in table 1. Table 1; Chemical composition C max Si Mn Cr Ni N W Co Cu Nb 0.1 0.2 0.5 22.5 25 0.23 3.6 1.5 3.0 0.5 2. Manufacturing Tubes have been produced in different dimensions which have been used for comprehensive material testing and for various test installations in coal fired power plants. Tubes can be supplied hot- or cold worked, bright annealed or solution annealed and white-pickled.

Figure 1; Sandvik Sanicro 25 tubes, Ø60.3x9.5 mm and Ø38x6.3 mm

3. Mechanical properties 3.1 Tensile strength Tensile tests have been performed at room temperature and at 50°C intervals from 50°C (100°F) up to 800°C (1500°F). Average values of proof- and tensile strength for both cold worked and hot extruded tubes are shown in figure 2. The results are derived from double tests at each temperature from three different heats of cold worked and two different heats of hot extruded tubes.

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Figure 2; Tensile properties for Sandvik Sanicro 25

The tensile tests values are lower for the tubes which are hot extruded compared to the values for the cold pilgered. The tensile properties seem to be about 10% lower for the hot extruded tubes.

3.2 Creep properties Creep rupture tests have been carried out on both bar and tube material. The target set when developing the material was a creep strength of 100 MPa (14,5 ksi) at 700°C (1300°F) after 100 000 hours. Ongoing creep tests indicate that the creep strength may be somewhat lower than the original target. The estimated values at different temperatures are given in tables 2 and 3. Table 2; Creep rupture strength, Sandvik Sanicro 25* Temperature[°C] 10 000h 100 000h 500 500 MPa 405 MPa 550 385 MPa 325 MPa 600 300 MPa 230 MPa 650 230 MPa 155 MPa 700 145 MPa 95 MPa 750 85 MPa 50 MPa 800 50 MPa 30 MPa *Extrapolated values; August, 2007 Table 3; Creep rupture strength, Sandvik Sanicro 25, imperial units* Temperature [°F] 10 000h 100 000h 900 80 ksi 65 ksi 1000 60 ksi 50 ksi 1100 45 ksi 35 ksi 1200 35 ksi 20 ksi 1300 25 ksi 15 ksi 1400 15 ksi 5 ksi *Extrapolated values; August, 2007 The creep test results come from specimens tested at four different laboratories. There are also several creep tests ongoing. Figure 3 show creep rupture properties for Sandvik Sanicro 25 and some other high performing boiler tube materials. The data correspond to extrapolated 100 000 hour values. The graphs show that Sandvik Sanicro 25 has notable higher creep strength compared to literature data for other austenitic materials.

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Figure 3; Creep test data, 100 000 hours

The master curve for Sandvik Sanicro 25 is shown in figure 4. The data comes from creep tests at 600, 650, 700 and 750°C.

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Figure 4; Master curve, Sandvik Sanicro 25

3.3 Impact strength Due to its austenitic microstructure, Sandvik Sanicro 25 has very good impact strength both at room temperature and at cryogenic temperatures. Tests have demonstrated that the steel fulfils the requirements (60 J (44 ft-lb) at -196°C (-320°F)) according to the European standards EN13445-2 (UFPV-2) and EN10216-5. Impact tests have been performed on creep specimens aged at 700°C (1300°F) for 1000, 3000, 10 000 and 30 000 hours. Average values from impact tests are given in figure 5.

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4. Microstructural examination 4.1 Aging Samples aged for 1000, 3000 and 10 000 hours at 700°C (1300°F) has been studied in light optical microscope. Figures 6 to 9 show optical images of the samples (magnification 200x). Scanning electron microscope (SEM) and transmission electron microscope (TEM) has also been used to study samples aged for 1000 and 3000 hours [1]. A SEM image of one sample is shown in figure 10 and a TEM image in figure 11.

Figure 6; as received Figure 7; aged for 1000 hours Figure 8; aged for 3000 hours Figure 9; aged for 10 000 hours The results from the optical examinations showed that samples in as delivered condition has Nb-rich precipitates enriched with Cr. The aged samples contained small needle-shaped M23C6 in both the grains and grain boundaries. Sigma-phase and Cr2N were not detected in any of the samples. Figure 10; SEM image, microstructure Figure 11; TEM image, spherical and with Nb-rich precipitates needle-like M23C6 particles

4. Fabricability tests 4.1 Bending Two different 180° bends and two different 90° bends were performed by Doosan Babcock [1]. The bending radius ratio was 3.1 and 2.3, corresponding to 16% and 22% deformation, for both of the 180° and 90° bends. The bends were cold bent at 20°C (70°F). The operations were successfully performed without any cracks in the tube wall. The maximum measured ovality on the tube was 1.6%. Figure 11 and 12 show samples of the bends. Figure 11; 90° bends Figure 12; 180° bends 4.2 Welding Currently the filler metal alloy 617 is recommended for welding of Sandvik Sanicro 25. The literature data for alloy 617 indicates that the corrosion and mechanical data are satisfactory [1]. Creep tests of welded tubes have been performed at 650°C (1200°F) and 700°C (1300°F). The results show that the time to rupture was lower than the estimated targets. A matching Sanicro 25 filler metal, 23.25.HT, has been manufactured and welding trials as well as creep tests are currently in progress. Results from tensile tests with 23.25.HT at 700°C show similar values as for the base material. 5. Corrosion properties 5.1 Oxidation tests in air Oxidation test at 650°C (1200°F) and 750°C (1400°F) were carried out at Sandvik Materials Technology. Three heats of Sandvik Sanicro 25 have been tested together with TP347HFG, TP310HCbN, UNS30432 and Alloy 617 modified. Two test samples of each material were tested and two samples from each heat of Sanicro 25. Isothermal tests were the weight of the test samples was registered after 1000 and 2000 hours were carried out with two heats of Sandvik Sanicro 25, TP310HCbN and UNS30432 as well as tests for 1000 and 3000 hours for one heat of Sandvik Sanicro

25, TP347H and Alloy 617 modified. Furthermore discontinuous tests where the weight was measured after 24, 50, 100, 200, 500, 1000, 1500, 2000, 2500 and 3000 hours were carried out with all materials. All materials had a very low mass change after the tests at 650°C. There were also small differences between the results from the isothermal tests compared to the discontinuous tests but a rather higher weight change was measured for Alloy 617 modified and TP347H during the discontinuous tests at both test temperatures, indicating that those materials may be more sensitive to thermal cycling. One of the TP347H test pieces showed a considerable increase in weight after 1500 hours during the discontinuous test at 750°C which also can be an indication of sensitivity to thermal cycling. The difference in mass change between the materials was larger for the tests at 750°C. Average values of the results from the discontinuous tests at 750°C are shown in figure 13, showing that Sandvik Sanicro 25 is the material with the best oxidation resistance of the tested materials.

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Figure 13; Discontinuous test at 750°C (1400°F)

5.2 Oxidation tests in aqueous steam The Corrosion and Metals Research Institute in Sweden performed oxidation tests in an atmosphere of aqueous steam where four different heats of Sandvik Sanicro 25 was tested together with the austenitic alloys NF709, TP310HCbN, UNS30432 and TP347H. Tests were carried out at 600°C (1100°F) and 700°C (1300°F) for 500 and 1000 hours

[2]. De-ionized water was fed into the test ovens corresponding to a gas flow of 10 mm s-1. The test results showed that very thin oxides (<1μm) had formed on the surfaces of the Sandvik Sanicro 25 test coupons. The oxides were analyzed using GDOES (Glow Discharge Optical Emission Spectroscopy). The oxides on the Sanicro 25 material consisted mainly of chromium oxide, with a minor enrichment in manganese and at 600°C also silicon. The same type of oxides was formed on UNS30432 and NF709. The oxides on the TP310HCbN and TP347H samples were thicker and contained also a considerable amount of iron which indicates that a non-protective oxide was about to form, causing breakaway oxidation. The average increase in weight of the Sandvik Sanicro 25 test coupons after 1000 hours at 700°C was 0.028 mg cm-2. About the same increase in weight was measured for UNS30432. The value for NF709 was approximately 0.07 mg cm-2, for TP310HCbN about 0.18 mg cm-2 and for TP347H 0.23 mg cm-2. Figure 14 show the results from the test at 700°C, 1000h.

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Figure 14; Oxidation tests in aqueous steam, 700°C, 1000h

5.3 Hot corrosion Hot corrosion tests were carried out at 700°C (1300°F) for an exposure time of 3000 hours by Ansaldo ricerche s.r.i. [1]. Reference material was the austenitic alloy NF709. Test specimens of both NF709 and Sandvik Sanicro 25 were covered with ash and exposed for a gas flow during the test time. The ash composition was 5% Na2SO4, 5%

K2SO4, 30% Fe2O3, 30% Al2O3 and 30% SiO2 and the gas composition was 0.25% SO2, 3.5% O2, 15% CO2 and balance of N2. The samples were covered by ashes, which were renewed every 100 hour for the first 500 hours of testing and then every 500 hour until the end of the test. The mass change as a function of test time is shown in figure 15.

Figure 15; Weight change after descaling as a function of test duration

As shown in the graph, Sandvik Sanicro 25 has much lower corrosion rate than NF709. The corrosion morphology also differed between the two materials. Localized corrosion with some pits was found on Sandvik Sanicro 25 after longer exposure times whereas NF709 had a uniform corrosion front with a great loss of thickness. 6. Test installations Sandvik Sanicro 25 is tested in two units in Germany and one in Denmark. In August 2004, smaller tests were installed in the Dong Energy plant in Esbjerg and in the RWE plant in Weisweiler. In July 2005 this was followed by a bigger installation in the E.ON plant in Scholven Germany. Feedback so far is promising. A smaller test in a brown coal fired unit was started in June 2007 and a bigger installation in another plant is planned for the spring 2008.

7. Material approvals A PMA, Particular Material Appraisal (i.e. an approval for use in pressure vessels in the European community), for tubes made of Sandvik Sanicro 25 was issued in March 2007 (TÜV file n.o. 3537W137650). A vd TÜV-blatt is ordered and will be available during 2007. A UNS designation has been applied for and will likely be available in October 2007. 8. Summary The austenitic alloy Sandvik Sanicro 25 is developed by Sandvik Materials Technology for use as superheater and reheater tubes up to about 700°C (1300°F). The alloy has excellent strength at elevated temperatures, high oxidation resistance combined with high structural stability and good fabricability. The obtained test results indicate that Sandvik Sanicro 25 has higher creep strength than any other known commercial austenitic material. The ongoing creep testing indicate that the results only will be slightly lower than the original target of 100 MPa (14.5 ksi) at 700°C (1300°F) after 100 000 hours. References: [1] Sandvik Sanicro 25, A new material for ultrasupercritical coal fired boilers. R.Rautio, S.Bruce. [2] Oxidation av rostfria stål I vattenånga. J.Flyg, P.Viklund, R.Pettersson. 2007-06-21. KIMAB:s ref.nr: 708002. [3] Material and values developed in the German MARCKO DE2 program, ref. mail ru_husemann@hitachi-power.de, 2007-08-29.