advanced.qxd 6/14/04 2:26 PM Page 1 ADVANCED NICKEL ALLOYS FOR COAL-FIRED BOILER TUBING Boiler tubing for ultra-supercritical coal-fired power generation plants must maintain strength while resisting corrosion at high temperatures. Gaylord Smith* Lewis Shoemaker* Special Metals Corp. Huntington, West Virginia

ltra-supercritical coal-fired boilers can generate electric power with very low emissions even while burning high-sulfur coal, and can provide efficiency of 46 to U48%, compared with today’s ~41%. The challenge is to develop boiler tubing alloys that provide at least Incoclad 671/800HT tubing is produced by co-extrusion of a duplex billet. It pro- 100,000 hours creep strength at 750°C (1380°F)/100 vides the corrosion resistance of alloy 671 and the strength of alloy 800HT. MPa (14.5 ksi); and that provide coal-ash corrosion 48 resistance of less than 2 mm (0.079 inches) metal loss

in 200,000 hours. Only an alloy with aerospace 47 283 bar (3500 psi) USC steam strength properties coupled with the corrosion 272 bar (4000 psi) U.S. goal resistance of the best of the high-nickel solid solu- 306 bar (4500 psi) 46 340 bar (5000 psi) Increasing tion alloys could meet these new requirements. pressure The effect of improvements in plant efficiency is shown in Fig. 1. Raising steam temperature pro- 45 vides far greater benefits to efficiency than raising 44 steam pressure, as is clearly shown in this graph. +5.33%

Unfortunately, raising steam temperature disqual- Plant efficiency, % ifies even the most advanced ferritic tube steels at 43 temperatures much above 600°C (1110°F), because of their lack of strength and resistance to coal ash 42 corrosion. The best of the current austenitic solid- Current U.S. norm solution alloys enables raising the steam temper- 500 550 600 650 700 750 800 850 ature to 650°C (1200°F), but steam pressure is lim- Steam temperature, °C ited to about 295 bar (4350 psi). Resistance to coal-ash corrosion is also questionable for many of Fig. 1 —Increasing steam temperature improves efficiency more than increasing these alloys. pressure. Efficiencies are calculated using Gilbert/Commonwealth ASPEN model for supercritical steam cycles. This article covers the composition of advanced nickel alloys, and details the development of In- cobalt are added to confer additional solid solution conel alloy 740, which is designed for applications strengthening. These alloys, such as Inconel alloy in the very hottest section of the superheater. It also 617, are relatively easy to weld and generally do describes clad tubing and discusses several suc- not require post-fabrication heat treatment. cessful applications in power plants. However, these materials lack the creep strength for long life in service as an advanced ultra- Advanced nickel alloys supercritical superheater tubing alloy. Aluminum, Advanced nickel-base high strength alloys are titanium, and niobium are added to provide high- traditionally composed of nickel and chromium. temperature strength via precipitation hardening,. Elements such as molybdenum, tungsten, and Alloys such as Inconel alloy 718, Nimonic alloy *Member of ASM International 263, and Waspaloy are typical examples. ADVANCED MATERIALS & PROCESSES/JULY 2004 23 advanced.qxd 6/14/04 2:27 PM Page 2

300 300 ■ ■ 250 250 200 ■■ ■ 200 ■ ■ 150 ■■ ■ ■ 150 100 ■ mils/year ■ ■■ 100 50 ■

Metal loss, mils/year ■ ■ Maximum metal loss, ■ 0 50 ■■ -50 10 20 30 40 50 60 20 40 60 80 100 Chromium content, wt. % Chromium + nickel content, wt.% Fig. 2 —A. This graph shows chromium content versus wastage rate. (Logarithmic fit.) B. This graph shows chromium+ nickel content versus wastage rate. (Linear regression fit.) Table 1 — Nominal composition of Inconel alloy 671 and Incoloy alloy 800HT Alloy C Ni Cr Al Ti Cu Mn Fe Si 671 0.05 53.5 46.0 — 0.4 ———— 800HT 0.08 32.5 21.0 0.4 0.4 0.4 0.8 46.0 0.5

Where tubing must resist coal ash corrosion, in- as shown in Fig. 2. The best resistance is provided creased levels of chromium are mandated for most by alloys containing 50% chromium/50% nickel. nickel-base superalloys. The contribution of The wrought nickel alloy closest to this composi- chromium to coal ash corrosion resistance has re- tion is Inconel alloy 671, which typically contains cently been demonstrated in the study “Coal Ash 48% chromium and 52% nickel. Although this alloy Corrosion Resistant Materials Testing Program.” This offers excellent resistance to coal-ash corrosion, its study was conducted by the Babcock & Wilcox strength at elevated temperatures is not sufficient Company, the U.S. Department of Energy, and the for the application. Ohio Coal Development Office at the Reliant En- On the other hand, excellent strength at elevated ergy plant in Niles, Ohio. temperature is provided by Incoloy alloy 800HT To carry out the study, a test panel was placed (UNS N08811), an iron-nickel-chromium alloy. It in the superheater section of this 120 megawatt sub- has served successfully as a boiler tube alloy under critical plant. The plant burns coal containing 3 to moderately corrosive conditions. However, its 3.5% sulfur, and operates at 593°C (1100°F). The chromium content of 21% is not sufficient to resist panel was evaluated after 21,200 hours of opera- fuel ash corrosion under the conditions anticipated tion (11,288 hours of exposure at full temperature). for future ultra-supercritical boilers. Figure 2 plots the metal loss in mils/year vs. Obviously, an alloy offering the corrosion resist- chromium content in 2A, and vs. chromium-plus- ance of alloy 671 along with the strength of alloy nickel in 2B. Chromium is a strong indicator of life 800HT would be ideal for this service. Incoclad expectancy, as shown by the results of 2A. Figure 671/800HT was designed by Special Metals to offer 2B suggests that an even better predictor of service just this combination of properties. life may be the sum of chromium plus nickel. Incoclad tubes are produced by co-extrusion of a duplex billet. The process creates a metallurgical Clad tubes bond between the Incoloy alloy 800HT substrate and Higher chromium content enables alloys to re- the Inconel alloy 671 cladding. The clad tube pro- sist corrosion better than those with less chromium, vides good heat transfer properties and sufficient ductility for cold fabrication. The nominal cladding Initiatives to raise efficiency thickness is typically 1.9 mm (0.075 in). The compo- Historically, supercritical steam boilers have typically delivered steam sitions of both alloys are shown in Table 1. to the turbine at temperatures up to 566°C (1050°F) and pressures up to The duplex product can be formed and ma- 238 bar (3500 psi) with fuel efficiency between 42 and 43%. Notable efforts to improve efficiency began in the late 1950s at American Electric Power chined by conventional techniques. The tubing is to raise the conditions to 621°C/310 bar (1150°F/4500 psi) and by the readily weldable, and welding products are avail- Philadelphia Electric Company in 1962 to 649°C/340 bar (1200°F/5000 able with strength and corrosion resistance com- psi). Unfortunately, materials issues and other problems ultimately forced parable to those of the tubing. these plants to scale back their operating conditions. For the next three For installations constructed according to the decades, the best state of the art supercritical boiler stood at 593°C/310 rules of the ASME Boiler and Pressure Vessel Code, bar (1100°F/4570 psi). the design stresses specified by the code for Incoloy By mid-to-late 1990, utilities, their construction companies and govern- alloy 800HT should also be used for Incoclad ments, decided to make the necessary developments to achieve a 700°C 671/800HT. However, tube wall thickness re- (1290°F) ultra-supercritical technology for coal-fired power plants. Three no- quirements should be based only on the thickness table initiatives that have developed are THERMIE AD700 and MARKO in Europe in the late 1990s and the U. S. DOE Vision 21, “Material Develop- of the Incoloy alloy 800HT substrate. ment and Qualification” in 2002. The target of the European projects is a 400-1000 MW, 700°C/350 bar (1290°F/5150 psi) power plant; the target Inconel alloy 740 for the DOE project is a more aggressive 760°C/ 380 bar (1400°F/5580 psi). With the technical challenge of the European Thermie AD 700 project requirements in mind, Spe- 24 ADVANCED MATERIALS & PROCESSES/JULY 2004 advanced.qxd 6/14/04 2:27 PM Page 3

Incoclad Case Study: Philadelphia Electric Co. Philadelphia Electric Company (PEC) Eddystone Station, Unit 2, is a 350,000 kW unit with double reheat. After 37,000 hours of op- eration, up to 35% of the wall had been lost in the 9% Cr-1% Mo steel reheater tubes. This forced a downrating of the unit, with steam temperature lowered by 28°C (50°F) from the start-up throttle of 565°C (1050°F). Searching for a material that would resist the environment, PEC installed Incoclad 671/800HT tubes for evaluation. Tubes were installed in an area where the risk of corrosion was highest, a location directly struck by the flue gas. A wide range of coal was burned during the exposure period. The average sulfur content was 2.44%. The coal mixture exhibited an average ash content of 10.76% and high metallic contaminants (3100 ppm iron, 330 ppm sodium, and 1190 ppm potassium). Laboratory evaluation after nine years of service verified that the tubing had performed well. The Inconel alloy 671 cladding was essentially unaffected by coal-ash corrosion. No deterioration of the metallurgical bond between the cladding and the substrate was observed. The alloy 800HT had shown excellent resistance to steam-side corrosion and had retained good tensile properties, stress-rupture strength, and ductility. Furthermore, it was not sensitized to intergranular corrosion even after the long exposure at 595°C (1100°F). Table 2 — Nominal composition of Inconel alloy 740 CNiCrMoCoAlTiNbMnFeSi 0.03 Bal 25.0 0.5 20.0 0.9 1.8 2.0 0.3 0.7 0.5

cial Metals Corporation was asked to develop a law average curve shown in Fig. 2 predicts that a nickel-base alloy for the very hottest section of the stress of 145 MPa (21 ksi) is required to produce superheater. creep rupture in 100,000 hours at 750°C (1380°F). Nimonic alloy 263 was selected as the starting Test specimens were annealed 1150°C (2100°F)/30 point. Variations of the gamma-prime hardener el- minutes/water quenched and aged at 800°C ements were explored to assure that the strength (1470°F)/16 hours/air cooled. target was met for 750°C/100 MPa (1380°F/14.5 To determine the corrosion resistance of the ex- ksi) and 100,000 hours. This was achieved with the perimental Inconel alloy 740 compositions, flue gas composition shown in Table 2. and coal ash compositions were chosen as shown A Larson-Miller parameter plot for Inconel alloy in Fig. 5. This combination produces a Type II hot- 740 data is presented in Fig. 3. Comparative 100,000 corrosion mode of attack when the alkali trisulfates hours creep rupture data with other typical steam are molten between about 600°C and 750°C (1110°F boiler materials is shown in Fig. 4. The fitted power and 1380°F). Attack is reduced when the trisulfates are solid below 600°C (1110°F). Attack is also re- 1000 duced above 750°C (1380°F), as the trisulfates tend

■ to evaporate upon formation. ■ ■ Figure 5 depicts both a linear and a parabolic ■■■ ■■ projection for the metal loss of Inconel alloy 740 in ■ ■ ■ ■ ■ ■ ■■ ■ 200,000 hours based on corrosion data at 700°C Stress, MPa ■■ ■ ■ ■ ■ 100 Incoclad Case Study: United Kingdom 31 32 33 34 35 36 Larson-Miller parameter, x 1000 In the United Kingdom, the Central Electricity Generating Board (CEGB) operated a trial reheater using Incoclad co-extruded tubes in a Fig. 3 —Stress-rupture data compilation for hot-rolled, 550 MW power station for over six years. The tubes exhibited trouble- solution-annealed and aged Inconel alloy 740 bar and plate, free service. In previous service, stainless steel tubes corroded at the rate showing a power law fit. Larson-Miller parameter is defined of 3.5 mm/y (0.04 in/y) due to the particularly aggressive fuel. The as T x [30 + log (tr)], where T = temperature in Kelvins, and tr Incoclad tubing was found to have corroded at only one fortieth of this = time to rupture in hours. rate. This assessment led to a full replacement of 32 x 24 chevron bends on the reheater with Incoclad tubes. Incoclad 617/800HT tubes were installed in a superheater unit oper- Dashed line is projected value ating at 570°C (1055°F) and 14.8 MPa (2,150 psi) under conditions that pro- 300 Inconel 740 Nickel-base duced fuel ash corrosion in previous boiler tubes. The boiler, rated at 540,000 ■ superalloys kg/h (1,200,000 lb/h) was fired by high-sulfur pulverized coal. After 32 Standard 617 ■ Nimonic 263 months, a sample was removed and evaluated. ■ The cladding thickness remained uniform, circumferentially and longi- ■ tudinally. Cladding exposed on the fire side showed only minimal corrosion, ■ the most severe attack being pitting to a depth of 0.05 mm (0.002 in). The 100 back side of the tube was essentially free of coal-ash corrosion and pitting. 80 The Incoloy alloy tube interior showed only a uniform, adherent oxide 60 about 0.05 mm (0.002 in) thick. Ferritic steels A clad tube sample from the same installation was examined after 66 (9-12Cr) Austenitic months. Again, the cladding thickness remained uniform with only su- 40 alloys perficial gas impingement erosion on the fire side. The worst attack Creep-rupture stress after 100,000 hours, MPa showed fissures in the cladding to a maximum depth of about 0.30 mm 550 600 650 700 750 800 Temperature, °C (0.012 in). However, about 1.5 mm (0.060 in) of cladding still remained unattacked beneath the fissures. The inside of the tubing was unchanged Fig. 4 — Creep-rupture strength for various steam boiler from the earlier review and evaluation. alloys during 100,000-hour test. ADVANCED MATERIALS & PROCESSES/JULY 2004 25 advanced.qxd 6/23/04 3:35 PM Page 4

(1290°F) to 4000 hours. A linear projection equates 10,000 to a metal loss of about 1.3 mm (0.051 in), and par- ■ Inconel alloy 740 abolic projection leads to a metal loss of about 0.25 Linear projection Parabolic projection mm (0.01 in), both in 200,000 hours. Figure 6 shows weight change (in mg/cm2) in 1000 steam oxidation at 750°C (1380°F) for times up to 10,000 hours. Measurement of the scale at the end of the test revealed a surface film of about 1.22 mi- crons thickness. 100

Maximum attack, microns Future boilers ■ • Efforts to increase the efficiency of power gen- ■ ■ eration boilers will undoubtedly result in more de- 10 manding conditions within the units. Thus, ad- 100 1000 4000 10,000 100,000 200,000 1,000,000 vanced alloys will be required to meet the rigors of Time, hours this service. Fig. 5 — Simulated coal-ash corrosion at 700°C (1290°F) for Inconel alloy 740. Gas • Inconel alloy 740 exhibits the properties required is N2 –15%CO2 – 3.5%O2 - 0.25%SO2 at 700°C. Salt coating is 5%Na2SO4 – for developmental boilers designed to operate at sig- 5%K2SO4 – 90%(Fe2O3-Al2O3-SiO2 in 1:1:1 ratio). nificantly higher pressures and temperatures. • Incoclad 671/800HT tubes have proven ca- 2 2 pable of resisting fuel ash corrosion and providing ■ Inconel 740 long term service (20+ years) in boilers fueled with 1.5 low-grade coal. ■ ■ ■ ■ 1 ■ ■ For more information: Lew Shoemaker is Industry Man- ■ ■ ■ ager at Special Metals Corporation, 3200 Riverside Drive, 0.5 ■ ■■ Huntington, WV 25705-1737; tel: 304/526-5664; e-mail: ■■ Weight change, mg/cm [email protected]; Web site: www.specialmetals. 0 2000 4000 6000 8000 10,000 com. Test duration, hours Inconel, Incoloy, Nimonic, Incoclad, and 800HT are trade- Fig. 6 —This graph shows steam oxidation of Inconel alloy 740 marks of the Special Metals Corporation group of com- at 750°C (1380°F). panies.

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