10.1002==9781118495223.ch72

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Cast Alloys for Advanced Ultra Supercritical Steam Turbines Gordon R. Holcomb, Ping Wang, Paul D. Jablonski, and Jeffrey A. Hawk Surface Stability of Materials in High-Temperature Aggressive Environments, Vail, CO, May 16-20, 2010
Outline Ult S iti l St T bi Introduction • Ultra Supercritical Steam Turbines • Research Aims Results • Casting and Homogenization • Oxidation Behavior Summary 2
Impact of Advanced Coal Generation on CO 2 Emissions CO 2 Emissions Improved Plant Efficiency Efficiency 3 EPRI, 2007
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Increasing Efficiency Each 1% increase in efficiency US-DOE Advanced Power Systems: Each 1% increase in efficiency eliminates ~1,000,000 tons of CO 2 emissions over the lifetime of an 800-MW plant 46%-48% efficiency from coal generation Steam condition: 760 °C - 35MPa ~ 5ksi ements 800 MW plant subcritical ncy Improve subcritical 540 °C-14.5MPa 35% eff. mature t h l current market introduction 600 °C-28MPa Efficien Subcritical: < 22 MPa Supercritical (SC): > 22.1 MPa, 538 to 565 °C Ultra Supercritical (USC): 565 to ~675 ° C (advanced ferritic & austenitic steels required) technology 4 Adapted from: Viswanathan, et al , 2005 and Swanekamp, 2002 Ultra Supercritical (USC): 565 to 675 C (advanced ferritic & austenitic steels required) Advanced Ultra Supercritical (A-USC): > ~675 ° C (nickel-base superalloys required)
Technological Issues There is an immediate and continuing need for increased power production. Increases in Temperature and Pressure increase efficiency and decrease CO 2 production along with other efficiency and decrease CO 2 production along with other pollutants. Hi h T t d P l t Higher Temperature and Pressure place greater demands upon the Materials. Large castings are required for some components— many technical issues. 5
Example Components • Castings – 1-15 tons – Up to 100 mm in thickness V l B di 6 Valve Bodies Turbine Casing Courtesy Alstom
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Challenges for A-USC Castings Traditionally wrought alloys are being considered due to proven weldability in thick sections • Large pour weights (1 15T) • Large pour weights (1-15T) • Thick section components • Slow cooling rates • Segregation prone alloys Our approach is to examine a suite of traditionally wrought Ni- based superalloys cast under conditions designed to emulate the full sized casting • A computationally optimized homogenization heat treatment was developed to improve the performance of these materials Steam oxidation resistance • Compare the steam oxidation behavior of cast versions of candidate Ni- 7 based superalloys with their wrought counterparts
Alloys Under Consideration Solid Solution Age Hardenable H230 N105 IN617 H263 IN625 H282 IN740 8
Our Model Casting Geometry The actual component is nominally 4in thick Our casting is nominally 4in in and “infinite” in the other directions. diameter and 4-5in tall. 9
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“Enhanced” Slow Cooling Casting layout shown schematically in cross Graphite Mold schematically in cross section on the left A permanent graphite ld d Thi Ingot mold was used. This mold was surrounded by loose sand such that the top of the casting was top of the casting was below the sand line This attempts to emulate Loose Sand the “semi-infinite” plate model of the turbine casing 10
First Ingot Chemistries C Cr Mo Co Al Ti Nb Mn Si B W Nimonic 105 0.15 14.85 5.00 20.00 4.70 1.10 0.50 0.50 0.05 0.16 14.61 5.02 20.04 4.43 1.10 0.51 0.51 0.05 Aims Results 0.16 14.61 5.02 20.04 4.43 1.10 0.51 0.51 0.05 Haynes 230 0.120 22.00 2.00 0.35 0.70 0.50 14.00 0.12 21.59 2.01 0.37 0.69 0.50 13.91 Haynes 263 0.070 20.00 5.80 20.00 0.35 2.10 0.50 0.35 Results 0.07 19.68 5.74 19.89 0.40 2.04 0.50 0.34 Haynes 282 0.070 19.50 8.50 10.00 1.50 2.10 0.25 0.15 0.005 0.07 19.22 8.48 9.84 1.44 2.08 0.24 0.15 0.01 IN617 0.120 22.00 9.00 12.50 1.10 0.30 0.50 0.50 0.12 21.73 8.96 12.35 1.04 0.31 0.50 0.49 IN625 0.070 21.00 9.00 0.10 0.10 3.60 0.50 0.35 0.07 20.71 8.92 0.15 0.089 3.58 0.49 0.34 IN740 0.030 25.00 0.50 20.00 1.30 1.50 1.50 0.30 0.30 Fe: 0.70 0.04 24.71 0.50 20.03 1.24 1.48 1.50 0.30 0.31 0.57 11
N105—Solidification Equilibrium Solidification Solidification Note 1200 °C Note 1200 °C partial liquid fraction Segregation Induced Melt Depression 12
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Weight Fraction Cr in FCC Phase—IN740 The weight fraction of Cr in the FCC phase Cr in the FCC phase changes with temperature in IN740. This is on a Scheil This is on a Scheil calculation basis. The irregularity in the curve occurs at a second phase formation temperature. 13
Weight Fraction Nb in FCC Phase—IN740 The weight The weight fraction of Nb changes in the FCC phase with temperature in IN740. This is on a Scheil This is on a Scheil calculation basis. 14
Critical Microstructural Features Secondary Dendrite A Arm Spacing A very useful measurement is the secondary dendrite arm spacing (sdas) (sdas) Solidification modelers use sdas to estimate the local cooling (solidification) rate We used sdas as a characteristic diffusion distance to base 15 homogenization heat treatments: d=0.5x(sdas)
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H282 Secondary Dendrite Arm Spacing Columnar zone ~65 μ m ~46 μ m zone Equiaxed zone ~92 μ m ~82 μ m 16
N105—1100 °C Heat Treatment Since the largest measured sdas<100 μ m; we used 50 μ m as the characteristic diffusion distance characteristic diffusion distance 1 2 3 4 ( seconds 1 2 3 4 ( seconds 1 1 2 3 3 4 2 3 4 f f °C (m) (m) 17 Neither Mo or Ti are fully homogenized even after 22.4h at 1100 °C with this basic heat treatment
Section Summary: As-Cast Profiles The refractory elements W, Mo, and Nb do not homogenize after ~22h/1100 °C Significant segregation of the second phase strengthening elements Al, Nb and Ti were observed in many alloys…to the point that ½ to ⅔ of the casting would be considered “lean” In some cases, Cr poor regions are predicted Significant Co segregation was observed in some alloys Significant partitioning of Mn and Si to the interdendritic region was predicted. This result suggests that a turn down in the levels of these elements may be beneficial (e g for welding) 18 these elements may be beneficial (e.g., for welding)
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N105—Homogenization Heat Treatment Comparison Comparison 1 2 3 4 1 1 2 3 4 2 3 4 4 Isothermal at 1100 °C 1100 °C/10,000s+1200 °C/remaining time Patent Pending (m) (m) 19 Patent Pending Metall. Trans. B, 40B , (2009) 182.
Nimonic 105 Qualitative Confirmation of the Effectiveness of the Homogenization Heat Treatment As-Cast Homogenized 20
800 °C Creep Results H282 ksi) 100 Cast alloy results: data points Average wrought f li Stress (k performance: line LM = T[K](C[20]+log(t)) 20000 21000 22000 23000 24000 25000 26000 27000 10 21 LM = T[K](C[20]+log(t))
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Solid Solution Alloys All 800C H230 si) 100 H230 H263 IN617 IN625 Stress (ks S 20000 21000 22000 23000 24000 25000 26000 27000 10 22 LM = T[K](C[20]+log(t))
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Gamma Prime Formers All 800C N105 si) 100 N105 H282 IN740 H263 Stress (ks S 21000 22000 23000 24000 25000 26000 10 23 LM = T[K](C[20]+log(t))
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Alloys Under Consideration for Steam Oxidation Resistance Oxidation Resistance Solid Solution Age Hardenable H230 N105 H230 N105 IN617 H263 IN625 H282 IN740 Isothermal tests in deaerated steam Isothermal tests in deaerated steam 760 and 800 °C 60 a d 800 C 250 hour cycles for 1000 and 2000 total hours 24 hours
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Oxidation Results H263 No significant differences between cast & homogenized and wrought 25
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Oxidation Results H282 No significant differences between No significant differences between cast & homogenized and wrought 26
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Oxidation Results N105 Significant differences between cast & homogenized and wrought Wrought versions show much less mass gain with time 27
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Exposure in Steam for 1000 hr—H263 Cast and Homogenized Wrought Cast and Homogenized Wrought C E t l 760 ° C External chromia scales Internal 800 ° C Internal oxidation of Ti and Al was more d i 8 pronounced in the C&H alloys 28
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Exposure in Steam for 1000 hr—H282 Cast and Homogenized Wrought C Over much the same as in H263 760 ° C as in H263 External chromia scales 800 ° C Internal oxidation of Ti and Al was more 8 Ti and Al was more pronounced in the C&H alloys 29
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Exposure in Steam for 1000 hr—N105 Cast and Homogenized Wrought Two types of structures were found 760 ° C found • One with a chromia external scale and internal oxidation of Al internal oxidation of Al and Ti • One with a very thin alumina external scale— f d ti f th 800 ° C found on sections of the wrought alloys at both temperatures Al i l 8 Alumina scales are much more protective 30
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Scaling Behavior External Scale and Internal Oxidation Behavior, 760 ° C Alloy T hr External Scale Internal Oxidation Thickness μm std % Thickness μm std % Maximum μm C 1000 2.81 ± 0.63 22% 4.70 ± 4.08 87% 28.20 Haynes 263 Cast 1000 2.81 0.63 22% 4.70 4.08 87% 28.20 2000 3.34 ± 0.97 29% 9.79 ± 5.35 55% 42.62 Wrought 1000 2.79 ± 0.37 13% 7.11 ± 3.49 49% 16.44 2000 4.36 ± 0.64 15% 5.18 ± 2.68 52% 23.94 Haynes 282 Cast 1000 2.54 ± 0.43 17% 6.92 ± 3.73 54% 34.22 2000 4.53 ± 1.36 30% 6.26 ± 4.73 76% 38.00 Wrought 1000 1.13 ± 0.35 30% 1.81 ± 1.07 59% 10.21 2000 3.00 ± 0.55 18% 5.09 ± 2.20 43% 25.13 2000 3.00 0.55 18% 5.09 2.20 43% 25.13 Nimonic 105 Cast 1000 1.52 ± 0.50 33% 5.95 ± 0.69 12% 9.90 2000 2.86 ± 0.81 28% 10.09 ± 1.90 19% 17.58 Wrought 1000 0.39 ± 0.47 119% 0.60 ± 1.22 201% 9.25 Wrought 2000 0.98 ± 0.86 89% 2.46 ± 2.75 112% 14.86 H263 & H282—External scaling similar between C&H and Wrought H263 & H282—Internal scaling more pronounced in C&H N105 Wrought clearly shows partial alumina coverage 31 N105 Wrought clearly shows partial alumina coverage Alumina coverage shrinking with time as shown by scaling kinetics
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Microstructure after 1000 hr at 800 °C—H263 ogenized Light precipitates are Mo-rich t and Homo carbides Dark Cast Dark precipitates are Cr-rich carbides ought More gb precipitate Wro precipitate coverage in C&H alloy 32
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Microstructure after 1000 hr at 800 °C—H282 mogenized Light precipitates are Mo-rich carbides t and Hom Large dark precipitates are Ti- i h bid Cast rich carbides Smaller dark ought precipitates are Cr-rich carbides M b Wro More gb precipitate coverage in C&H alloy 33
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Microstructure after 1000 hr at 800 °C—N105 mogenized Light precipitates are Mo-rich carbides t and Hom Dark precipitates are Ti and Cr-rich carbides in C&H Cast alloy Dark precipitates are Cr-rich ought are Cr rich carbides in wrought alloy More gb Wro More gb precipitate coverage in C&H alloy 34
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Summary I The development of high creep strength and steam oxidation resistant cast alloys for use in A-USC steam turbines is required to meet the need of some of the turbines is required to meet the need of some of the large components that comprise the turbine A computationally optimized homogenization heat treatment was developed to improve the performance of these materials Haynes 263, Haynes 282, and Nimonic 105, were l t d b d d t ll i t selected based on good cast-alloy creep resistance for further examination in terms of steam oxidation resistance 35
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Summary II Haynes 263 and Haynes 282 • External chromia scale (both the C&H alloys and the wrought alloys) • Internal oxidation of Al and Ti (both the C&H alloys and the wrought alloys) • The overall mass gain and parabolic kinetics were similar • The depth of internal oxidation was greater in the C&H alloys • More Mo-rich and Cr-rich carbides were found along the grain boundaries of the C&H alloys C&H alloys • Ti-rich carbides were found along grain boundaries of the C&H Haynes 282 alloys Nimonic 105 • Wrought alloys exhibited lower oxidation kinetics than the C&H alloys • Some of the surface of the wrought alloys was covered by a very protective alumina scale Wh l i t t h i l t ith i t l • Where alumina was not present, a chromia scale was present with internal oxidation of Al and Ti • The fraction of the surface protected by the alumina scale decreased with time • C&H grain boundaries contained Mo-rich carbides, Ti-rich carbides, and Cr-rich bid 36 carbides • Only Cr-rich carbides on the wrought Nimonic 105 grain boundaries
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