The Effect of Heat Flux on the Steam Oxidation Kinetics and Scale Morphology of Low Alloy Materials

Transcription

1 The Effect of Heat Flux on the Steam Oxidation Kinetics and Scale Morphology of Low Alloy Materials Tony Fry 6th International Conference on Advances in Materials Technology for Fossil Power Plants, La Fonda, Santa Fe 30 th August 3 rd September 2010

2 Overview Introduction Air blown heat flux tests Steam cooled heat flux tests Comparison with service components Summary & Conclusions

3 Introduction NPL is the UK s national measurement laboratory and represents the pinnacle of measurement science in the UK. Government funded research into new and better measurement techniques Solve measurement problems for industry Advanced Engineered Materials Innovative Metals Engineering Liquid Metal Processing, Residual Stress and Strain, High Temperature Degradation Performance of Engineered Surfaces Nano Mechanics, Tribology, Powder Route Materials (PRM), Microscopy Standardised measurement methods Steam Oxidation Fireside Corrosion Mechanical testing Modelling

4 Introduction Two main drivers for power generation Emissions Costs Improved efficiency, reliability, availability and maintainability Improved material and material systems UltraGen: A Proposed Initiative by EPRI to Advance Deployment of Ultra-Supercritical Pulverized Coal Power Plant Technology with Near-Zero Emissions and CO 2 Capture and Storage, J. Wheeldon, D. Dillon, and J. Parke, EPRI, Conf. On Adv Mat. Tech for fossil Power Plants

5 Improved materials need validation Mechanical properties Heat Transfer Processing Microstructural characterisation Corrosion Corrosion Simplistic lab based isothermal tests Typically on coupons Exposed to gaseous atmospheres

6 Effect of a temperature gradient a x10-4 (m 2 s -1 ) Temperature ( C) As-received 200h 400h 600h 800h 1000h

7 More realistic test method Effect of heat flux has been observed experimentally and is often included in models There is a requirement therefore for a more complex test method for the evaluation of material which includes this parameter Effect of heat flux on iron oxide deposition rate with different water temperatures (iron concentration being the same). Notes: C; x 316 C Ref: O. Povarov, T. Petrova et at, Deposition in Boilers: Review of Soviet and Russian Literature, EPRI, Pali Alto, CA:

8 Heat flux test air blown 500 C +50 mm +25 mm Centre -25 mm -50 mm Hotter Cooler 540 Heat Flux Pipe Profile 520 Compressed Air Temperature, C Outer Inner Position, mm from centre

9 Performance trials AISI mm OD tube with 3 mm wall thickness. Temperature gradient of 40 C was achieved using compressed air as the cooling medium and a 3 kw wire wound furnace. Assuming a thermal conductivity of 16.3 W m -1 K -1, this is equivalent to a conductive heat flux of 217 kw m -2. In coal fired boilers heat fluxes of around 400, and 200 kwm -2 are found in furnace section evaporator tube walls and Superheater walls respectively and even higher values can be expected in the turbine blades [Cutler et al] Temperature, C Outer Temp 460 Inner Temp Time, h Cutler, A. J. B. and Raask, E., External Corrosion in Coal-Fired Boilers: Assessment From Laboratory Data, Corrosion Science, Vol. 21, No. 12, pp, , 1981.

10 Effect of Heat Flux on 15Mo3 Isothermal tests Specimen size (mm) 10 x 10 x 3 Test duration (h) 100, 300, 500, 1000, 2000 Temperatures ( C) Specific Mass Change, mgcm C 475 C 500 C 450, 475, 500 Heat Flux tests Specimen size (mm) 700 x 39 OD x 29 ID (5 mm wall thickness) Test duration (h) 500, 1000, 2000 Temperatures ( C) 450, 480, 500 Scale Thickness (mean), microns Sqrt Time, h 1/ Exposure Time, h 450 C 475 C 500 C Heat Flux 500 C Heat Flux 480 C Heat Flux 455 C

11 Comparison of scales (1000h)

12 Electron Back Scatter Diffraction Maps obtained with 0.1µm step size 1nA current used to minimise the probe size Hough resolution of 80 used to improve indexing accuracy Phase identification was poor but orientations indexed accurately

13 EBSD Images Air Cooled (500 C 2000h) Isothermal Average Width, µm Metal/Oxide Interface Intermediate Layer Oxide/Air Interface Isothermal (2000 h) Heat Flux (2000 h) NA 0.4 to to µm Heat Flux

14 It is a consequence of a thermal gradient or something else? Geometry Isothermal samples were flat heat flux tests are on tubes Additional isothermal test on tube section did not result in banded structure Temperature, C Thermal fluctuations Thermal fluctuations during the test caused different grain growth rates The temperature was monitored during the test and was controlled to ± 2 C 460 Inner Temp Time, h Outer Temp

15 Heat flux rig steam cooled Control Thermocouple Steam out Steam Out 50 mm Furnace Condenser 50 mm Water In Sealed tubular sample Water in Reservoir Pump Mean Oxide Thickness, microns Isothermal Air-cooled Steam-cooled

16 Mechanism? Increase in rate is due to a gradient in the point defect concentration in the oxide, as this varies as a function of T (Covino). Hence at higher temperatures one might expect there to be more vacancies which would biase the atomic jumps. This mechanism is dependant on point defects and so relies on lattice diffusion being the dominant mechanism. However, the finer grain structure does support the dominance of grain boundary diffusion. However, neither of these account for the actual formation of this fine-grained structure, and further fundamental studies are needed to address this.

17 Effect of Heat Flux Increase in oxidation rate Change in oxide morphology Accelerated by the presence of steam Does the laboratory test reproduce the structures observed on service exposed materials?

18 Service Exposed Sample Two samples of 2¼Cr1Mo were received from E.On Engineering (UK) Sample A - from a primary platen superheater operated for 53,000 hours, with a design temperature of C 160 µm Sample B - from a final stage reheater operated for 88,000 hours 25µm

19 Primary platen superheater, 53,000 hours, C 25µm 5µm

20 T23 Heat Flux Tests T23 tube ~37 mm OD, 5 mm wall thickness, 500 mm long Furnace control temperature 600 C, internal temperature set point 550 C Thermocouples at the centre section and ± 25 mm either side Temperature, C Distance from centre of tube length, mm

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22 Heat Flux Tests Steam Cooled 700 Temp v. Elapsed Time Temp, C Top of tube Furnace control Bottom of tube Pump control Elapsed Time, Hrs

23 Variation with Temperature 160 Average Total Oxide Thickness, microns Temperature, C

24 T23 Oxide Thickness Average Total Oxide Thickness, microns Isothermal Flowing Steam Steam Cooled Heat Flux Time, h

25 550 C Flowing Steam 40 microns 300h

26 Variation with Temperature 160 Average Total Oxide Thickness, microns Temperature, C

27 Banding? EBSD measurements have been inconclusive 1000h exposure complete metallography underway

28 Summary Simple test method has been developed Comparison with isothermal tests show Increased oxidation rate Modified scale morphology Mechanisms described in the literature tend to favour the effect of a thermal gradient on point defects 15Mo3 results suggest the dominant mechanism could be grain boundary diffusion Initial T23 results have not shown evidence of the banding seen on 15Mo3 and service exposed 2.25Cr steel