Experiences on Aluminising of Strip Components for PFBR Applications

1. Experiences on Aluminising of Strip Components for PFBR Applications G. Srinivasan, V.Shankar*, A.K. Bhaduri Materials Technology Division Indira Gandhi Centre for Atomic Research, Kalpakkam (* formerly with MTD, IGCAR) WS&FT, IPR-Gandhinagar

2. Aluminising • Surface modification process • Layer of NiAl intermetallic formed at 1123–1373K • Aluminide Coatings • Very high & stable hardness • Excellent resistance to oxidising environments • Used in Turbine blades, aircraft engine components • Attractive for wear resistance • High resistance to impact fretting under flow-induced vibration of tubes in liquid sodium – water steam generators • Required for SG tube support strips • Nickel Alloy 718 (53Ni-19Cr-18Fe-5Nb-3Mo) • Excellent compatibility with liquid sodium • Most commonly employed coating process • Pack cementation

3. Requirement for PFBR • NiAl coating • Thickness: ~80 micron • Minimum: 50 micron • Resistant to self-welding in flowing liquid sodium • Oxygen: ~0.5 ppm • Hardness: 900-1000 VHN • Chemical stability in sodium • Between aluminised Inconel 718 & Cr-Mo ferritic steel tubes • Coefficient of friction = 0.3 • Minimum damage to tubes after large number of testing cycles

4. Available Aluminising Processes • Slurry spraying, brushing, dipping etc. followed by high temperature diffusion, electrolysis • Pack cementation • Treated at 1123–1323 K in a Pack consisting of • Al source: Ni-Al, Ti-Al, or Cr-Al • Activator: Halide • Inert filler: Alumina • Limitations • Handling of large quantities of alumina & metal powders • Long furnace time cycles & inherently reduced throughput • Vapour phase aluminising • Largely eliminates limitations of Pack Cementation process • Requires specialised vacuum furnaces & fixtures • Need for alternate process • Both processes involve exposure of operators to corrosive halide activators (environmental hazard)

5. Thermal Spray – Diffusion Process • Process • Molten / semi-molten particles applied by impact on surface • Diffusion treatment in vacuum • Formation of Aluminide coating involves • Melting of Aluminium • Its reaction with Ni-Fe base alloy • Results in formation of the B2 phase • Major advantages • Can be used to form coating e.g. only on the inner bearing surfaces • No masking required for areas where coating is not desired • Much lower cost & Higher productivity • Environmentally clean • Steps in our Process Development • Pilot-scale aluminising using pack cementation process • Pilot-scale development of thermal-spray–diffusion process • To match properties of coating by pack cementation process • Industrial-scale Technology demonstration • Aluminising of 1100 corrugated strips using thermal-spray–diffusion process.

6. Step 1:Pack Cementation based Aluminising • Process used • Pre-purging of argon for 1 h before loading retort boxes in furnace at 873 K • Argon flow maintained during entire process • To avoid excessive generation of fumes • AlF3 used instead of NH4F • Does not affect aluminising kinetics • XRD analysis of coatings • Major phase present: NiAl-type • Inter-substitution of Fe & Ni • Structure: NiAl (B2 structure) • ~ 20 a/o of Fe & Cr substituted in nearly equal amounts in Ni sites of B2 structure

7. Aluminising of Flat Strips of Nickel Alloy 718 using Pack Cementation Process • Uniform coating thickness • ~50 micron • Coatings showed features typical of low-activity process • Reaction zone • Cr-rich interlayer • Hardness • Un-aluminised Ni alloy strip: 303–315 VHN • Nickel aluminide layer: 860–990 VHN

8. Step 2: Development Trials for Thermal-Spray–Diffusion Process • Steps involved • Degreasing & Grit blasting (using alumina grits) • Standardising of Procedures • Spraying of Aluminium (commercial grade aluminium wire) • Optimising spraying parameters • Diffusion heat treatment(in Vacuum) • Optimising temperature (1223–1323 K) & time (1-2h) • Distortion removal wherever necessary

9. Aluminising by Thermal-Spray–Diffusion Process • XRD of aluminised coating • NiAl phase • Nb3Al • Coating consists of 2 layers with similar microstructure • Separated by discontinuous layer of intermetallic compounds containing Cr, Nb & Mo that are insoluble in the NiAl • Coating thickness: 90 microns • Variation: within 20 micron • Hardness • Un-aluminised substrate: 290–305 VHN • Aluminide layer: 870–1030 VHN • Marginally higher than that obtained by pack cementation

10. Aluminide Coating Growth Mechanism • Coating growth from sprayed Al complex • Initially, rapid reaction & inward diffusion of Al • Outward diffusion on Ni close to substrate interface • Stoichiometry shifts to Ni-rich coating • Diffusion barrier layer forms • Contains Cr, Nb, Mo – insoluble in NiAl

11. Step 3: Aluminising of Corrugated Strips Using Thermal-Spray–Diffusion Process • Results of aluminising trials with flat strips used to optimise aluminising procedure • Procedure optimisation with 100% inspection • Random inspection for dimensional checking on production strips • 100% inspection on qualification coupons • Optimised procedure implemented for aluminising actual components made of corrugated strips • All of 1100 corrugated strips coupons aluminised • In 3 batches used fabrication of Technology Development Steam Generators

12. Aluminising byThermal-Spray–Diffusion Process • Developed in collaboration with industrial partner M/s G&M, Chennai • Advantages • Uniform 80  20 micron thick NiAl coating • Very low cost compared to pack cementation coating • 10 times more productive than pack cementation process • Low cycle times • Line-of-sight – no need for masking unwanted areas • Embedment of pack particles eliminated

13. Aluminising byThermal-Spray–Diffusion Process • 1100 strips aluminised as part of PFBR technology development • 350 strips aluminised for SGTF SG • 9500 strips being aluminised done for PFBR SG for BHAVINI by M/s G&M-Chennai • Strips size: 180-890 mm • Process now under Patenting • A Process for Producing Body Centred Cubic (B2) Nickel Aluminide (NiAl) Coating of Controlled Thickness on Nickel-base Alloys, PCT/IN07/00514

14. On-going Developments on Aluminising for PFBR • NiAl coating on ferritic & austenitic SS • Ni content < 1% in mod. 9Cr-1Mo steel • Ni content ~ 12% in austenitic SS • Methodology • Enrichment of substrate with Ni • Optimising • Coating composition & properties • Parameters for thermal spraying & diffusion heat treatment (temperature & time) • Challenges • Coating free of diffusion barrier • Cr-rich second phases may be present • Coating structure may be rich in Fe-Al • Coating may be considerably soft • Higher coating stresses in FeAl coating due to CTE mismatch

15. Aluminising for TBM • FeAl + Al203 coating on RAFMS • Modify the Thermal-spray – Diffusion based Aluminising procedure for NiAl coating on IN-718 • Objective • Achieve FeAl + Al203 coating simultaneously (both with controlled thickness) in a single diffusion heat treatment • Methodology • Surface Preparation (Grit blasting) • Standardising of Procedures • Spraying of Aluminium • Optimising spraying parameters • Diffusion heat treatment (in Oxidizing environment) • Optimising temperature, time & oxidizing environment

16. Summary • Systematic approach in optimising different parameters of aluminising led to successful development of aluminised coatings on Ni-alloy 718 corrugated strips for PFBR • Similar approach for development of modified aluminising procedures for • NiAl coating on ferritic & austenitic SS • FeAl + Al203coating on RAFMS

17. Thank You