1. 21-22 July 2008 WS&FT, IPR-Gandhinagar 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)

2. 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 Pack cementation

3. 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. 4 Available Aluminising Processes Slurry spraying, brushing, dipping etc. followed by high temperature diffusion, electrolysis Pack cementation Pack 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. 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 1.Pilot-scale aluminising using pack cementation process 2.Pilot-scale development of thermal-spray–diffusion process To match properties of coating by pack cementation process 3.Industrial-scale Technology demonstration Aluminising of 1100 corrugated strips using thermal-spray–diffusion process.

6. 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 AlF 3 used instead of NH 4 F  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. 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. 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. 9 Aluminising by Thermal-Spray–Diffusion Process XRD of aluminised coating NiAl phase Nb 3 Al 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. 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. 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. 12 Aluminising by Thermal-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. 13 Aluminising by Thermal-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 A Process for Producing Body Centred Cubic (B2) Nickel Aluminide (NiAl) Coating of Controlled Thickness on Nickel-base Alloys, PCT/IN07/00514

14. 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. 15 Aluminising for TBM FeAl + Al 2 0 3 coating on RAFMS Modify the Thermal-spray – Diffusion based Aluminising procedure for NiAl coating on IN-718 Objective Achieve FeAl + Al 2 0 3 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. 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 modified aluminising procedures Similar approach for development of modified aluminising procedures for NiAl coating on ferritic & austenitic SS FeAl + Al 2 0 3 coating on RAFMS

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