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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 ). Aluminising. Surface modification process

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experiences on aluminising of strip components for pfbr applications

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

aluminising
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
requirement for pfbr
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
available aluminising processes
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)
thermal spray diffusion process
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.
step 1 pack cementation based aluminising
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
aluminising of flat strips of nickel alloy 718 using pack cementation process
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
step 2 development trials for thermal spray diffusion process
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
aluminising by thermal spray diffusion process
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
aluminide coating growth mechanism
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
step 3 aluminising of corrugated strips using thermal spray diffusion process
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
aluminising by thermal spray diffusion process12
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
aluminising by thermal spray diffusion process13
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
on going developments on aluminising for pfbr
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
aluminising for tbm
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
summary
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
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