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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

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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)

WS&FT, IPR-Gandhinagar


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

  • 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

  • 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

    • 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

    • 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

    • 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

    • 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

    • 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

    • 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

    • 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 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 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

    • 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

    • 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

    • 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


    Thank You


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