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

Experiences on Aluminising Experiences on Aluminising of Strip Components of Strip Components

for PFBR Applicationsfor PFBR Applications

G. Srinivasan, V.Shankar*, A.K. Bhaduri

Materials Technology DivisionIndira Gandhi Centre for Atomic Research,

Kalpakkam(* formerly with MTD, IGCAR)

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AluminisingAluminising 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 cementationPack cementation

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Requirement for PFBRRequirement 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

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Available Aluminising ProcessesAvailable Aluminising Processes

Slurry spraying, brushing, dipping etc. followed by high temperature diffusion, electrolysis

Pack cementation Treated at 1123–1323 K in a PackPack 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)

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Thermal Spray – Diffusion ProcessThermal 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 Development1. Pilot-scale aluminising using pack cementation process2. Pilot-scale development of thermal-spray–diffusion process

To match properties of coating by pack cementation process3. Industrial-scale Technology demonstration

Aluminising of 1100 corrugated strips using thermal-spray–diffusion process.

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Step 1Step 1::Pack Cementation based AluminisingPack 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

25 30 35 40 45 50 55 60 65 70 75 80 850

500

1000

1500

2000

2500

3000

3500

4000

Nb

3Al (200

) N

iAl

(211

) N

iAl

(111

) N

iAl

(100

) N

iAl

(110

) N

iAl

INT

EN

SIT

Y (

arb.

uni

ts)

2 (o)

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Aluminising of Flat Strips of Nickel Alloy 718 using Aluminising of Flat Strips of Nickel Alloy 718 using Pack Cementation ProcessPack 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 20 m20 m

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Step 2Step 2: Development Trials for : Development Trials for Thermal-Spray–Diffusion ProcessThermal-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

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Aluminising by Thermal-Spray–Diffusion Aluminising by Thermal-Spray–Diffusion ProcessProcess 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 20 30 40 50 60 70 800

200

400

600

800

1000

1200

1400

1600

1800

Nb 3A

l

(11

1)

NiA

l

(10

0)

NiA

l

(20

0)

NiA

l

(11

0)

NiA

l

INT

EN

SIT

Y (

arb

.un

its)

TWO THETA (o)

20 m20 m20 m

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Aluminide Coating Growth MechanismAluminide 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

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Step 3Step 3: Aluminising of Corrugated Strips : Aluminising of Corrugated Strips Using Thermal-Spray–Diffusion ProcessUsing 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

Degreasing

Surface Finish

Diffusion Treatment

Grit Blasting

Dimensional Check

Thermal Spray

Dimensional Check

Degreasing

Surface Finish

Diffusion Treatment

Grit Blasting

Dimensional Check

Thermal Spray

Dimensional Check

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Aluminising byAluminising byThermal-Spray–Diffusion ProcessThermal-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

Al sprayed + diffusion treated

Al sprayed

Al sprayed + diffusion treated

Al sprayed

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Aluminising byAluminising byThermal-Spray–Diffusion ProcessThermal-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) A Process for Producing Body Centred Cubic (B2)

Nickel Aluminide (NiAl) Coating of Controlled Nickel Aluminide (NiAl) Coating of Controlled Thickness on Nickel-base AlloysThickness on Nickel-base Alloys, PCT/IN07/00514

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On-going Developments on Aluminising On-going Developments on Aluminising for PFBRfor 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

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Aluminising for TBMAluminising 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

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SummarySummary

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 proceduresmodified aluminising procedures for NiAl coating on ferritic & austenitic SS FeAl + Al203 coating on RAFMS

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