Types of Surface TreatmentTypes of Surface Treatment

•• AluminizingAluminizing

•• Gas AnnealingGas Annealing

•• Overlay CoatingsOverlay Coatings

Oxidation of AluminaOxidation of Alumina--forming ODS forming ODS Alloys Alloys

Gordon J. Gordon J. TatlockTatlockCentre for Materials and StructuresCentre for Materials and Structures

School of EngineeringSchool of EngineeringUniversity of Liverpool, UKUniversity of Liverpool, UK

Outline of the PresentationOutline of the Presentation

•• Introduction Introduction –– factors affecting the oxidation factors affecting the oxidation of of ferriticferritic stainless steelsstainless steels

•• Scale growthScale growth

•• Scale adhesionScale adhesion

•• Breakaway effectsBreakaway effects

•• Improvements that can be made to mitigate Improvements that can be made to mitigate degradation due to oxidation degradation due to oxidation

Major constituents of some Major constituents of some commercial commercial FeCrAlFeCrAl alloys alloys

5000-4100--021.05.9PM2000 (ODS)

5000-5000--020.04.6MA956 (ODS)

0-200100020020021.05.9Kanthal APM

01900940580334021.05.2Kanthal AF

029009932040548020.05.5Aluchrom YHfY2O3SiTiZrHfYCrAl

Ppm balance FeWt%

Alloy

Scale GrowthScale Growth•• Want to form protective, stable Want to form protective, stable αα-- alumina alumina

scale as soon as possiblescale as soon as possible

•• Subsequent oxide growth should be as slow Subsequent oxide growth should be as slow as possible to conserve aluminium supplyas possible to conserve aluminium supply

Pre treatment of alloy to ensure formation Pre treatment of alloy to ensure formation of of αα--alumina scalealumina scale

Use reactive element additions to control Use reactive element additions to control scale growth ratescale growth rate

Add a nonAdd a non--ODS layer to minimize lack of ODS layer to minimize lack of compliance between oxide and substratecompliance between oxide and substrate

Start with more aluminium Start with more aluminium in base alloyin base alloy

Scale crossScale cross--sectionsection

5050µµm foil m foil AluchromAluchrom YHfYHf 3000h at 9003000h at 900°°C C A A –– initially formed outward growing transient alumina initially formed outward growing transient alumina

B B –– inward growing inward growing αα--aluminaalumina

Columnar growth of aluminaColumnar growth of alumina

Model Model FeCrAlYFeCrAlY alloy oxidised alloy oxidised for 500 h at 1200for 500 h at 1200°°CC

Scale GrowthScale Growth•• Can Can preoxidisepreoxidise alloys at T> 1000alloys at T> 1000°°C to ensure that C to ensure that αα--

alumina results alumina results –– but but ‘‘wastewaste’’ aluminiumaluminium

•• Work by Work by QuadakkersQuadakkers et al has shown that gas annealing et al has shown that gas annealing at 1200at 1200°°CC in an in an ArAr + 4%H+ 4%H2 2 + 2%H+ 2%H22O mixture quickly gives O mixture quickly gives an excellent protective scale for many alloysan excellent protective scale for many alloys

•• Although alloys with more than 6% Al have low ductility, Although alloys with more than 6% Al have low ductility, extra Al can be incorporated into the finished extra Al can be incorporated into the finished components by a gas phase reaction followed by a components by a gas phase reaction followed by a diffusion treatment.diffusion treatment.

•• Can modify the surface layer of the alloy by mechanical Can modify the surface layer of the alloy by mechanical treatment or add an treatment or add an overlayeroverlayer

•• Additions of a Additions of a ‘‘softsoft’’ FeCrAlFeCrAl layer to ODS alloys appears layer to ODS alloys appears to have a beneficial effect.to have a beneficial effect.

0

0.5

1

1.5

2

2.5

0 500 1000 1500 2000Time (hours)

Mas

s ch

ange

(mg/

cm2)

as-received

fine ground

coarse ground

pre-oxidised (fine+coarse)

PM2000+sputtered FeCrAlY

PM2000+sputtered FeCrAlY+EBPVD TBC

Mass change data from PM2000 slow cycle / ‘isothermal’ exposuresin combustion gas at 1200°C

PM2000 + sputtered FeCrAlY+ EBPVD TBC

500 hours at 1200°C in combustion gas (N2+14%O2+3.2%CO2+1%Ar+5%H2O)

Scale AdhesionScale Adhesion•• Presence of reactive elements such as yttrium, Presence of reactive elements such as yttrium,

hafnium and titanium help scale adhesionhafnium and titanium help scale adhesion

•• Reactive elements also act as scavengers for tramp Reactive elements also act as scavengers for tramp elements such as sulphur, which otherwise segregate elements such as sulphur, which otherwise segregate to metal to metal -- oxide interface and affect adhesionoxide interface and affect adhesion

•• Even carbon can form chromium carbides at the scale Even carbon can form chromium carbides at the scale metal interface, although may be tied up with titanium, metal interface, although may be tied up with titanium, for examplefor example

•• The build up of oxide growth stresses or accumulation The build up of oxide growth stresses or accumulation of point defects at the scale metal interface can also of point defects at the scale metal interface can also affect adhesion affect adhesion

10µm

FeFe--20Cr20Cr--5Al with S

Auger spectrum, FeAuger spectrum, Fe--20Cr20Cr--5Al with S

Alumina scaleAlumina scale

SubstrateSubstrateCr present at the Cr present at the metal metal -- oxide oxide interface as well as interface as well as the grain boundarythe grain boundary

Carbide growth at different cooling rates:Carbide growth at different cooling rates:(a) furnace cooled, (b) air cooled, (c) liquid N(a) furnace cooled, (b) air cooled, (c) liquid N22 quenchedquenched

KochubeyKochubey, , NaumenkoNaumenkoet al (2006)et al (2006)

Oxidation of an Oxidation of an FeCrAlYFeCrAlY alloy alloy with 530ppm carbon at 1200with 530ppm carbon at 1200°°C C

Distortion of parallel sided coupon of Distortion of parallel sided coupon of FeFe--20Cr20Cr--5Al alloy after oxidation for 528 h at 10505Al alloy after oxidation for 528 h at 1050°°CC

Breakaway EffectsBreakaway Effects

•• Lack of aluminium in sufficient quantities to Lack of aluminium in sufficient quantities to rehealrehealprotective alumina scale that has fractured can lead to protective alumina scale that has fractured can lead to growth of voluminous iron and chromium rich oxides growth of voluminous iron and chromium rich oxides

•• Iron/chromium scale then formed offers little Iron/chromium scale then formed offers little protection and sample quickly goes into protection and sample quickly goes into ““breakawaybreakaway””oxidationoxidation

•• Related to the amount of aluminium remaining in the Related to the amount of aluminium remaining in the sample and the size of the aluminium reservoir sample and the size of the aluminium reservoir

100µmSEM image of breakaway oxidation in SEM image of breakaway oxidation in PM2000, oxidised at 1300PM2000, oxidised at 1300°°C for 140h containing C for 140h containing critical remnant aluminium concentrationcritical remnant aluminium concentration

6.5º

20mm0.2-0.3mm

MA956 oxidised at 1350MA956 oxidised at 1350°°C for 24h.C for 24h.

A schematic diagram showing the cut face from which elemental A schematic diagram showing the cut face from which elemental profiles were recorded during the microprobe analysis.profiles were recorded during the microprobe analysis.

Metal

α-alumina

00.5

11.5

22.5

33.5

44.5

5

1 3 5 7 9 11 13 15 17 19 21Distance(mm)

Rem

aini

ng A

l lev

el (w

t%)

77h149h221h

Al profiles along wedges of Al profiles along wedges of KanthalKanthal APM alloy,APM alloy,oxidised at 1350oxidised at 1350°°C for three different times.C for three different times.

Wedge-shaped sample of an FeCrAl model alloy oxidised at 1300°C for 96h in laboratory air.

5mm

Spallation Spallation Breakaway OxidationBreakaway Oxidation

Time

Al Level

Possible breakaway

Al break

Cr2O3Formation

Spalling

Al crit

Upper limit for breakaway

After H. E. Evans et alAfter H. E. Evans et al

00.20.40.60.8

11.21.41.61.8

22.2

PM

2000

KA

F

YH

f

Mre

f

M2

M3

M4

M5

M6

M7

AP

M

MA

956

A a

nd C

Sample Reference

Al w

t%Remnant Al content at onset of breakaway (1200°C)

Control of Oxidation of Control of Oxidation of ODS Alloys ODS Alloys

•• Gas annealing at 1200Gas annealing at 1200°°CC in an in an ArAr + 4%H+ 4%H2 2 + 2%H+ 2%H22O O mixture quickly forms an excellent protective mixture quickly forms an excellent protective αα--alumina alumina scale for some alloys.scale for some alloys.

•• Incorporate more than 6% Al into the finished Incorporate more than 6% Al into the finished components by a gas phase reaction followed by a components by a gas phase reaction followed by a diffusion treatment may prolong lifetime.diffusion treatment may prolong lifetime.

•• Addition of a Addition of a ‘‘softsoft’’ FeCrAlFeCrAl layer to ODS alloys may have layer to ODS alloys may have a beneficial effect on the control of scale a beneficial effect on the control of scale spallationspallation..

•• Sufficient quantities of reactive elements and titanium Sufficient quantities of reactive elements and titanium are needed to control any tramp elements and excess are needed to control any tramp elements and excess carbon which may be present.carbon which may be present.

The help of The help of Hameed AlHameed Al--Badairy, Mike Bennett, Hugh Evans, and Badairy, Mike Bennett, Hugh Evans, and

other members of the other members of the IMPROVE, LEAFA, SMILERIMPROVE, LEAFA, SMILER teams teams is gratefully acknowledged is gratefully acknowledged

The financial assistance of both the EU and EPSRC The financial assistance of both the EU and EPSRC is also gratefully acknowledgedis also gratefully acknowledged

AcknowledgementsAcknowledgements