“A New Ferritic Stainless Steel with Improved Thermo-mechanical Fatigue Resistance for Exhaust Parts”

Laurent Faivre

Aperam Isbergues

France

CHARLES HATCHETT SEMINAR 2015

CBMM ©

1

2

• Sous-titre 18pt Texte texte texte, texte, texte,texte, texte.

Charles Hatchett Seminar – 15/07/2015 © Aperam

Laurent Faivre, Pierre-Olivier Santacreu, Nicolas Dujardin

A NEW FERRITIC STAINLESS STEEL WITH IMPROVED

TMF RESISTANCE FOR EXHAUST PARTS

APERAM Isbergues, France

3

OUTLINE

• Context

• Materials & procedure

• Laboratory tests results

• Numerical approach

• Durability tests results (exhaust components)

• Conclusions

4

CONTEXT

Automotive pollution regulation standards

• More demanding depollution requirements

• Higher durability required

Solutions : downsizing, catalytic conversion

Consequence : increase of gas temperature, more severe requirements for materials

Diesel Gasoline

European standard Particulate

MatterNOx

Total

hydrocarbonsNOx

Euro 3 (2000) 50 500 200 150

Euro 4 (2005) 25 250 100 80

Euro 5 (2009) 5 180 100 60

5

CONTEXT

Hot end exhaust parts :

• 1000-1050°C could be reached in a near future (manifold, turbocharger, etc.)

• Stainless steel manifold : lightweight & high temperature solution (comparedto cast iron)

• Common stainless steel grades limited to 950°C (e.g. EN 1.4509)

Need for material with higher TMF resistance

• Motor bench tests necessary but long/expensive tests

• FE calculations → reduction of motor bench tests

Need for corresponding numerical models (behavior, damage)

6

MATERIALS & PROCEDURE

Materials

K44X

• ferritic stainless steel grade with 19%Cr, 2%Mo and Nb-stabilization

• designed for hot end exhaust applications up to 1000°C (manifold, turbo, etc.)

Typical stainless steel grades for exhaust applications :

Flat specimens (~2mm-thick, CR) were used for all presented tests

Ferritic

Austenitic

Grade C Cr Si Mn Mo Ni N Nb Ti

1.4512 0.01 11.5 0.5 0.3 - - 0.01 - 0.2

1.4510 0,02 16,5 0,35 0,3 - - 0,015 - 0.4

1.4509 0,02 17,8 0,6 0,3 - - 0,015 0.5 0.15

K44X – 1.4521M 0,02 19 0,6 0,3 1,9 - 0,015 0.6 -

1.4828 0,05 19,3 1,6 1,3 - 11,4 0,03 - -

MATERIALS & PROCEDURE

Materials

Stabilizers

• Added to avoid intergranular sensitization

• Prevent the formation of Cr carbides at high temperature (welding, heattreatment, in service)

• Ex:Nb, Ti

7

Ti Ti(C,N)

Nb Nb(C,N)instead of Cr23C6

%Cr

18%

10%

Grain boundary (Cr23C6)

x

Cr23C6

Cr-depleted zone

8

MATERIALS & PROCEDURE

Procedure : lab test overview

Deflection (mm)

Fatigue (HCF & LCF)

20 to 850°C

HT Tensile tests750 to 1000°C

CreepSAG-test

750 to 1000°C

CreepUniaxial in tension

850 to 950°C

Compressed air

Possibility of

adding solutions

Ttest - 10°C

t (min)

T °C

20 min

0

5 minTtest

Cyclic oxidation20 to 950/1000°C

Thermal fatigueV-shaped specimen

100 to 950/1000°C

ISOTHERMAL ANISOTHERMAL

9

LAB TESTS

Tensile properties at high temperature

Ferritics (1.4509, K44X, etc.) < austenitics (1.4828)

K44X : 30% higher YS than 1.4509

HT Tensile tests•0.54mm.min-1

•L0=25mm

•EN 6892-2

→ Yield/tensile strength

0

20

40

60

80

100

120

140

160

180

700 800 900 1000

0,2

% P

roo

f str

ess [M

Pa]

Temperature [°C]

1.4828

K44X-1.4521

1.4509

1.4510

1.4512

0

10

20

30

40

800 900 1000 1100

0,2

% P

roo

f str

ess (M

Pa)

Temperature (°C)

1.4828K44X-1.45211.45091.45101.4512

LAB TESTS

Tensile properties at high temperature

10* Fujita et al. - Scripta Materialia, Vol. 35, No. 6, pp. 705-710, 1996

K44X : increased mechanical properties thanks to 2%Mo & 0.6%Nb (solid solution hardening)

*

11

LAB TESTS

Fatigue - HCF

HCF•Tension/comp

•R=-1

•2 million cycles

•20Hz

→ Endurance limit

0

50

100

150

200

250

300

200 300 400 500 600 700 800 900

Fati

gu

e li

mit

[M

Pa]

Temperature [°C]

K44X-1.4521

1.4509

1.4510

1.4512

HCF properties correlated to HT strength (same ranking)

K44X exhibits 50% higher fatigue limit than 1.4509 at 850°C

12

LAB TESTS

Cyclic oxidation

Compressed air

Possibility of

adding solutions

Ttest - 10°C

t (min)

T °C

20 min

0

5 minTtest Cyclic oxidation

•Immersion/extraction

•Compressed air cooling

•30 min per cycle

→Mass gain

Austenitic grades : poor resistance at high temperature (>900°C)

25% higher mass gain for 1.4509 than for K44X

0

50

100

150

200

250

0 100 200 300 400 500

Mass g

ain

[g

/m2]

Time [h]

Cyclic oxidation - 950°C - air

1.4828K44X-1.45211.4509

0

50

100

150

200

0 100 200 300 400 500 600

Mass g

ain

[g

/m2]

Time [h]

Cyclic oxidation - 1000°C - air

1.4828K44X-1.45211.4509

LAB TESTS

Cyclic oxidation

13

Coefficient of thermal expansion 1.4828 1.4509 K44X Cr2O3

Between 20-800°C [10-6/°C] 19 12,5 12 6-8

Austenitic grades : poor resistance due to higher CTE mismatch with oxide

14

LAB TESTS

Creep - sagtest

Creep - sagtest•No external force

(weight only)

•200mm specimen

→ Deflection after 100h

1.4509 comparable to 1.4828 for T≤950°C

K44X has the best resistance at every temperature

0

10

20

30

40

409

1.4512

430Ti

1.4510

441

1.4509

K44X 309

1.4828

304

1.4301

Cre

ep d

efle

ctio

n a

fter

100h [

mm

]

850°C

950°C

1000°C

Best

Worst

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LAB TESTS

Uniaxial creep tests

Higher resistance of austenitic grade (1.4828) than ferritic

Improved resistance of K44X compared to 1.4509

1

10

100

0.01 0.1 1 10 100

Str

ess (

MP

a)

t1% (h)

Time to 1% creep - 850°C

1.4828

1.4509

K44X

1

10

100

0.01 0.1 1 10 100

Str

ess (

MP

a)

t1% (h)

Time to 1% creep - 950°C

1.4828

1.4509

K44X

Creep - tension•Constant force applied

•Uniaxial tensile loading

→ Time to 1% creep strain

LAB TESTS

Uniaxial creep tests

Intermetallic precipitation :

• Laves phase Fe2Nb (hexagonal) in 1.4509

• Type Fe2Nb3 (cubic) in K44X :

– higher density and finer precipitates

– more stable at high temperature

More resistance to creep and grain growth

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Nb-rich intermetallic precipitation

at grain boundaries (pining effect)

K44X - Annealed condition

K44X - Heat-treated at 850°C for 10h

Finer and more stable precipitation for K44X -> increased creep resistance

Grain boundaries linear covering rate of precipitates [%]

Sa

gte

st

cre

ep

rate

[m

m/h

]

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LAB TESTS

Thermal fatigue

• Test to approach TMF conditions of a manifold in service

• Thermal fatigue coupled with oxidation & creep (depending on dwell time)

• Lifetime criterion : 50% drop of stabilized clamping force

Thermal fatigue•V-shaped specimen

•Resistive heating

•Clamped ends

•with/without dwell time

→ Number of cycles

0

100

200

300

400

500

600

700

800

900

1000

0 100 200 300 400

Tem

pe

ratu

re [

°C]

Time [s]

: dwell time

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LAB TESTS

Thermal fatigue

1.4828K44X

K44X : 10-40% longer lifetime than 1.4509

Different cracking mechanisms for austenitic and ferritic grades

heating

0

1000

2000

3000

4000

5000

6000

7000

1.4828 K44X 1.4509 1.4512

Th

erm

al fa

tig

ue

lif

eti

me

(c

yc

les

)

100-950°C

0s

180s

0

1000

2000

3000

4000

5000

6000

7000

1.4828 K44X 1.4509 1.4512

Th

erm

al fa

tig

ue

lif

eti

me

(c

yc

les

)

100-1000°C

0s

180s

K44X

1.4509

1.4828

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LIFETIME PREDICTION

FE calculation

Behavior model

Stress, strain, T fields

Damage models

Predicted lifetime

Model MATERIAL BEHAVIOR TF CRITERION CREEP CRITERION

Type Chaboche (EVP) Taira* Larson-Miller*

Experimental base Isothermal LCF tests Th. Fat. V-shaped tests Uniaxial creep tests

Model integrationSubroutines in

FE calculations (ε,σ,T)

Lifetime calculation through

XHAUST-LIFE

* Modified model

Geometry, loading

Lifetimecalculation

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LIFETIME PREDICTION

MATERIAL BEHAVIOR : CHABOCHE - type

Elasto-viscoplastic model

)( XJ

Xpvp

22

3

n

K

RXJp

)(2

pXCX p 3

2

Identification : isothermal LCF test

• Strain-controlled

• 0.2% < /2 < 0.6%

• strain rate :10-3 or 10-4.s-1

• T: 20, 300, 650 and 850°C

vpelth

F19MNb - 750°C - 0,1%/s - relaxation (100s)

-120

-80

-40

0

40

80

120

-0,0025 -0,0015 -0,0005 0,0005 0,0015 0,0025

Strain

Str

ess (

MP

a)

simulation

experience

Norton

Kinematic

hardening

• Identification through Zebulon software :

→ E, K, n, C, D, R

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LIFETIME PREDICTION

Identification :

• V-shape TF tests with a wide range of conditions :

– V geometry

– Sheet thickness

– Dwell time

• p determined from FE calculation

→ λ, t, a, b

N = l(Teq) pt

)ln(max

max

baT

TTeq

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TF DAMAGE : TAIRA – type

Non isothermal LCF strain-based law

• : dwell time

• Oxidation/creep effect through Teq

(coupling with TF)

dtvpvp

cycleeq

2/1

:3

2.

0

2000

4000

6000

8000

10000

12000

14000

0,0% 0,1% 0,2% 0,3% 0,4% 0,5%

Strain Amplitude

TF

Lif

e (C

ycl

es)

1000°C

950°C

900°C

850°C

750°C

Tmax

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LIFETIME PREDICTION

Identification :

• Uniaxial creep test in tension

– at constant load

– Time to 1% of εcreep (t<100h)

– Test at 750, 850 & 950°C

• Identification of LM parameters

→ B, C, m

CREEP DAMAGE : LARSON MILLER - type

• : dwell time

• Stress triaxiality effect

Tmeq

BCt .

1

%1 ).10.(10

)(..1 TrVMeq

%1tNC

CtKTP log][

PmB log

0

0,2

0,4

0,6

0,8

1

0 1 2 3 4

Duration (h)

Str

ain (

%)

AISI 441 EN 1.4509

K44X

Creep test @ 22 MPa - 850°C

Larson Miller modelisation

0

0,5

1

1,5

2

2,5

10000 20000 30000 40000 50000 60000

P

Ln

(M

Pa

)

K44X

1.4509

1.4828

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DURABILITY TESTS AND WORKABILITY

Motor bench tests

Forming

Weldability

+20% in lifetime in this case & up to +30% observed on bench tests

Manifold #1 First Leakage

(cycles)

TMF Crack

(Cycles)

1.4509 1600 2400

K44X 2900 No (>3000)

K44X’s formability is equivalent to 1.4509

TIG, MIG, Laser welding and brazing

24

CONCLUSIONS

K44X, in comparison to common stainless steel grades exhibits:

Higher HT strength & HCF compared to ferritic grades (%Mo)

Better creep resistance (Nb precipitation)

Good cyclic oxidation resistance up to 1000°C

Increased TF lifetime on lab & motor bench tests

Higher service temperature (up to 1000°C)

Cost-effective solution (in comparison to austenitic refractory grades)