“A New Ferritic Stainless Steel with Improved Thermo-mechanical Fatigue Resistance for Exhaust Parts”
Laurent Faivre
Aperam Isbergues
France
CHARLES HATCHETT SEMINAR 2015
CBMM ©
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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
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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
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Ti Ti(C,N)
Nb Nb(C,N)instead of Cr23C6
%Cr
18%
10%
Grain boundary (Cr23C6)
x
Cr23C6
Cr-depleted zone
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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
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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)
*
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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
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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
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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
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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
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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)