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Improved Forged Crankshaft Performance Utilizing Deep Rolling
Michael BurnettThe Timken Company
Mark RichardsColorado School of Mines
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• Steel has made significant inroads in automotive crankshafts applications based on:– Improved stiffness relative to cast iron– Improved fatigue strength– Lower weight– Improved Noise Vibration Harshness (NVH)
• Annual US steel automotive crankshaft tonnage now nearly 200,000 tons/yr out of ~500/000 tons/yr total
STEEL CRANKSHAFTS IN AUTO INDUSTRY
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• Crankshaft main/pin fillet fatigue strength enhanced by – Induction hardening– Nitriding– Deep rolling
• For automotive crankshafts, deep rolling is the preferred/common method
• Effect of deep rolling parameters on fatigue strength is currently in-complete
ENHANCING CRANKSHAFTFATIGUE STRENGTH
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DEEP ROLLINGINTRODUCTION
H. Naumann, 1968 T. Watmough and M. Malatesta, 1984
• Deep Rolling is a radially symmetric deformation process used primarily for surface finish, hardness, and residual stress control.
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DEEP ROLLING IN CRANKSHAFTS
T. Watmough and M. Malatesta, 1984
• Deep Rolling is Utilized in Crankshaft Production to Strengthen Journal Fillets.
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• High interest to understand how material and rolling parameters effect fatigue strength
• Varying material and rolling parameters and fatigue testing of crankshafts is difficult and expensive
• Need for a laboratory deep rolling and testing procedure to simulate crankshaft tests
DEEP ROLLING METHOD INVESTIGATION
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DEEP ROLLINGHISTORY
• Process Control Equations Developed in 1950/60’s
• Empirically Determined that Contact Stress Fall Within 3-5 Times the UTS
• UTS Specific to Tensile Loading, Deep Rolling is Complex Compression with Multiple Loading and Unloading Cycles
G = Contact Stress (psi)F = Rolling Force (lbs.)E = Modulus of Elasticity (psi)B = Contact Length (in.)D1 = Diameter of Work Roller (in.)D2 = Diameter of Workpiece (in.)
( )⎟⎟⎟⎟
⎠
⎞
⎜⎜⎜⎜
⎝
⎛
+×⎟⎠⎞
⎜⎝⎛ ×
×=
2D1
2D1
BEF0.175G
21
Stress = f (Loading, Geometry)
H. Naumann, 1968
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• Colorado School of Mines commissioned the Steel Research Center in 1984 with steel makers and users as members
• Center now has over 20 supporting members who direct the work of 21 graduate students
• Mark Richards initiated a deep rolling project in 2001 to both develop a meaningful test and began studying the effects of the various material/rolling parameters
ADVANCED STEEL PROCESSING &PRODUCTS RESEARCH CENTER
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MECHANICAL AND METALLURGICAL ISSUES
Geometry Due to Deformation
Deformation Volume
Strain
Notch Constraint
• Material
• Deformation
• Strain Hardening
• Residual Stress Stability
• Geometry
• Notch Effect Material Flow
• Burnishing
• Residual Stress Development
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PROJECT SCOPE
Deep Rolling ResponseDeformation Behavior
Microstructure
• Materials Representing 0.4 wt. pct. C Bar / Forging Steels
• Different Deformation Behaviors
• Simulative Test
• Maintain Constant Test Methodology / Test Parameters
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TESTING METHODS
Fatigue Dynamics, Inc. , 1985T. Watmough and M. Malatesta, 1984
• Rotating Bending
• Laboratory Scale
• 6.9 mm Diameter Samples
• Reversed Bending
• Industry Practice
• Production Crankshaft Journals
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ASPPRC DEEP ROLLING FATIGUE SPECIMEN
2 0 3 .2 m m
2 5 .4 m m
(Radius 4.2 mm)1 5 2 .4 m m
1 7 .0 m m
• Based on Rotating Bending and Flex Beam Literature• Flex Beam Test Method• Fully Reversed (R=-1) • Larger Scale Than Previous Studies
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FATIGUE TEST SETUP
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MATERIAL SELECTION
Alloying Element in wt. pct.
Steel C Mn S Si Ni Cr
4140 0.45 0.85 0.041 0.20 0.10 0.85
NTB 0.38 1.21 0.011 0.73 0.12 0.09
C38M 0.41 1.30 0.067 0.56 0.09 0.11
M.D. Richards et al., SAE 2004-01-1528, 2004
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MATERIAL SELECTION
• C38M- Ferrite/Pearlite
- Crankshaft Grade
• NTB-15% Retained
Austenite
- Direct Cooled
• 4140- Quenched
and Tempered
- Forging GradeEng. 0.2% Yield
Stress (MPa) UTS (MPa)
848 946
Eng. 0.2% Yield Stress (MPa) UTS (MPa)
579 881
Eng. 0.2% Yield Stress (MPa) UTS (MPa)
552 835
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MECHANICAL TESTING
0 0.01 0.02 0.03 0.04 0.05True Strain (inch/inch)
400
500
600
700
800
900
1000
True
Stre
ss (M
Pa)
60
70
80
90
100
110
120
130
140
TrueStress
(ksi)4140 Compression4140 TensileNTB CompressionNTB TensileC38M CompressionC38M Tensile
4140
NTB C38M
• Monotonic True Stress vs. True Strain
• Tension and Compression
• Close-Up of Low Strain Region
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FATIGUE DATABASELINE
Cycles
200
300
400
500
600
700
800
900
1000
1100
Nom
inal
Rev
erse
d Be
ndin
g St
ress
(MPa
)
30
4050607080
90100110120
130140150
Nom
inal
Rev
erse
d Be
ndin
g St
ress
(ksi
)
104 105 106 107
C38M Baseline
(3)
• C38M Baseline• Endurance Limit:
241 MPa (35 ksi)• Reproducible Data
at Any Stress Level• Characteristic of All
Materials Tested
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DEEP ROLLING SETUP
• 108 mm Diameter
• 3.9 mm Crown Radius
• Horizontally Opposed
• Dual Roller
Deep Rolling Provided By
Hegenscheidt-MFD Corp. Sterling Heights, MI
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ROLLING LOADOPTIMIZATION
0 2 4 6 8 10 12 14 16 18 20 22 24Rolling Load (kN)
10000
100000
Life
at S
tress
(cyc
les)
0 1000 2000 3000 4000 5000Rolling Load (lbf)
C38M - 524 MPaNTB - 552 MPa4140 - 572 MPa
• Similar Baseline Fatigue Behavior and UTS Values
• Different Strain-Hardening Behavior
• Deep Rolled Behavior
• 4140 – 17.5 kN• NTB – 15 kN• C38M – 10.5 kN
M.D. Richards et al., SAE 2004-01-1528, 2004
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FATIGUE DATADEEP ROLLED
Cycles
200
300
400
500
600
700
800
900
1000
1100
Nom
inal
Rev
erse
d Be
ndin
g S
tress
(MP
a)
30405060708090100110120130140150
Nom
inal
Rev
erse
d Be
ndin
g St
ress
(ksi
)
104 105 106 107
Deep Rolled
Baseline
(3)
(3)
C38M
• C38M Deep Rolled• Endurance Limit:
386 MPa (56 ksi)
• Reproducible Data at Any Stress Level
• Characteristic of All Materials Tested
Steel
Nominal Endurance Limit
Change (MPa) ΔSf
Percent Increase 100%*ΔSf/Sf(BL)
4140 159 51
NTB 172 62
C38M 145 60
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CHARACTERIZATION
Depth (mm)
250
300
350C38M
250
300
350
Vick
er's
Har
dnes
s
250
300
350
NTB
4140
0 0.5 1 1.5 2 2.50 0.01 0.02 0.03 0.04 0.05True Strain (inch/inch)
400
500
600
700
800
900
1000
True
Stre
ss (M
Pa)
60
70
80
90
100
110
120
130
140
TrueStress
(ksi)4140 Compression4140 TensileNTB CompressionNTB TensileC38M CompressionC38M Tensile
4140
NTB C38M
• Hardness Profiles• Highlight Differences in
Deformation Behavior
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CONCLUSIONS
• Laboratory Test Methodology Successfully Developed and Implemented
• Deformation Behavior Influences Response to Deep Rolling
• Deep Rolling Provides:1. Significant Increase in Fatigue Performance in all Materials Tested2. Highly Reproducible Fatigue Data
• Ongoing Research at CSM
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ACKNOWLEDGEMENTS
• Advanced Steel Processing and Products Research Center (ASPPRC)
• Hegenscheidt-MFD Corp. of Sterling Heights, MI– In Particular Mike Hoeppner