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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