Transcript Effect of Retained Austenite and Residual Stress on Rolling Contact Fatigue Yi Shen Research Assistant Mechanical Engineering Tribology Laboratory (METL) November 14, 2013

Effect of Retained Austenite and
Residual Stress on Rolling Contact
Fatigue
Yi Shen
Research Assistant
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
2
Outline
•
•
•
•
Background
Motivation
Objective
Analytical Work
– Introduction to 2-D FEM rolling contact fatigue model
– Voronoi tessellations
– 2-D crack initiation and total life of fatigue incorporating residual stress
• Experimental Work
– Three-ball-on-rod rolling contact fatigue test
– Torsion fatigue test
• Summary and Future Work
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Background of Rolling Contact Fatigue (RCF)
•
Fatigue: Failure of a component subject to
repeated loads that are often well below the
ultimate strength or even yield strength of the
material
Over-rolling components
RCF in ball bearing
(Rosado et al., 2009)
RCF in tribo-components occurs by surface and subsurface initiated spalling
Surface originated pitting
Subsurface originated spalling
–
–
–
micro-cracks originate below the surface
propagation is towards the surface to form a
surface spall
leads to the formation of deep cavities
–
–
–
cracks initiate at a surface irregularity such as a
scratch or dent
propagation is at a shallow angle until some
critical length or depth and branching towards
the surface, removing a piece of material
leads to the formation of shallow craters
Subsurface originated spalling is dominant when the bearing is operating under lubricated
conditions and free of any surface irregularities such as scratch or dents or any defects
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Motivation
• Retained austenite (RA) does not transform to martensite upon quenching.
The amount of retained austenite has a significant influence on the rolling
contact fatigue (RCF) life of steel (SAE 8620)
• In addition to any direct effect on life, retained austenite influences the
residual stress (RS) profile, which also affects the RCF life of steel
• There is no general agreement about the effect of the retained austenite on
component durability
 Should it be at high (>35%) or low (<5%) levels?
 Is there any optimum choice?
Retained Austenite (light-colored areas) present
in a case carburized component (Daniel, 2005)
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Objectives
• Determine the optimum amount or range of retained austenite
in SAE 8620 steel for rolling contact fatigue (RCF)
• Investigate how residual stresses profile influence RCF life
• Explore the relationship between retained austenite and
residual stresses
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Modeling of Rolling Contact
6b
Microstructure of steel
b=100μm
7b
10b
•
•
•
All physical materials are discontinuous at some level and failure in bearing contacts
originates at a micron scale (comparable to the scale of discontinuities)
Rolling contact is modeled by moving a Hertzian Pressure (2GPa - width 2b) across
the surface in 21 analytical steps
Induce residual stress (RS) field into the RCF model
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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2-D Voronoi Element
•
A set of points (seeds, sites, or generators) is specified and for each seed there
will be a corresponding region consisting of all points closer to that seed than
to any other
o The region is thus referred to as a Voronoi cell[1]
2b
1b
Voronoi is a good representation of material microstructure
• 33 domains with different Voronoi mesh are generated to
statistically investigate the effects of residual stresses on RCF
[1] B. Jalalahmadi, F. Sadeghi, 2009, A Voronoi Finite Element Study of Fatigue Life Scatter in Rolling Contacts, ASME J. Trib., 131(2) (2009).
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Damage Mechanics
[3]
Elastic Damage Law
[2]
Where 0<D<1
dD   


dN  R (1  D ) 
m
• N is number of cycles
• Δτ is shear stress reversal along
the grain (Voronoi) boundary
• τR and m are material dependent
parameters
• τ = 6113MPa
• m = 10.0
R
•
Apply damage law to RCF model
[2] Robotnov, Y.N., 1969, Creep Problems in Structural Mechanics, North-Holland
[3] Xiao, Y.C., Li, S., Gao, Z., 1998, “A Continuum Damage Mechanics Model for High Cycle Fatigue,” Int J Fatigue, 20(7)
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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2-D Weibull Life Plot without RS
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

Slope of current Weibull plot:
Initiation: 7.8
Total: 4.4 (within 0.51 – 5.7 by Harris and Barnsby, 2001)



Slope of Jalalahmadi’s result:
Initiation: 5.11
Total: 4.08

Portion of propagation: 64% (within 60%-80%)

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
Slope of Anurag’s result:
Initiation: 4.81
Total: 5.13
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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2-D Initiation and Total Life Plot
constant residual stress
Weibull plot for cases with different residual stresses



linear residual stress
RS type
Without RS
Linear RS
V-shape RS
Constant RS
Slope of initiation life
7.8
9.3
9.03
9.29
Slope of propagation life
4.4
3.3
3.7
4.2
Residual stresses have very limited influence on crack initiation life
Different kinds of residual stresses have different level of influence on total life
Generally, compressive residual stress will increase the total life of RCF
V-shape residual stress
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Effect of residual stress on life
Increase of life =
Ltotal with RS − 𝐿𝑡𝑜𝑡𝑎𝑙 (without RS)
Ltotal (without RS)
RS type
Linear RS
V-shape RS
Constant RS
Max. increase of life
92.3%
99.1%
94.8%
Min. increase of life
1.6%
2.8%
2.8%
Average increase of life
20.1%
35.6%
43.6%
RS type
Without RS
Linear RS
V-shape RS
Constant RS
Portion of propagation life
64.0%
69.8%
73.7%
75.4%
The 2-parameter Weibull cumulative distribution
function, has the explicit equation:
F(t) = Probability of failure at time t;
t = time, cycles, miles, or any appropriate parameter;
η= characteristic life or scale parameter;
also it is the life at which 63.2% failure probability
β= slope or shape parameter.


L10, L50, L63.2 and L90 under residual stresses
LX means the life at probability of failure X% (0<X<100)
Besides L63.2, we also investigate L10 and L50, which are important parameters for RCF life
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Three-ball-on-rod Test Rig
• Federal Mogul three-ball-on-rod RCF machine
Loading Principle:
S W /3
F  F  S / tan 25
L
LB
UB
L
F  F  F  2S / tan 25  4.28S
BR
LB
UB
L
L
P  1.5F /(ab)
BR
Where a and b are the semi-axes of the contact area[3]
Parameters
•
Rod (8620 steel)
○
•
Roughened Steel Ball
○
•
Turbine oil (MIL-PRF23699F)
Rotation velocity
○
•
Diameter: 12.7mm (0.5in)
Oil
○
•
Diameter: 9.5mm (0.374in)
3600 rpm
Hertzian Pressure
○
3.5 GPa
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Three-ball-on-rod Rolling Contact Test
Low RA (RA<5%) Specimen Test Results


Currently 16 data points have been recorded
Slope of Weibull plot of three-ball-on-rod test: 1.95 (within range 0.51-5.7)
L
cycl es
 2.389
cycles
rev
min
 3600
 60
L
rev
min
hour
h o u rs
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Torsion Testing
Experiment Setup
MTS Torsion test rig
Custom mechanical interface between MTS rig and specimen
Bearing Steel
Torsion Specimen
Rotary
Actuator
Custom grips
Torque cell
Objective of this study : To obtain static and fatigue data in shear for modern bearing steels with
different amounts of retained austenite
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Torsion Fatigue Test Results
• In torsion fatigue test, 8620 steel with high level of retained austenite
has greater life than the one with low level of retained austenite
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013
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Summary and Future Work
Summary:
• Developed and used damage model for 33 domains to research on
the effect of different residual stresses on RCF life
• Finished torsion fatigue test for 8620 steels under high and low
RA level
• Continued three-ball-on-rod test on rods with low-level retained
austenite
Future Work:
• Get more data in three-ball-on-rod test to form the final Weibull
plot
• Develop the code to model the crack propagation in RVE
• Investigate and initiate the model on effect of retained austenite
on RCF
Mechanical Engineering Tribology Laboratory (METL)
November 14, 2013