Effect of Retained Austenite and Residual Stress on Rolling Contact Fatigue

Mechanical Engineering Tribology Laboratory (METL)November 14, 2013

Yi ShenResearch Assistant

Effect of Retained Austenite and Residual Stress on Rolling Contact

Fatigue

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

3


Background of Rolling Contact Fatigue (RCF)

Over-rolling components RCF in ball bearing (Rosado et al., 2009)

• Fatigue: Failure of a component subject to repeated loads that are often well below the ultimate strength or even yield strength of the material

Subsurface originated spalling Surface originated pitting– 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

RCF in tribo-components occurs by surface and subsurface initiated spalling

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

4


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)

5


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

6


Modeling of Rolling Contact

6b

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

b=100μm

Microstructure of steel

7


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 othero The region is thus referred to as a Voronoi cell[1]

Voronoi is a good representation of material microstructure

1b

2b

[1] B. Jalalahmadi, F. Sadeghi, 2009, A Voronoi Finite Element Study of Fatigue Life Scatter in Rolling Contacts, ASME J. Trib., 131(2) (2009).

• 33 domains with different Voronoi mesh are generated to statistically investigate the effects of residual stresses on RCF

8


Damage Mechanics

Where 0<D<1

m

R DdNdD

)1(

• N is number of cycles• Δτ is shear stress reversal along

the grain (Voronoi) boundary• τR and m are material dependent

parameters• τR = 6113MPa• m = 10.0

[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)

[2]

Elastic Damage Law[3]

• Apply damage law to RCF model

9


2-D Weibull Life Plot without RS

Slope of current Weibull plot: Initiation: 7.8 Total: 4.4 (within 0.51 – 5.7 by Harris and Barnsby, 2001) Portion of propagation: 64% (within 60%-80%)

Slope of Jalalahmadi’s result: Initiation: 5.11 Total: 4.08 Slope of Anurag’s result: Initiation: 4.81 Total: 5.13

10


2-D Initiation and Total Life Plot

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

constant residual stress

linear residual stress

V-shape residual stress

Weibull plot for cases with different residual stresses

11


Effect of residual stress on life

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 probabilityβ= slope or shape parameter.

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

L10, L50, L63.2 and L90 under residual stresses

Increase of life

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%

12


Three-ball-on-rod Test Rig• Federal Mogul three-ball-on-rod RCF machine

LLUBLBBR

LUBLB

L

SSFFFSFF

WS

28.425tan/225tan/

3/

ab)/(1.5FP BR

Loading Principle:

Where a and b are the semi-axes of the contact area [3]

Parameters• Rod (8620 steel)

○ Diameter: 9.5mm (0.374in)

• Roughened Steel Ball○ Diameter: 12.7mm (0.5in)

• Oil○ Turbine oil (MIL-PRF-23699F)

• Rotation velocity○ 3600 rpm

• Hertzian Pressure○ 3.5 GPa

13


Three-ball-on-rod Rolling Contact Test Low RA (RA<5%) Specimen Test Results

hourscycles Lhour

revrev

cyclesL

min60

min3600389.2

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)

14


Torsion TestingExperiment Setup

Custom mechanical interface between MTS rig and specimen

Bearing Steel Torsion Specimen

Custom gripsRotary Actuator Torque cell

MTS Torsion test rig

Objective of this study : To obtain static and fatigue data in shear for modern bearing steels with different amounts of retained austenite

15


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

16


Summary and Future WorkSummary:• 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

austeniteFuture 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

Documents

Effect of Retained Austenite and Residual Stress on Rolling Contact Fatigue