1.
Effect of Surface Hardening
Technique and Case Depth on
Rolling Contact Fatigue
Behavior of Alloy Steels
1 BRP US, Inc. – Marine Propulsion Systems Division, Sturtevant, Wisconsin, USA
2 Center for Surface Engineering and Tribology, Northwestern University, Evanston, Illinois, USA
3 State Key Laboratory of Metal Matrix Composites, Shanghai Jiaotong University, Shanghai, China
4 Midwest Thermal-Vac, Kenosha, Wisconsin, USA
5 State Key Laboratory of Mechanical Transmission, Chongqing University, Chongqing, China
David Palmer1, Lechun Xie 2,3, Frederick Otto4, Zhanjiang Wang5,
Q. Jane Wang2

2.
Introduction
Two-stroke outboard engine components
used in rolling contact fatigue (RCF)
2
 Crankshafts
 Connecting rods
 Driveshafts
 Propeller shafts
 Gears
 Bearings
 etc.

3.
Introduction
Typical manufacturing process
Forging → Rough machining → Heat treatment
Straightening → Grinding
3

4.
4
Surface hardening processes
Introduction
Atmosphere carburizing Induction hardening
Vacuum carburizing
(with or without
high-pressure gas
quenching)

5.
5
Introduction
Carburizing vs. induction hardening
Jones et al (2010):
 Spiral bevel gears
 Induction hardened AISI 4340 had lower distortion
 Atmosphere carburized AISI 9310 had higher strength
Townshend et al (1995):
 Straight spur gears
 Induction hardened AISI 1552 had longer RCF life than
atmosphere carburized AISI 9310
 Carburized gears were ground after heat treatment; induction
hardened gears were not

6.
6
Introduction
Atmosphere vs. vacuum carburizing
Lindell et al (2002):
 Helical spur gears
Atmosphere carburizing (with oil quench) vs. vacuum
carburizing (with gas quench) for AISI 8620
 Vacuum carburized AISI 8620 had:
 Higher surface hardness
 Higher surface compressive residual stress
 Greater depth of high hardness (>58 HRC) at same case
depth

7.
7
Introduction
Case depth
 Traditionally measured as depth to 50 HRC (513 HV)
 May vary due to non-uniform heating, non-uniform carbon
pickup, and/or non-uniform stock removal during grinding

8.
Introduction
Problems
 How do different surface hardening methods affect RCF life?
 Atmosphere carburizing with oil quenching
 Vacuum carburizing with oil quenching
 Vacuum carburizing with gas quenching
 Induction hardening
 What is the effect of case depth on RCF life?
 Can elasto-plastic modeling shed light on this problem?
8

9.
Experimental
Design and
Procedure
9

10.
10
RCF
life
Case
depth
Hardening
method
Alloy
Experimental Design

11.
11
RCF
life
Case
depth
Hardening
method
Alloy
Experimental Design
AISI 8620
AISI 9310
AISI 4140

12.
12
RCF
life
Case
depth
Hardening
method
Alloy
Experimental Design
Atmosphere carburize / oil quench
Vacuum carburize / oil quench
Vacuum carburize / gas quench
Induction harden / water quench

13.
13
RCF
life
Case
depth
Hardening
method
Alloy
Experimental Design
Low (0.50 – 0.75 mm)
Medium (0.75 – 1.00 mm)
High (1.00 – 1.25 mm)

14.
14
Procedure
Rough
machining
Heat
treatment
Centerless
grinding
RCF testing
(ball-on-rod)
Microhardness
/ case depth
Metallography
Elasto-plastic
modelling

15.
AISI 8620, 9310, 4140
Vacuum degassed
Certified AQ per SAE AMS 2301
Materials
Material C Mn Si Ni Cr Mo Cu P S Al Fe
8620 0.21 0.83 0.24 0.48 0.54 0.21 0.12 0.008 0.010 0.031 Balance
9310 0.11 0.62 0.26 3.06 1.16 0.10 0.15 0.007 0.020 0.033 Balance
4140 0.41 0.90 0.25 0.06 0.99 0.22 0.07 0.007 0.019 0.030 Balance
15
Length: 3.00 ± .05” (76.2 ± 1.3 mm)
Diameter: 0.400 ± .005” (10.16 ± 0.13 mm) – before grind
Diameter: 0.3750 ± .0001” (9.525 ± 0.002 mm) – after grind
Surface finish: 2 – 4 µin. (0.05 – 0.10 µm)

16.
16
Heat treatment
Carburizing
Temp. (°C)
Hold
Temp.
(°C)
Hold Time
(min.)
Quench
Temp (°C)
Tempering
Temp. (°C)
Tempering
Time
(hours)
Atmosphere 954 815 20 80 177 3
Vacuum 940 815 20 80 177 3
Carburizing
Temp. (°C)
Hold
Temp.
(°C)
Hold Time
(min.)
Quench
Pressure
(kPa)
Tempering
Temp. (°C)
Tempering
Time
(hours)
Vacuum 940 815 20 1500 177 3
AISI 8620
AISI 9310

17.
17
RCF testing
Spring load:
310 N
Rotation speed:
3600 rpm
Lubrication:
0.3 mL/min TC-W3
Ball diameter:
0.500 in. (12.7 mm)
Ball material:
AISI 52100

18.
18
Case depth measurement
Rough
machined
diameter Hardened
layer
Ground
diameter

19.
19
Case depth measurement
Microhardness
traverse
Circumscribed
circle
Vickers scale
(500 g load)
Measurements
made every
.005” (.127 mm)
beginning from
surface
Eccentricity = distance
from specimen axis to
center of circumscribed
circle
Average case depth =
specimen radius – radius
of circumscribed circle

20.
20
Elasto-plastic
Modelling

21.
21
Elasto-plastic modelling
Elasto-plastic modelling for plastically-graded materials (Wang
et al, 2013)
Assumptions:
• Yield strength gradient based on experimentally-determined
hardness gradient
• Tabor approximation: H = 3σY0
• Isotropic strain hardening: σY = σY0(1+εp)n
• Strain hardening exponent:
n = 0.140 for AISI 8620
n = 0.035 for AISI 4140
n = 0.113 for AISI 9310
Values from AISI Bar
Steel Fatigue database

22.
22
Results and
Discussion

23.
23
Microhardness profiles
AISI 8620 – Atmosphere carburized
Hardness at 0.005”: 711 HV
Depth to 56 HRC: 0.54 mm
Depth to 50 HRC: 0.99 mm

24.
24
Microhardness profiles
AISI 8620 – Vacuum carburized
Hardness at 0.005”: 819 HV
Depth to 56 HRC: 0.60 mm
Depth to 50 HRC: 1.02 mm

25.
25
Microhardness profiles
AISI 9310 – Vacuum carburized
Hardness at 0.005”: 776 HV
Depth to 56 HRC: 0.78 mm
Depth to 50 HRC: 1.06 mm

26.
26
Microhardness profiles
AISI 4140 – Induction hardened
Hardness at 0.005”: 639 HV
Depth to 56 HRC: 0.24 mm
Depth to 50 HRC: 1.05 mm

27.
27
Surface hardness
Surface hardness vs. case depth

28.
28
Surface hardness
Cycles to failure vs. surface hardness

29.
29
Surface hardness
Cycles to failure vs. hardness at 0.005 in.

30.
30
Case depth
Cycles to failure vs. case depth

31.
31
Case depth
Cycles to failure vs. depth to 56 HRC

32.
RCF life
Material
Surface
treatment
Average life
(cycles)
Average
case depth
(mm)
Eccentricity
(mm)
Depth to
56 HRC
(mm)
Hardness at
0.005”
(HV)
8620 vacuum 2,300,400 0.813 0.066 0.484 777
8620 vacuum 29,095,200 1.083 0.075 0.718 812
8620 vacuum 37,778,400 1.500 0.066 0.942 866
8620 atmosphere 2,049,300 0.578 0.067 0.287 693
8620 atmosphere 8,812,800 1.009 0.093 0.545 716
8620 atmosphere 21,772,800 1.332 0.141 0.560 673
9310 vacuum 26,568,000 0.677 0.030 0.470 749
9310 vacuum 43,372,800 1.078 0.033 0.793 782
9310 vacuum 52,401,600 1.257 0.027 0.931 793
4140 induction 237,600 0.502 0.041 0.080 605
4140 induction 410,400 0.720 0.060 0.342 631
4140 induction 604,800 1.115 0.086 0.465 635
32
Average cycles to failure

33.
33
Metallography
AISI 8620 atmosphere carburized
Case depth: 1.03 mm
Cycles to failure: 5,248,880
Dark etching area: 0.99 mm wide × 0.15 mm deep

34.
34
Metallography
AISI 8620 vacuum carburized
Case depth: 1.14 mm
Cycles to failure: 29,872,800
Dark etching area: 0.49 mm wide × 0.15 mm deep

35.
35
Metallography
AISI 9310 vacuum carburized
Case depth: 1.10 mm
Cycles to failure: 51,256,800
Dark etching area: 0.44 mm wide × 0.16 mm deep

36.
36
Metallography
AISI 4140 induction hardened
Case depth: 1.17 mm
Cycles to failure: 874,800
Light etching area: 0.78 mm wide × 0.28 mm deep

37.
37
Stress and plastic deformation
Maximum von Mises stress:
3638 MPa at 0.148 mm
depth (0.006”)

38.
38
Stress and plastic deformation
Material
Surface
treatment
Case
depth
Maximum
von Mises
stress (MPa)
Depth of
maximum
stress (mm)
Volume of plastic
deformation zone
(mm³)
Average life
(cycles)
8620 vacuum low 3566 0.138 0.065 2,300,400
8620 vacuum medium 3603 0.148 0.056 29,095,200
8620 vacuum high 3653 0.148 0.042 37,778,400
8620 atmosphere low 3279 0.128 0.113 2,049,300
8620 atmosphere medium 3356 0.138 0.085 8,812,800
8620 atmosphere high 3229 0.128 0.093 21,772,800
9310 vacuum low 3556 0.138 0.078 26,568,000
9310 vacuum medium 3638 0.148 0.057 43,372,800
9310 vacuum high 3647 0.148 0.057 52,401,600
4140 induction low 3531 0.128 0.189 237,600
4140 induction medium 3665 0.148 0.148 410,400
4140 induction high 3636 0.189 0.120 604,800
Maximum stress and plastic zone size

39.
39
Conclusions

40.
Conclusions
(1.) Longest RCF lives: vacuum carburized 8620 and
9310
Shortest RCF lives: induction hardened 4140
More than 2 orders of magnitude!
(2.) Higher surface hardness → longer RCF life
(except for atmosphere carburized 8620)
(3.) More important than surface hardness:
hardness at depth of maximum von Mises stress
(in this case, approximately 0.13 mm)
40

41.
41
Conclusions
(4.) RCF life correlates more closely with depth of
high hardness (>56 HRC) than with case depth
as traditionally defined (>50 HRC)
(5.) Vacuum carburizing → greater depth of high
hardness → smaller plastic deformation zone →
longer RCF life
(6.) Metallographically light- and dark-etching areas
correspond with plastic deformation zones as
predicted by elasto-plastic simulation

42.
42
Dissemination activities
STLE Annual Meeting & Exhibition
Lake Buena Vista, Florida
May 22, 2014
Design News
“Understanding Case Hardening of Steel”
September 4, 2014
Tribology Transactions
Volume 58, Issue 1
2015 (forthcoming)

43.
Acknowledgements
• BRP US, Inc. – Marine Propulsion Systems Division, Sturtevant,
Wisconsin, USA
• Center for Surface Engineering and Tribology, Northwestern
University, Evanston, Illinois, USA
• Midwest Thermal Vac., Kenosha, Wisconsin, USA
• China Scholarship Council (No. 2011623074), Beijing, China
• Shanghai Jiao Tong University, Shanghai, China
• Chongqing University, Chongqing, China
43

44.
References
Jones, K.T., et al. (2010), "Gas Carburizing vs. Contour
Induction Hardening in Bevel Gears," Gear Solutions 8 (82),
pp. 38-44, 51, 54.
Lindell, G.D, et al. (2002), "Selecting the Best Carburizing
Method for the Heat Treatment of Gears (AGMA Technical
Paper 02FTM7)," American Gear Manufacturers
Association, Alexandria, VA, ISBN 1 55589 807 6.
Townsend, D.P., et al. (1995), "The Surface Fatigue Life of
Induction Hardened AISI 1552 Gears (AGMA Technical Paper
95FTM5)," American Gear Manufacturers Association,
Arlington, VA, ISBN 1 55589 654 5.
44

45.
45
References
Wang, Z.J., et al. (2013), “An Efficient Numerical Method
With a Parallel Computation Strategy for Solving Arbitrarily
Shaped Inclusions in Elasto-Plastic Contact Problems,"
ASME Journal of Tribology 135 (3), pp. 031401.