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SAE TECHNICALPAPER SERIES 2002-01-0918
Induction Hardened Ductile Iron Camshafts
Alan P. Druschitz and Steve ThelenIntermet Corp
Reprinted From: Designing and Achieving Lightweight Vehicles(SP1684)
SAE 2002 World CongressDetroit, Michigan
March 4-7, 2002
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ISSN 0148-7191Copyright 2002 Society of Automotive Engineers, Inc.
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Copyright 2002 Society of Automotive Engineers, Inc.
ABSTRACT
The General Motors L850 world engine uses an induction
hardened, ductile iron, camshaft. Unlike most induction
hardened camshafts that are machined first and then
hardened, this camshaft is deep hardened first and then
machined. Using this process, the beneficial compressive
surface residual stresses are extremely high.
During the development of the L850 camshaft, the casting
process was optimized to produce material of sufficient
quality to resist quench cracking during the hardening
process and to resist mechanical cracking during the
machining process. Retained austenite content, residual
stress profiles, hardness, microstructure and chemical
composition were all characterized and optimized.
This paper reviews the material and process development
for this unique automotive application.
BACKGROUND
A wide variety of materials and processes are used to
produce camshafts for the automotive industry. One of
the most versatile material-process combinations is cast
ductile iron and induction hardening. Cast ductile iron
produces a low cost, near net shape component that is
suitable for subsequent precision machining and heat
treatment. Induction hardening can be economically used
to heat the casting and subsequent quenching produces
hard, wear resistant martensite or ausferrite (austempered
ductile iron). Machining can be performed before or after
induction hardening.
INTERMET Corporation currently produces L850 camshaft
castings for General Motors Corporation. To develop a
firm understanding of the interactions between casting
chemistry (manganese level), processing history
(induction hardening and machining) and final component
characteristics (microstructure, hardness and residua
stress distribution), a comprehensive research program
was performed. INTERMETs Radford Foundry produced
L850 camshafts with three levels of manganese. Genera
Motors Tonawanda Engine Plant induction hardened and
machined these camshafts. INTERMET Materials R&D
performed a preliminary examination of the camshafts and
then sent samples to Lambda Research, Cincinnati, OH
for determination of residual stresses and retained
austenite by x-ray diffraction. This paper details the
results of that study. The exhaust and intake camshafts
for the L850 engine are shown in Figure 1.
Figure 1. The camshafts for the L850 engine.
Since a camshaft is a multi-purpose component, different
properties are needed at different locations; for example,
the desirable characteristics for the cam lobes are a hardwear resistant surface with high load carrying capacity,
however the bearing surfaces need to be readily
machinable, dimensionally accurate and smooth
Therefore, a strong, readily machinable material, such as
ductile iron, combined with localized induction hardening
is often used. Induction hardening produces a tempered
martensite layer at the surface and, when combined with
the appropriate grinding process, very high beneficia
compressive surface residual stresses. A cross section
through a cam lobe is shown in Figure 2.
2002-01-0918
Induction Hardened Ductile Iron Camshafts
Alan P. Druschitz and Steve ThelenIntermet Corporation
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Figure 2. Cross section through a hardened L850 cam lobe.
A hard, dimensionally accurate mold, clean metal plus a
well designed gating system that minimizes the formation
of inclusions is mandatory for the production of camshaft
castings since inclusions will cause cracking during
induction hardening and/or machining.
EXPERIMENTAL METHODS
Since many automotive specifications require high Mn
contents (>0.45 wt%), L850 camshafts were poured atthree different Mn contents that covered the typical range
and significantly lower, Table I. For each Mn level, five to
ten castings from one mold cavity were sent to GM
Tonawanda for induction hardening on production
equipment. Two castings of each Mn level were then fully
machined at Tonawanda and all castings were returned to
INTERMET Materials R&D for analysis.
Table I. Composition of L850 Camshafts (wt%)
Sample ID Mn C Si Cu Mg Cr Ni Mo
Low Mn 0.35 3.87 2.07 0.73 0.035 0.075 0.04 0.011
Med Mn 0.46 3.80 2.10 0.78 0.040 0.081 0.04 0.014
High Mn 0.64 3.80 2.01 0.83 0.032 0.080 0.04 0.017
The surface hardness (Rockwell C) of the induction
hardened camshafts, before and after machining, were
measured on the nose, ramps and base circle. The depth
of the hardened layer was determined by visual
examination and the retained austenite content was
estimated using a fully automated, Clemex Image
Analyzer on camshaft lobes that were sectioned, polished
and etched. Retained austenite content was determined
by averaging the results from 200 fields of view at 200x for
each sample.
Residual stresses and retained austenite contents as a
function of distance from the surface were measured at
the base circle for samples of lobe 3 in the induction
hardened only condition using X-ray diffraction techniques
Residual stresses and retained austenite contents at thesurface and at a depth of 0.1 mm (near surface) were also
measured at the base circle for samples of lobe 3 and
lobe 6 in the induction hardened then fully machined
condition using X-ray diffraction techniques
RESULTS AND DISCUSSION
The induction hardening process used in this investigation
used RF frequency (30 KHz) to simultaneously heat four
of the camshaft lobes, quenching in a water-polyme
solution and immediately tempering using inductionheating. The camshaft was then indexed and the
remaining four lobes hardened and tempered in a similar
fashion. During induction hardening, the camshaft was
rotated slowly to produce more uniform heating.
During induction hardening, the surface of the camshaft
heats rapidly and the heat soaks into the component
Since silicon strongly affects the temperature at which
ferrite transforms to austenite, silicon must be closely
controlled to insure consistent hardening response. For a
camshaft, the heat soaks farther into the nose than into
the base circle due to the difference in mass. The nose of
the cam lobe also reaches higher temperatures since it is
closer to the induction coil. Care must be taken not to
overheat the casting since the amount of carbon that
dissolves into the high temperature phase of iron
(austenite) is a function of temperature, time, chemical
composition and starting microstructure. A starting
matrix microstructure of >80-90% pearlite is desirable to
minimize the carbon diffusion distance. Since its
desirable to keep cycle times short, exceptionally high
temperatures (as high as 1038oC) are often used. The
amount of carbon that goes into solution at these hightemperatures later determines the temperature at which
martensite forms during quenching. For example, if the
maximum temperature reached during heating is 900oC
martensite will start to form at 195oC during quenching.
However, if the maximum temperature reached during
heating is 1038oC, martensite will not start to form until
162oC during quenching. During quenching, the rate of
cooling must be controlled since too slow of a quench wil
not produce martensite and too fast of a quench wil
cause the formation of excessive amounts of retained
austenite and may cause cracking. Tempering afte
Tempered martensite
Pearlite
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quenching is necessary to improve toughness and relieve
excessive residual stresses.
In the induction hardened only condition, low Mn (0.35
wt%) castings had the highest hardness values at the
nose for all lobes, Figure 3a. In the induction hardened
and machined condition, only the low Mn castings met
the hardness specification at all lobe noses, Figure 3b.
Average surface hardness data for all cam lobes taken on
the nose, ramps and base circle are shown in Figures 3-5.All locations showed a characteristic wide variation in
hardness in the induction hardened only condition and a
greatly reduced variation after machining. The wide
variations in hardness were due to variations in the
retained austenite content near the cam lobe surfaces
since these were the regions that reached the highest
temperatures during heating.
Induction Hardened Only
46
48
50
52
54
56
58
60
1 2 3 4 5 6 7 8
Lobe
NoseHardness,RockwellC
0.35% Mn
0.46% Mn
0.64% Mn
Figure 3a. Average nose hardness after induction hardening only
Induction Hardened then Fully Machined
46
48
50
52
54
56
58
60
1 2 3 4 5 6 7 8
Lobe
NoseHardness,RockwellC
0.35% Mn
0.46% Mn
0.64% Mn
spec. min.
spec minimum
Figure 3b. Average nose hardness after induction hardening andmachining.
Induction Hardened Only
46
48
50
52
54
56
58
60
1 2 3 4 5 6 7 8
Lobe
RampHardness,RockwellC
0.35% Mn
0.46% Mn
0.64% Mn
Figure 4a. Average ramp hardness after induction hardening only.
Induction Hardened then Fully Machined
46
48
50
52
54
56
58
60
1 2 3 4 5 6 7
Lobe
RampHardness,RockwellC
0.35% Mn
0.46% Mn
0.64% Mn
spec. min.
spec minimum
Figure 4b. Average ramp hardness after induction hardening andmachining.
Induction Hardened Only
46
48
50
52
54
56
58
60
1 2 3 4 5 6 7 8
Lobe
BaseCircleH
ardness,RockwellC
0.35% Mn0.46% Mn
0.64% Mn
Figure 5a. Average base circle hardness after induction hardeningonly.
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Induction Hardened then Fully Machined
46
48
50
52
54
56
58
60
1 2 3 4 5 6 7 8
Lobe
Bas
eCircleHardness,RockwellC
0.35% Mn
0.46% Mn
0.64% Mn
spec. min.
spec minimum
Figure 5b. Average base circle hardness after induction hardeningand machining.
As expected, increasing Mn caused higher average
retained austenite contents in induction hardened only
camshafts, Table II. This was caused by Mn segregation
and was the reason for falling hardness with increasing
Mn level. The reason for the poor agreement between
image analysis and X-ray diffraction for the 0.46 wt% Mn
sample is not known. After grinding, the variation in
retained austenite content was greatly reduced, Table III.
Table II. Average Retained Austenite Content as a
Function of Mn Content at the Surface of Induction
Hardened Only Camshafts.
Retained Austenite Content, vol%
Measurement Technique 0.35% Mn 0.46% Mn 0.64% Mn
Image analysis (lobe 3) 28 41 38
X-ray diffraction (lobe 3) 32.4 27.5 40.7
Table III. Retained Austenite Content as a Function of Mn
Content at the Surface of Induction Hardened Then Fully
Machined Camshafts.
Retained Austenite Content, vol%
Measurement Technique 0.35% Mn 0.46% Mn 0.64% Mn
X-ray diffraction (lobe 3) 28.5 30.8 30.5
X-ray diffraction (lobe 6) 25.0 30.5 30.4
Representative microstructures for the three compositions
are shown in Figures 6-8.
Figure 6. Representative microstructure of the nose of the inductionhardened camshaft with 0.35 wt% Mn before grinding. Light coloredareas are retained austenite.
Figure 7. Representative microstructure of the nose of the inductionhardened camshaft with 0.46 wt% Mn before grinding. Light coloredareas are retained austenite.
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Figure 8. Representative microstructure of the nose of the inductionhardened camshaft with 0.64 wt% Mn before grinding. Light coloredareas are retained austenite.
Manganese, over the range investigated (0.35-0.64 wt%),
had no measurable effect on hardenability (depth of the
hardened layer). The measured depth of the hardened
layer before machining was 6 6.5 mm at the nose and
~4 mm at the base circle for all samples. Therefore, the
lack of a measurable Mn effect on hardenability indicated
that other elements were present in more than sufficient
quantity to provide full hardening, for the hardening
process used.
In the induction hardened only condition, surface residual
stresses were tensile in the hoop and axial directions.
This explained the tendency for lobe cracking when
defects were present or if the induction hardening
conditions were not properly controlled. As manganese
content increased, the magnitude of the surface residual
stress in the axial direction decreased but the depth at
which the residual stress changed from tension to
compression increased (an undesirable trend). The
residual stress distributions in the induction hardened only
condition are shown in Figures 9 and 10. Retained
austenite content did not correlate with residual stress,Table IV.
Figure 9. Hoop residual stress distributions for induction hardenedonly (lobe 3).
Induction Hardened Only
(axial stress)
-250
-200
-150
-100
-50
0
50
100
150
0 1 2 3 4
Depth Below Surface, mm
ResidualStress,MPa
0.35 wt% Mn
0.46 wt% Mn
0.64 wt% Mn
Figure 10. Axial residual stress distributions for induction hardenedonly (lobe 3).
Induction Hardened Only
(hoop stress)
-250
-200
-150
-100
-50
0
50
100
150
0 1 2 3
Depth Below Surface, mm
ResidualStress,
MPa
0.35 wt% Mn
0.46 wt% Mn
0.64 wt% Mn
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Table IV. Residual Stress and Retained Austenite Content
as a Function of the Depth Below the Surface for Cams in
the Induction Hardened Only Condition.
Sample
Depth
Below
Surface
(mm)
Hoop
Stress
(MPa)
Axial
Stress
(MPa)
Retained
Austenite
Content
(vol %)
0.35 wt% Mn lobe 3
0.02
0.99
2.05
3.00
62
-121
-184
-130
108
-76
-163
-181
32.4
28.8
26.3
28.7
0.46 wt% Mn lobe 3
0.04
1.17
1.98
3.01
59
-139
-185
-192
80
-96
-104
-205
27.5
27.4
23.8
26.5
0.64 wt% Mn lobe 3
0.03
1.06
2.17
3.02
52
-71
-214
-163
45
11
-105
-199
40.7
29.5
27.9
24.2
In the induction hardened then fully machined condition,
surface residual stresses were highly compressive in the
hoop and axial directions. Residual stress as a function
of depth below the surface was not determined for this
condition since they were expected to be shallow.
However, since the compressive residual stresses were
much higher than expected, residual stresses at 0.1 mm
below the surface were determined for lobe 3. At 0.1 mm
below the finished machined surface, the residualstresses were much lower but still compressive. The
residual stresses in the induction hardened then fully
machined condition are shown in Figures 11 and 12.
Induction Hardened then Fully Machined
(hoop stress)
-800
-700
-600
-500
-400
-300
-200
-100
0Surface of Lobe 6 Surface of Lobe 3 0.1 mm Below Surface of Lobe 3
ResidualStress,
MPa
0.35 wt% Mn
0.46 wt% Mn
0.64 wt% Mn
Figure 11. Hoop residual stresses at surface of lobes 3 and 6 and at
0.1 mm below the surface of lobe 3. Note: the scale has been
reversed from Figures 9 and 10.
Figure 12. Axial residual stresses at surface of lobes 3 and 6 and at0.1 mm below the surface of lobe 3. Note: the scale has beenreversed from Figures 9 and 10.
CONCLUSIONS
1. The induction hardening process alone produced
tensile surface residual stresses and compressive
subsurface residual stresses. The tensile surface
residual stresses caused cracking to occur if casting
defects were present.
2. Manganese did not have an effect on residua
stresses in the hoop direction, but did have a
significant effect on residual stresses in the axia
direction. Increasing manganese decreased the
magnitude of the surface residual stress in the axial
direction and increased the depth at which the axial
residual stress changed from tension to compression.3. The grinding process used after induction hardening
resulted in a final product that had significant
beneficial compressive surface residual stresses.
4. Increasing manganese content produced highe
retained austenite content and lower hardness in the
cam lobe nose, which reached the highes
temperature during induction heating and had the
fastest cooling rate during quenching.
5. Retained austenite content did not correlate with
residual stress.
6. Manganese, over the range investigated (0.35-0.64
wt%), had no measurable effect on hardenability.
ACKNOWLEDGMENTS
INTERMET Radford Foundry made the camshaft castings
General Motors Tonawanda Plant induction hardened and
machined the camshafts. Melanie Folks of INTERMET
Materials R&D performed the hardness measurements
and metallography. Lambda Research, Cincinnati, Ohio
performed the residual stress and retained austenite
measurements using X-ray diffraction techniques.
Induction Hardened then Fully Machined
(axial stress)
-800
-700
-600
-500
-400
-300
-200
-100
0
Surface of Lobe 6 Surface of Lobe 3 0.1 mm Below Surface of Lobe 3
Resid
ualStress,
MPa
0.35 wt% Mn
0.46 wt% Mn
0.64 wt% Mn
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CONTACT
Dr. Alan P. Druschitz received his PhD in Metallurgical
Engineering in 1982 from the Illinois Institute of
Technology, Chicago, Illinois. He is currently the Director
of Materials Research and Development for INTERMET
Corporation. He is located at the INTERMET Technical
Center, Lynchburg, Virginia 24502. He can be reached at
[email protected] or (434) 237-8749. Before
joining INTERMET Corporation, he was a staff researchengineer for General Motors Corporation for fourteen
years. He has been a member of the American Foundry
Society for thirteen years, the Society of Automotive
Engineers for twenty years and ASM International for
twenty-five years. He is currently the Vice President of the
Ductile Iron Society, a member of the Industrial Advisory
Board for the Central Virginia Governors School, and a
member of the Governors Board of Transportation Safety
for the Commonwealth of Virginia.