Ductile Induction

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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.


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.


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.


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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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



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.



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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).


(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).


(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.


(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.


(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.

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Ductile Induction