The Development of Cast Iron Materials for Brake Discs

� Marc Walz, Reiner Becker,CastTec 2012, Krefeld

Fritz Winter Eisengießerei GmbH & Co.Kg

� Today's demands on brake rotors

� Metallurgical basics

� Types of load during braking

� Development of a new disk material

Topics

� Test procedure

� Test results

� Conclusiones and preview

Today´s demands on brake rotors

market price / competitiveness

light weight

natural frequencies /damping /

NVH behaviour

design

thermal performance /thermal shock vs.

thermal fatigue

corrosion / wear

During braking kinetic energy needs to be changed into heat. Different load profiles lead to various forms of damage mechanisms like

thermal shock thermal fatigue thermal shock fatigue

Modern brake disks in Europe are predominantly produced in the material category named EN-GJL 150

A lot of different test work makes it more and more clear that there is no ‘universal’ material for the brake disk at present.

Today's demands on brake disks

However to face the high expectations it will increasingly be necessary to design new brake disk materials according to these increasing demands .

‘universal’ material for the brake disk at present.

The question is how to develop and modify a material that can cope with all the different tasks that cast iron brake discs have to sustain, under economic aspects.

Grey iron is a multi alloy system. Main elements are Fe, C and Si

Metallurgical Basics

area of steelarea of grey iron

area of high carbon brake disk material

Metallurgical Basics

Advantages of grey cast iron:

• high compressive strength

• good damping capacity

• good castability

• good self feeding potential (graphite expansion)

• good thermal conductivity

• good machinability

• low material costs • low material costs

• 100 % recyclable

Disadvantages of grey cast iron:

• limited tensile strength (strong wall thickness dependency)

• bad ductility (< 0,8% elongation, at room temperature)

• low corrosion resistance

Types of load during braking

� inhomogeneous, unsteady temperature distribution like

� thermal shock� thermal fatigue� thermal shock fatigue

getting more and more problematic due to weight reduction of disks and more high performance cars

The life time of a brake disk is mainly dominated by two different loading conditions:

� mechanical forces� centrifugal forces� torsional forces� preasure forces

no longer a problem these days,not object of this paper

Both load components cooperate in a very complex and still today not fully analyzed way.

Types of load during braking

Thermal stress is thermally induced mechanical stress caused by transient temperature gradients in a part. This leads either to contraction- or elongation constraints.

The different types of load could be divided into:

� Thermal shockhigh temperature gradient, high thermal stress, crack initiation, instablecrack propagation and a rapid loss of mechanical strength

� Thermal fatigue

In reality we see a mixture of thermal shock and thermal fatigue

� Thermal fatiguecyclic load profil with lower temeperatur gradients on a middle orhigh temperature level.

� Thermal shock fatiguea mixture of both, thermal stress and thermal fatigue.

A good resistance against thermal fatigue is based on:

A good resistance against thermal shock is based on:

� low tensile strength� low Young´s -modulus

Types of load during braking

These mixture of thermal shock and thermal fatigue causes the problems of the right material selection for brake disks.

A good resistance against thermal fatigue is based on:

� high tensile strength� high Young´s-modulus

Therefore it`s necessary to make compromises or to choose special materials for special applications.

What is destroying the disk? Thermal stressWhat is thermal stress? σ th = E x α x ∆TWhat can be influenced? E and ∆T

E α ∆T σth

� Increasing amount of graphite

Development of a new disk material

� Elimination of disturbing alloy elements

The result was a new brake disk material with a high carbon and low silica content, a pure matrix with a more homogenious graphite distribution and

improved material parameters, we called

VARIFER®

� Elimination of disturbing alloy elementsespecially silica

Development of a new disk material

Matrix VARIFER® after extrem thermal load Matrix GJL 150 after extrem thermal load

Test results show:

� a better crack resistance � less wear� a better corrosion behaviour

Development of a new disk material

Due to the permanantly increasing demands on brake disks it was decided to start a huge test program for all high performance brake disk materials produced at FW.

The aim of these test programm was to get a more deeper understanding of the influence of different alloying elements at different temperature levels and in combination with VARIFER®.

Concerning elongation at higher temperatures we found big differences in

between different alloying elements.

Tests were done with proportional test bars on a tensile test maschine with heating device in a temperature range up to 700°C.

The validation of the test results were done on a full scale dynamometer.

This gives an indication why the Mo alloyed VARIFER®

material works best on parts with high thermal loads as truck disks and high performance car disks. 0 200 400 600 800

Elo

ngat

ion

Temperature in °C

Development of a new disk material

Of particular interest was the influence of Niobium and Molybdenum.

The influence of Nb at 500°C is similar to the influence of Mo at 700°C.

Elon

gatio

n]

x% Mono Mo z% Moy% MoNb Mo

400 °C500 °C700 °C

3,5

0

20

40

60

80

100

120

140

20 100 200 300 400 500 600 700

Varifer

EN-GJL 150

0

50

100

150

200

250

20 100 200 300 400 500 600 700

Rm Varifer

Rm EN-GJL 150

Rp 0,2 Varifer

Rp 0,2 EN-GJL 150

Rm and Rp 0,2 (N/mm²)Tensile strength, Yield strength

Elongation (%)

Young´s modulus

Development of a new disk material

0

0,5

1

1,5

2

2,5

3

3,5

20 100 200 300 400 500 600 700

Varifer

EN-GJL 150

Thermal expansion coeff. ; thermal conductivity, density, thermal capacity

Varifer EN-GJL 150 Varifer EN-GJL 150 Varifer EN-GJL 150 Varifer EN-GJL 150

T °C α 10-6 / K λ W / mK ρ kg / m³ Cp J / g K

20 - - 56,1 60,1 7195 7153 - -

100 10,8 10,8 51,3 54,2 7176 7135 0.511 0.507

200 11,4 11,4 48,4 50,3 7151 7109 0.553 0.556

300 12,1 12,1 45,1 46,2 7122 7081 0.586 0.592

400 12,7 12,6 42,2 43,0 7092 7051 0.623 0.631

500 13,3 13,4 39,8 40,6 7065 7025 0.675 0.690

600 13,6 13,7 37,0 37,5 7037 6997 0.745 0.754

700 14,0 14,1 34,3 34,6 7009 6970 0.879 0.926

Test procedureCurrent serial production parts made of EN-GJL-150 HCare compared with dimensional identic parts made of VARIFER® . The objective of the tests was to find out if there are differences in the behaviour of both disk materials and what are the reasons for it.

Test facility:� Horiba Dyno Giant 6200 at Fritz Winter� 6200 Nm max. brake torque� 2800 Rpm max.� 8 rotating temperature sensores� 4 static temperature sensores� 4 static temperature sensores� 6 capacitive sensores for deformation� Cooling air with constant temperature and

speed arround the test piece

Test procedure:�Acc. FW-Spec. Tape-F-0001

� Deformation / Coning� Crack testing

Test part:�18“ Frt axle brake disc OD 355 mm

Test Results - Coning

∆T= 27,6°C

∆T= 40,6°C ∆T= 24,4°C

∆T= 32,1°C ∆T= 27,9°C

∆T= 19,9°C

One complete coning test run

∆T= 40,6°C ∆T= 24,4°C

VARIFER®EN-GJL-150

- 0,2080 mm- 0,2190 mm - 0,2220 mm - 0,2060 mm

- 0,1620 mm

- 0,1850 mm

Test Results - Coning

Coning results

EN GJL Varifer

inboard / GJL outboard / GJL Delta / GJL inboard / Varifer outboard / Varifer Delta / Varifer

Bedding 0,1060 0,1200 0,0140 0,0960 0,1260 0,0300

Stop 1/1 0,1070 0,1400 0,0330 0,0980 0,1490 0,0510

Stop 1/2 0,1950 0,2190 0,0240 0,1490 0,2080 0,0590

Stop 2/1 0,1700 0,1860 0,0160 0,0949 0,1280 0,0331

Stop 2/2 0,2040 0,2220 0,0180 0,1570 0,2060 0,0490

Stop 3 0,1600 0,1850 0,0250 0,1250 0,1620 0,0370

0,0000

0,0500

0,1000

0,1500

0,2000

0,2500

Bedding Stop 1/1 Stop 1/2 Stop 2/1Stop 2/2 Stop 3

inboard / GJL

outboard / GJL

Delta / GJL

0,2500

Test Results - Coning

� There are different behaviorsof the two materials during the coning test run.

� There is a more homogenousbehaviour between inboard and outboard rotor face at EN-GJL 150 compared to VARIFER®.

0,0000

0,0500

0,1000

0,1500

0,2000

Bedding Stop 1/1 Stop 1/2 Stop 2/1 Stop 2/2

Stop 3

inboard / Varifer

outboard / Varifer

Delta / Varifer

� Between the first and seconddouble stop the VARIFER®

disk recovers nearly back to the starting position.

� The coning level itself of VARIFER® is lower than ofEN-GJL 150 GJL.

Test Results Crack Test

Varifer

EN-GJL-150 HCStop 1 Tmax: 388°CStop 2 Tmax: 532°CT-diff.: 144°CTime: 612 sec

VariferStop 1 Tmax: 408°CStop 2 Tmax: 511°CT-diff.: 103°CTime: 625 sec

Test Results

0

5

10

15

20

25

30

35

inboard / GJL

outboard / GJL

Delta / GJL

inboard / Varifer

outboard / Varifer

Delta / Varifer

EN GJL Varifer

15

35

20

3

22

18

Length of cracks after 500 stops

mm

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

EN GJL Varifer

0,79

0,55

gr / stop

Length of cracks after 500 stops

VARIFER®

Wear gr / stop after 500 stops

EN-GJL-150

Test Results

To explain what`s happening it seems that we need to look back to an old figure called „Eichelberg Faktor K“ which gives an indication about the resistance of an material against thermal cracks. The higher „K“ is the better is the resistance against thermal cracks.

K = 6,00

8,00

10,00

12,00

Eic

he

lbe

rg

-Fa

kto

r K

GJL150HC

The main influence is coming out of the relation of the tensile strength at room temperature compared to higher temperatures

* E is divided by 1000

K

0,00

2,00

4,00

0 200 400 600 800

Eic

he

lbe

rg

Temp. In °C

GJL150HC

Varifer I

Conclusion

� VARIFER® and EN-GJL 150 show a different behaviour under thermal load.

� VARIFER® is more resistant against thermal cracks due to a higher Eichelberg-factor. Main reason therefore is a higher tensile strength also on highertemperatures.

� VARIFER® is more resistant against wear. The reason could be the differencein Young´s-modulus, tensile strength and yield strength and the resultingbehaviour during stress and strain on high temperatures.

� The belief that especially in high carbon grey iron a relatively low tensile and

� It is necessary to find the right combination of thermal conductivity, thermal expansion, density, Young`s-modulus, tensile strength and 0,2 yield strength to a specific application which could be done with different alloying elements in the right concentration.

� The belief that especially in high carbon grey iron a relatively low tensile andyield strength is good for the thermal behavior of the disks is right but only at room temperature. It is also necassarry to have a stable tensile strenght level over a maximum temperature range up to 700°C.

Preview

� The next step will be a more intensive test programm with VARIFER® brakedisks and special alloying elements in different concentrations.

� There is a need to find solutions how to stabalize the tensile strength on higher temperature levels.

� and what is the impact of the different OEM test specifications to the results.

� An additional aim of the tests is to find out which weight reduction could berealized by using this new material for conventionel brake disks

� Another aim, which is not subject of this paper, is to investigate the corrosionbehavior of VARIFER® which is much better as it is for current EN-GJL 150.

� and what is the impact of the different OEM test specifications to the results.

Thank you very much for your attention.

Any questions ?

Authors

Dipl-Ing. (FH) Reiner BeckerEisengiesserei Fritz Winter GmbH & Co.KgAlbert Schweitzer Straße 15D-35260 StadtallendorfPhone: +49 6428 78 546Fax: +49 6428 78 77 546Mobil: +49 171 4988263E-Mail: Reiner.Becker@FritzWinter.de

Dipl-Ing. Marc WalzEisengiesserei Fritz Winter GmbH & Co.KgEisengiesserei Fritz Winter GmbH & Co.KgAlbert Schweitzer Straße 15D-35260 StadtallendorfPhone: +49 6428 78 840Fax: +49 6428 78 77 840Mobil: +49 160 7106370E-Mail: Marc.Walz@FritzWinter.de