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Mikrostruktur –leistungsbestimmend für Makro-BauteileMicrostructure – the Performance Determining Factor in Macro-Components

Metallurgie-KolloquiumTU Clausthal, 04.11.2016

Dr. Martin Göbel,SKF Group Technology Services – Manager Global Testing

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SKF Produkte reduzieren die Reibung und machenProdukte und Prozesse schneller, sauberer und sicherer.Indem wir diese Aufgabe mit größtmöglicher Effizienz,Produktivität und Nachhaltigkeit angehen, ist die SKFGruppe zu einem der führenden Global Player beiProdukten, Lösungen und Leistungen in den BereichenLager und Lagereinheiten, Dichtungen, Mechatronik,Dienstleistungen und Schmiersysteme geworden.Weitere Serviceangebote der SKF Gruppe sind:Technische Beratung, Zustandsüberwachung,Steigerung der Anlageneffizienz und Schulungen.

www.skf.com

Die SKF Gruppe2015

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0 - Bearing steel

1 - Operatingconditions ofrolling bearings

2 - Fatigue testing ofrolling bearing steel

3 - Microstructuralalterations underrolling contact fatigue

4 – The new challengesof large components …

Bearing steel

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© SKF Group 2016-10-28Slide 5

Bearing steel microstructures

Martensite – S0 stabilised Bainite – Normal transformation temp.

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© SKF Group 2016-10-28Slide 6

Carburised ComponentsSurface Structure High Ret. Austenite

Induction Hardened ComponentsHardened and annealed

Bearing steel microstructures

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© SKF Group 2016-10-28Slide 7

Special bearing steel microstructure(very fine and uniform)

• High hardness

• High toughness

• High cleanliness

• High thermal andmechanical stability

• High corrosionresistance

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© SKF Group 2016-10-28Slide 8

Bearing steel requirements

Impa

ct to

ughn

ess

Hardness

Hardness and toughnessneed to be optimizedaccording to the bearingtype requirements

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© SKF Group October 30, 2007 © SKF GroupSlide 9

Bearing steel requirements -evolution of material cleanliness

Reduction of Oxygen, Sulphur and Inclusions à Longer Fatigue Life

0

5

10

15

20

25

30

1960 1965 1970 1975 1980 1985 1990 1995 2000

Oxy

gen

cont

ent [

ppm

]

0

5

10

15

20

25

30

35

0 5 10 15 20Oxygen Content (ppm)

Rel

ativ

e L1

0 Br

g. L

ife

0.005 wt% Sulphur

0.010 wt% Sulphur 0.015 wt% Sulphur

Rel

ativ

e be

arin

g lif

eOxygen content [ppm]

Operatingconditions ofrollingbearings

© SKF Group

© SKF Group

Operating conditions of gears vsrolling bearings

Gears - large radius (small volume)one stress cycle per revolution

typical Nf stress cycles 107

typical contact stress 400 – 800 MPa

Bearings - reduced radius of curvature~10 ~20 stress cycle per revolution

~ 50 for LSB much large volume at riskrequired Nf of stress cycles about 109

typical contact stress 1000 – 3000 MPa

© SKF Group

© SKF Group 2016-10-28Slide 12

Rolling bearings operate under very highcycle fatigueà Nf = 109 – 1011 stress cycles

Dynamicloading

Range of high Nf load cyclesof bearing applications

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+09

1.E+10

1.E+11

1.E+12

Car Engines H.S. Trains Turbine Engines Compressors

Num

ber o

f stre

ss c

ycle

s

Very High Cycle Fatigue (GCF)High Cycle Fatigue (HCF)Low Cycle Fatigue (LCF)

1011

109

Quasi-staticLoading (in relationto rolling bearingoperations)

107

108

1010

© SKF Group

© SKF Group 2016-10-28Slide 13

Rolling bearing

Rolling bearings are the

most stressed

(Ph ~ 1 - 3 GPa) and the

most dynamically

loaded components

(up to 1011 cycles) in a

mechanical system!

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© SKF Group

Orthogonal shear stress –Rolling Contact Fatigue (RCF)

Fatigue testingof rollingbearing steel

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© SKF Group

The best machine to measure the fatigue strength of bearingmaterial microstructure is .... the rolling bearing

• Bearing material fatigue testing needs toreproduce the shear stress status as found inHertzian contacts.

• Dimensional precision and finishing of thebearing components provide an importantcomponent of the Hertzian stresses

• The material microstructure resulting from themanufacturing process and heat treatment isreproduced well in a bearing

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© SKF Group

The fatigue strength of a bearing material microstructureis measured using bearing population samples

The fatigue life L10 of the bearing isthe 90% reliability of the bearingpopulation under test

L10

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© SKF Group

Example of fatigue failure (spalling) of 6309inner ring used in material fatigue testing

Endurance test machine R2

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© SKF Group

Test rigs: Bearing size:

Ø R0 rigs diam. 20 mm

Ø R1 rigs diam. 12 - 25 mm

Ø R2 rigs diam. 25 - 45 mm

Ø R3 rigs diam. 100 - 150 mm

Ø R4 rigs diam. 100 - 180 mm

Ø R5 rigs diam. up to ~ 500 mm

Rolling Contact Fatigue is affected by thematerial volume at riskàbearing size matters

R2

R3

© SKF Group

© SKF Group 2016-10-28Slide 20

Large bearingsà the larger the volume at riskà the shorter the life

Fatigue life ൎଵ

ெ௧ ௩௨ ௧ ௦

Bearing size

Rel

ativ

e B

earin

g lif

e

© SKF Group

© SKF Group

Bearing RCF life – size matters

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© SKF Group

Large size life testing - dm ~ 500

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© SKF Group

and for larger bearings ....

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© SKF Group

Large size testing - R5 test rig

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© SKF Group

Nautilus bearing life testing – SW1

2.5 m

Microstructuralalterations underrolling contactfatigue

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© SKF Group 2016-10-28Slide 27

Microstructural alterations in RCFLocal fatigue damage Matrix wide changes

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© SKF Group

Local microstructural alterations

Localized fatigue damagearound inclusions (butterfly)

Fraunhofer butterfly mappingafter running RCF at 3 GPa

Fraunhofer butterfly distribution: 3GPa vs. 2.75 GPa

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© SKF Group

Matrix wide microstructural alterations

DER (Dark Etching Region)

Steel microstructure change in the subsurface of a ballbearing after a very long over-rolling. (a) Dark etchingregion (DER) developed in the subsurface. (b)Longitudinal section showing the high angle band(HAB) and low angle band (LAB) visible in the DER.(c) A cross-section of the ring showing the DER.

Voskamp A.Microstructural change during rollingcontact fatigue, PhD thesis, DelftUniversity of Technology/SKFEngineering & Research Centre; 1997.

108 cycles; 3.3 GPa; 70 °C Z0=184 um

360 MPa

490 MPa

825 MPa

© SKF Group

© SKF Group 2016-10-28Slide 30

Microstructure fatigue damage in RCF

Microstructuralfatigue damage of

the material

No microstructuralalterations (good

performance)Transition line

ISO 281 Fatigue Limit (dm=100 mm)

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© SKF Group

Localized microstructure fatigue damage(Stress above the fatigue limit)

Slide 31

Stress • Cracks grow from weak links wherelocal stress exceeds local strength.

• Crack growth rate increases withstress (σ- σu ) and crack length.

Fatigue limit σu depends on bearing size

Number of stress cycles

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© SKF Group

Ultrasonic measure of microstructure fatiguedamage of a large size bearing after testing

Slide 34Result of LSB ultrasonic scanning

The newchallenges oflargecomponents...

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© SKF Group 2016-10-28Slide 37

Example:Siemens WindPower (6MW)

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© SKF Group 2016-10-28Slide 38

Example: Siemens Wind Power (6MW)

Rotor blade manufacturing …

… and transportation.

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© SKF Group 2016-10-28Slide 39

Example: Siemens Wind Power (6MW)

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© SKF Group 2016-10-28Slide 40

SKF2-rowtaper rollerbearing:Nautilus

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© SKF Group

Rotor bearing:Nautilus > 10.000 kgOuter Diameter 4 m6 MegaWatt Turbine

HALT - Highly Accelerated Life Testing oflarge size bearings

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© SKF Group

Large Size Bearing Test Center

Slide 42

MSTR - dedicated to wind main shaft applications

DDTR - dedicated to general bearing development

40 Mio € InvestmentBiggest LSB test center in the worldTwo new huge LSB test rigsExtremely high loads and dynamicsStart of test rig assembly June-2016Start of operation H1-2017

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© SKF Group Slide 43

The building

Sven Wingquist Test Center

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© SKF Group

The location - SKF Werk 3 in Schweinfurt

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© SKF Group Slide 45

MSTR – Main Shaft Test Rig

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© SKF Group Slide 46

MSTR - purposeTesting of single bearings Testing of shaft arrangements

à Dynamic testing, validation and development of single wind main bearings andfull wind main shaft arrangements (optionally including customer parts)à Application of relevant load conditions based on field information andsimulations

© SKF Group

© SKF Group Slide 47

Main Shaft Test Rig• MSTR (dedicated to wind main shaft applications)

• Dynamic application of high load, especially bending moments• Radial load: 8 MN• Axial load: 8 MN• Bending moment: ≥ 40 MNm (typically)• Rotational speed: 30 rpm• Bearing size: ≥ 4 m• Dynamic capabilities: 5 Hz

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© SKF Group

DDTR – Dynamic Development Test Rig

© SKF Group

© SKF Group Slide 49

DDTR- Purposeà Dynamic testing for development purposes up to high rotation speedsà For large size bearings in wind, pulp & paper, cement, steel industry, marine, etc

© SKF Group

© SKF Group Slide 50

Dynamic Development Test Rig• DDTR (dedicated to general bearing development)

• High speed, dynamic application of medium load, „medium“ sized LSB• Radial load: 7 MN• Axial load: 3 MN• Bending moment: 10 MNm• Rotational speed: 250 rpm• Bearing size: ≥ 2.5 m• Dynamic capabilities: 5 Hz

© SKF Group

© SKF Group

Start of operation in H1-2017

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© SKF Group

Special thanks to

Dr. Antonio Gabelli

SKF Engineering & ResearchCenter - The Netherlands

2016-10-28Slide 52

Thank you very much foryour attention!

© SKF Group

© SKF Group 2016-10-28Slide 53

The microstructures of heat treated bearing steels are discussed and their main characteristics

reviewed. Rolling bearings are machine components operating under very high cycle fatigue,

exceeding 1011 overrolling cycles in some very demanding applications. Contact pressures found in

rolling bearings can be up to the plasticity limit of the material, making rolling bearings the most

dynamically loaded components of a mechanical system.

Metallurgical development of bearing steels and the determination of overrolling fatigue properties are

primarily based on endurance testing of bearing population samples. Fatigue testing by using

bearings offers two major advantages:

i) Fatigue cycling in a bearing can be 20 times faster than a common rotating bending approach.

ii) In bearings the material’s microstructure which results from the manufacturing processes and

heat treatment is reproducible very well (a prerequisite for mass production). Therefore

conclusions how to improve the material’s performance by optimizing the metallurgical processes

can be derived and verified from bearing fatigue testing directly.

Bearing fatigue predictions are based on the probability of failure of the weakest link. Large material

volumes at risk, as found in large size bearings, have a strong impact on the fatigue properties of the

bearing itself: size matters! The development of advanced material microstructures for large wind

turbine main shaft bearings is discussed in terms of the experimental methods that are needed and

the gigantic testing infrastructure required to carry out such a task.

Abstract