Fatigue and High Strength Bainitic Steels · 2014. 6. 30. · 2 . Confidential | CR/APM2-Kuntz |...

1 Confidential | CR/APM2-Kuntz | 16/05/2012 | © Robert Bosch GmbH 2012. All rights reserved, also regarding any disposal, exploitation, reproduction, editing, distribution, as well as in the event of applications for industrial property rights.

Fatigue and High Strength Bainitic Steels Matthias Kuntz


Modern Bainitic Steels Workshop, May 16th, Paris

… in the next 20 minutes Why are fatigue properties important for engine components?

Why are bainitc steels show advantages concerning fatigue properties?

Why uses Bosch high strength bainitic steels for automotive components?

Why could carbide free bainites be a solution for fatigue requirements?

Need for further research!



Cyclic Load of diesel Components P

ress

ure

Load Cycle

Exemplary Start-Stopp-Cycle of Rail

Start Stopp

Cycles from 0 to maximum pressure

Stress ratio R > 0 Total number of cycles

up to 109

Source: Bosch Pressebild; FVV Heft 876 - 2009



ideal (specimen, laboratory)

load

abili

ty

Material and component conditions real

(component, product)

High pressure borehole heat treated surface

s

Static strength

Cyclic strength of notched specimen

s

Loading conditions Anisotropy Surface roughness Residual stresses

Comparison of specimen and real condition



Material Fatigue Behaviour – Schematic S/N-Curve

•Surface Quality • Tensile Residual Stress • Surface Material Defects • Microstructural features

Cycles to Failure (log)

Loca

l Str

ess

Am

plitu

de (l

og)

Interiour Surface

Internal Material Defects

Best Worse

Final 2 Final 1

Concurring damage processes and influences are leading to final S/N-Curve

Scource: Bacher-Höchst et al.: Materialprüfung 2009

Corrosion Fatigue



Schematic phases of fatigue fracture

Crack propagation Material

parameter Microstructure

dependent

Cra

ck le

ngth

Time, Cycles

Fracture Crack initiation Damage accumulation Defects (surface, internal)



Classification of Failure Mechanisms

Surface

Surface layer

Fracture Origin Crack Initiation

Surface

Surface Layer

Bulk area

Inclusion

Manufacturing abnorm

ality

without

abnormality Description of Fracture Origin

1 X X Surface without abnormality

2 X X Surface with manufacturing

3 X X Surface with cutted inclusion

4 X X Inclusion in surface layer

5 X X Inclusion in bulk area

6 X X „Matrix failure“



Requirements of High Stressed Components

Loads • high pressure up to 2500 bar • internal fatigue loads • direction of load perpendicular to

rolling direction • wear, cavitation • eventually corrosion • eventually rolling contact fatigue

Environmental Conditions • temperatures -40 to 140°C, partly higher • eventually internal and external corrosion

Construction • small dimensions • low wall thickness • notched • different cross sections,

partly hardened • low tolerances

Source: Bosch Pressebild



Requirements for High Strength Materials

Production • deformability (cold, warm, forging) • ability for heat treatment

(case hardening, quenching…) • machining (deep hole drilling, turning, …)

Chemistry • low amount of tramp elements • best hardenability with

low tolerances

Microstructure • homogeneous structure • no/few bandings • no/few segregations

Cleanliness • adjusted microscopic degree of purity • 0-defect-goal for macroscopic

internal defects










Properties of bainitic steels Good compromise between strength and ductility leads to good

fatigue properties.

Great variation of mechanical properties in dependence of transformation temperature.

Good process stability through isothermal heat treatment process

Homogenous distortion due to phase transformation during heat treatment.

Compressive residual stress after phase transformation during heat treatment.




fatigue properties.





0

500

1000

1500

2000

2500

3000

180 200 220 240 260 280 300

Transfomation temperature [°C]

Tens

ion

[MPa

]

0

2

4

6

8

10

12

14

Elon

gatio

n at

frac

ture

[%]

Yield StrengthTensile StrengthElongation at fracture

Scource: Kerscher (2005) Dissertation Uni Karlsruhe

Austenitising: 855°C, 20 min, Transformation 180 - 300°C, 6 h




fatigue properties.





Residual stress profile for a bainitic microstructure (e.g.: fatigue specimen)

Scource: Linkewitz (1998) VDI Reihe 18, Nr. 233



Bainite Steel applications at Bosch- Common Rail Pump

cylinder head










Fatigue Properties of CFB High Strength Steels Within the European project “Nanobain” different high strength

bainitic steels were develop Fatigue test with notched specimens (Kt = 2) tension-tension fashion

(R = 0,1) tests were performed

280

320

360

400

440

480

1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

Nom

inal

Str

ess

Am

plitu

de S

a/ M

Pa

Number of Cycles to Failure Nf



Fatigue Properties of Nanobain 0,6C Steels (R = 0,1; Kt = 2) failure mode inclusion surface not evaluated runout Interiour Surface

S/N-curve shows same characteristic as for other high strength bainitic steels

fatigue crack originate from surface

fatigue crack originate subsurface from inclusion (TiCN)

180

220

260

300

340

380

1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

Nom

inal

Str

ess

Am

plitu

de S

a/ M

Pa




Fatigue Properties of Nanobain 1C Steels (R = 0,1; Kt = 2) failure mode inclusion surface not evaluated runout

Interiour Surface

S/N-curve occurs different, but both failure mechanisms are present

fatigue crack originate from surface

fatigue crack originate subsurface from inclusion










Nanobain – Comparison of Tensile and Fatigue Properties

0

5

10

15

20

25

0

500

1000

1500

2000

2500

Elo

ngat

ion

/ %

Str

engt

h / M

Pa

Yield Strength / MPaTensile Strength / MPaSa,50% at N = 107 cycles / MPaUniform elongation / %Elongation at fracture / %

180

220

260

300

340

380

420

460

1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

Nom

inal

Str

ess

Am

plitu

de S

a/ M

Pa




Nanobain – Fatigue Properties Comparison

180

220

260

300

340

380

420

460

1,0E+04 1,0E+05 1,0E+06 1,0E+07 1,0E+08

Nom

inal

Str

ess

Am

plitu

de S

a/ M

Pa


Nanobain 0,6C Nanobain 1C

Interiour Failure origin from

internal material defects occurs at lower numbers of cycles to failure for Nanobain 1C steel

Inclusion failure at lower numbers of cycles are due to lower cleanliness level in the steel.

Surface Failure origin from the

specimen surface occurs at lower strength levels for Nanobain 1C steel

Surface quality, residual stresses and surface material defects are the same for both steels.

Decrease must be due to microstructural features

Decrease of fatigue strength is due to lower cleanliness level and microstructural feature



Nanobain 1C Steels

Cleavage fracture areas

Remaining Questions Are the “cleavage fracture

areas” the microstructural features influencing fatigue strength of Nanobain 1C steel?

What are the reason for the occurance of the “cleavage fracture areas” in Nanobain 1C steel?



Contact Details:

Dr.-Ing. Matthias Kuntz [email protected] +49(0)711/811-34413

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Fatigue and High Strength Bainitic Steels · 2014. 6. 30. · 2 . Confidential | CR/APM2-Kuntz |...

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