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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
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2 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.
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!
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3 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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
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Modern Bainitic Steels Workshop, May 16th, Paris
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
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Modern Bainitic Steels Workshop, May 16th, Paris
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
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Modern Bainitic Steels Workshop, May 16th, Paris
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)
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Modern Bainitic Steels Workshop, May 16th, Paris
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“
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Modern Bainitic Steels Workshop, May 16th, Paris
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
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9 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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
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10 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.
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!
Page 11
11 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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.
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12 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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.
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
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13 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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.
Residual stress profile for a bainitic microstructure (e.g.: fatigue specimen)
Scource: Linkewitz (1998) VDI Reihe 18, Nr. 233
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Modern Bainitic Steels Workshop, May 16th, Paris
Bainite Steel applications at Bosch- Common Rail Pump
cylinder head
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15 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.
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!
Page 16
16 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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
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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
17 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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)
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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
Number of Cycles to Failure Nf
18 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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
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19 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.
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!
Page 20
20 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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 / %
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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
Number of Cycles to Failure Nf
21 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.
Modern Bainitic Steels Workshop, May 16th, Paris
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
Number of Cycles to Failure Nf
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
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Modern Bainitic Steels Workshop, May 16th, Paris
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?
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Modern Bainitic Steels Workshop, May 16th, Paris
Contact Details:
Dr.-Ing. Matthias Kuntz [email protected] +49(0)711/811-34413