graded graded highhigh---strength steels for -strength ... · improved corrosion resistance through...
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Graded Graded Graded Graded HighHighHighHigh----Strength Steels for Strength Steels for Strength Steels for Strength Steels for
Improved Corrosion Resistance Improved Corrosion Resistance Improved Corrosion Resistance Improved Corrosion Resistance
through Inductive Heat Treatment through Inductive Heat Treatment through Inductive Heat Treatment through Inductive Heat Treatment Materials Science Engineering (MSE) 2014
22.09.2014
Dipl.-Ing. Alexander Tump, Dr.-Ing. Timm Bauschke [Mubea Fahrwerksfedern GmbH]
Prof. Dr. Robert Brandt [Universität Siegen – Lehrstuhl für Werkstoffsysteme und Fahrzeugleichtbau]
� Reviewed Material
� Fatigue strength of the material
� Correlation fatigue limit and hardness
� Surface Layer Modification (SLM) Concept
� Mechanisms behind the Surface Layer Modification
� Fatigue life results of tested components (coil springs)
� Summary
24.11.2014 Materials Science Engineering (MSE) 2014 2
Contents
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
Materials Science Engineering (MSE) 2014
Reviewed Material
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 3
� Tensile strength 1700 MPa up to 2200 MPa
� Silicon-Chromium-Steel
� Tempered by “Inductive-Heat-Treatment”
Properties
Usage
� High dynamically stressed parts
� Environmental conditions (e.g. salt-water)
coil spings forautomotive application
24.11.2014 Materials Science Engineering (MSE) 2014 4
Fatigue strength of the material
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
� corrosion starting at the surface layer (I)
� compressive residual stresses at the wire surface through shot-peening (positive)
� non metallic inclusions (III)
� cracks and decarburization at the surface layer (I)
Wire cross section
Fatigue strength is influenced by
30 µm
wire
not grinded
IIII
500 µm
non metallic
inclusion
500 µm
corrosion pit
resid
ua
l str
ess
σ
depth
ten
sio
nco
mp
ressio
n
24.11.2014 Materials Science Engineering (MSE) 2014 5
Fatigue strength of the material
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
Enhancement of the stress capacity
� optimized residual stresses in (III)
� removal of surface cracks and surface decarburization in (I)
� avoiding crack initiation (I)
SLM� reduction of the crack propagation rate (II)
� Surface Layer Modification (SLM) allows to enhance the stress capacity by optimization of the surface layer
� corrosion starting at the surface layer (I)
� compressive residual stresses at the wire surface through shot-peening (positive)
� non metallic inclusions (III)
� cracks and decarburization at the surface layer (I)
Fatigue strength is influenced by
SLM
IIII
II
HPP2
wire
grinded
resid
ua
l str
ess σ
depth
ten
sio
nco
mp
ressio
n
250 µm
Materials Science Engineering (MSE) 2014
Correlation fatigue limit and hardness
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 6
Rotational bending test (smooth, unnotched specimen)
* DIN EN ISO 18265
� Enhancement of the fatigue limit with increased hardness→ Higher tolerable stress level
� Critical hardness value due to the decrease of fracture toughness
source:
Garwood et al., 1951
tensile strength MPa*
maximumfatigue limit
coil springs Requirements:- high fatigue life- sagloss resistance
(high tensile strength)
Materials Science Engineering (MSE) 2014
Surface Layer Modification (SLM) Concept
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 7
Concept: Surface Layer Modification (SLM)
� Reduction of hardness in the surface layer
� Elevated hardness in the core area
* DIN EN ISO 18265
SurfaceHardness = 520 HV
(Rm ~ 1.700 MPa*)
Tensile TestRm = 2.025 MPa
CoreHardness = 650 HV
(Rm ~ 2.100 MPa*)
wire-Ø
500
550
600
650
0 1 2
Distance from surface in mm
Hard
ness
in H
V Core
Soft layer
T2
tempering SLM cooling station
H2OT1
Target
� Enhancement of the stress capacity
Creation of the hardness gradient
� Adjustment of the core hardness in the tempering process T1
� Adjustment of the surface hardness via an additional inductive heat treatment process T2
Standard
SLM
Materials Science Engineering (MSE) 2014
Mechanism behind the Surface Layer Modification
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 8
Fracture toughness and tensile strength
� Improvement of fracture toughness KIC
→ The fracture toughness increases significantly with decreasing tensile strength
0
10
20
30
40
50
60
70
80
1500 1600 1700 1800 1900 2000 2100 2200 2300
fractu
reto
ughness
KIc
[MP
a√m
]
tensile strength MPa
material: 54SiCr6 / SAE 9254
induktive tempered /SENB
piece tempered / CT (source: Holland, D.; Dahl, W. 1992)
Materials Science Engineering (MSE) 2014
Mechanism behind the Surface Layer Modification
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 9
Correlation inclusion size to failure
� Failure caused by inclusions→ Inclusions become more critical in materials with higher tensile strength
0
50
100
150
200
250
300in
clu
sio
nsiz
eto
failu
rein
µm
tensile strength of the used wire MPa
Analysis of fatigue life tests (dry conditions) on coil springs
N=268 Material: 54SiCr6 / SAE 9254
Inductive heat treated material
Materials Science Engineering (MSE) 2014
Mechanism behind the Surface Layer Modification
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 10
Crack propagation rate and crack initiation
� Enhancement of the crack threshold ∆Kth
→ Lower tensile-strength tends to higher ∆Kth-values (crack initiation)
1 2 3 5 6 7 8 910 20
30
40
50
60
70
8090
1,E-10
1,E-09
1,E-08
1,E-07
1,E-06
1,E-05
cra
ck p
rop
ag
ati
on
rate
da
/dN
[m]
(lo
g)
cyclic loading ∆K [MPa√m] (log)
2200 MPa2100 MPa2000 MPa1900 MPa1700 MPa
material: 54SiCr6 / SAE 9254 ## R = 0,2
10
0
� Reduction of crack propagation rate da/dN→ The crack propagation rate increase with higher tensile strength
(parallel shift of the paris-line)
delayed crack initiation
increasing crack propagation rate
Materials Science Engineering (MSE) 2014
Results of tested componentsGraded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 11
Dynamic-Fatigue-Corrosion-Test (coil springs)
0
100.000
200.000
300.000
400.000
500.000
600.000
700.000
cyc
les N
0
100.000
200.000
300.000
400.000
500.000
600.000
700.000
cyc
les N
Standard: Integral: 580 HV
SLM: Integral: 580 HV
Surface:515 HVCore: 590 HV
Standard: Integral: 620 HV
SLM: Integral: 620 HV
Surface:515 HVCore: 640 HV
n = 4 n = 4
standard spring-steel
0
100.000
200.000
300.000
400.000
500.000
600.000
700.000
cyc
les N
0
100.000
200.000
300.000
400.000
500.000
600.000
700.000
cyc
les N
Standard: Integral: 580 HV
SLM: Integral: 580 HV
Surface:515 HVCore: 590 HV
corrosion resistant spring-steel
n = 4n = 4
standard spring-steel
� Surface Layer Modification (SLM) enhances the fatigue life (stress capacity)
� A substitution of expensive materials (e.g. corrosion resistant materials) is possible
� SLM can be used with different materials
Materials Science Engineering (MSE) 2014
Summary
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 12
� SLM is based on ...
– reduced hardness of the surface layer.
– elevated hardness level in the core area.
� SLM...
– avoids a supercritical hardening at the surface.
– increases the fracture toughness of the surface layer.
– decreases the crack propagation rate in the surface layer.
– reduces the risk of fracture induced by inclusions.
� SLM offers ...
– potential to enhance the stress capacity (mass reduction is possible).
� SLM-mechanism is investigated on...
– specimen with different hardness but homogenous structure.
– parts with applied SLM-Technology (coil springs).
enhances the fatigue life of dynamically loaded components
Materials Science Engineering (MSE) 2014
Graded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment
24.11.2014 13
THANK YOU
Special thanks to the ˮEuropäischer Fonds für regionale Entwicklung” (EFRE) and “EuropäischerSozialfonds” (ESF) for supporting the project.
Special thanks to the ˮLehrstuhl für Materialkunde und Werkstoffprüfungˮ (Prof. Dr.-Ing. H.-J. Christ) for supporting the fracture mechanical investigations.