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


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� 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


Fatigue strength of the material


� 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


Fatigue strength of the material


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


Correlation fatigue limit and hardness


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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)


Surface Layer Modification (SLM) Concept


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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


Mechanism behind the Surface Layer Modification


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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)




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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




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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


Results of tested componentsGraded High-Strength Steels for Improved Corrosion Resistance through Inductive Heat Treatment

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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




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




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


Summary


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� 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



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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.

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