designing new damage tolerant forging steels by icmpe

01.10.15

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Designing new damage tolerant forging steels by ICMPE

Wolfgang Bleck

Werkstoffwoche Dresden 2015

ICMPE: Integrated Computational Materials and Process Design

Designing new forging steels by ICMPE - Wolfgang Bleck 2

Topics

Steels

Trends

Outline

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Topics   New processes

  Optimal material utilisation

  Damage tolerance

  Integrated Computational Materials Engineering (ICME)   Nano-characterisation

Steels

Trends

Outline


T

Heating

Austenitisation Forging

Conventional process chain

tconventional t

Hardening Tempering

Straightening

Stress relief

Topics: new processes

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tconventional

T

Heating


Isothermal

holding

Controlled cooling

Surface treatment

Air cooling

Machining at

medium heat


EcoForge process chain

t tEcoForge



T

Heating

Austenitisation Forging Controlled cooling



tEcoForge tconventional t

PHFP-Mod. process chain

tPHFP-Mod.


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T

Heating


Hardening



t

PHFP-Mod. process chain

High temperature carburising

process chain

tEcoForge tconventional tPHFP-Mod.

tHT-carburising

Carburising



High pressure components

Source: Bosch, Schaeffler

Gear components

Topics: material utilisation

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Conventional 100Cr6 as bainite

m

axim

um

syste

m p

ressu

re [b

ar]

Source: DVM-Bericht 135 "Optimierungspotentiale in der Betriebsfestigkeit", Sindelfingen 2008

High pressure components




Year

1994 1989 1976 1966 1952

400

200

0

600

500

300

100

Dynamic load rating C

Nominal life expectancy

Relative capacity:

in %

Cylinder roller bearing NU 1010 Gear components



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load capacity load

„ppm-failure“

safety

increased damage tolerance

σ2 < σ1

with reduced medium load capacity

with increased load capacity

Source: HiPerComp

Topics: damage tolerance


* Source: T. Hilditch et al: Metallurgical and Materials

Transactions A, Vol. 40 (2009), pp. 342-353

TRIP 780 EBSD image

red: ferrite, martensite green: retained austenite

with reduced medium load capacity

with increased load capacity

σ2 < σ1

increased damage tolerance

Topics: damage tolerance

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Transformation of retained Austenite into Martensite in front of a fatigue crack :

EBSD picture of a crack tip of a TRIP 780:

Ferrite and Martensite are red, retained

Austenite is green [1]

Volume fraction of retained Austenite

as a function of distance from crack

tip measured by EBSD [2]

Source: [1] T. Hilditch et al.: Cyclic Deformation of Advanced High-Strength Steels: Mechanical Behavior and Microstructural analysis,

Metallurgical and Materials Transactions A, Vol. 40 (2009), pp. 342-353

[2] X. Cheng et al.: Fatigue crack growth in TRIP steel under positive R-ratios, Engineering Fracture Mechanics, Vol. 75 (2008)

No. 3-4, pp. 739–749

Steels: Q&T steels


Hot rolling Forging FP annealing Machining Carburising

A → F+P precipitation

grain growth F+P → A precipitation,

dissolution

mic

ro

grain growth

carburising cooling forging rolling FP annealing

ma

cro

machining

Topics: ICMPE Integrative Computational Materials and Process Engineering

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Comprehensive, standardised, modular and extendable modeling platform

being efficiently adaptable to a specific material, process-chain and product

ma

cro

m

icro

su

b-µ

Thermodynamic databases

Hot rolling Forging FP annealing Machining Carburising

Topics: ICMPE Integrative Computational Materials and Process Engineering


Topics: ICMPE

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1 m 10-3 mm 10-6 µm 10-9 nm

Source: Müller Weingarten AG

TMP

UFG

NS Nanostructured steels

Ultra Fine Grain

Thermo Mechanically Processed

conventional steel

Topics: nano-characterisation


1 m 10-3 mm 10-6 µm

Source: Müller Weingarten AG

10-9 nm

Atom probe

DE-Synchotron

Topics: nano-characterisation

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  Integrated Computational Materials Engineering (ICME)

  Nano-characterisation

Steels   Case-hardening steels

  Quenched and Tempered Steels   Bearing steels

  High Mn steels

Trends

Outline


Pro

ce

ss

ch

ain

fo

r g

ea

r c

om

po

ne

nts

Heat

Treatment Rolling

Melting/

Casting Processing

Semi-

Products

cold warm

Deformation

Forging

Heat

Treatment Cutting

Case

Hardening

Final

Product

Post

Treatment

Steels: Case hardening steels

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loading heating vacuum carburising diffusion gas quenching decrease to

hardening temperature

1000

0,1

pressure, mbar

930

20

930 930 890

40

air

time

930 890

20

Temperature, °C

1000

0,1

nitrogen propane vacuum nitrogen

air

1000

0,1

vacuum

Source: ALD



High temperature carburizing

decreases the production time

and costs of gear components.

Grain size control of austenite is

needed.

Grain growth can be prevented by

microalloying of case hardening

steels.

Prediction and optimization of particle size and amount is

performed by numerical

simulation of particle evolution.


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Steels: Al-free steel

120 ppm N 220 ppm N

We

igh

t fr

actio

n

We

igh

t fr

actio

n

T in ºC T in ºC

1: MLE (C,N)

2: MnS

3: AlN


r

fPz

2

3 σ=

CASTS carburising

MICRESS Grain growth

Zener pinning pressure, MPa

Abnormal grain growth

No grain growth

Normal grain growth

Hot rolling

Carburising

Forging

FP Annealing

Machining

precipitation

evolution

Casting


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Al reduction is requested for cleanliness improvement.

The same pinning force at 1050 °C in Nb modified 25CrMo4 steel can be obtained

by an increase of Nb content to 850 ppm.

Additionally, an increase in solution temperature is needed.

Conclusion: a combined development of alloy and process parameters is needed in order to realize this new steel concept.

Reference

Al-reduced

HT-Carburizing

Temperature, °C

Vo

lum

e fra

ctio

n

Vo

lum

e fra

ctio

n (

Ti,N

b),

(C

,N)

+ (

AL

N)

Temperature, °C



100 µm

C Si Mn Cr Mo Ni V Al Nb Ti N

REF 0.24 0.22 0.89 0.92 0.43 0.18 0.008 0.023 0.034 0.0090 0.0170

Al-free 0.24 0.22 0.90 0.92 0.42 0.18 0.008 0.002 0.082 0.0005 0.0137

chemical composition in mass-% Steels: Al-free steel

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PHFP-M = Precipitation Hardening Ferritic-Pearlitic - Modified

PHFP-M

pearlite lamellae spacing λ

Ti, V, Nb (C,N)

ferrite fraction

Steels: Quenched & Tempered steel


PHFP-M = Precipitation Hardening Ferritic-Pearlitic - Modified

HDB = High Strength Ductile Bainitic

HDB

bainitic ferrite FB + retained austenite γR

PHFP-M

pearlite lamellae spacing λ

Ti, V, Nb (C,N)

ferrite fraction

Steels: Quenched & Tempered steels

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Bainitic steels: Phase field simulation

Kinetics calculations and microstructure evolution simulations

Phase-field simulated bainitic sheaves

SEM observation of bainitic sheaves


Bainitic steels: Phase field simulation of precipitates

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Microstructural description of bainite

Form

Polygonal1

Quasi-Polygonal1

Granular

Widmanstätten

Acicular

Lath-like2

Basic structure (LOM) Sub structures (≤LOM)

2nd phase form

Round1

Elongated2

Lath-like2

Film like2

Clustered

Crystal structure

bcc

Location

Boundary

Intragranular

2nd phase

Fe3C-Carbides

ε -carbide

Martensite

Austenite

None

Definitions: 1Roundness (Diff. of enclosing/enclosed ellipse) 2Aspect ratio (Length/width)


Microstructure characterisation by SEM

Quelle: F. Gerdemann: Bainite in medium carbon steel, Dr.-Ing. Disseration RWTHAachen (2010)

Primärphase Sekundärphase

kristallographische Struktur

Morphologie Entstehungsort kristallographische

Struktur Morphologie

krz (Ferrit) Polygonal Intergranular hex (Karbide) Rund

Quasi-polygonal Intragranular trz (Martensit) Gestreckt

Blockartig

kfz (Austenit) Lattenartig

Widmannstätten Nicht vorhanden Filmartig

Azikular

Inselförmig

Lattenartig Blockartig

F-L Inter-M/A-I

A-L A-B

F-B

TRIP1 Tiso=375°C tiso=30 min 4000-fache Vergrößerung

lattenartiger bainitischer Ferrit

lattenartiger Restaustenit

blockartiger bainitischer Ferrit

blockartiger Restaustenit

Martensit/Austenit

TRIP1 Tiso=375°C tiso=30 min 4000-fache Vergrößerung

15 von 48

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Steels: bearing steels

Source: Schaeffler

ab initio Modelling


Mechanical properties and microstructures in 100Cr6

Mechanical properties

•  High tensile strength (> 2 GPa)

•  High hardness (> 700HV)

•  Fatigue strength

•  Wear resistance

•  Ductility

•  Toughness

•  ……

Microstructure

•  Martensitic matrix + precipitates

•  Bainitic ferrite matrix + precipitates

•  types •  vol. fractions

•  morphology •  strain energy

•  bound./matrix

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3-Dimensional Atom Probe approach – Sample preparation

•  Electropolishing + FIB sharpening

•  FIB Lift-out + TEM + FIB resharpening

(a) (b) (c)

(e) (d) (f)


Atom Probe Tomography: sample and results

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APT results: Precipitates in lower bainite

Selected box 1

Selected box 2

Carbon atom map and 13 at% isoconcentration surface in lower bainite in 100Cr6 isothermally heat treated at 260 °C for 2500 s


ε and θ carbides in ferrite have almost identical thermodynamic stability. In austenite, however,

cementite formation is clearly preferred. This indicates that ε carbide is more prone to

precipitation from lower bainite than from upper bainite.

Atom Probe Tomography Ab initio calculation

θ and ε carbide precipitation in lower

Bainite in 100Cr6 isothermally heat-

treated at 260 °C for 2500 s.

Gibbs free reactions energies between ε

(Fe2.4C) and cementite θ (Fe3C) as a function of

temperature in a ferritic and austenitic matrix.

Bearing steel 100Cr6: carbide analysis in bainitic matrix

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Processing of forging steels: cooling strategy

1

2

3

4

Temperature

Time

1: Forging

2: Dependent on component size: acceleration of cooling by fan

3: Isothermal transformation step Tiso=375°C für 15 Minuten

4: Quenching in water


Industrial tests

•  Inhomogeneous cooling in different component areas impedes homogeneous microstructure development. Processing adjustments

dependent on size and geometry are required.

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C Si Mn S Cr Mo B Nb Ti V N

42CrMo4 0.440 0.30 0.80 0.022 1.15 0.190 - - - - -

AFP-M 0.360 0.68 1.44 0.026 0.15 0.030 - 0.029 0.022 0.19 0.0210

HDB 0.226 1.62 1.50 0.019 1.30 0.079 0.0030 0.030 0.024 - 0.0105

TRIP 0.181 0.97 2.50 0.017 0.20 0.096 0.0018 - 0.032 - 0.0069

Automotive applications:

~ 2 kg

Common Rail Axle Leg

~ 5 kg

Steering Link

~ 1 kg

chemical composition in mass-% Steels: Q&T steels


Forging steels

0

10

20

30

40

50

60

400 500 600 700 800 900 1000 1100 1200 1300

Av

at

RT

in

J

Rp0,2 in MPa

AFP

+ MLE

Q + T

+micro-alloying elements

Q + T

AFP-M

HDB

TRIP TRIP

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Forging steels

0

10

20

30

40

50

60

400 500 600 700 800 900 1000 1100 1200 1300

Av

at

RT

in

J

Rp0,2 in MPa

AFP

+ MLE

Q + T


Q + T

AFP-M

HDB

+bainitic

microstructure

TRIP TRIP


Forging steels

0

10

20

30

40

50

60

400 500 600 700 800 900 1000 1100 1200 1300

Av

at

RT

in

J

Rp0,2 in MPa

AFP

+ MLE

Q + T


Q + T

AFP-M

HDB

+bainitic

microstructure

+retained austenite

in bainitic

microstructure TRIP TRIP

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Forging steels with TRIP - effect

Austenite stability

Intrinsic Parameters:

Chemical Composition

C: increases γ stability

Si: solid solution strengthening; carbide formation retarded

Morphology

film-like austenite parallel to bainite sheaves shows slowlier transformation

Size

Austenite grains <0,01 µm are very stable

Crystallographic Orientation

Extrinsic Parameters:

Location of austenite grains

Different C enrichment dependent on diffusion conditions

Hardness of matrix

High hardness impairs transformation due to

volume change

Stress state

Kinetic of transformation depends on stress state

Residual stresses

Source: Jacques, P.; Furnémont, Q.; Pardoen, T.; Delannay, F.: On the Role of Martensitic Transformation on Damage and Cracking

Resistance in TRIP-Assisted Multiphase Steels, Acta Mat. 49 (2001)p. 139–152


0,0 0,2 0,4 0,6 0,8 1,0

0

200

400

600

800

1000

Rp0,2

=658MPa

Sp

an

nu

ng

sa

mp

litud

e σ

a [M

Pa

]

Dehnungsamplitude εa,t

[%]

TRIP

zyklisch

zügig

42CrMo4

zyklisch

zügig

Rp0,2

=834MPa

0,0 0,2 0,4 0,6 0,8 1,0

0

200

400

600

800

1000

Rp0,2

=658MPa

R'p0,2

=705MPa

R'p0,2

=602MPa

Sp

an

nu

ng

sa

mp

litud

e σ

a [M

Pa

]


[%]

TRIP

zyklisch

zügig

42CrMo4

zyklisch

zügig

Rp0,2

=834MPa

Static and cyclic properties of Q+T steels and TRIP steels

0,0 0,2 0,4 0,6 0,8 1,0

0

200

400

600

800

1000

42CrMo4

zyklisch

zügig

TRIP1

zyklisch

zügig

Spannungsam

plit

ude σ

a [

MP

a]


[%]

•  Static tensile properties Q+T > TRIP

•  Cyclic properties Q+T < TRIP (constant loading)

•  Cyclic properties Q+T < TRIP (variable loading)

 Service fatigue life of TRIP forging steel is improved compared to conventional

Q+T steel

TRIP

zyklisch

zügig

42CrMo4

zyklisch

zügig

01.10.15

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Notch sensitivity

Formzahl Kt = σ1,max

σn

σ1,max

σn

F

A

F

•  Smaller notch sensitivity of TRIP steel compared to Q+T steels

•  Notch sensitivity of TRIP comparable to mild steels

Advantages for components with complex geometrical shapes

F A

Nennspannung σn =








  Quenched and Tempered Steels   Bearing steels

  High Mn Steels

Trends

Outline

01.10.15

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High Mn Steels: Mechanical properties and deformation modes

S. Hoffmann; W. Bleck; B. Berme, Steel Research International 2012, 83, 79-384. T. Labudde; T. Ingendahl; M. Wildau; A. Saeed-Akbari; S. Hoffmann; W. Bleck, Materials Testing, 2013, 55, 636-642. .

strain / -

Str

ess, M

Pa

Str

ess, M

Pa

Technical strain / %


Deformation Mechanisms in High Manganese Steels

γ

κ κ

κ

κ

κ

κκ

T κ T T

T

T T

T

T T

T T T T T T

T

T T

T

T T T

Transformation during

deformation

Twinning during

deformation

Dislocation generation and

reactions

Dislocation channels by

intermetallics

SLIP TRIP TWIP MBIP

SLIP: Dislocation slip TRIP: transfomation induced plasticity TWIP: twinning induced plasticity MBIP: Microband induced plasticity

T T T

T T T

T T T T

T T T

01.10.15

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Material design

Mechanismenorientiertes Werkstoff- und Prozessdesign Christian Haase

O. Bouaziz, C.P. Scott, G. Petitgand: Scripta Mater., 60 (2009) 714-716. H.T. Wang, N.R. Tao, K. Lu: Acta Mater., 60 (2012) 4027-4040.

C. Haase et al.: Met. Mater. Trans. A, 44 (2013) 4445-4449.

Deformation Twins

before cold rolling after cold rolling after recovery annealing


Material design

50 % + 550 °C/30 min 40 % + 550 °C/1 h

1 µm 1 µm

RX

Twins

Twin boundaries are heat resistant C. Haase et al.: Acta Mater., 80 (2014) 327-340.

01.10.15

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0 10 20 30 40 50 60 700

200

400

600

800

1000

1200

1400

Technische S

pannung (MPa)

T echnis che D ehnung (% )

Material design

Variation of rolling degree and annealing parameters

allows the adjustment of final mechanical properties

CR: cold rolled RC: recovered RX: recrystallized

C. Haase et al.: Acta Mater., 80 (2014) 327-340. C. Haase et al.: Adv. Mater. Res., 922 (2014) 213-218.

0 10 20 30 40 50 60 70

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60 70

0

200

400

600

800

1000

1200

1400


In situ bending test in large chamber SEM

•  Twins and twin intersections are

visible

•  Surface roughening

•  Fracture occurs on surface at

shear bands

shear

bands

twin

intersections

Crack initiation at characteristic microstructural features

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Damage development

Initial state: polished

WR

30µm

-25µm

0µm

WR -25µm WR

X60Mn23

50% strain

Steel: X60Mn17 60µm

-50µm

0µm

WR

RD RD RD

Tensile strain

Investigation of surface defects by confocal microscopy I

90° to RD 45° to RD 0° to RD


Damage related to Hydrogen – Identification of hydrogen related mechanisms in steels

grain boundary

diffusion

hydrogen

embrittlement

bulk

diffusion

bulk with

vacancies

HΨ = EΨ

Investigation of H effects in AHHS

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Nanostructured Steels Stähle

γ

κ κ

κ

κ

κ

κκ

T κ T T

T

T T

T

T T

T T T T T T

T

T T

T

T T T

Transformation during

deformation

Twinning during

deformation

Transformation +

precipitation

Dislocation channels by

intermetallics

SLIP TRIP TWIP MBIP

SLIP: Dislocation slip TRIP: transfomation induced plasticity TWIP: twinning induced plasticity MBIP: Microband induced plasticity MMnS: Mittel-Mn-Stähle

T T T

T T T

T T T T

T T T

α‘ retained γ

Bainit MMnS

  Interfaces Transfer✓

γ α’

Process








  Quenched and Tempered Steels

  Bearing steels

Trends   ICMPE

  Damage tolerant design   Nanostructuring

Outline

01.10.15

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Integrative Computational Material and Process Engineering m

acro

m

icro

su

b-µ

AixViPMaP

Integration of machining

Ex

ten

sio

n t

o

life

tim

e/r

ep

air

Ex

ten

sio

n t

o

desig

n o

pti

mis

ati

on

Standardised data format / data generation ab initio

Hot rolling Forging FP annealing Carburising Machining

Thermodynamic databases

Trends: ICMPE


Damage prevention approach:

Avoid any damage occurrence by local stress / strain accumulation that results in

local failure

Damage tolerance approach:

Self healing approach:

Allow local metal degradation but limit its impact on the component structural behavior

Allow local metal degradation but enable inherent repair

Avoid critical features

Accept local plasticity

Allow diffusion repair

Trends: damage tolerance

01.10.15

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C

Cr

Mn

Si (Fe,Cr)3C / Fe3C boundary

Fe3C/αB

boundary

Z zone

(Fe,Cr)3C + Fe3C αB

(Fe,Cr)3C + Fe3C

~12nm Cr atoms map

αB

αB

αB (Fe,Cr)3C Fe3C

(Fe,Cr)3C Fe3C

(Fe,Cr)3C

Fe3C αB

Steels: bearing steels


(Fe,Cr)3C

Fe3C

αB

1-Dimensional concentration profile showing the

distribution of C, Si, Cr, Mn in undissolved

spheroidized carbide (Fe,Cr)3C, newly formed

cementite at 500 °C and bainitic ferrite matrix.

  Cr exhibits a gradual

chemical gradient from

surface to the core in

spheroidized carbides.

  S i e x h i b i t s a l a r g e

enrichment at the growth

front of cementite, which

hinders the coarsening of

cementite particles.

  The spheroidized carbide

may exist as a nucleation

site for the precipitation of

cementite within bainite.

  This microstructral feature

might be beneficial for

wear res i s tance and

fatigue properties of the

material.

Trends: Nanostructuring

01.10.15

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The authors thank

•  Exzellenzcluster - Integrative Production Technology for High-Wage Countries

RWTH Aachen University

•  Industrieverband Massivumformung e. V. (IMU)

•  Forschungsvereinigung Stahlanwendung e. V. (FOSTA)

•  Arbeitsgemeinschaft industrieller Forschungsvereinigungen Otto von Guericke

e.V. (AiF)

•  Arbeitsgemeinschaft der eisen- und metallverarbeitenden Industrie e.V (AVIF)

•  Deutsche Forschungsgemeinschaft (DFG)

•  Partners of the projects „TRIP-Schmiedestahl“, „EcoForge“, „ICMPE“, „HDB-

Schmiedestahl“, „Al-freie Einsatzstähle“, „HiPerComp“ and „SFP 761 – ab

initio“.

Acknowledgement

Welcome at the Steel Institute IEHK

Materials knowledge, you can rely on!

‹Nr.›

designing new damage tolerant forging steels by icmpe

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