V. Smanio, T. Sourmail Ascometal CREAS - CENIM - CSIC · 3 Introduction xHow to shorten bainitic treatment ? – Process changes (Dong, Vetters et al, Smanio et al publications) Time

1

Bainite Bainite kineticskinetics of of highhigh carboncarbon steelssteels

V. Smanio, T. SourmailAscometal CREAS

2

IntroductionIntroduction

x Bainitic treatment– High carbon steel (100Cr6)

• High hardness (>700HV)• Reducing distortions associated with quenching• Low retained austenite content in-service dimensional stability

– “low” temperature (<250°C) of bainitization for hardness requirements• Long transformation times

Time

Tem

pera

ture

Austenitizing

Austempering""Bainitizing /

3

IntroductionIntroduction

x How to shorten bainitic treatment ?– Process changes (Dong, Vetters et al, Smanio et al publications)

Time

Tem

pera

ture

Austenitizing

Double stepbainitizing

Time

Tem

pera

ture

Austenitizing

Pre-quenching / bainitization

Time

Tem

pera

ture

Austenitizing

Double stepbainitizing

4

A: 100Cr6 (52100)B: 1C-1.5Si-2.5CoC / D: 1C-1.5Si-X

Time

Tem

pera

ture

3°C/s

950°C/15min

50°C/s

20°C/s

400°C

T_end °C30°C/s

T_hold (220-250) / 10-20h

Ms

1°C/s

Time

Tem

pera

ture

3°C/s

950°C/15min

50°C/s

20°C/s

400°C

T_end °C30°C/s

T_hold (220- -20h

1°C/s

ProcessProcess changeschanges

x Martensite formation before bainitizing (Smanio et al, 2010)

5

0,00E+00

1,00E-03

2,00E-03

3,00E-03

4,00E-03

5,00E-03

6,00E-03

0 10000 20000 30000 40000 50000 60000Time / s

Stra

in

Alloy C

Very significant impact on early stages

x Martensite formation before bainitizing– Kinetics

15% Martensite

0% Martensite

30% Martensite

Reduction bainitizing time up to 50%

ProcessProcess changeschanges

6

0,00E+00

1,00E-03

2,00E-03

3,00E-03

4,00E-03

5,00E-03

6,00E-03

0 10000 20000 30000 40000 50000 60000 70000Time / s

Stra

in

Alloy C

Bainite formation kinetics appears virtually identical in the remaining austenite

x Martensite formation before bainitizing (Smanio et al, 2010)

ProcessProcess changeschanges

7

Hardness is unchanged

600

650

700

750

800

0 5 10 15 20 25 30 35Target martensite %

Har

dnes

s (H

V30)

x Martensite formation before bainitizing (Smanio et al, 2010)– Martensite amount vs hardness

ProcessProcess changeschanges

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ProcessProcess changeschanges

x Modified bainitic treatments– Reduction time up to 25-50% – Added complexity to industrial process

Time

Tem

pera

ture

Austenitizing

Double stepbainitizing

Time

Tem

pera

ture

Austenitizing

Pre-quenching / bainitization

Time

Tem

pera

ture

Austenitizing

Double stepbainitizingSteel chemical

composition design

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MethodMethod

x Development of steels for bainitic treatment– Based on high carbon steel like 100Cr6 and variants– Effect of alloying elements on bainitic kinetics

• Carbon Austenitization temperature and time variations• Manganese• Silicon• Chromium• Molybdenum

x Methods– Dilatometry isothermal treatment on Bähr 805 quenching dilatometer

Lab casts with different chemical compositions

Time

Tem

pera

ture

Austenitizing

Bainitizing

10

100

150

200

250

300

0,1 1 10 100 1000

Austenitization time (min)

Ms

(°C

)

AusteniteAustenite compositioncomposition

x Austenite composition of hypereutectoid steels– Carbon in solid solution in austenite depends on

• Austenitization temperature• Austenitization time (equilibrium)

– Carbon in solid solution in austenite influences properties

T°aust 860°C

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AusteniteAustenite compositioncomposition

x Method for determination of austenite composition– At equilibrium using Matcalc time to reach equilibrium ?

– In defined austenitization conditions using Ms formula • Dilatometry quenching after defined austenitization conditions Ms• Calculation of austenite Carbon content using Ms formula

T°aust 860°C

100

150

200

250

300

0,1 1 10 100 1000

Austenitization time (min)

Ms

(°C

)

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AusteniteAustenite compositioncomposition

x Validation of the method : 100CrMnSi6-4– Austenitization at 890°C– All carbon in solid solution at this temperature (Matcalc + SEM observations)

0

50

100

150

200

250

300

0 20 40 60 80 100 120 140 160 180 200

Austenitization time (min)

Ms

(°C

)

Andrews 2

Andrews 1

Steven Haynes

Nehrenberg

Measurement

1 µm

13

Austenitization/C content Austenitization/C content effecteffect

x Austenite composition of 100CrMnSi6-4– Austenite carbon content depending on austenitization temperature and time

0,700

0,750

0,800

0,850

0,900

0,950

1,000

820 830 840 850 860 870 880 890 900Austenitization temperature (°C)

Aus

teni

te c

arbo

n co

nten

t (%

)

60 min

60 min60 min

20 min

2071478261241180596958bulkCuCrMoNiMnSiC

140

20

40

60

80

100

120

0 20000 40000 60000 80000Time (s)

Bai

nitic

tran

sfor

mat

ion

(%)

e

100CrMnSi6-4 830°C/60 min 100CrMnSi6-4 850°C/60 min

100CrMnSi6-4 860°C/60 min 100CrMnSi6-4 890°C/20 min

↓ Austénitisation T°

890°C/20 min860°C/60 min850°C/60 min830°C/60

Austenitization/C content Austenitization/C content effecteffect

x Kinetics– Isothermal transformation at 220°C

740

760

780

800

820

840

860

880

900

0 5000 10000 15000 20000 25000 30000 35000Time (s)

Car

bon

cont

ent (

10-3

%)

10% transformation 50% transformation 90% transformation

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Austenitization/C content Austenitization/C content effecteffect

x Hardness– Isothermal transformation at 220°C

70888220890703842608607098326085071075560830

Hardness(HV30)

C content (10-3 %)

Austenitization time (min)

AustenitizationT (°C)

No difference in hardness

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AlloyingAlloying elementselements effecteffect

x Manganese content– Isothermal transformation at 220°C

860°C/60 min860°C/60 min

Aust

189185Cu

244175612132476864884100CrMo7-3 low Si low Mn261174011871074852873100CrMo7-3 low Si MoCrNiMnSiC austC bulk

0

10

20

30

40

50

60

70

80

90

100

0 10000 20000 30000 40000Time (s)

Bai

nitic

tran

sfor

mat

ion

(%)

100CrMo7-3 low Si low Mn100CrMo7-3 low Si

↓ Mn content

17

AlloyingAlloying elementselements effecteffect

x Molybdenum content– Isothermal transformation at 220°C

0

10

20

30

40

50

60

70

80

90

100

0 10000 20000 30000 40000Time (s)

Bai

nitic

tran

sfor

mat

ion

(%) 100CrMo7-3 low Si low Mn

100CrMo7 low Si

189191Cu

860°C/70 min860°C/60 min

Aust

244175612132476864884100CrMo7-3 low Si low Mn151177012230760862908100CrMo7 low Si MoCrNiMnSiC austC bulk

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AlloyingAlloying elementselements effecteffect

x Chromium content– Isothermal transformation at 220°C

860°C/20 min860°C/60 min

Aust

189189Cu

253149212131566845898100CrMo6-3 low Si low Mn244175612132476864873100CrMo7-3 low Si low MnMoCrNiMnSiC austC bulk

0

10

20

30

40

50

60

70

80

90

100

0 10000 20000 30000 40000Time (s)

Bai

nitic

tran

sfor

mat

ion

(%)

100CrMo7-3 low Si low Mn100CrMo6-3 low Si low Mn

↓ Cr content

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AlloyingAlloying elementselements effecteffectx Silicon content

– Determination of austenitization conditions

860°C/60 min860°C/10 min860°C/20 min

Aust

189191189Cu

26517441182951198793899100CrMo7-3 high Si low Mn2301744119321599796889100CrMo7-3 medium Si low Mn244175612132476794884100CrMo7-3 low Si low MnMoCrNiMnSiC austC bulk

0,700

0,720

0,740

0,760

0,780

0,800

0,820

0,840

0,860

0,880

0,900

0 20 40 60 80 100 120 140 160 180 200Austenitization time (min)

Car

bon

cont

ent (

%)

100CrMo7-3 low Si low Mn100CrMo7-3 medium Si low Mn100CrMo7-3 high Si low Mn

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AlloyingAlloying elementselements effecteffectx Silicon content

– Isothermal transformation at 220°C

0

10

20

30

40

50

60

70

80

90

100

0 10000 20000 30000 40000Time (s)

Bai

nitic

tran

sfor

mat

ion

(%)

100CrMo7-3 low Si low Mn100CrMo7-3 medium Si low Mn100CrMo7-3 high Si low Mn

↓ Si content

0

200

400

600

800

1000

1200

1400

0 2000 4000 6000 8000 10000 12000 14000 16000

Time (s)

Si c

onte

nt (1

0-3%

)

10% transformation 50% transformation 90% transformation

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DiscussionDiscussion

x Alloying elements effect on bainitic transformation kinetics in hypereutectoid steels

– No influence : Molybdenum– Small influence : Chromium– Strong influence : Manganese, Silicon and Carbon

Quidort et al, Acta Materialia 2001

↓Chromiumcontent

Quidort et al, Acta ISIJ 2002

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DiscussionDiscussion

x Design of new steels chemical compositions

253149231566898100CrMo6-3 low Si low Mn151177030760908100CrMo7 low Si

2601752639239973100CrMo7-3MoCrMnSiC bulk

Time reduction up to 50%/ standard

100CrMo7 low Si

100CrMo7

100CrMo6-3 low Si low Mn

100CrMo7-3

-60

-50

-40

-30

-20

-10

0700 720 740 760 780 800

Hardness (HV30)

Tim

e re

duct

ion/

100C

rMo7

-3

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ConclusionsConclusions

x Conclusions– Parametric study to evaluate influence of alloying elements on bainitic kinetics

of 100Cr6 and variants

– Main influence of :• Carbon (austenitization conditions)• Manganese• Silicon

– Design of new steels chemical compositions dedicated to bainitic treatment • Time reduction up to 50% / standard steel without loss in hardness