development of high chromium ferritic steels … · lt-lp n k. properties: tensile ... no...

The Role of Niobium in Advanced Ferritic Stainless Steels – Building a Sustainable Future

The Royal Society of Chemistry, Burlington House, Piccadilly, London, 15th July, 2015

Development of High Chromium Ferritic Steels

Strengthened by Intermetallic Phases

Mitg

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de

r H

elm

ho

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em

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sch

aft

B. Kuhn1, M. Talik1,2, L. Niewolak1, J. Zurek1, H. Hattendorf3, P.J. Ennis4, W. J.

Quadakkers1, T. Beck1*, L. Singheiser1

1Forschungszentrum Jülich GmbH

Institute of Energy and Climate Research (IEK)

Microstructure and Properties of Materials (IEK-2)

2Czech Technical University, Praha, Czech Republic

3VDM Metals GmbH, Altena, Germany

4visiting Professor at University of Leicester, Leicester, United Kingdom

*now at: Technical University of Kaiserslautern, Institute of Materials Science and Engineering

Outlook

Motivation

• Worldwide energy demand

• The German “Energiewende” and what it means for material selection

and development

• Steam oxidation resistance and creep strength: An antagonism?

High chromium ferritic steels strengthened by intermetallic phases

• Alloying philosophy, strengthening

• - High performance ferritic steels

• The role of Niobium for

thermomechanical treatment, microstructure and mechanical

properties

Conclusion and outlook

• Perspectives, development goals

• Partnerships

Motivation

Worldwide electric energy demand: A projection

Until 2030 a rise in international energy demand by about 50 % is

estimated.

Improvement in generation efficiency is mandatory for resource

protection and mitigation of emissions.

A rise in operation parameters of thermal power plants is

necessary!

Source: FZ Jülich, IEK-STE

Motivation

German “Energiewende” means…

… strongly increased requirement for control energy because of 40 GW

of installed intermitting solar and wind power in 2013 (and growing).

• load flexibility (low minimum load, strong gradients) with further

• increased efficiency

… enhanced thermal cycling capability is needed.

• frequent start-up and shut-down cycles

• longer and more frequent downtimes

… the processes will remain, but operation parameters will significantly

tighten.

… new challenges for plant maintenance, design and material development..

Group 1 Group 2 Group 3

600 °C

10

-5 h

cre

ep s

tren

gth

[M

Pa]

100

50

6 8 10 12 14 16

Oxid

atio

n r

ate

/ lo

g K

p

Cr content [wt.-%]

Steam oxidation resistance rises with Cr content, but

creep strength drops.

P92

E911

VM

12

X20

Z-

phase

d-

ferrite

Steam oxidation resistance and creep strength: An antagonism?

Motivation

Group 1 Group 2 Group 3

650 °C

600 °C

10

-5 h

cre

ep s

tren

gth

[M

Pa]

100

50

6 8 10 12 14 16

Ox

idat

ion

rat

e /

log

Kp

Cr content [wt.-%]

Steam oxidation resistance rises with Cr content, but

creep strength drops.

at higher T: Shift to higher Cr content necessary.

An antagonism in case of creep strength enhanced ferritic-martensitic

(CSEF) steels!

P92

E911

VM

12

X20

Z-

phase

d-

ferrite

Steam oxidation resistance and creep strength: An antagonism?

Motivation

Materials for “cyclic” application – Ranking by properties

+ -- (Ni > 50)

- (Ni > 10)

+ Cost

? O(?) O + “Technology”

? + + O Creep

+ (Cr > 15)

+ (Cr > 20)

+ (Cr > 15)

- (Cr < 12)

Downtime-

corrosion

+ (Cr > 15)

+ (Cr > 20)

+ (Cr > 15)

- (Cr < 12)

HT-oxidation

/ -corrosion

? -

(TEC↑, l↓)

-

(TEC↑, l↓)

? TMF

ferritisch Nickel-base austenitic ferritic-

martensitic

Motivation

ferritic-

martensitic

austenitic Nickel-base ferritic

TMF ? -

(TEC↑, l↓)

-

(TEC↑, l↓)

?

HT-oxidation

/ -corrosion

- (Cr < 12)

+ (Cr > 15)

+ (Cr > 20)

+ (Cr > 15)

Downtime-

corrosion

- (Cr < 12)

+ (Cr > 15)

+ (Cr > 20)

+ (Cr > 15)

Creep O + + ?

“Technology” + O O(?) ?

Cost + - (Ni > 10)

-- (Ni > 50)

+

… an alternative ?

… for sure for science !

Motivation

Materials for “cyclic” application – Ranking by properties

Alloying philosophy

High chromium ferritic steels

Inherent creep strength

of the matrix

Solid solution strengthening

Cr, W, Nb

Thermomechanical

treatment

Base:

Fe

Str

ess

Time to rupture

Base

Solution

strengthening

Creep strength of

matrix

Microstructural

stability

?

Precipitation strengthening

W, Nb, Si, N, C

(Fe,Cr,Si)2(Nb,W)

Microstructural

change

no martensitic

transformation

Alloying philosophy – Typical chemical compositions

TAppl.: 700 – 900 °C:

SOFC

Base: Fe +

> 18 % Cr

Crofer® 22 H ~ 45 trial alloys

High chromium ferritic steels (CHA)

Batch ID C N Cr W Nb Si Mn La Ti

2.02W0.48Nb 0.002 0.007 22.32 0.24 0.43 0.06 0.06

2.5W0.57Nb0Ti 0.004 0.008 22.95

2.02 0.48

2.5 0.57 0.20 0.46 0.03 0.004

2.1W0.49Nb0Ti 0.002 0.004 23.08 2.1 0.49 0.24 0.45 0.12 0.003

18 Cr 0.002 0.005 18.50 1.98 0.51 0.24 0.44 0.12 0.057

Group

Crofer® 22 H

“Zero Ti”

“Low Cr”

“Zero Ti”

described in the

CHA winning paper

The specific role of Nb: Alloying, processing and application


7 W (Fe,Cr)2W / Chi 2.5W0.57Nb0Ti (Fe,Cr,Si)2(Nb,W) only

Niobium

“replaces” W, is incorporated into the Laves phase and refines it.

boosts precipitation kinetics of the Laves phase

(Si seems to be incorporated into the (Fe,Cr,Si)2(Nb,W) phase only).

“forces” an HT-Laves phase with higher TSolvus better long-term stability.

no Chi-phase formation

400 600 800 1000

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

-Cr

LT

-Lav

es

HT-Laves

(FeCr)-

Phas

e fr

acti

on

Temperature [°C]

-Fe

0,000

0,005

0,010

0,015

0,020

0,025

400 600 800 1000

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

-Cr

Laves(FeCr)-

Chi

Phas

e fr

acti

on

Temperature [°C]

-Fe

0,00

0,05

0,10

0,15

0,20

0,25

Tappl. Tappl.

cred

it: M

. T

alik

cred

it: M

. T

alik


Increasing the Nb content

rises the Laves phase fractions,

refines the particles and

stabilizes them.


1 10 100 1000

20

40

60

80

100 17CrWxNb 17CrW2xNb

Eq

uiv

alen

t ci

rcle

dia

met

er [

nm

]

Time [h]

650 °C

17

CrW

xN

b

17

CrW

2x

Nb

650 °C

3 h

3 h

1000 h

1000 h

cred

it: M

. T

alik


Increasing the Nb content

rises the Laves phase fractions,

increases TSolvus and for this reason

has direct impact on processing, too.


400 600 800 10000,000

0,005

0,010

0,015

0,020

0,025

0,030

0,035

0,040

0,045

sum

sum

HT-LP

Phas

e V

olu

me

frac

tion

Temperature [°C]

17CrWxNb

17CrW2xNb

LT-LP

app

lica

tion

cred

it: M

. T

alik

Properties: Tensile strength


No martensitic transformation!

moderate tensile properties in comparison to 9 Cr CSEF grade 92

but: Can be altered by processing!

will be demonstrated in the following…

0 200 400 600 800

0

100

200

300

400

500

600

700

800 YS 2.02W0.48Nb

UTS 2.02W0.48Nb

UTS grade 92

YS

, U

TS

[M

Pa]

Temperature [°C]

at intermediate stress (120 MPa):

Competitive to 12 Cr CSEF steel VM-12.

Reduction to 18 Cr does not have negative implications on creep strength.

Alloys with enhanced W and Nb-contents perform better,

because of higher volume fractions of strengthening Laves phase precipitates.

Properties: Creep resistance (600 °C)


at high stress (145 MPa):

Competitive to 9 Cr CSEF steel

T/P92.

20000 40000

110

120

130

140

150

160

170

600 °C

VM-12

Crofer® 22 H Spec. zero Ti low Cr

2.02W0.48Nb 2.1W0.48Nb0Ti 18Cr

2.5W0.57Nb0Ti

Str

ess

[MP

a]

Time [h]

T/P92

Microstructure: Long-term precipitate stability (600 °C)


SEM: 2.5W0.57Nb0Ti

600 °C, 100 h

Laves phase particles

grow rapidly in the short-term (up to ~1000 h).

0 10000 20000 30000

70

80

90

100

110

120

130

EC

D [

nm

]

Time [h]

18Cr1,98W0.5Nb

2.5W0.57Nb0Ti

Microstructure: Long-term precipitate stability (600 °C)


SEM: 2.5W0.57Nb0Ti

600 °C, 100 h SEM: 2.5W0.57Nb0Ti

600 °C, 26032 h, head section

Laves phase particles

grow rapidly in the short-term (up to ~1000 h).

coarsen slowly in the mid- and long-term.

are stabilized by enhanced W and Nb-contents.

0 10000 20000 30000

70

80

90

100

110

120

130

EC

D [

nm

]

Time [h]

18Cr1,98W0.5Nb

2.5W0.57Nb0Ti

Microstructure: Secondary phase formation – (Fe,Cr) -phase

(Fe,Cr)--phase at grain boundaries

might deteriorate ductility in the long-term.

forms in all the 22 Cr alloys.

SEM: 2.02W0.48Nb



Microstructure: Secondary phase formation – (Fe,Cr) -phase

(Fe,Cr)--phase at grain boundaries

might deteriorate ductility in the long-term.

forms in all the 22 Cr alloys.

Reduction to 18 Cr does

mitigate formation of the (Fe,Cr)--phase.

not have negative impact on creep strength.

What about steam oxidation resistance?

SEM: 2.02W0.48Nb

600 °C, 28224 h, head section SEM: 18Cr



600 °C

Ar50Vol.-%H2O

Properties: Steam oxidation (600 °C)

mass gain rates appr. 1-2 orders of magnitude lower than 9-12 Cr CSEF grades

comparable to high Cr austenitic grades

cut in Cr content to 18 wt.-% does not negatively affect steam oxidation resistance


2.02W0.48Nb

18Cr

cred

it: J.

Zure

k,

L.

Nie

wola

k

Preliminary conclusion


Ferritic steels strengthened by intermetallic phase particles offer promising creep strength and

favorable steam oxidation resistance.

Questions beyond: • Further potential in compositional changes?

• Role of thermomechanical processing?

• Properties relevant to application (tensile, impact and creep strength, resistance to

TMF, weldability)?

hot-rolling parameters (TMT) govern tensile properties

YS can be increased above UTS of solution annealed material in the whole temperature

range (elongation: 20 / 30 / > 50 % @ ambient / 600 °C / 700 °C +)

Tensile properties comparable to ferritic-martensitic grades are achievable!

0.007C

0.009

0.5

0.25

0.5

2

22

R

N

Mn

Si

Nb

W

Cr

Fe

0.007C

0.009

0.5

0.25

0.5

2

22

R

N

Mn

Si

Nb

W

Cr

Fe


Processing: Tensile strength

0 200 400 600 800

150

300

450

600

750

900

solution annealed, YS0.2

solution annealed, UTS

hot-rolled (moderate), YS0.2

hot-rolled (moderate), UTS

hot-rolled (enhanced), YS0.2

hot-rolled (enhanced), UTS

grade 92, UTS

YS

, U

TS

[M

Pa]

Temperature [°C]

Processing: Particle size and creep (600 °C)

Initial state:

solution annealed

HV0.1 ~ 200 2 mm

600 °C, 120 MPa,

106 h

0.007C

0.009

0.5

0.25

0.5

2

22

R

N

Mn

Si

Nb

W

Cr

Fe

0.007C

0.009

0.5

0.25

0.5

2

22

R

N

Mn

Si

Nb

W

Cr

Fe


0.1 1 10 100 1000 10000

0.01

0.1

1

10

100

Crofer® 22 H

pre-deformed

Crofer 22 model steel

Str

ain

[%

]

Time [h]

Cro

fer®

22

H

600 °C, 120 MPa tr = 106 h

Initial state:

solution annealed

HV0.1 ~ 200 2 mm

600 °C, 120 MPa,

106 h

Initial state:

rolled (moderate)

HV0.1 ~ 225 2 mm

600 °C, 120 MPa,

100 h

0.007C

0.009

0.5

0.25

0.5

2

22

R

N

Mn

Si

Nb

W

Cr

Fe

0.007C

0.009

0.5

0.25

0.5

2

22

R

N

Mn

Si

Nb

W

Cr

Fe


TMT-state has an enormous effect on particle

microstructure and thus on strength

Freedom to adjust property profiles, but

increased complexity!

0.1 1 10 100 1000 10000

0.01

0.1

1

10

100

Crofer® 22 H

pre-deformed

Crofer 22 model steel

Str

ain

[%

]

Time [h]

Cro

fer®

22

H

600 °C, 120 MPa

Processing: Particle size and creep (600 °C)

tr = 106 h

tr = 31 kh

Alloying philosophy

Base: Fe +

> 15 % Cr

~ 45 trial alloys

TAppl.: 550 - 650 °C:

steam power plants

petrochemical plants

The future:

Transfer of

alloying and

processing knowledge

Hi perF ritemanceoHigh er

TAppl.: 700 – 900 °C:

SOFC

Crofer® 22 H

The past


Fe Cr W Nb Si Mn C N

wt.-% R ≤ 18 ≥ 2.5 ≤ 1 ≤ 0.25 ≤ 0.2 < 0.01 < 0.015

Advanced trial alloys combine

further reduced Cr content (17 Cr),

enhanced W content (2.5 W) for increased Laves-phase formation,

optimized rolling parameters and thus yield

creep strength potential beyond CSEF grade 92

without (Fe,Cr)--phase embrittlement.

Creep (650 °C) – Hot-rolled condition

100 1000 10000

60

80

100

120

in progress

650 °C

T/P

9217Cr1_1

17Cr1_2

17Cr1_3

17Cr2_4

Str

ess

[MP

a]

Time [h]

Cr W Nb Si Mn N C

17 2.5 0.6 0.2 0.2 < 0.01 < 0.01

composition of the new “17Cr” batches (wt.-%):


Creep (650 °C) – Heat treatment

same batch (17Cr1_1)

solution annealed (SA) plus

• low temperature treated (LTT) or

• high temperature treated (HTT)

Creep strength can be regained from the

solution treated state by simple heat

treatment.

These heat treatments are effective in

homogenizing hardness profiles after

welding and do not harm creep strength

of the hot-rolled material.

Thermomechanical processing is not

mandatory, but beneficial.

Subject to further optimization.

0,1 1 10 100 1000 10000

0,01

0,1

1

10

SA+LTT

650 °C / 100 MPa

Str

ain [

%]

Time [h]

HR

SA+HTT


cred

it: M

. T

alik

Thermomechanical fatigue

0 1000 2000

400

500

600

700

800

900

1000AISI316L

Nf = 1320

HiperFer

17Cr2_4

Nf = 1635

T92

Nf = 1510

50 - 650 °C, 10 Ks-1

no holding times

mech.

= -th.

Str

ess

ran

ge

[MP

a]

Cycles (-)

Ferritic-martensitic steel: T92: Nf ~ 1510

not very “forgiving” after damage

initiation

Austenitic steel: AISI316L: Nf ~ 1320 much higher stress range, “unforgiving”

Ferritic steel: HiperFer 17Cr2_4: Nf ~ 1635 higher lifetime with

higher stress range than T92 and

good natured failure

(Dmech. ≈ 0.79 %)

(Dmech. ≈ 1.12 %)

(Dmech. ≈ 0.71 %)


Thermomechanical fatigue – Microstructure stability

20 mm

vir

gin

conditio

n

fatig

ue

d c

on

ditio

n

20 mm

Ferritic-martensitic steel

50 mm 0 100 mm

50 mm 0 100 mm

Austenitic steel Ferritic steel

HiperFer ferritic steel combines

microstructural stability,

low thermal expansion and high heat conductivity.


Impact strength – Heat treatment and alloying

Impact strength can be increased by • heat treatment alone or

• heat treatment and combined

alloying

DBTT shift of -70 °C by heat treatment

alone

combined alloying gives another boost,

but is not yet fully characterized

subject to optimization

to be implemented into lab scale

production 0 50 100 150 200 250

0

50

100

150

200

250

300

350

400 solution annealed (SA)

SA + heat treated

Impac

t E

ner

gy [

Jcm

-2]

Temperature [°C]

DBTT (SA):

~ 135 °C

DBTT (HT):

~ 65 °C



Hiperfore

Hiperforgee

Future applications

• ease of processing,

• weldability and

• impact strength

play the major role:

Thin wall components (tube, sheet, …)

- grades for applications, where

- grades with optimized thermomechanical processing for

applications, where

• weldability does not play a role,

• increased short-term / low temperature tensile properties

are beneficial:

Blading application

• good stress relaxation properties

are desired:

Bolting application

Recent work

New trial melts with

• doubled Laves phase content (~ 7 vol.-%) and thus enhanced mechanical,

• further improved impact strength and

• further decreased -phase stability range (< 550 °C)

were produced.

Matching SMAW electrodes were developed and first SMAW / TIG trial welds

were produced by voestalpine Boehler Welding Gmbh (Germany) / ORNL

(USA).

Optimization of trial steels concerning

• thermomechanical treatment (by rolling and / or forging) and

• heat treatment

is underway in cooperation with ORNL (USA) and RWTH Aachen (Germany).

A lot of characterization work to come …


Hiperfore

Hiperforgee

Acknowledgments

Thank you …

… to the (former) colleagues at Forschungszentrum Jülich:

M. Talik, J. Lopez Barrilao, L. Niewolak, J. Froitzheim, P. Huczkowski, W. J.

Quadakkers, J. Zurek, C. Li, Z. W. Hsiao, N. Jing, W. Chen, Y. Wang, W. Lange, M.

Braun, H. Esser, A. Moser, L. Lobert, H. J. Penkalla, E. Wessel, P. J. Ennis, T. Beck,

L. Singheiser …

ORNL: Y. Yamamoto, Y. Yang, P. Tortorelli

voestalpine Boehler Welding Germany: M. Schmitz-Niederau, O. Trunova

RWTH Aachen, Institute of Ferrous Metallurgy: H. H. Dickert, M. Schulte, A. Stieben, W. Bleck, G. Hessling

… and you for your kind attention !

development of high chromium ferritic steels … · lt-lp n k. properties: tensile ... no...

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