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Advanced Materials Opportunities and Challenges

Jonathan ParkerJonathan ParkerStructural Integrity Associates, IncStructural Integrity Associates, Inc

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Summer Workshop 2005 Annapolis, MD

Advanced Boiler Steels

Greater Strength with good ductility allows higherallowable stresses, thinner wall and higher operating

temperatures and pressures

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Advanced 2%Cr Steels

Element Grade 22 Grade 23 Grade 24

C % 0.15 max 0.04-0.10 0.05-0.10

Si % 0.50 max 0.50 max 0.15-0.45

Mn % 0.30-0.60 0.10-0.60 0.30-0.70

P % 0.030 max 0.030 max 0.020 max

S % 0.030 max 0.010 max 0.010 max

Cr % 1.9 -2.6 1.9 - 2.6 2.2 - 2.6

Mo % 0.87-1.13 0.05-0.30 0.90-1.10

Ti % - - 0.05-0.10

Nb % - 0.02-0.08 -

V % - 0.20-0.30 0.20-0.30W % - 1.45-1.75 -

Al % - 0.030 max 0.020 max

N % - 0.030 max 0.012 max

B % - 0.0005-0.0060 0.0015-0.0070

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C Si Mn P S Al Cu Cr Ni Mo V W B Nb N

0,07 0,28 0,54 0,008 0,004 0,018 0,041 2,08 0,047 0,08 0,22 1,65 0,002 0,026 0,011

Grade 23

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C Si Mn P S Al Cu Cr Ni Mo V B Nb Ti N

0,08 0,21 0,53 0,004 0,004 0,004 0,09 2,44 0,18 0,95 0,26 0,004 0,002 0,053 0,0073

Grade 24

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Grade 23 has shown problems of Stress Relief Cracking

0

200

400

600

800

1000

Te

mperature

[C

]

cooling rate100 - 150C/h

540 - 560Cmin. 1h

250 - 300C

740C

heating rate80 - 120C/h

uniformwall thickness

pre-

hea

ting

we

ldin

g

intermediate

stressrelieving

PWHT-time

dependson

wallthickness

time

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Can occur where there are significant changes insection, where beads overlap etc

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Tube Cracking in Service

Cracking on tube side of weld typically due to

excessive systems or thermal stress

T23T23

P91

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Creep Strength

Temperature [C]

10

5 h

Creepr u

pturestren

gth

[MPa

]

T/P24

T/P23T/P22

500 510 520 530 540 550 560 570 580 590 6000

50

100

150

200

250

300

T/P91

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Advanced 9 to 12 %Cr Steels

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Advanced 9 - 12%Cr Steels

Grade 91

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Grade 91 Metallurgy

Martensitic structure brittle so

tempering requiredNormalizing @1040 1100oC,

typically 1050oC.

Need to control cooling toensure completetransformation

Tempering @ 740 780oC,typically 750 to 760oC ~ 1-2 hr

Need to avoid nucleation ofnew austenite during temper

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9 12%Cr Steel Metallurgy

RGB-Image with EELS-

and EDX-analysis

blue: Z-Phasered : M23C6black: matrix

E /eV

E /eV

E /eV

N K V L2,3 Cr L2,3

Cr L2,3Fe L2,3

C K

Cr L2,3

Fe L2,3

O K

400 500300

600500400300

500 600 700400

EELS

E/eV

E/eV

E/eV

Nb

V

Cr

Cr

Fe

Fe

C

Cr

Cr

Fe

FeW Mo

Fe

FeCrCr

Fe

C

C

0 4000 8000

0 4000 8000

0 4000 8000

Cr

Cr

EDXchemical analysis

G-X12, HAZ (soft zone), 600C / 90 MPa / 26.000 h

Cr

Investigation of Z-Phase - Cr(V,Nb)N

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G-X12, HAZ (soft zone), 600C / 90 MPa / 26.094 h.Thedevelopment of Cr,V,N Z phase coincides with a loss

of creep strength

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as received condition 3.919 h / 150 MPa 8.065 h / 130 MPa 12.271 h / 120 MPa 26.094 h / 90 MPa

as received condition 3.919 h / 150 MPa 8.065 h / 130 MPa 12.271 h / 120 MPa 26.094 h / 90 MPa

G-X12 CrMoWVNbN 10.1.1, HAZ (soft zone)

Cromocord 10M, weld deposit

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M23C6

(Cr, Mo, W, Fe)

Duration at 600C, h

0 20 000 40 000

MX(V, Nb), (Nb, V)

Laves Phase(Mo, W, Cr, Fe)

Z-Phase(Cr, V, Nb, N)


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Creep Behaviour

Life with Larson Miller

Constant of 20 is onethird life with LarsonMiller Constant of 32

Alternative to describelife with stress and

temperature.

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Analysis with all rupture data indicates over estimation of long-term strength

0

100

200

300

0 - 1000 h (276) 0 - 3000 h (334) all data (408) > 1000 h (132) > 3000 h (74)

range of rupture times in extrapolation

(number of data points in brackets)

100000

h

rup

ture

stress,

M

Pa

550C

600C

650C

(Ref Ennis 2004)

Stress for 100 000 hr Rupture P92

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Creep Cavitation Damage

Long term failure occurs by the development, growthand link up of creep cavities on prior austenite grain

boundaries, lath boundaries and inclusions

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Monitoring the

development ofcavities offersthe opportunityfor assessment

- must takeappropriate care

C C i i D

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Monitoring the development of cavities offers theopportunity for assessment - must take appropriate care

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Oxidation Behaviour

Significant improvement in oxidation resistance above about 12%CrUseful operating metal temperature for Grade 91 600oC, 1112oF?

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Oxidation Behaviour

Rate influenced by alloying elements such as SiliconVariation of 2 times for Si levels within specification

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Oxidation Behaviour

Rate influenced by trace elements such as Sulphur

O id i B h i

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Oxidation Behaviour

Exfoliation can cause- tube blockages-rapid overheating and failure- erosion in downstream components

Thick scale will cause tube overheating & accelerated creep

Eg Ennis 2002

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Oxidation Behaviour

ID scale can also act asinitiation site for cracking

Bends

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Bends

History of in-service failures in pipe bends withprecipitation strengthened alloys such as CrMoV steel

problems typically due to bending and subsequent HT

Bends

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Bends

Pictures Alstom

Problems of cracking, prior to, or very soon afterstart of service with Grade 91 tube bends due to

stress corrosion cracking,

Bends

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Bends

Problems of cracking very soon after service withGrade 91 tube bends due to very rapid creep becauseof incorrect heat treatment

Post bend heat treatment frequently requires a fullrenormalization and tempering.Important to ensure proper control of heat treatment

Examples of the microstructure in nominally identical bends

Creep - Fatigue

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Creep - Fatigue

Interaction leads to similar life reduction to P22

Data N.Taylor et al

Creep - Fatigue

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Creep - Fatigue

In Grade 91interaction leads

to similar lifereduction to P22i.e.

Df + Dc = 0.25

Creep with cyclingreduces life by75% (or more)

Welds

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Welds

Control of thermal cycles - required microstructureand precipitation. Typical PWHT ~ 750 760oC

00,511,522,533,544,555,566,577,588,599,51010,51111,51212,51313,5

Time

0

200

400

600

800

1000

Tem

peratu

re[

C]

250 - 300C

760C

Heating rate

80 - 120C/h

Cooling rate

100 - 150C/h

< 400Ccooling in still air

+ 0- 10

2 - 4 h

slowcooling rate

20 - 100C

PreheatingWelding

necessary cooling down after welding

or soaking

Welds

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Welds

E911 multipass weldfusion line red line

Ac1 green line

Ac3 blue line

between Ac1 and Ac3 black area

Weld Creep

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Weld Creep

Properties below parent, P91 40-60% reduction factor ?

HAZ Weak Zone

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HAZ Weak Zone

Type IV cracking - weak zone caused by overagingduring welding in a narrow zone within the HAZ

Weld Creep

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Weld Creep

Weld HAZ verysusceptible toType IV creepCracking

Life reductionproposed of40 60%


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Stub Tube-to-Header Welds

Recent experience shows > 100 welds crackedin P91 header after around 60 000 hrs

Header side cracking of stub tube welds

Type IV Cracking in Service

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Plant failures have been reported: In US, early failures associated with high system loading,

poor heat treatment

In Europe, first failure after 20 000 hrs was attributed toincorrect heat treatment,

Others in header end plate welds occurred after 36 500 hrs(with 469 hot and 72 cold starts).

Nominal operating conditions for the headers were 568C1055oF, and 160bar.

The failed weld had been inspected with standard UT and

found clear 8,663 hrs prior to failure. Following the failure, High sensitivity UT was carried out onthe three other modified 9Cr headers on the affected unitand the equivalent headers on another unit. This led to the

discovery of further damaged end plate welds


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Cracks after 36,500h, 469 hot + 72 cold starts

Brett RWE npower


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ype C ac g Se ce

Creep Strength of Grade91 linked to N : Al ratio

With High Al, limited NAvailable to form Carbo Nitrides and Nitrides,lower Hardness and,more importantly, Lowcreep strength

Data RWE npower

Summary

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y

Advanced Steels typically exhibit excellent parentcreep properties in straight sections with correct

composition and heat treatment, but Long Term life - Larson Miller Constant

Oxidation temperature limit, thick scale leads to

problems with

Exfoliation

Cracking

Over temperature

Creep Fatigue interaction gives Life at Df + Dc=0.25

Summary

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y

Bending introduces complexities which in Grade91 can severely reduce life

Hot bend , need full renormalize and temper

Cold bend, heat treatment depends on strain

Welding can lead to problems due to

Residual Stress

PWHT

Type IV cracking

Repair, Joining to P22 Inspection, need to use validated high sensitivity

ultrasonic technique to ensure accuratevolumetric damage assessment


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yp g


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yp g

Weld Issues - P91/ P22

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