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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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Advanced 2%Cr Steels
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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Advanced 2%Cr Steels
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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Advanced 2%Cr Steels
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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Advanced 2%Cr Steels
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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Advanced 2%Cr Steels
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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Summer Workshop 2005 Annapolis, MD
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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9 12%Cr Steel Metallurgy
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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9 12%Cr Steel Metallurgy
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)
9 12%Cr Steel Metallurgy
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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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Creep Cavitation Damage
Monitoring the
development ofcavities offersthe opportunityfor assessment
- must takeappropriate care
C C i i D
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Creep Cavitation Damage
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%
Tube Cracking in Service
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Tube Cracking in Service
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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Type IV Cracking in Service
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
Type IV Cracking in Service
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Type IV Cracking in Service
Cracks after 36,500h, 469 hot + 72 cold starts
Brett RWE npower
Type IV Cracking in Service
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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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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
Type IV Cracking in Service
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yp g
Type IV Cracking in Service
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yp g
Weld Issues - P91/ P22
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