boiler materials
DESCRIPTION
Boiler SteelHigh TemperatureOxidation resistantTRANSCRIPT
1
Boiler Materials
P.Sundaramoorthy
BHEL, Tiruchirappalli
2
The following topics are planned to be covered in this talk:
1. Selection of materials for high temperature service
2. Limitations of the conventional materials
3. Development of new materials
4. Some of the problems due to the weldments during high temperature service
5. Some of the weldability aspects of the newer grades
Introduction
3
General Considerations: Materials
The following major properties of materials is of interest in the choice of materials for Boiler and Pressure Vessel applications:
• Strength at room temperature and elevated/ service temperature
• Corrosion/ Oxidation resistance
• Stability of structure over a service period normally about 30 years
• Ease of fabrication including welding
4
Effect of Common Alloying Elements
Carbon: This is the main element which provides strength. For considerations of weldability the carbon content is restricted to 0.25% in IBR and in many of the European codes.
American Code (ASME B&PV) allows carbon up to 0.35%.
The purchase specifications of BHEL restricts the carbon to a maximum value of 0.30%.
Carbon has a major bearing on the high temperature strength also, for example a minimum of 0.04% of carbon is required as per ASME B&PV code to ensure the high temperature creep properties of austenitic SS grades.
5
Chromium: This is the major alloying element conferring the oxidation /corrosion resistance to the steel. This element also provides resistance to corrosion in sulphur rich flue gases.
Continuous oxidation temperature vs the Chromium content0
1 2.255
9
1217
27
400
500
600
700
800
900
1000
1100
0 5 10 15 20 25 30
Weight % Chromium
Ox
ida
tio
n t
em
pe
ratu
re (
un
de
r
flu
e g
as
es
) d
eg
.C
These temperatures are based on
oxidation/ corrosion by flue gases
wherever applicable.
In case of plain air as in pent
house region, higher metal
temperatures can be tolerated.
Effect of Common Alloying Elements
6
Molybdenum: The main alloy element which confers creep resistance for the steel. 100,000 hrs rupture strength is used in these presentations for the purpose of various comparisons
Effect of Mo on 100,000hrs, rupture strength
0
50
100
150
200
250
300
440 450 460 470 480 490
Temperature deg. C
100,0
00h
rs.
rup
ture
str
en
gth
,
N/
sq
.mm
Carbon 0.30Mo Steel,
15Mo3
Carbon Steel,
St35.8,45.8
Effect of Common Alloying Elements
7
The other common alloying elements used for enhancing the creep resistance are Nb, V, and W. Similar to Mo these are strong carbide formers, providing a fine network of carbides in the matrix impeding the dislocation movement thus enhancing resistance to creep deformation.
Nitrogen is used in order to substitute the carbon and form nitrides which provide creep resistance similar to carbides.
Effect of Common Alloying Elements
8
Effect of De-oxidation Practice & Grain Size
Fully killed steels are preferred for high temperature application in view of their homogeneity.
The higher creep strength of silicon-killed steels has been attributed to the free nitrogen available in these. This superiority is seen only in short term tests. In the long term, there is no difference.
9
Aluminum killed steels because of their fine grain size have better toughness as well as matching strength at higher temperatures with silicon killed steels, hence can be used at higher temperatures.
Higher proneness to graphitisation of aluminum treated steels, however, is to be kept in mind.
Effect of De-oxidation Practice & Grain Size
10
The problem of graphitisation
There had been failures in Carbon and Carbon Moly steel piping operating at temperatures beyond 425 deg C by this phenomenon.
Graphite being the more stable phase than cementite there is a tendency during high temperature service after long times for the carbides in these steels to separate out as iron and carbon (graphite)
11
Weld HAZ of multi layer joints where the metal temperature has reached just above the lower critical temperature (7500C inter critical temperature zone) are the preferred regions for graphitisation.
Cold worked bands in base materials are also locations where chain type graphitisation has been observed.
Based on a study of various failures of this type and also examination of piping, working in this temperature range, the time temperatures required for such material degradation has been worked out.
The problem of graphitisation
12
Time-Temperatures for different levels of graphitisation10000
100000
100000
01E
+07
320 370 420 470 520 570
Temperature deg C
Lo
g t
ime (
ho
urs
)
Graphitisation level 20%
Initiation of graphitisation
Graphitisation level 30%
Graphitisation level 50%
The problem of graphitisation
13
14
Area of
Application
Material type Typical spec. for
Plates, Tubes,
Pipes
Upper limit Temp.
deg C(Heat
Absorbing
Surface)
Guiding Reason
for Upper Limit
Drum C Steel/ Low
Alloy Steel
SA299 425
Water walls,
Economiser
C Steel SA192, SA210,
SA106
425 Graphitisation
Superheater and
Reheater
C ½ Mo steel A209 T1 465 Graphitisation
1Cr ½ Mo SA213T11,
SA335P11
565 Oxidation/
corrosion, Flue
gas
2 ¼ Cr 1Mo SA213T22,
SA335P22
580 Oxidation/
corrosion, Flue
gas
18 Cr 8 Ni SA213 TP304 H 704
18 Cr 10 Ni Cb SA213 TP347 H 704
Modified 9Cr SA213 T91, T92
SA335 P91, P92
650 ASME code
12%Cr X20CrMoV12 1 700 German Code
Conventional Boiler Materials
15
ROOF
To withstand higher
temperatures expected
inside the gas path,
higher grade material,
T91, is given inside the
flue gas path,
(as compared to T22
material inside the
penthouse, i.e. above
roof) Gas Flow
Heat Absorbing Surfaces
16
Creep rupture strength of conventional
ASTM materials
100 000 hrs Creep rupture stress of conventional ASTM materials
0
50
100
150
200
250
300
325 375 425 475 525 575 625 675 725
Temperature deg C
Ru
ptu
re
str
es
s M
Pa
Carbon steel
1.25 Cr 0.5 Mo Si Steel (T11/ P11)
2.25 Cr 1 Mo Steel (T22/ P22)
Carbon 0.3 Mo Steel (German Steel 15 Mo3)
304 H Steel (min)
17
Following are some of the issues which led to the development of newer grades.
1. Beyond 6000C only austenitic stainless grades have the necessary corrosion/ oxidation resistance and creep strength.
2. However austenitic stainless steels have the following limitations:
• Higher thermal expansion and lower thermal conductivity
• Higher affinity for carbon of austenitic grades causes carbon migration to austenitic area, causing decarburisation in the ferritic side HAZ, leading to poorer creep strength of this region
Reasons for development of newer grades
18
Steel Type Type α
X10-6 °C
λ at 20°C
W/m.°C (Conductivity)
Ω at 20°C
nΩm
E at 20°C
kN/mm²
Carbon
Steel
1016 13 47 150 205
Ferritic S44400 12.5 24 600 225
Ferritic
Austenitic
329 13.5 20 850 205
Austenitic 304 19.5 15 700 200
19
T22
304H
T22
304H
304H T22
20
Stresses due to the differential thermal expansion and also lower high temperature strength of the decarburised zone leads to creep fracture along this zone and this type of failure is called Dissimilar Metal Weld failures or DMW failures.
Use of Ni base (inconel) filler has been found to improve the situation by delaying the onset of failure, and the failure situation was not fully eliminated.
The other problem is the proneness of austenitic stainless steel to SCC.
Development of ferritic grades of steel with improved creep strength, matching that of austenitic grades was necessitated for the above reasons.
Reasons for development of newer grades
21
SS347H Tube
Ø 63.5 x 8.0 mm
SS347H Insert
Ø 63.5 x 12.5 mm
(8mm near STBW)
SS347H Insert
Ø 51 x 6 mm
STBW
joint
Weld
joint
SS 347H
Bifurcate
Ø 51 x 7.5 mm
SS347H Insert
SS347H Tube
OD
SH Bifurcate Assembly Less than one month service- SCC
due to welding stress in HAZ
22
ID
SS347H Insert
Ø 51 x 6 mm
23
24
2.25 Cr 1 Mo
ASME T/P22
(STBA24)
2.25Cr-1.6WVNb
HCM2S
(ASME T23, STBA 24J1)
60-80 MPa 80-100 MPa 140-180 MPa30 MPa
2.25 Cr 1MoV
9Cr 1Mo
ASME T9
(STBA26)
9Cr 2Mo
HCM9M
(STBA27)
9Cr2MoVNb
EM12
NFA 49213
9Cr1MoVNb
Tempaloy F-9
9Cr1MoVNb
ASME T91
STBA28
9Cr0.5Mo1.8WVNb
E911, NF616
ASME T92, STBA 29
12Cr
AISI 410
12Cr-0.5Mo
12Cr0.5Mo1.8WVNb
(TB12)
12CrWCoNiVNb
(NF12)
12Cr1MoV
HT91
X20CrMoV121
12CrWCoVNb
SAVE 12
12Cr0.5Mo2WCuVNb
HCM12A, ASME T122
SUS410J3TB
12Cr1Mo1WVNb
HCM12
SUS410J2TB
12Cr1MoWV
HT9
X20CrMoWV121
120-140 MPa
Evolution of Ferritic Steels for Boilers
+V-C +W-Mo +Nb
+ Mo
+ Mo
+ V
+ Nb
+ V+ Nb
105 h Creep Rupture Strength at 6000 C
+ V
+ Nb
Optimised
+ Mo
-Mo+ W
- W+ Co
+Mo+V +W
- Cu
+ W
+ Nb- Mo
+ W
+ Cu + W
+ Co
Ref: Cerjak H., Letofsky E., Schuster F., “Basic aspects of the weldability of 9-10% Cr Steels for advanced Power Generation”, Indian Welding Journal, 1999, pp. 17-24
25
Sub 80 MPa 80-120 MPa 140-190 MPa
18Cr-8Ni
AISI 302
18Cr-8Ni, C<0.08
AISI 304
120-140 MPa
Evolution of Stainless Steels for Boilers
+ Ti
+ Mo
105 h Creep Rupture Strength at 6000 C
- C
18Cr-8Ni, Ti
AISI 321
18Cr-8Ni, Nb
AISI 347+ Nb
18Cr-8Ni, Mo
AISI 316
+ Cr+ Ni
22Cr-12Ni
AISI 309
18Cr-8Ni,
C-0.04-0.10
H Grade
+ C
25Cr-20Ni
AISI 310
+ Cr+ Ni
21Cr-32NiTiAl
Alloy 800H
18Cr-8NiNb
ASME TP347 HFGHeat Treatment
18Cr-8NiNbTi
Tempaloy A-1SUS321J1HTB
Chem. Optimisation
18Cr-8NiCuNbN
Super 304HSUS304J1HTB
Cu Addition
25Cr-20NiNbN
HR3CSUS310J1HTB
20Cr-25NiMoNbTi
NF709SUS310J2HTB
22Cr-15NiNbN
Tempaloy A-3SUS309J4HTB
26
Table 2: Chemical composition of the new materials being used C Si Mn P S Ni Cr Mo V Nb N Al W Cu
2.25 Cr Steels
Min - 0.25 0.30 - - - 1.90 0.87 - - - - - - T22
Max 0.15 1.00 0.60 0.030 0.030 - 2.60 1.13 - - - - - -
Min 0.04 - 0.10 - - - 1.90 0.05 0.20 0.02 - - 1.45 - T23*
Max 0.10 0.50 0.60 0.030 0.010 - 2.60 0.30 0.30 0.08 0.03 0.03 1.75 - Min 0.05 0.15 0.30 - - - 2.20 0.90 0.20 - - - - -
T24** Max 0.10 0.45 0.70 0.020 0.010 - 2.60 1.10 0.30 - 0.012 0.020 - -
* T23 material additionally contains 0.0005-0.0006 Boron. ** T24 has Boron in the range of 0.0015 to 0.0070 and Ti in the range of 0.05 to 0.10.
9 Chrome Steels
Min 0.08 0.20 0.30 - - - 8.00 0.85 0.18 0.06 0.03 - - - T91
Max 0.12 0.50 0.60 0.020 0.010 0.40 9.50 1.05 0.25 0.10 0.07 0.04 - - Min - 0.20 0.80 - - - 8.50 1.70 0.20 0.30 - - - -
EM12 Max 0.17 0.65 1.30 0.030 0.030 0.30 10.50 2.30 0.40 0.55 - - - -
Min 0.09 - 0.30 - - - 8.00 0.30 0.15 0.03 0.03 - 1.50 - T92
Max 0.13 0.50 0.60 0.020 0.010 0.40 9.50 0.60 0.25 0.10 0.07 0.04 2.50 -
12 Chrome Steels
Min 0.17 - - - - 0.30 10.00 0.80 0.25 - - - - - X20
Max 0.23 0.75 1.00 0.030 0.030 0.80 12.50 1.20 0.35 - - - - - Stainless Steels
Min 0.07 - - - - 7.5 17.00 - - 0.30 0.05 - - 2.5 Super 304H Max 0.13 0.30 0.50 0.045 0.030 10.5 19.00 - - 0.60 0.12 - - 3.5
Min - - - - - 17.0 23.00 - - 0.20 0.15 - - - HR3C
Max 0.10 1.50 2.00 0.030 0.030 23.0 27.00 - - 0.60 0.35 - - -
Min 0.04 - - - - 9.00 17.00 - - Nb+Ta 8xC
- - - - 347 HFG Max 0.10 0.75 2.00 0.040 0.030 13.00 20.00 - - Nb+Ta
1.0 - - - -
27
Specn C Si Mn P S Ni Cr Nb N Fe Co Ti Al B Cu
HR3C 0.10
max 1.50
max 2.00
max 0.030
max 0.030
max 17.0
23.0 23.0
27.0 0.20
0.60 0.15
0.35 3.0
max 10.0
15.0 0.60
max 0.80
1.50 0.006
max 0.50
max
IN617 0.05
0.15 0.50
max 0.50
max 0.015
max 0.015
max Base 20.0
24.0
IN740
Nominal 0.034 0.45 0.27 Bal 24.31 1.83 1.02 19.63 1.58 0.75
Chemical Composition of High Temperature Materials
28
T22 T23 T91
F91 P91 T92
MICROSTRUCTURES OF NEW MATERIALS
29
X20
ASS – Proper SAHT ASS – Improper SAHT
MICROSTRUCTURES OF NEW MATERIALS
30
ASS Pipe Material ASS HAZ + Weld Metal Weld Metal + Inc Pipe
Inc Pipe Material
MICROSTRUCTURES OF NEW MATERIALS
31
100000 hrs creep rupture strengths of
different high temperature steels
100 000 hrs Creep rupture strength of different high temp. steels
0
50
100
150
200
500 550 600 650 700
Temperature deg C
Str
ess M
Pa
10 CrMo 910 (Eq. P22)
X20 CrMoV12 1
P91
NF 616 (Eq P92)
347 SS
ASME 304H
32
0
20
40
60
80
100
120
140
400 450 500 550 600 650
Temperature (Deg.C)
Allo
wab
le s
tress (
MP
a)
T 22
T 23
T 91
T 24
Allowable Stress Levels of different high
temperature ferritic steels
33
0
20
40
60
80
100
120
140
550 575 600 625 650 675 700
Temperature (deg.C)
Allo
wab
le s
tress (
MP
a)
T91
T92
TP 347H
TP347HFG
SUPER304H
Allowable Stress Levels of different high
temperature stainless steels
34
35
Nickel Alloys are having good amount of corrosion
resistance and strength at temperatures above 600°C
36
AC1: 820°C
AC3: 851°C
37
Welding cycle for X20CrMoV12 1 Steel
38
AC1: 810°C
AC3: 930°C
39
If PWHT is possible within 8Hrs
40
If PWHT is not possible within 8Hrs
41
Hardness vs. tempering temperature-T/ P91 weld
345332
322
304
283
254
200
220
240
260
280
300
320
340
360
380
500 550 600 650 700 750 800 850
Temperature Deg C (2hours)
Ha
rdn
es
s H
V1
0
As welded Hardness 473 HV10
42
Hardness vs Tempering temperature (T/ P91 HAZ)
336 333 336
302
268254
200
220
240
260
280
300
320
340
360
380
500 550 600 650 700 750 800
Temperature deg C (2hours)
Ha
rdn
es
s H
V1
0
As Welded Hardness 380 HV 10
43
Different types of damages in high
temperature weldments
Type IV cracking in FGHAZ of P91 and P92 steels
44
45
Candidate Materials for Advanced Supercritical Plants for various Steam Conditions
Component
31MPa,
565/565/565 C
31 MPa,
593/593/593 C
31 MPa,
620/620/620°C
34.5 MPa,
650/650/650C
35Mpa
732/760/760C
Headers/
Steam Pipes
P22, HCM2S
(P23),
P91, P92, P122
P91, P92, P122,
E911
P92, P122,
E911, NF12,
SAVE12
SAVE12, NF12
Haynes 230,
INCO 740,
CCA617
Finishing
Superheater
/ Reheater
non-
corrosive
T91, 304H, 347
TP347HFG
Super 304H,
P-122
NF709, Super
304H
NF709,
Inconel 617
Haynes 230,
INCO 740,
CCA617,
HR6W,
Super 304H
Corrosive
310NbN (HR3C)
HR3C
SS347/IN72 (Weld
overlay)
HR3C
Super304H/IN7
2 (Weld
overlay)
CR30A
NF709/IN72(Wel
d overlay)
Waterwall
Lower Wall
Upper Wall
For low NOx
Boilers +
High S Coal
C Steel
T11, T12, T22
Clad with alloy
containing >20%
Cr or chromised
T11, T12, T22
T23 (HCN12)
Clad with alloy
containing >20%
Cr or chromised
Clad with alloy
containing
>20% Cr or
chromised
Clad with alloy
containing >20%
Cr or chromised
T92, T23
46
12 Cr Steels
15Cr-15Ni
Tempaloy
F12M
HCM12
HCM12A,
T122
Save 12
NF12
12Cr-Mo-W
12Cr-1Mo-1W-VNbN
12Cr-0.5Mo-2W-
VNbBN
12Cr-W-Co-V-Nb-N
12Cr-W-Co-V-Nb-N
17-14CuMo
Eshete1250
Tempaloy A2
17Cr-14Ni-2Mo-Nb-Ti-B-
3Cu
15Cr-10Ni-6Mn-1Mo-W-
1V-Ti
18Cr-14Ni-Mo-Nb-Ti
20-25Cr
High Cr-High Ni
Alloy 800H
Tempaloy A3
NF707
NF709
SAVE25
HR3C
32Ni-21Cr-Ti-Al
15Ni-22Cr-Nb-B-N
35Ni-21Cr-Mo-Nb-Ti
25Ni-20Cr-Mo-Nb-Ti
18Ni-23Cr-W-Nb-
3Cu-N
25Cr-20Ni-Nb-N
HR6W
CCA617/ Inconel
617
INCO 740
Haynes 230
43Ni-23Cr-6W-Nb-Ti-
B55Ni-22Cr-0.3W-8Mo-
11Co-Al
50Ni-25Cr-20Co-2Ti-2Nb-
V-Al
57Ni-22Cr-14W-2Mo-La
Materials for Advanced Supercritical Plants
47
THANK YOU