Boiler Materials

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.


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


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.


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.


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




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


30

ASS Pipe Material ASS HAZ + Weld Metal Weld Metal + Inc Pipe

Inc Pipe Material


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

Documents

Boiler Materials