Hot cracking of welded joints of the 7CrMoVTiB 10-10 (T/P24) steel

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Hot cracking of welded joints of the 7CrMoVTiB 10-10 (T/P24) steel This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2011 IOP Conf. Ser.: Mater. Sci. Eng. 22 012001 (http://iopscience.iop.org/1757-899X/22/1/012001) Download details: IP Address: 74.125.126.80 The article was downloaded on 15/05/2012 at 15:03 Please note that terms and conditions apply. View the table of contents for this issue, or go to the journal homepage for more Home Search Collections Journals About Contact us My IOPscience

Transcript of Hot cracking of welded joints of the 7CrMoVTiB 10-10 (T/P24) steel

Hot cracking of welded joints of the 7CrMoVTiB 10-10 (T/P24) steel

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

2011 IOP Conf. Ser.: Mater. Sci. Eng. 22 012001

(http://iopscience.iop.org/1757-899X/22/1/012001)

Download details:

IP Address: 74.125.126.80

The article was downloaded on 15/05/2012 at 15:03

Please note that terms and conditions apply.

View the table of contents for this issue, or go to the journal homepage for more

Home Search Collections Journals About Contact us My IOPscience

Hot cracking of welded joints of the 7CrMoVTiB 10-10

(T/P24) steel

J Adamiec

Department of Materials Science, Silesian University of Technology, Krasińskiego 8,

40-019 Katowice, Poland

E-mail: [email protected]

Abstract. Bainitic steel 7CrMoVTiB10-10 is one the newest steels for waterwalls of modern

industrial boilers [1]. In Europe, attempts have been made to make butt welded joints of pipes

made of this steel of the diameter up to 51 mm and thickness up to 8 mm. Many cracks have

been observed in the welded joint, both during welding and transport and storage [2-4]. The

reasons of cracking and the prevention methods have not been investigated. No

comprehensive research is carried out in Europe in order to automate the welding process of

the industrial boiler elements made of modern bainitic steel, such as 7CrMoVTiB10-10. There

is no information about its overall, operative and local weldability, influence of heat

treatment, as well as about resistance of the joints to cracking during welding and use. The

paper presents experience of Energoinstal SA from development of technology and

production of waterwalls of boilers made of the 7CrMoVTiB 10-10 steel on a multi-head

automatic welder for submerged arc welding.

1. Introduction

The 7CrMoVTiB 10-10 steel, designated also as T/P24, has been designed by Vallourec &

Mannesmann and is a modification of the T/P23 (HCM2S) steel [1]. It belongs to the group of low-

alloy ferritic-bainitic steels and is an alternative for martensitic steels for power engineering with 9%

chromium content. It is used to build new power unit with supercritical parameters and to modernize

the existing units, particularly membrane wall elements, pipelines, and thick-wall boiler parts with

operating temperature in the 500-550 oC range. Table 1 includes the comparison of chemical

composition of the T/P24 and T 22 steels. In comparison to the T22, the carbon contents is reduced

which has improved weldability. The carbon content is 0.05% to 0.10% which ensures correct

strength properties, good weldability and plasticity of the steel [5]. Low carbon contents allows to

eliminate the post-welding heat treatment. Reduced carbon contents results also in reduced hardness

related to the amount of martensite in the steel structure which is below 350 HV10 [6].

The T/P24 is stabilized with carbide forming elements, i.e. titanium, vanadium and niobium. This

has significantly improved its creep resistance. The nitrogen content has been reduced to maximum

0.010% to avoid formation of titanium nitrides [7]. Heat treatment of the T/P24 steel includes

normalizing at 1000 oC and tempering at 750

oC. Chromium carbides of the M23C6 type and small

carbonitrides are released during the tempering. These releases stabilize the bainitic structure which

improves high-temperature creep resistance [8]. Mechanical properties of the 7CrMoVTiB 10-10 are

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

Published under licence by IOP Publishing Ltd 1

similar to the T23, but are much better than those of the T22. Comparison of basic mechanical

properties for typical steels for power engineering is presented in table 2.

Table 1. Comparison of chemical compositions of T22 and T24 steels [1].

Steel Chemical composition (wt. %)

C Mn Si Cr Mo V Ti B

T22 0.15 0.30-0.60 0.25-1.00 1.9-2.6 0.87-1.13 - - -

T24 0.05- 0.10 0.30-0.70 0.15-0.45 2.2-2.6 0.9-1.1 0.2-0.3 0.06-0.10 0.0015-

0.007

Table 2. Comparison of mechanical properties of high-temperature creep resistant bainitic and

martensitic steels used to make parts of boilers for supercritical parameters [9].

Steel grade Rp0,2 (MPa) Rm (MPa) A5(%) KV (ISO-V) (J)

HCM2S (T23) 400 510 20 -

7CrMoVTiB10-10 (TP24) 450 585-840 17 41

X20 (1.4922) 500 700-850 16 39

P91 (1.4903) 450 620-850 17 41

E911 (1.4905) 450 620-850 17 41

P92 450 620 20 27

VM12 450 620 20 27

Due to similar structure of the 7CrMoVTiB 10-10 (T/P24) and the T/P23, better strength

properties have been achieved by addition of alloy ingredients – Ti, V i B. Comparison of operating

temperature of the wall materials is shown in figure 1.

Figure 1. Maximum allowed operating temperatures of

materials for waterwalls in the boilers [10].

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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Comparison of creep resistance of the waterwall materials is shown in figure 2. The 7CrMoVTiB10-

10 and HCM12 steels have the highest creep resistance. Figure 3 presents the creep strength of the

7CrMoVTiB10-10 steel after 100 000 h in comparison with 10CrMo9-10 (T/P22), T/P23 and T/P91.

Figure 2. Creep resistance of materials for waterwalls

in the boilers [11].

Figure 3. Creep strength of the T/P91, T/P24, T/P23 and T/P22 steels [12].

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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Tight walls made of the T24 steel have been tested, amongst others in the following power plants:

Weisweiler, Unit G (Germany), Neckar 2 (Germany), Thierbach, Unit D (Germany) and Asnaes, Unit

4 (Denmark).

The 7CrMoVTiB 10-10 is used to make components of modern power generation installations and

to modernize the existing components. Comparison of the wall thickness of a tee made of the T/P22,

T/P23, T/P24 and T/P91 is shown in figure 4 (assumed tee parameters: pressure 191 bar, temperature

T= 545 oC, main diameter Di = 450mm, nozzle diameter Di = 300mm). Walls made of the

7CrMoVTiB 10-10 steel are marked in green.

Figure 4. Comparison of the T-connection wall thicknesses: steels T/P22,

T/P23, T/P24 and T/P91 [12].

An advantage of the HCM2S (T/P23) and 7CrMoVTiB 10-10 (T/P24) steels is a relatively low

hardening in the heat affected zone after welding (maximum hardness 350HV). The maximum

allowed wall temperature for the 7CrMoVTiB 10-10 steel is 550 oC. The creep strength of materials

used for membrane walls is presented in figure 5.

Figure 5. Creep strength of power engineering steels used for membrane

walls [13].

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GTAW, SAW and SMAW methods are used to weld the parts made of the T24 steel, and the

choice of method has no significant influence on the mechanical properties of welded joints.

Chemical analysis and mechanical properties of the deposited metals of the T24 steel made with the

GTAW, SMAW and SAW methods are presented in table 3.

In the untreated post-welding condition, the deposited metal exhibits high strength and hardness.

When the wall thickness is <10 mm, the tempering is not needed [1]. Absence of heat treatment after

welding of thin-walled pipes of water screens with the GTAW method is caused by low carbon

content in the deposited and parent metal which prevents the hardness increase above 350HV. With

the SMAW and SAW methods used for larger wall thicknesses (> 10mm), the post-welding heat

treatment is necessary (740 oC/2h or 740

oC/4h for the SAW method) [1]. Alloy additives, such as Ti

and B, are burned during the process due to their high affinity to oxygen. Therefore, the additional

material for welding includes also Nb, contrary to the parent metal which has the titanium addition.

Replacing titanium with niobium is advantageous because niobium has a low affinity to oxygen and

because it significantly increases the creep resistance.

Table 3. Chemical analysis and mechanical properties of deposited metals of the T24 steels made

with the GTAW, SMAW and SAW methods [14].

Chemical composition of deposited metal for various welding methods (wt. %)

Dia C Si Mn Cr Mo V Ti Nb N B

Wire 0.061 0.24 0.53 2.39 1.01 0.24 0.073 0.008 0.016 0.0037

GTAW 2.4 0.061 0.23 0.49 2.29 1.00 0.24 0.034 0.007 0.014 0.0020

SMAW 3.2 0.064 0.47 0.56 2.38 0.97 0.24 0.043 0.008 0.022 0.0030

SAW 4 0.050 0.20 0.72 2.26 0.98 0.22 0.015 0.007 0.009 0.0010

Mechanical properties of deposited metal for various welding methods

Welding

method

mm

Test

temperature

(oC)

Heat

treatment

(oC/h)

Rp0,2

(MPa)

Rm

(MPa)

A5

(%)

KV

(ISO-V)

(J)

Hardness

(HV10)

GTAW

Union I

CrMoVTiB

2.4

-20 - 664 803 19.1 298 332

+600 - 457 561 18.6 - -

+20 740/2 595 699 20.3 278 230

SMAW

Thyssen

CrMoVTiB

4.0

+20 740/2 507 626 21.9 161 233

+600 740/2 306 366 25.6 - 192

SAW

Union S

CrMoVTiB/

UV 430

TTR-W

4.0 +20 740/2 495 600 23.8 269 206

2. Welded joints of the 7CrMoVTiB 10-10 made with the submerged arc method

Submerged arc welding was used at Energoinstal S.A. to weld the 7CrMoVTiB 10-10 (T/P24) pipes

of the 44.5 mm diameter and the 7.1 mm wall thickness, and the 8.0 mm x 75.9 mm flat. The

chemical composition of the steel according to the heat No. 41037 at Vallourec & Mannesmann is

presented in table 4. For comparison, the table includes also the required chemical composition and

some mechanical properties according to the EN 10204:2004 standard.

Panels of the boiler waterwalls are built on the basis of a two-sided pipe-flat joint made with the

submerged arc method. Length of a single panel may reach 24 m, and its width 2 m. Individual panels

are butt-welded also with the submerged arc method. The geometry requirements for the pipe-flat

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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joint have been specified according to the PN EN 12952 and VGB - R 501 H standards, and the

maximum hardness for such joints should not exceed 350 HV.

Welding was performed with the submerged arc method (121), using the P24-UP wire

(SZCrMo2VNb acc. to EN 12070), in the shield of the BB 305 flux (SA AR 1 76 AC H5 acc. to EN

760) on the four-head Deuma welder (figure 6). Designation of samples and range of parameters is

presented in table 5. The samples were preheated to 100 oC before welding. The criterion was

meeting the rigorous requirements according to PN-EN 5817 (class B) and the requirements

according to PN EN 12952, as well as the VGB – R 501 H regulations. Face of welds and cross-

sections are presented in figure 7.

Table 4. Chemical composition and mechanical properties of the 7CrMoVTiB10-10 (T/P 24) steel.

Steel

grade Chemical composition (wt. %) according to 3.2 EN 10204:2004

T/P 24 C Si Mn P S Al Cr Mo V Ti

min 0.050 0.150 0.30 - - - 2.200 0.900 0.200 0.050

max 0.100 0.450 0.70 0.020 0.010 0.020 2.600 1.100 0.300 0.100

T/P 24

Chemical composition according to the mill certificate – heat 41037

0.074 0.283 0.55 0.014 0.001 0.014 2.422 0.986 0.249 0.076

Mechanical properties according to the mill certificate – heat 41037

Rp0.2 (MPa) Rm (MPa) A5 ( %) HV10

523 617 19.5 220.0

Table 5. Designation of samples and range of parameters for submerged arc welded waterwall panels

of the 7CrMoVTiB 10-10 steel at Energoinstal SA.

Sam

ple

des

ignat

ion Parameters of submerged arc welding of waterwall panels

Method

Filler

metal

diameter

(mm)

Current

(A)

Arc

voltage

(V)

Current type

polarity

Welding

speed vs,

(m min-1

)

Linear arc

energy

El

(kJ cm-1

)

1 121 2 400 30 DC + 1.0 6.5

2 121 2 400 30 DC + 0.9 7.2

3 121 2 400 30 DC + 0.8 8.1

4 121 2 400 28 DC + 0.7 8.6

5 121 2 400 28 DC + 0.6 10.0

Visual inspection of welded joints revealed transversal cracks of the weld face in sample No. 1

(El = 6.5 kJ/cm, vs =1.0 m/min) and sample No. 2 (El = 7.5 kJ/cm, vs =0.9 m/min) (figure 7a).

Observation of the crack fracture surface in sample No. 1 indicate that the crack develops through the

whole joint and propagates to the heat affected zone (figure 7b). In sample No. 2, it has been noted

that the crack initiates in the incomplete penetration area (figure 7c). Such incomplete penetration is

acceptable according to the regulations (up to 2 mm). However, in case of the 7CrMoVTiB 10-10

steel it is appropriate to have full penetration welding. As a result of slower cooling of the welded

joint under the flux, a thin plastic layer is formed on the face surface (figure 7b).

Visual inspection of sample No 3 has shown a correct face, and no shape irregularities and cracks

have been found. Macroscopic examination of the surface perpendicular to the welding direction has

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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revealed a correct shape of the weld. This welded joint meets the geometry requirements of EN

12952 and the VGB requirements.

Figure 6. Four-head Deuma welder.

The face of weld No. 4 made with the 8.6 kJ/cm linear arc energy at the 0.7 m/min speed was also

correct. Macroscopic examination has revealed correct shape and full penetration. Similarly,

evaluation of the welded joint No. 5 (El = 10 kJ/cm, vs =0.6 m/min) indicates a correct face. However,

a few pores have appeared on the face surface. This is caused by air drawing from the root side if the

joined parts do not abut correctly. In terms of geometry and penetration, the joint No. 5 meets the

acceptance requirements. Therefore, it can be stated that the best face geometry and penetration was

achieved with the 8.6 kJ/cm linear arc energy at the 0.7 m/min welding speed (table 3).

Figure 7. Face and transversal section of the weld: (a) face of weld No. 1, welding speed 1 m/min

with visible crack, (b) crack surface in sample No. 1, (c) crack macrostructure in sample No. 2 – hot

crack.

Macroscopic observations to determine the crack propagation in the welded joint and in its

individual zones were performed on the Olympus SZX9 stereoscopic microscope, at magnification

from 5÷50x. Typical crack is shown in figure 8. Examination of crack propagation in samples 1 and 2

have indicated that the crack develops in the weld, then through the heat affected zone it propagates

to the parent metal in both the flat and the pipe (figure. 8). In the weld area, the crack develops

c) a) b)

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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mainly as an intercrystalline crack, and in the heat affected zone as a mixed crack – both

intercrystalline and transcrystalline.

Figure 8. Crack trajectory in the submerged arc welded joint in the waterwall panel: WM – Weld

Metal, HAZ – Heat Effected Zone, BM – Base Metal.

Metallographic examination to show the microstructure in the cross section of the pipe-flat joint

was performed on the OLYMPUS GX-71 light microscope at magnification from 100x to 500x.

Light field observation technique was used (figure 9).

Figure 9. Structures of the submerged arc welded joint of the 7CrMoVTiB 10-10 steel, sample No. 1;

(El = 6.5 kJ/cm, vs =1.0 m/min) : (a) parent metal structure, pipe dia 44.5 x7.1 mm, (b) heat affected

zone, (c) bainitic-martensitic structure of non-crystallized weld as a result of the heat introduced by

the second run, (d) crack trajectory on the weld-heat affected zone line.

a) b)

c) d

)

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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Structures of a two-sided pipe-flat joint in the 7CrMoVTiB 10-10 steel, made with the submerged

arc welding method, are typical (figure 9). The parent metal area has a bainitic microstructure (figure

9a). Normalization area and overheating area have been found in the heat affected zone (figure 9b).

Column crystals of the weld grow orthogonally to the crystallization surface on the weld line (figure

9b). These crystals have a bainitic-martensitic structure (figure 9c). Figure 9d shows how the crack

crosses the weld line to the HAZ.

In order to determine how the cracks develop, the metallographic tests in the light microscope

were supplemented with the fracture tests performed on the HITACHI S3400N electronic scanning

microscope. Figure 10 presents the crack surfaces, after full breaking was made on a tensile tester.

The examination was carried out with the SE (secondary electrons) technique. Such imaging shows

well the surface topography.

The crack surface analysis reveals typical hot cracking features, as exemplified by smooth

surface of crystallites which forms as a result of solidification of a thin layer of fluid on the

crystallites during welding (figure 10a). Areas with brittle, transcrystalline fractures have been found

on the borders of crystallites. These areas are formed in the high-temperature brittleness range as a

result of cracking of already solidified and adjacent crystals caused by welding stresses and strains

(figure 10b). These cracks develop along the borders of the weld crystals which is confirmed by the

fact that individual crystals do not match which is shown in figure 10a. This forms a hot cracking net

(figure 10a). Change of the fracture type from intercrystalline along the crystallites to the mixed

along the grain borders is clearly seen on the weld line. Numerous cracks, mainly with crystalline

trajectory have been found in the heat affected zone (figure 10b), and the main crack has a strongly

developed with features of mixed brittle fracture – intercrystalline and transcrystalline (figure 10b).

This proves that the cracking mechanism changes in the HAZ.

Figure 10. Crack surface of the submerged arc welded joint of the 7CrMoVTiB 10-10 steel:

(a) weld area, (b) heat affected zone.

3. Hardness measurement

The hardness was measured on the Vickers HPO250 hardness tester under the 98 N load. Test

locations and results are presented in figure 11. Hardness distribution analysis of the welded joints in

the 7CrMoVTiB10-10 waterwall panels indicates that the hardness decreases with reduced welding

speed (figure 11). In the joints made with the submerged arc welding method, significant hardness

increase is noted in the weld (to 380 HV) and in the heat affected zone to about 370 HV. Increase

hardness of the welded joint and the HAZ is caused by excessive cooling rate which results in

formation of untempered martensite in the weld (figure 11). Only in case of the welded joint made at

a) b)

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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the 0.7 m/min speed, the hardness did not exceed 350 HV (figure 11). Therefore, the waterwall panels

should be submerged arc welded with the 0.7 m/min maximum speed and preheated to 100 oC to dry

the elements.

Figure 11. Hardness distribution submerged arc welded joint of the

7CrMoVTiB 10-10 steel.

4. Conclusions

The performed tests and examinations and analysis of results allow formulation of the following

conclusions:

Damage of welded joints in the submerged arc welded 7CrMoVTiB 10-10 waterwall panels

is caused by hot cracking. The hot cracks are formed in the high-temperature brittleness

range when the plasticity of the joint is exceeded during crystallization. Main factors

contributing to cracking are excessive welding speed (above 0.7 m min-1

) and joint strain

caused by the welding technology.

Hot cracking of the 7CrMoVTiB10-10 takes place by loss of cohesion of the thin

crystallizing fluid layer between the growing crystals of the welded joint. The development

of cracks in solid state is caused by broken bridges between crystals, eg as a result of eg

transporting finished elements.

High hardness and low impact strength of the joints is caused by presence of untempered

martensite in the structure of the welded joint. This is a result of excessive joint cooling rate

after welding.

5. References

[1] Each Arndt J et al 2000 The T23/T24 Book – New Grades for Waterwalls and Superheaters

(Germany: Vallourec & Mannesmann Tubes)

[2] Tasak E Adamiec J 2008 Seminarium Nowe Żarowytrzymałe stale dla energetyki (Rafako -

Rudy Raciborskie)

[3] Adamiec J Gawrysiuk W Więcek M 2008 Międzynarodowa Konferencja Spawanie

w Energetyce (Opole Jarnołtówek) pp 30

[4] Tasak E Adamiec J Ziewiec A 2008 Międzynarodowa Konferencja Spawanie w Energetyce

(Opole Jarnołtówek) pp 55

[5] Pasternak J Heuser H Repelowicz A 2004 Proc. of University of Technology Opole:

Electric 53 No.295

[6] Bendick W Hahn B Schindler W 2001 Zeitschrift 3R international No 5 pp 264

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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[7] http://www.zelpo.sk/zelpo/home_uk.nsf/page/steel_grade_T_P_24

[8] Pasternak J Kiełbus A 2006 Proc. of University of Technology Opole Electric 56 No 315

pp 520

[9] Gross V Hauser H Jochum C 2005 Nowe Żarowytrzymałe stale dla energetyki (Rafako - Rudy

Raciborskie

[10] http://www.cbmm.com.br/portug/sources/techlib/science_techno/table_content/sub_4/images/p

dfs/040.pdf

[11] http://www.msm.cam.ac.uk/phase-trans/2005/LINK/103.pdf

[12] http://www.t-put.com/english/files/SZ_Kraftwerk_E.pdf

[13] Huseman R 2005 Advanced Material for AD700 Boilers A Clean Coal European Technology

(Milano)

[14] Macura T 2003 Energy Gigawat Supercritical boilers pp. 5

Technologies and Properties of Modern Utilised Materials IOP PublishingIOP Conf. Series: Materials Science and Engineering 22 (2011) 012001 doi:10.1088/1757-899X/22/1/012001

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