Modern martensitic steels for power industry

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Mar 24, 2012 - X10CrWMoVNb9-2 (T/P92), X11CrMoWVNb9-1-1 (E911) and .... restrictive test of susceptibility to cold cracking (Tekken test) and presented in ...
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Author's Personal Copy archives of civil and mechanical engineering 12 (2012) 49–59

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Modern martensitic steels for power industry M. Łomozikn, M. Zeman, J. Bro´zda Institute of Welding, Testing of Materials Weldability and Welded Constructions Department, Błogosławionego Czesława 16-18, 44-100 Gliwice, Poland

art i cle info Available online 24 March 2012

ab st rac t The article presents general characteristics of heat-resisting and creep-resisting steels

Keywords:

intended for operation at increased temperature and having martensitic microstructure i.e.

Structural steels

X10CrWMoVNb9-2 (T/P92), X11CrMoWVNb9-1-1 (E911) and X12CrCoWVNb12-2-2 (VM12-

Power industry

SHC). The weldability of the aforesaid steels is discussed on the basis of CCT diagrams of

Welding

austenite transformation in welding conditions and the results of simulation tests aimed at

Welded joint

determining the resistance of the steels to brittle and annealing cracking. The paper also

Weldability of steels

contains information on test results related to mechanical (strength and plastic) properties of joints welded with various methods. The description of each of the steels under analysis has been supplemented with technological recommendations on welding and heat treatment of joints. The article is addressed to researchers who are interested in problems of heat- and high-temperature creep resisting constructional steels. Furthermore, the article is intended for designers, technologists and welding engineers engaged in constructional jobs in power industry. & 2012 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z.o.o. All rights reserved.

1.

Introduction

The emission of pollutants into the atmosphere in the form of carbon dioxide (CO2), sulphur dioxide (SO2), nitric oxides (NOx) and various kinds of dusts imposes on Poland their reduction to the level not exceeding 300 000t in 2012. Directive 2001/80/WE of the European Parliament and of the Council of 23 October 2001 on the limitation of emissions of certain pollutants into the air from large combustion plants specifies the path for reduction of allowed values. Large combustions plants are power boilers of power amounting to or exceeding 50 MW, which means that the aforesaid directive applies to the whole of Poland’s utility power industry as well as to a significant part of industrial and public/municipal power engineering, where 97% (140.8 TWh) of total energy is generated using solid fuels (carbon and brown coal). The emission of pollution into the air can be decreased by applying supercritical

parameters of fresh steam. The notion of ‘‘supercritical’’ comes from thermodynamics and characterises the state of the matter in which there is no clear distinction between the liquid and gaseous phases i.e. there is homogenous fluid. Water reaches such a state at pressure above 22.1 MPa. By increasing steam temperature and pressure it is possible to improve the efficiency of power units and obtain better conditions of combustion of solid fuels with simultaneous significant reduction of pollution emission into the air. Supercritical steam parameters, however, require the application of structural materials characterised by higher hightemperature creep resistance and higher heat resistance (resistance to oxidation). The group of modern martensitic creep-resisting steels with 9–12% chromium content includes the following steels X20CrMoV12.1 (X20), X10CrMoVNb9-1 (T/P91), X10CrWMoVNb92 (T/P92), X11CrMoWVNb9-1-1 (E911) as well as the latest

n

Corresponding author. E-mail address: [email protected] (M. Łomozik).

1644-9665/$ - see front matter & 2012 Politechnika Wrocławska. Published by Elsevier Urban & Partner Sp. z.o.o. All rights reserved. http://dx.doi.org/10.1016/j.acme.2012.03.010

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Table 1 – Chemical composition of new-generation creep-resisting martensitic steels. Element

X20CrMo12-1 (X20)/No. W. 1.4922

X10CrMoVNb9-1 (T/P91)/No. W. 1.4903

X11CrMoWVNb91-1 (E911)/No. W. 1.4905

X10CrWMoVNb92 (T/P92)/No. W. 1.4901

HCM12A

X12CrCoWVNb12-2-2 (VM12-SHC)/No. W. 1.4915

C Si Mn Cr Ni Mo V W Nb Others

0.17–0.23 o0.50 o1.0 10.0–12.50 0.30–0.80 0.80–1.20 0.25–0.35 – – –

0.08–0.12 0.20–0.50 0.30–0.60 8.00–9.50 o0.40 0.85–1.05 0.18–0.25 – 0.06–0.10 N: 0.03–0.07

0.09–0.13 0.10–0.50 0.30–0.60 8.50–9.50 0.10–0.40 0.90–1.10 0.18–0.25 0.90–1.10 0.06–0.10 N: 0.05–0.09

0.07–0.13 o0.50 0.30–0.60 8.50–9.50 o0.40 0.30–0.60 0.15–0.25 1.50–2.00 0.04–0.09 N: 0.03–0.07

0.07–0.13 r0.50 r0.70 10.0–12.50 r0.50 0.25–0.60 0.15–0.30 1.50–2.50 0.04–0.10 Cu: 0.30–1.70

0.10–0.14 0.40–0.60 0.15–0.45 11.00–12.00 0.10–0.40 0.20–0.40 0.20–0.30 1.30–1.70 0.03–0.08 Co: 1.40–1.80

B: 0.001–0.006 Working tempa (1C)

r560

r585

r600

r620

r630

N: 0.03–0.07 B: 0.003–0.06 r620 (650)

Note: for steels P91, P92, E911 and VM12-SHC total content MnþNio1.5%. Construction-dictated boundary values for working temperature in power plants.

a

X12CrCoWVNb12-2-2 (VM12-SHC). The chemical composition of the aforesaid steels is detailed in Table 1. At present, the greatest importance and application is associated with the steels designated with the symbols of P92, E911 and VM12-SHC (manufactured by Europe’s steelworks of Vallourec & Mannesmann). These steels along with their weldability constitute the subject of this article. The test results presented are a common work of Instytut Spawalnictwa, Przedsi˛ebiorstwo Modernizacji Urza˛dzen´ Energetycznych REMAK S.A. in Opole and Fabryka Kotło´w Rafako S.A. in Racibo´rz. On the basis of available publications it was also possible to contain in this article information on results of works obtained in other research centres. Table 1 presents also the chemical composition of the HCM12A martensitic steel manufactured by Japan’s Sumitomo Steelworks. The properties and weldability of the X20CrMoV12-1 and X10CrMoVNb9-1 steels have been described in publications by Instytut Spawalnictwa numerous times and thus are not covered by the scope of this article. Fig. 1 presents the dependence between creep strength and temperature for steels listed in Table 1. The creep strength of the VM12-SHC steel is similar to the corresponding values of the P92 steel, yet owing to higher chromium content the former is characterised by higher oxidation resistance. The application of creep-resisting steels of higher creep strength makes it possible to considerably decrease the wall thickness of power plants (Fig. 2).

2.

Fig. 1 – Creep strength of new-generation heat-resisting steels [1].

Steel T/P92 and its weldability

One of important structural materials applied in the construction of modern power units is the T/P92 steel developed in Japan in 1990 as the NF616 steel. In comparison to the P91 steel, P92 contains tungsten (up to 2%), less molybdenum (up to 0.6%) and micro-addition of boron; at 600 1C the P92 steel also demonstrates approx. 30% higher creep strength. The primary application of the T92 steel (tube) is in the secondary

Fig. 2 – Reduction of wall thickness of superheated steam pipeline depending on grade of applied martensitic steel.

heaters of steam in power plants of supercritical parameters, whereas heavy-wall large-diameter tubes made of the P92 steel (pipe) find application in fresh and superheated steam chambers and pipelines operating under extreme temperature

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Fig. 3 – CCT diagram in welding conditions for T92 steel with microstructures for selected cooling-down times t8/5.

Fig. 4 – Dependence of impact toughness KCV and hardness HV10 of T92 steel-simulated HAZ on cooling-down time t8/5. Maximum temperature of thermal cycle Tmax ¼1250 1C.

and pressure conditions. The said steel was, among other applications, used in the construction of power units in Denmark’s power plants of Vestkraft Unit 3, Nordjyllandsværket (580 1C/29 MPa) and Aved½re 2 (600 1C/30 MPa), Germany’s power plants of GK Kiel, Westfalen (650 1C/18 MPa) and a Japan’s power plant of Tachibanawan 2 (613 1C/26.4 MPa). The basic characteristics of the T/P92 steel is presented in publication [2]. The CCT diagram of austenite transformations for welding conditions (austenitisation temperature of 1250 1C, cooling times between 800 and 500 1C—t8/5 in the 2–200 s range) (Fig. 3) and results of tests of HAZ simulated on the T/P92 steel (Figs. 4 and 5) reveal that for cooling-down times t8/5 characterising a wide range of welding conditions, the HAZ area adjacent to the fusion line will be characterised by the presence of martensite of hardness exceeding 400 HV, low impact toughness and resultant low fracture toughness. The results of the simulation tests carried out with a simulator of welding thermal cycles are confirmed by the results of the hardness measurements of a welded joint made of the T92 steel not subjected to heat treatment (Fig. 9), in which the maximum hardness in HAZ reaches 435 HV5.

The butt welded joints of a pipe (+ 51  7 mm) made of the T92 steel were produced with the TIG method, in the PC and PF positions, and with the P92-IG wire of diameter of 2.4 mm, whereas the welded joint of a pipe (+ 219  20 mm) was produced with the TIG method (penetration with the P92-IG wire) and the FOX P92 covered electrode (+ 2.5 and 3.2 mm). The preparation of the edges of the pipes made of the T92 and P92 steels is presented in Fig. 6. Also the microstructure of the HAZ weld area of the welded joint not subjected to heat treatment (Fig. 7a) corresponds to the martensitic structure present in the simulated HAZ (Fig. 5). The pipe welded joint made of the T92 steel, not subjected to heat treatment, reveals poor plastic properties in the weld area and HAZ, particularly visible in a bend test (Fig. 8). In the light of the foregoing, welded joints made of the T/P92 steel, irrespective of the thickness of elements to be joined, must undergo heat treatment in order to temper hard and brittle martensite, reduce remaining welding stresses and ensure required impact toughness. Heat-treated welded joints of the T92 and P92 steels (760 1C/ 1.5 h and 3 h) are characterised by hardness not exceeding 280 HV5 (Fig. 9) and 285 HV10, respectively.

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Fig. 5 – Dependence of absorbed energy KV, hardness HV10 and structure of P92 steel-simulated HAZ on cooling-down time t8/5. Maximum temperature of thermal cycle Tmax ¼1250 1C.

Fig. 6 – Pre-weld preparation of edges and macrostructure of welded joint in forced (b) and transverse (c) positions of welded joint made of T92 (A) and P92 (B) steels.

The mechanical properties of the joints meet the requirements as set for the parent metal (rupture of specimens outside the weld). Also the plastic properties of the joints, verified in a bend test, prove to be good (Fig. 10). The impact toughness results revealed sufficiently high fracture toughness of the joints of pipes made of the T92 and P92 steels welded in various positions. The highest average impact toughness detected in the HAZ contained in the ranges 165–197 J/cm2—for the joint made of the T92 steel, and 199–219 J/cm2—for the joint made of the P92 steel. The impact toughness values for the welds were slightly lower and amounted to 85–140 J/cm2 (Fig. 12)—for the joint made of the T92 steel, and 67–101 J/cm2—for the joint made of the P92 steel. The impact toughness values fully meet the require¨ V and ment of technical inspection bodies i.e. those of TU Poland’s Technical Inspection Office (min 51 J/cm2).

The CCT diagram (Fig. 3) and the simulated HAZ test results (Figs. 4 and 5) revealed that martensitic structures of high hardness (up to 470 HV) are present in the whole HAZ area of the welded joint, in the whole range of cooling time t8/5, irrespective of welding conditions. In order to prevent cold cracking it is necessary to heat a joint prior to welding and maintain the temperature during welding as well as limit the amount of diffusing hydrogen entering the weld pool. According to tests carried out in Japan, applying a very restrictive test of susceptibility to cold cracking (Tekken test) and presented in publication [5], the pre-heating temperature amounts to approx. 200 1C. Fig. 11 presents the said test results compared with other grades of creep-resisting steels. The minimum pre-weld pre-heating temperature of the P92 steel preventing the cold cracking is similar to the pre-heating temperature of the P91 and HCM12A steels. A similar

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Fig. 7 – Microstructures of welded joints of pipes made of T92 steel (etchant FeCl3).

Fig. 8 – Specimens cut out of welded joint made of T92 steel, not subjected to heat treatment after bend test.

Fig. 10 – HV5 hardness distribution in cross-section of joint of pipes (+ 219  20 mm) made of P92 steel, welded in transverse (PC) position.

Fig. 9 – Hardness distribution in welded joint of pipes made of T92 steel, not subjected to heat treatment, after annealing (760 1C/1.5 h) [3,4].

temperature value is provided by the manufacturer of the P92 steel [6,7]. The test results of simulated HAZ indicate that the high impact toughness of the parent metal in the initial state

(normalisingþtempering) significantly decreases in the HAZ (Figs. 4 and 5) with the lower impact toughness decrease for longer t8/5 times. Subjecting the simulated HAZ to stress relief annealing restores its high impact toughness, therefore heat treatment of welded joints aimed at tempering hard martensite present in the HAZ and the weld is absolutely essential. As the tests carried out with a simulator of heat and strain cycles revealed, the P92 steel is not susceptible to annealing cracking (Fig. 12) as the contraction Z of the simulated HAZ

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significantly exceeds the value of 20% in the range of the applied heat treatment temperature. The T/P92 steel is welded with the TIG method, covered electrodes and, automatically, with submerged arc under flux.

Fig. 11 – Dependence between fraction of fractures C in Tekken test and temperature of pre-heating P92 steel and other grades of creep-resisting steels [5].

The chemical composition of fillers metals and that of deposited metal corresponds to the composition of the parent metal. Table 2 presents the mechanical properties of the deposited metal of welding consumables manufactured by Bo¨hler Welding [8]. Pursuant to suggestions of Bo¨hler Welding it is recommended to apply a welding technology as for the steels with 9–12% chromium content. Thus, the conditions for welding the P91 steel may be applied for welds made of the T/P92 steel [2]. Fig. 13 presents a typical post-weld thermal cycle of welding and heat treatment pursuant to the P92 steel manufacturer’s recommendations [6]. The pre-weld heating temperature amounts to approx. 200 1C, whereas the interpass temperature should not exceed 250 1C. After welding, in order to ensure the completion of martensitic transformation, the joint should slowly cool down to below 100 1C. The stress relief annealing of welded joints, during which hard and brittle martensitic structure undergoes tempering, is carried out at 760 1C. Prior to heat treatment the welded joint is characterised by low plastic properties and therefore should be handled with care. On grounds of positive results of conducted tests the ¨V company of REMAK in Opole became authorised by RWTU Polska and Poland’s Technical Inspection Office, Opole’s branch, to weld pipes made of the T92 and P92 steels.

Fig. 12 – Dependence of contraction Z of simulated HAZ on test temperature for P92 steel.

Table 2 – Mechanical properties of deposited metal of welding consumables for welding T/P92 steel. Welding method/consumable

Heat treatment

R0.2 (MPa)

Rm (MPa)

A5 (%)

KV (J)

TIG/P92-IG

760 1C/2 h with furnace up to 300 1C/air 760 1C/6 h with furnace up to 300 1C/air

710

820

19

77

650 (230)

770 (340)

20 (21)

70

690

810

19

55

630 (230)

760 (330)

20 (22)

80

660

780

20

60

Covered electrode/FOX P92

Submerged arc under flux/ wire: P92-UP flux: BB 910

760 1C/2 h with furnace up to 300 1C/air 760 1C/6 h with furnace up to 300 1C/air Stress relief annealing

Note: results in brackets refer to tests carried out at 650 1C.

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The obtained test results confirm worldwide-published information that the new-generation creep-resisting steel T/P92 of high creep strength is characterised by good weldability. For this reason, the steel should find more common application (than so far) in the construction of power units of supercritical steam parameters, also in Poland’s power sector.

3.

Steel E911 and its weldability

The E911 steel belongs to the group of creep-resisting steels containing 9% of chromium and an additive of tungsten; its chemical composition being similar to that of Japan’s NF616 (P92) steel. In comparison with the P92 steel, the welding of which was discussed in publication [2,9] the E911 steel contains a slightly higher additive of molybdenum and a slightly lower additive of tungsten. The steel is applied in the production of elements of boilers of supercritical parameters (secondary heaters, heater chambers and fresh steam pipelines) and is supplied after toughening (hardening: 1050 1C/10 min/air and tempering: 760 1C/60 min/air). The chemical composition of the steel and its creep strength is presented in Table 1 and Fig. 1.

Fig. 13 – Thermal cycle for welding of T/P92 steel and heat treatment of welded joints according to [7].

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The CCT diagrams of austenite decomposition in welding conditions (Fig. 14) indicate that, irrespective of welding conditions, the HAZ of welded joints will contain martensitic structures of hardness exceeding 400 HV (similarly as in case of the P92 steel). The fracture toughness of the HAZ areas heated to high temperature will be low, as is indicated by the low absorbed energy values of the simulated HAZ (Fig. 15). The foregoing indicates the necessity of pre-weld pre-heating, maintaining the required pre-heating temperature also during welding and the application of low-hydrogen welding processes in order to prevent cold cracking; the pre-heating and inter-pass temperatures should be lower than the initial temperature of martensitic transformation MS. The parent metal of tempered martensite structure is characterised by high absorbed energy (177 J) and low hardness (215 HV10)—Fig. 15. In order to reduce high hardness of the HAZ and that of the weld as well as to improve plastic properties and reduce welding stresses, it is necessary to subject welded joints to stress relief annealing, during which hard and brittle martensite undergoes tempering. In order to obtain martensitic

Fig. 15 – Dependence of absorbed energy KV and HV10 hardness of simulated HAZ on E911 steel on cooling-down time t8/5 (maximum temperature of thermal cycle Tmax ¼ 1250 1C).

Fig. 14 – CCT diagram in welding conditions for E911 steel [9].

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Fig. 16 – Specimen for testing steel susceptibility to annealing cracking and dependence of contraction Z of simulated HAZ on test temperature for E911 steel.

structure in the whole area of the joint, after welding and prior to stress relief annealing, the welded joint should be cooled down to a temperature lower that the final temperature of martensitic transformation MF. The tests carried out with a simulator of thermal cycles revealed that the E911 steel is not susceptible to annealing cracking as the contraction Z of the specimens with the simulated HAZ, subjected to tension in the 550–750 1C temperature range, significantly exceeds the minimum value of 20% (Fig. 16). The E911 steel must be welded in such a manner, that the metal of the weld and HAZ, each time during cooling-down, should undergo martensitic transformation. The pre-heating and inter-pass temperatures should be lower than the initial temperature of martensitic transformation MS; after welding, the joint should be cooled down to room temperature or that not exceeding 100 1C. If pre-heating and/or inter-pass temperature is higher than that of MS and the joint, after welding and before stress relief annealing, is not cooled down to the temperature lower than the final temperature of martensitic transformation MF, the martensitic transformation will take place during the cooling-down of the joint, after heat treatment. The correct course of welding cycle and heat treatment of the E911 steel (according to publications [10,11]) is presented in Fig. 17. The introduction of the E911 steel to industrial applications was accompanied by the development of appropriate consumables for its welding. Table 3 contains some data characterising the properties of the deposited metal of welding consumables applied in the welding of the E911 steel. The addition of nickel to the filler metal aims to compensate the negative impact of tungsten and niobium on the impact toughness of deposited metal; in order not to excessively decrease transformation temperature AC1 (essential for proper post-weld heat treatment of the joints), it is important to follow the principle: MnþNio1.5% [11]. The impact toughness of the weld depends also on the amount of supplied heat (welding linear energy), welding method and heat treatment of the joint. The recommended value of the absorbed energy of the weld metal is KV¼50 J at þ20 1C [12]; it can be obtained by applying welding linear energy not exceeding 1.2 kJ/mm and 2-h annealing at 750 1C. The required value of the absorbed energy can also be obtained

Fig. 17 – Cycle of welding and heat treatment of E911 steel [10].

by applying higher welding linear energy (e.g. 2.1 kJ/mm), yet in such case the time of annealing should be extended even up to 10 h. The publication [13] specifies the minimum value of required absorbed energy (KV) at the level of 41 J. The test butt joints of pipes (+ 405  60 mm) welded in the forced position (PF) and transverse position (PC) applying TIG method (wire C9 MVW-IG, root run) and Bo¨hler-manufactured FOX C9 MVW covered electrodes, subjected to stress relief annealing (760 1C/3 h) are characterised by tensile strength not lower than that of the parent metal (the specimens underwent rupture outside the weld) and good plastic properties confirmed in a bend test. The welded joints demonstrate the structure of tempered martensite of hardness not exceeding 222 HV10—for the parent metal, 283 HV10—for the weld and 297 HV10—for the HAZ. The absorbed energy (KV) of the parent metal and HAZ is high (Fig. 15). Lower KV values were detected in case of the weld (49 J for the weld cut out of the joint welded in the transverse position and 64 J for the weld cut out of the joint welded in the forced position). Both values, however, meet the minimum E911 ¨ V and Poland’s steel absorbed energy requirements of TU Office of Technical Inspection assumed on the level of 41 J. On grounds of positive results of conducted tests the company of REMAK S.A. in Opole became authorised by ¨ V Polska to produce butt joints made of the E911 steel RWTU using the TIG method (141) and covered electrodes (111).

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Table 3 – Mechanical properties of deposited metal of some welding consumables for welding of E911 steel [10–12]. Chemical composition of wire or desposited metal (%)a

Welding method/ welding consumable TIG/Thermanit MTS 911b

Covered electrode/Thermanit MTS 911b

Subm. arc/wire: Thermanit MTS 911bflux: Marathon 543

TIG/C9 MVWIGc

Covered electrode/ FOX C9MVWc

C Si Mn Cr Mo Ni W V Nb N Heat treatment (1C/h) Mechanical R0.2 (MPa) Rm (MPa) properties A5 (%) of KV (J) deposited metal

0.10 0.38 0.45 9.00 1.00 0.70 1.00 0.20 0.06 0.04 750–760/Z2 560 720 16 41

0.11 0.20 0.60 8.80 0.50 0.70 1.60 0.20 0.05 0.05 750–760/Z2 560 720 15 41

0.11 0.35 0.45 9.00 1.00 0.75 1.00 0.20 0.06 – 760/4 638 761 19 75

0.11 0.35 0.45 9.00 0.98 0.75 1.05 0.20 0.06 0.07 760/2 660 790 16 50

0.10 0.25 0.70 8.50 1.00 0.70 1.00 0.20 0.05 0.05 760/2 Z630 Z700 Z15 Z27

a b c

Chemical composition of deposited metal specified for covered electrode. Thyssen Schweisstechnik. Bo¨hler Welding.

Table 4 – Properties of parent metal and deposited metals produced with wire (TIG) and covered electrode [11]. Material and welding method

Heat treatment

Re (MPa)

Rm (MPa)

A5 (%)

KV (J)

Hardness HV10

Parent metal TIG (141) + 2.4 mm EO (111) + 4.0 mm

750–800 1C 770 1C/2 h 770 1C/2 h

4450 684 689

620–850 822 832

Z17 18.5 17.2

27–40 44 44

r260 o297 o281

Chemical composition of deposited metals made with method: 141: 0.17%C, 0.2%Si, 0.4%Mn, 11.6%Cr, 0.4%Ni, 0.3%Mo, 0.22%V, 1.44%W, 0.06%Nb, 1.64%Co, 0.003%B and 111: 0.13%C, 0.33%Si, 0.7%Mn, 11.2%Cr, 0.8%Ni, 0.3%Mo, 0.24%V, 1.48%W, 0.06%Nb, 1.59%Co, 0.003%B.

Fig. 18 – CCT diagram in welding conditions for VM12-SHC steel.

4.

Steel VM12-SHC and its weldability

The newly developed X12CrCoWVNb12-2-2 (VM12-SHC) steel is intended to be applied in the heavy-duty elements of power boilers; it contains 11–12%Cr, 1.4–1.8%Co and 1.3–1.7%W. The addition of chromium increases the heat resistance of the

Fig. 19 – Cycle of welding and heat treatment of VM12-SHC steel [11].

steel and causes the increase of ferrite in the structure. The addition of cobalt decreases the ferrite content as the former acts in a manner similar to nickel, but does not decrease the temperature of martensitic transformation. The chemical composition of the steel is presented in Table 1, whereas its mechanical and plastic properties are described in Table 4.

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Table 5 – Properties of joints of pipes (u 140  10 mm) made of VM12-SHC steel. Material and welding method

Heat treatment

Rm (MPa)

Place of rupture

KV (J)

Hardness HV10

Parent metal TIG (141) EO (111)

750–800 1C 770 1C/0.5 h 770 1C/2 h

620–850 745 728

– Parent metal Parent metal

27–40 37–73 31–51

r260 r351 r322

The table presents the properties and chemical composition of deposited metals produced with the TIG method and covered electrode/wire/electrode Thermanit MTS 5 CoT. The consumables were provided with more carbon – to limit the amount of ferrite and nickel – to increase the impact toughness of welds (accompanied, however, by the decrease in their ductility). For this reason the joints must be annealed at 770 1C for 4 h. The CCT diagram of austenite transformations for welding conditions (Fig. 18) indicates that the HAZ area of the welded joints made of the VM12-SHC steel will contain martensite microstructure in a wide range of cooling times t8/5. The 10 mm-thick pipes made of the VM12-SHC steel should be welded pursuant to the diagram presented in Fig. 19. After welding, welded joints can be safely stored (storage rooms must be dry in order to prevent stress corrosion; it is also advisable to avoid any dynamic action on the material). Recommendable welding positions include those of PA, PC and PF. The properties of the joints made with the TIG method and covered electrodes, after heat treatment, are presented in Table 5. The VM12-SHC steel and related welding consumables will be applied in design and construction of new power units in the EU and, possibly, in other countries. The developed welding consumables should meet creep strength-related criteria, which, however, can be confirmed only after carrying out a classical long-lasting creep test until the rupture of a specimen. The optimisation of welding conditions (allowing for a welding method, welding linear energy, pre-heating and inter-pass temperatures, type and structure of joints, thickness of layer/run, bevelling) should take into account high mechanical properties of the steel as well as lowered impact toughness of the parent metal and that of the weld. An issue of vital importance is strict collaboration of designers, subcontractors and sub-suppliers of elements in solving technical problems.

5.

Conclusion

This study has demonstrated the following: 1. Welded joints made of the T/P92 steel, irrespective of the thickness of elements to be joined, must undergo heat treatment in order to temper hard and brittle martensite, reduce remaining welding stresses and ensure required impact toughness. 2. Heat-treated welded joints of the T92 and P92 steels (760 1C/1.5 h and 3 h) are characterised by hardness not exceeding 280 HV5 and 285 HV10, respectively.

3. The impact toughness values of HAZ and weld areas in welded joints from T/P92 and E911 steels fully meet the requirement of technical inspection bodies i.e. those of ¨ V and Poland’s Technical Inspection Office. TU 4. The P92 and E911 steels are not susceptible to annealing cracking as the contraction Z of the specimens with the simulated HAZ, subjected to tension in the 550–750 1C temperature range, significantly exceeds the minimum value of 20%. 5. In order to obtain martensitic microstructure in the whole area of the joint, after welding and prior to stress relief annealing, the welded joints from E911 steel should be cooled down to a temperature lower than the final temperature of martensitic transformation MF. 6. In order to increase the impact toughness of the weld metal in welded joints of VM12-SHC steel must be annealed at a temperature of 770 1C for 4 h.

r e f e r e n c e s

[1] R. Viswanathan, W.T. Bakker, Materials for boilers in ultra supercritical power plants, in: Proceedings of the International Joint Power Generation Conference, Miami Beach, Florida, 2000. [2] J. Bro´zda, New-generation high-temperature creep resisting steels, their weldability and properties of welded joints. Part 1: Application objective of new-generation high-temperature creep resisting steels, their characteristics and resultant advantages, Bulletin of Instytut Spawalnictwa 48 (1) (2004) 41–49. [3] J. Bro´zda, J.G. Czaja, Creep-resisting steel T92/P92, welding of steel and properties of welded joints, in: Proceedings of the 11th Conference on Materials Testing for Power Plants and Power Industry, Zakopane, 2002. [4] J. Bro´zda, M. Zeman, J. Pasternak, The first supercritical power unit in Poland, Weldability evaluation of new martensitic chromium steels with tungsten additions and properties of welded joints, in: Proceedings of the 7th Liege Conference, European Commission, University de Liege, vol. 21, part III, 2002, pp. 1711–1720. [5] A.W. Marshall, Z. Zhang, G.B. Holloway, Welding consumables for P92 and T23 creep resisting steels. /http://www. metrode.com/docs/news/t23.pdfS. [6] D. Richardot, J.C. Vaillant, A. Arbab, W. Bendick, The T92/P92 book, Information materials of Vallourec & Mannesmann Tubes, 2000. [7] T92-P92 Vallourec Experience, Information Materials of Vallourec Industries, Power Generation Division, 1997. [8] Wissenswertes fu¨r den Schweisser, Bo¨hler Welding Catalogue, 2001. [9] J. Bro´zda, New-generation high-temperature creep resisting steels, their weldability and properties of welded joints.

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Part 4: Steel E911, Bulletin of Instytut Spawalnictwa 48 (4) (2004) 49–54. [10] B. Hand, H. Heuser, Schweisstechnische Verarbeitung der neuen Kraftwerksstahle P92 und VM12-SHC, in: Proceedings of the ‘‘Powerwelding’’ Conference, Ustron´, 2010. ˆ en neuartiger warmfester [11] H. Heuser, C. Jochum, SchweiO CrMo-legierter Sta¨hle fu¨r konventionelle Kraftwerke, Information materials of Thyssen Welding, 2000.

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[12] E911-Vallourec Experience, Information Materials of Vallourec Industries, Power Generation Division, 1997. [13] J. Orr, L. Buchman, K. Everson, The commercial development and evaluation of E911, a strong 9%CrMoNbVWN steel for boiler tubes and headers, in: Proceedings of the ‘‘Advanced heat resistant steels for power generation’’ Conference, San Sebastian, Spain, 1998.