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Effect of nickel content on mechanical properties and fracture toughness of weld metal of WWER-1000 reactor vessel welded joints
International Journal of Pressure Vessels and Piping 81 (2004) 713–717 www.elsevier.com/locate/ijpvp
Effect of nickel content on mechanical properties and fracture toughness of weld metal of WWER-1000 reactor vessel welded joints A.S. Zubchenko*, G.S. Vasilchenko, E.G. Starchenko, S.I. Nosov Central Scientific Research Institute for Machine Building Technology, SRC NPO TSNIITMASH 4, Sharikopodshipni kovskaya, 4 115088 Moscow, Russia
Abstract Welding of WWER-1000 reactor vessel of steel 15X2HMFA is performed using the CB-12X2H2MAA wire and Fq-16 or Fq-16A flux. Nickel content in the weld metal usually lays within the limits 1.2– 1.9%. The experimental data is shown on the weld metal with the nickel contents 1.28 – 2.45% after irradiation with fluence up to 260.1022n/m2 at energy more than 0.5 MEV. The embrittlement was measured by shift of critical brittleness temperature. Has appeared, that the weld metal with the low nickel content is the least responsive to irradiation embrittlement. The mechanical properties and fracture toughness of the weld metal with the contents of a nickel less than 1.3% are studied. Specimens CT-1T are tested, the “master-curve”, and its confidence bounds with probability of destruction 5 and 95% is built. “Master-curve” in the specified confidence interval is affirmed by CT-4T specimens test data. Is shown, that the mechanical properties and fracture toughness of the weld metal with the contents of nickel less than 1.3% satisfy the normative requirements. q 2004 Published by Elsevier Ltd. Keywords: Reactor vessel; Weld metal; Nickel content; Fluence, Irradiation embrittlement; Mechanical properties; Critical brittleness temperature; Fracture toughness; Normative values
Automated welding of serial reactor vessels WWER-1000 of 15X2HMFA and 15X2HMFAA steels was performed using CB-12X2H2MAA wire in combination with fluxes Fq-16 and Fq-16A. With this, the nickel content in the weld metal was secured in the range of 1.2– 1.9%. To clarify the role of nickel content in the formation of performance characteristics of the reactor vessel metal the investigation on evaluation of nickel content effect on tendency of weld metal to brittle fracture including those after neutron irradiation was carried out. For this investigation the welding wire CB-12X2H2MAA with nickel content of 1.25 and 1.6% (heats 178058 and 179687) was used. Additionally, an experimental lot of wire with 2.48% nickel (with the content of other elements being at the level of requirements for technical specifications for the wire CB-12X2H2MAA) was fabricated. Welded joints 190 mm thick were made satisfying the requirements of standard documentation for welding and heat treatment of 15X2HMFAA steel. Chemical composition of welded joint metal, mechanical properties and critical embrittlement temperature of studied joints are shown in Tables 1 and 2. * Corresponding author. Tel./fax: þ 7-95-277-1012. E-mail address: [email protected] (A.S. Zubchenko). 0308-0161/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ijpvp.2004.02.017
The value of irradiation embrittlement of weld metal with various nickel content was estimated by the shift of the critical brittleness temperature ðDTc Þ after irradiation with fluence (40 – 260 £ 1022 n/m2) at E . 0:5 MEV. The investigation results have shown that the observed correlating relation between the value of fluence and the Tc shift of weld metal is close to a linear one (Fig. 1). The data have confirmed that the shift DTc of weld metal with the nickel content more than 1.30% as well as with the low nickel content (under 1.30%) after irradiation with fluence corresponding to that of calculated for 40 years operation of WWER-1000 reactor (# 64 £ 1022 n/m2) does not exceed the guaranteed values. The influence of nickel content on irradiation embrittlement increases with the fluence considerably exceeding the calculated value. The decision was taken to correct the chemical composition of welding wire CB-12X2H2MAA so as to decrease the nickel content to the value of not more than 1.30% with simultaneous severity of the requirements as to the content of impurities (phosphorus and sulphur—each is not more than 0.006% and copper is not more than 0.06%). For the investigation of resistance to brittle fracture of weld metal, carried out with a production lot of wire CB-12X2H2MAA (ht. 22554) of a corrected composition, the welded joints of 15X2HMFA steel, with the thickness
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A.S. Zubchenko et al. / International Journal of Pressure Vessels and Piping 81 (2004) 713–717
Table 1 Chemical composition of welded joint metal made for evaluation of the effect of nickel on the weld metal susceptibility to irradiation embrittlement (weight—%) No. of heat, wire, flux lot
C
Si
Mn
Cr
Ni
Mo
S
P
Cu
ht. 178058, lot 139 ht. 179687, lot 69 lab. ht., lot 139 ht. 22554, lot 235
0.064 0.065 0.078 0.08
0.25 0.32 0.27 0.30
1.08 0.86 1.10 0.75
1.6 1.83 1.68 2.00
1.28 1.60 2.45 1.10
0.55 0.6 0.65 0.62
0.011 0.006 0.007 0.012
0.009 0.007 0.006 0.006
0.055 0.06 0.03 0.04
of 250 mm, were made in OAO ‘Izhorskie zavody’. From these welded joints the tensile test specimens for testing at 20 and 350 8C, Charpy notched impact-test specimens as well as compact specimens CT-1T and CT-4T were fabricated. After the process of welding, heat treatment at 620 8C for 25 h and 650 8C for 20 h was performed. The welded joints were cut into templets and the specimens were fabricated by machining in accordance with the technical requirements for the welded joints control. The results of the tensile specimens testing are given in Table 3. It can be seen from Table 3 that all the testing results for tensile test specimens meet the normative requirements. Stress –strain diagrams recorded at testing of the tensile specimens were used to plot the Ramberg – Osgood diagrams at 20 8C (Fig. 2) and 350 8C (Fig. 3). Charpy V-notch specimens of weld metal were tested on an impact testing machine PSW-300 at room temperature from þ 20 8C to 2 40 8C. The experimental values of all 20 tested specimens are shown in Fig. 4. There are plotted the temperature dependences of impact toughness mean values and of minimum fibre percentage on the fracture surface of V-notch specimens. By comparison of the plotted dependences with critical values according to the rules stated in the document [1], the critical brittleness temperature of weld metal Tc ¼ 220 8C was determined. It is considerably lower than the standard requirements (Tc ¼ 0 8C) for the weld metal of WWER-1000 reactor vessels. The test results of Charpy V-notch specimens made it also possible to p evaluate the temperature of the mean value ffiffiffi KJC ¼ 100 MPa m and that determined testing conditions of the compact specimens CT-1T in accordance with the equation recommended by the standard [2], Texp ¼ T28J þ C
ð1Þ
where for Charpy specimens the correction C ¼ 218 8C. Fig. 4 makes it possible to estimate the temperature T28J ¼ 232 8C and therefore, Texp ¼ 250 8C. After forming cracks from the notches in accordance with the requirements [3], the first group of CT-1Tpspecimens was ffiffiffi tested and the values of K1C . 140 MPa m were determined. Therefore, for the next group of CT-1T specimens, the testing temperature was decreased to Tc ¼ 270 8C. Testing of eight specimens at temperature 2 70 8C, all of which appeared to be acceptable, determined the range of values of K1C ; with the mean value of fracture toughness at
pffiffiffi about 100 MPa m; which is necessary for plotting the ‘master-curve’ [2]. The test results of all 12 specimens CT1T are given in Table 4. The experimental data obtained with CT-1T specimens were treated with the help of equations given in the calculation section of document [2]. The results of the calculation determined the reference temperature, To ¼ 260 8C for the CT-1T specimens of the tested welded joints. For plotting the master-curve the following equation was used: pffiffiffi KJC ¼ 30 þ 70 exp½0:019ðTexp 2 To Þ MPa m ð2Þ The tolerance limits 5 and 95% were calculated from the equation: KJC ¼ D1 þ D2 exp½0:019ðTexp 2 To Þ
ð3Þ
where D1 and D2 —reference coefficients from Ref. [2]. The master-curve and the tolerance limits for the tested weld metal with the nickel content 1.10% were obtained with the help of test results of the CT-1T specimens (Fig. 5). It is seen that actually all experimental points are within the chosen tolerance limit. The compact CT-4T specimens were fabricated of the same welding sample as the CT-1T specimens. For formation of fatigue cracks from the notches, a test machine qMM-200Py was used. Initial cyclic loading before the appearance of cracks on the side surfaces was performed under the maximum load Pmax ¼ 450 kN and minimum load Pmin ¼ 80 kN during about 60,000 cycles. The final 5 mm cracks were formed with Pmax ¼ 150 kN and Pmin ¼ 30 kN during 50,000 cycles. Testing of the CT-4T specimens after formation of cracks was carried out in the same machine qMM-200Py equipped with sensors for force and displacement, the system to record deformation of a specimen, and the cryocamera for specimen cooling with liquid nitrogen vapour. The control of cooling temperature of a specimen was carried out continuously Table 2 Critical embrittlement temperature of welds metal (after final tempering) No. of heat, wire, flux lot
Critical brittleness temperature (8C)
Impact toughness at 20 8C (J/cm2)
ht. 178058, lot 139 ht. 179687, lot 69 exp. heat, lot 139
230… 2 40 230… 2 40 220… 2 30
180–190 143–193 150–170
A.S. Zubchenko et al. / International Journal of Pressure Vessels and Piping 81 (2004) 713–717
715
Fig. 1. Irradiation embrittleness of weld metal with nickel content: B—1.28%, K—1.60%, X—2.45%. Table 3 Mechanical properties of weld metal Specimen stamp
1 2 3 Average 4 5 6 Average Standard requirements
Yield stress, RTp0:2 (MPa)
Tensile strength, Rm (MPa)
484 494 486 488
591 588 589 589
22.8 25.0 22.6 23.5
73.1 73.1 75.0 73.7
350
412 450 422 428
491 518 495 499
18.2 16.5 15,8 17.2
69.5 67.8 68.2 68.5
20 350
422 392
539 490
15.0 17.0
55.0 50.0
Testing temperature (8C) 20
20 350
Fig. 2. Diagram of deformation at 20 8C of weld metal with nickel content not greater then 1.3% and its approximation by Ramberg–Osgood method.
Elongation A (%)
Reduction of area Z (%)
Fig. 3. Diagram of deformation at 350 8C of weld metal with nickel content not greater then 1.3% and its approximation by Ramberg–Osgood method.
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Fig. 4. Determination of Tk of weld metal of 15Cr2NiMoVA steel welded joint made by conventional technology with the nickel content not greater than 1.3%.
using Chromel – alumel thermocouples, whose junctions were welded to the specimen surface in the area of an end of the crack. The KJC value for the CT-4T specimens was determined taking into account the value of the plastic component of fracture energy for only one specimen which had been tested at plus 20 8C, because by the moment of fracture it had 2.15 mm static growth from the initial fatigue crack. The rest eight specimens underwent fracture without static development of a crack and that made it possible to determine KJC only by the elastic component. The values of KJC obtained for the CT-4T specimens were also recalculated to the thickness of the CT-1T specimen from the equation: KJC ¼ Kmin þ ðK4JC 2 Kmin ÞðB4 =B1 Þ1=4
K4JC and B4 —fracture toughness and the CT-4T specimen thickness. The test results of the CT-4T specimens are shown in Table 5. The values of KJC are given in Table 5 (CT-4T specimens both actual and recalculated to 25 mm thickness are presented in Fig. 5). It can be seen in Fig. 5 that the 5 and 95% confidence intervals of the master-curve obtained by the test results of the CT-1T specimens are confirmed by the test results of the CT-4T specimens with the thickness of
ð4Þ
where
pffiffiffi Kmin ¼ 20 MPa m;
K1JC and B1 —fracture toughness and the CT-1T specimen thickness; Table 4 Test results of CT-1T specimens No. Test Crack size (mm) Breaking Fracture toughness pffiffiffi temperature (8C) load (N) KJC (MPa m) 1 2 3 4 5 6 7 8 9 10 11 12
250 250 250 260 270 270 270 270 270 270 270 270
24.15 24.4 24.5 24.7 27.9 24.7 24.5 24.8 26.1 24.5 24.8 25.0
6870 6600 6800 6400 6660 3470 5200 5330 5000 5200 4700 4200
160.0 142.3 170.0 129.0 121.4 58.0 96.1 104.4 103.2 98.0 83.5 80.3
Fig. 5. ‘Master-curve’ with 5 and 95% of the confidential intervals for weld metal.
A.S. Zubchenko et al. / International Journal of Pressure Vessels and Piping 81 (2004) 713–717 Table 5 Test results of the CT-4T specimens No. Temperature Crack size Breaking Fracture toughness KJC pffiffiffi (8C) (mm) load (kN) (Mpa m) Without Recalculated recalculation by 25 mm thickness 1 2 3 4 5 6 7 8 9
20 20 20 215 220 220 255 280 280
107.85 104.02 103.4 104.4 103.2 103.9 104.64 104.4 104.1
722 1080 830 465 500 676 385 374 312.5
202.9 369 203.9 103.5 109.6 149.8 86.4 80.6 69.5
276.9 512 279.3 138.3 146.7 203.1 113.6 105.4 89.6
Fig. 6. Comparison of test results for fracture toughness of the CT-IT and CT-4T specimens with the normative curve for welded joints of WWER1000 reactor vessels.
100 mm both by actual and recalculated by the thickness of 25 mm. In Fig. 6 the test results of the CT-1T and CT-4T specimens are shown as a function of the temperature equal to the difference between the test temperature of the specimen and the critical brittleness temperature of weld metal Tc ¼ 220 8C. It is seen that even the minimum values
717
of fracture toughness of the specimens both with 25 and 100 mm thickness satisfy the requirements of the document [1] standard curve for welded joints of vessel steels and exceeds values of basic curve [4].
1. Conclusions This work was intended for investigation of mechanical properties and fracture toughness of weld metal with the thickness of 250 mm performed on 15X2HMFA steel featuring an automated welding with CB-12X2H2MAA wire with nickel content up to 1.3% under flux Fq-16A. The mechanical properties of weld metal at 20 and 350 8C were determined and Ramberg – Osgood diagrams were plotted for these temperatures. Testing of Charpy Vnotch impact specimens determined the critical brittleness temperature Tc ¼ 220 8C of weld metal. The obtained mechanical properties of tension and impact toughness exceed the normative values. The results of CT-1T specimens testing made it possible to plot the master-curve and confidence intervals of 5 and 95%. Testing of the CT-4T specimens at room and lower temperatures confirmed the validity of the obtained confidence intervals that proves the possibility of obtaining reliable results for fracture toughness of materials using small specimens. Comparison of the obtained results with the normative curve of fracture toughness determined a considerable advantage of weld metal made by an automated welding with CB-12X2H2MAA wire with the nickel content not more than 1.3% under flux Fq-16A. The improved technology, resulting in making weld metal with less nickel content, is recommended for production of WWER-1000 and WWER-1500 reactor vessels, and the calculation of these reactors for resistance to the brittle fracture shall be made using the normative curve for fracture toughness of welded joints [1].
References [1] PHA4-7-002-86. Calculation norms for the strength of equipment and pipelines of nuclear power plants. M. Energoatomizdat; 1989. 528p. [2] ASTME 1921–97. Standard method for determination of reference temperature for ferric steels in transition range; 1997. 17p. [3] GOST 25.506-85. Methods for mechanical tests of materials. Determination of characteristics of cracking resistance (fracture toughness) at statistical Loading. Gosstandart USSR, Moscow; 1985. 61p. [4] Margolin BZ, Shvetsova IA, Rivkin EYu. Prediction procedure of dependency of fracture toughness of WWER-440 and WWER-1000 reactor vessel materials, St Petersburg; 2000.
Effect of nickel content on mechanical properties and fracture toughness of weld metal of WWER-1000 reactor vessel welded joints A.S. Zubchenko*, G.S. Vasilchenko, E.G. Starchenko, S.I. Nosov Central Scientific Research Institute for Machine Building Technology, SRC NPO TSNIITMASH 4, Sharikopodshipni kovskaya, 4 115088 Moscow, Russia
Abstract Welding of WWER-1000 reactor vessel of steel 15X2HMFA is performed using the CB-12X2H2MAA wire and Fq-16 or Fq-16A flux. Nickel content in the weld metal usually lays within the limits 1.2– 1.9%. The experimental data is shown on the weld metal with the nickel contents 1.28 – 2.45% after irradiation with fluence up to 260.1022n/m2 at energy more than 0.5 MEV. The embrittlement was measured by shift of critical brittleness temperature. Has appeared, that the weld metal with the low nickel content is the least responsive to irradiation embrittlement. The mechanical properties and fracture toughness of the weld metal with the contents of a nickel less than 1.3% are studied. Specimens CT-1T are tested, the “master-curve”, and its confidence bounds with probability of destruction 5 and 95% is built. “Master-curve” in the specified confidence interval is affirmed by CT-4T specimens test data. Is shown, that the mechanical properties and fracture toughness of the weld metal with the contents of nickel less than 1.3% satisfy the normative requirements. q 2004 Published by Elsevier Ltd. Keywords: Reactor vessel; Weld metal; Nickel content; Fluence, Irradiation embrittlement; Mechanical properties; Critical brittleness temperature; Fracture toughness; Normative values
Automated welding of serial reactor vessels WWER-1000 of 15X2HMFA and 15X2HMFAA steels was performed using CB-12X2H2MAA wire in combination with fluxes Fq-16 and Fq-16A. With this, the nickel content in the weld metal was secured in the range of 1.2– 1.9%. To clarify the role of nickel content in the formation of performance characteristics of the reactor vessel metal the investigation on evaluation of nickel content effect on tendency of weld metal to brittle fracture including those after neutron irradiation was carried out. For this investigation the welding wire CB-12X2H2MAA with nickel content of 1.25 and 1.6% (heats 178058 and 179687) was used. Additionally, an experimental lot of wire with 2.48% nickel (with the content of other elements being at the level of requirements for technical specifications for the wire CB-12X2H2MAA) was fabricated. Welded joints 190 mm thick were made satisfying the requirements of standard documentation for welding and heat treatment of 15X2HMFAA steel. Chemical composition of welded joint metal, mechanical properties and critical embrittlement temperature of studied joints are shown in Tables 1 and 2. * Corresponding author. Tel./fax: þ 7-95-277-1012. E-mail address: [email protected] (A.S. Zubchenko). 0308-0161/$ - see front matter q 2004 Published by Elsevier Ltd. doi:10.1016/j.ijpvp.2004.02.017
The value of irradiation embrittlement of weld metal with various nickel content was estimated by the shift of the critical brittleness temperature ðDTc Þ after irradiation with fluence (40 – 260 £ 1022 n/m2) at E . 0:5 MEV. The investigation results have shown that the observed correlating relation between the value of fluence and the Tc shift of weld metal is close to a linear one (Fig. 1). The data have confirmed that the shift DTc of weld metal with the nickel content more than 1.30% as well as with the low nickel content (under 1.30%) after irradiation with fluence corresponding to that of calculated for 40 years operation of WWER-1000 reactor (# 64 £ 1022 n/m2) does not exceed the guaranteed values. The influence of nickel content on irradiation embrittlement increases with the fluence considerably exceeding the calculated value. The decision was taken to correct the chemical composition of welding wire CB-12X2H2MAA so as to decrease the nickel content to the value of not more than 1.30% with simultaneous severity of the requirements as to the content of impurities (phosphorus and sulphur—each is not more than 0.006% and copper is not more than 0.06%). For the investigation of resistance to brittle fracture of weld metal, carried out with a production lot of wire CB-12X2H2MAA (ht. 22554) of a corrected composition, the welded joints of 15X2HMFA steel, with the thickness
714
A.S. Zubchenko et al. / International Journal of Pressure Vessels and Piping 81 (2004) 713–717
Table 1 Chemical composition of welded joint metal made for evaluation of the effect of nickel on the weld metal susceptibility to irradiation embrittlement (weight—%) No. of heat, wire, flux lot
C
Si
Mn
Cr
Ni
Mo
S
P
Cu
ht. 178058, lot 139 ht. 179687, lot 69 lab. ht., lot 139 ht. 22554, lot 235
0.064 0.065 0.078 0.08
0.25 0.32 0.27 0.30
1.08 0.86 1.10 0.75
1.6 1.83 1.68 2.00
1.28 1.60 2.45 1.10
0.55 0.6 0.65 0.62
0.011 0.006 0.007 0.012
0.009 0.007 0.006 0.006
0.055 0.06 0.03 0.04
of 250 mm, were made in OAO ‘Izhorskie zavody’. From these welded joints the tensile test specimens for testing at 20 and 350 8C, Charpy notched impact-test specimens as well as compact specimens CT-1T and CT-4T were fabricated. After the process of welding, heat treatment at 620 8C for 25 h and 650 8C for 20 h was performed. The welded joints were cut into templets and the specimens were fabricated by machining in accordance with the technical requirements for the welded joints control. The results of the tensile specimens testing are given in Table 3. It can be seen from Table 3 that all the testing results for tensile test specimens meet the normative requirements. Stress –strain diagrams recorded at testing of the tensile specimens were used to plot the Ramberg – Osgood diagrams at 20 8C (Fig. 2) and 350 8C (Fig. 3). Charpy V-notch specimens of weld metal were tested on an impact testing machine PSW-300 at room temperature from þ 20 8C to 2 40 8C. The experimental values of all 20 tested specimens are shown in Fig. 4. There are plotted the temperature dependences of impact toughness mean values and of minimum fibre percentage on the fracture surface of V-notch specimens. By comparison of the plotted dependences with critical values according to the rules stated in the document [1], the critical brittleness temperature of weld metal Tc ¼ 220 8C was determined. It is considerably lower than the standard requirements (Tc ¼ 0 8C) for the weld metal of WWER-1000 reactor vessels. The test results of Charpy V-notch specimens made it also possible to p evaluate the temperature of the mean value ffiffiffi KJC ¼ 100 MPa m and that determined testing conditions of the compact specimens CT-1T in accordance with the equation recommended by the standard [2], Texp ¼ T28J þ C
ð1Þ
where for Charpy specimens the correction C ¼ 218 8C. Fig. 4 makes it possible to estimate the temperature T28J ¼ 232 8C and therefore, Texp ¼ 250 8C. After forming cracks from the notches in accordance with the requirements [3], the first group of CT-1Tpspecimens was ffiffiffi tested and the values of K1C . 140 MPa m were determined. Therefore, for the next group of CT-1T specimens, the testing temperature was decreased to Tc ¼ 270 8C. Testing of eight specimens at temperature 2 70 8C, all of which appeared to be acceptable, determined the range of values of K1C ; with the mean value of fracture toughness at
pffiffiffi about 100 MPa m; which is necessary for plotting the ‘master-curve’ [2]. The test results of all 12 specimens CT1T are given in Table 4. The experimental data obtained with CT-1T specimens were treated with the help of equations given in the calculation section of document [2]. The results of the calculation determined the reference temperature, To ¼ 260 8C for the CT-1T specimens of the tested welded joints. For plotting the master-curve the following equation was used: pffiffiffi KJC ¼ 30 þ 70 exp½0:019ðTexp 2 To Þ MPa m ð2Þ The tolerance limits 5 and 95% were calculated from the equation: KJC ¼ D1 þ D2 exp½0:019ðTexp 2 To Þ
ð3Þ
where D1 and D2 —reference coefficients from Ref. [2]. The master-curve and the tolerance limits for the tested weld metal with the nickel content 1.10% were obtained with the help of test results of the CT-1T specimens (Fig. 5). It is seen that actually all experimental points are within the chosen tolerance limit. The compact CT-4T specimens were fabricated of the same welding sample as the CT-1T specimens. For formation of fatigue cracks from the notches, a test machine qMM-200Py was used. Initial cyclic loading before the appearance of cracks on the side surfaces was performed under the maximum load Pmax ¼ 450 kN and minimum load Pmin ¼ 80 kN during about 60,000 cycles. The final 5 mm cracks were formed with Pmax ¼ 150 kN and Pmin ¼ 30 kN during 50,000 cycles. Testing of the CT-4T specimens after formation of cracks was carried out in the same machine qMM-200Py equipped with sensors for force and displacement, the system to record deformation of a specimen, and the cryocamera for specimen cooling with liquid nitrogen vapour. The control of cooling temperature of a specimen was carried out continuously Table 2 Critical embrittlement temperature of welds metal (after final tempering) No. of heat, wire, flux lot
Critical brittleness temperature (8C)
Impact toughness at 20 8C (J/cm2)
ht. 178058, lot 139 ht. 179687, lot 69 exp. heat, lot 139
230… 2 40 230… 2 40 220… 2 30
180–190 143–193 150–170
A.S. Zubchenko et al. / International Journal of Pressure Vessels and Piping 81 (2004) 713–717
715
Fig. 1. Irradiation embrittleness of weld metal with nickel content: B—1.28%, K—1.60%, X—2.45%. Table 3 Mechanical properties of weld metal Specimen stamp
1 2 3 Average 4 5 6 Average Standard requirements
Yield stress, RTp0:2 (MPa)
Tensile strength, Rm (MPa)
484 494 486 488
591 588 589 589
22.8 25.0 22.6 23.5
73.1 73.1 75.0 73.7
350
412 450 422 428
491 518 495 499
18.2 16.5 15,8 17.2
69.5 67.8 68.2 68.5
20 350
422 392
539 490
15.0 17.0
55.0 50.0
Testing temperature (8C) 20
20 350
Fig. 2. Diagram of deformation at 20 8C of weld metal with nickel content not greater then 1.3% and its approximation by Ramberg–Osgood method.
Elongation A (%)
Reduction of area Z (%)
Fig. 3. Diagram of deformation at 350 8C of weld metal with nickel content not greater then 1.3% and its approximation by Ramberg–Osgood method.
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Fig. 4. Determination of Tk of weld metal of 15Cr2NiMoVA steel welded joint made by conventional technology with the nickel content not greater than 1.3%.
using Chromel – alumel thermocouples, whose junctions were welded to the specimen surface in the area of an end of the crack. The KJC value for the CT-4T specimens was determined taking into account the value of the plastic component of fracture energy for only one specimen which had been tested at plus 20 8C, because by the moment of fracture it had 2.15 mm static growth from the initial fatigue crack. The rest eight specimens underwent fracture without static development of a crack and that made it possible to determine KJC only by the elastic component. The values of KJC obtained for the CT-4T specimens were also recalculated to the thickness of the CT-1T specimen from the equation: KJC ¼ Kmin þ ðK4JC 2 Kmin ÞðB4 =B1 Þ1=4
K4JC and B4 —fracture toughness and the CT-4T specimen thickness. The test results of the CT-4T specimens are shown in Table 5. The values of KJC are given in Table 5 (CT-4T specimens both actual and recalculated to 25 mm thickness are presented in Fig. 5). It can be seen in Fig. 5 that the 5 and 95% confidence intervals of the master-curve obtained by the test results of the CT-1T specimens are confirmed by the test results of the CT-4T specimens with the thickness of
ð4Þ
where
pffiffiffi Kmin ¼ 20 MPa m;
K1JC and B1 —fracture toughness and the CT-1T specimen thickness; Table 4 Test results of CT-1T specimens No. Test Crack size (mm) Breaking Fracture toughness pffiffiffi temperature (8C) load (N) KJC (MPa m) 1 2 3 4 5 6 7 8 9 10 11 12
250 250 250 260 270 270 270 270 270 270 270 270
24.15 24.4 24.5 24.7 27.9 24.7 24.5 24.8 26.1 24.5 24.8 25.0
6870 6600 6800 6400 6660 3470 5200 5330 5000 5200 4700 4200
160.0 142.3 170.0 129.0 121.4 58.0 96.1 104.4 103.2 98.0 83.5 80.3
Fig. 5. ‘Master-curve’ with 5 and 95% of the confidential intervals for weld metal.
A.S. Zubchenko et al. / International Journal of Pressure Vessels and Piping 81 (2004) 713–717 Table 5 Test results of the CT-4T specimens No. Temperature Crack size Breaking Fracture toughness KJC pffiffiffi (8C) (mm) load (kN) (Mpa m) Without Recalculated recalculation by 25 mm thickness 1 2 3 4 5 6 7 8 9
20 20 20 215 220 220 255 280 280
107.85 104.02 103.4 104.4 103.2 103.9 104.64 104.4 104.1
722 1080 830 465 500 676 385 374 312.5
202.9 369 203.9 103.5 109.6 149.8 86.4 80.6 69.5
276.9 512 279.3 138.3 146.7 203.1 113.6 105.4 89.6
Fig. 6. Comparison of test results for fracture toughness of the CT-IT and CT-4T specimens with the normative curve for welded joints of WWER1000 reactor vessels.
100 mm both by actual and recalculated by the thickness of 25 mm. In Fig. 6 the test results of the CT-1T and CT-4T specimens are shown as a function of the temperature equal to the difference between the test temperature of the specimen and the critical brittleness temperature of weld metal Tc ¼ 220 8C. It is seen that even the minimum values
717
of fracture toughness of the specimens both with 25 and 100 mm thickness satisfy the requirements of the document [1] standard curve for welded joints of vessel steels and exceeds values of basic curve [4].
1. Conclusions This work was intended for investigation of mechanical properties and fracture toughness of weld metal with the thickness of 250 mm performed on 15X2HMFA steel featuring an automated welding with CB-12X2H2MAA wire with nickel content up to 1.3% under flux Fq-16A. The mechanical properties of weld metal at 20 and 350 8C were determined and Ramberg – Osgood diagrams were plotted for these temperatures. Testing of Charpy Vnotch impact specimens determined the critical brittleness temperature Tc ¼ 220 8C of weld metal. The obtained mechanical properties of tension and impact toughness exceed the normative values. The results of CT-1T specimens testing made it possible to plot the master-curve and confidence intervals of 5 and 95%. Testing of the CT-4T specimens at room and lower temperatures confirmed the validity of the obtained confidence intervals that proves the possibility of obtaining reliable results for fracture toughness of materials using small specimens. Comparison of the obtained results with the normative curve of fracture toughness determined a considerable advantage of weld metal made by an automated welding with CB-12X2H2MAA wire with the nickel content not more than 1.3% under flux Fq-16A. The improved technology, resulting in making weld metal with less nickel content, is recommended for production of WWER-1000 and WWER-1500 reactor vessels, and the calculation of these reactors for resistance to the brittle fracture shall be made using the normative curve for fracture toughness of welded joints [1].
References [1] PHA4-7-002-86. Calculation norms for the strength of equipment and pipelines of nuclear power plants. M. Energoatomizdat; 1989. 528p. [2] ASTME 1921–97. Standard method for determination of reference temperature for ferric steels in transition range; 1997. 17p. [3] GOST 25.506-85. Methods for mechanical tests of materials. Determination of characteristics of cracking resistance (fracture toughness) at statistical Loading. Gosstandart USSR, Moscow; 1985. 61p. [4] Margolin BZ, Shvetsova IA, Rivkin EYu. Prediction procedure of dependency of fracture toughness of WWER-440 and WWER-1000 reactor vessel materials, St Petersburg; 2000.