Integrity Assessment of Aged and On-Site Welded Joints … · Based on the mechanical test results expected to ... Keywords: Liquid metal fast reactor, welded joint ... Integrity

OMMI (Vol.4, Issue 1) April 2007 www.ommi.co.uk

Integrity Assessment of Aged and On-Site Welded Joints in LMFR Takashi WAKAI, Takashi ONIZAWA, Masanori ANDO, Kazumi AOTO, Advanced Material Research Group, O-arai Engineering Centre, Japan Nuclear Cycle Development Institute, Japan Laurent MARTIN, Phenix, France Abstract This paper studies the structural integrity of the on-site welded joints expected future long life fast breeder reactors. It is essential to clarify the influence of heat input during on-site welding on the structural integrity of aged materials. Some welded joints made of austenitic stainless steels are sampled from sodium pipes and the microstructures of those are examined using optical and electron microscopes. Comparison of the microstructures of aged factory welded joints and the on-site welded joints, suggested that the heat input during repair welding process may have little harmful effect on the structural integrity of aged material as far as this study examines. Based on the mechanical test results expected to be obtained in future, the structural integrity of such on-site welded joints will be demonstrated. Keywords: Liquid metal fast reactor, welded joint, microstructure, carbide, plant life management

Introduction

In order to enhance the economic competitiveness of the fast breeder reactors (FBRs) with future LWRs and other electric resources, the total cost reduction by prolongation of plant service period. In the long term service of the plant, e.g. for 60 years, as damaged pipes and/or components may be repaired or replaced, some on-site repair welds may take place. A number of on-site welds are actually used in the predecessor plant, such as EBR-II and Phenix, and few troubles have been experienced. However, it is essential to establish the damage assessment techniques applicable to such on-site welds to assure the structural integrity of the components for the future long life FBR plants. In order to establish structural integrity assessment methods for the on-site welds, this study examines some aged welds sampled from sodium pipes using optical and scanning electron microscopes to clarify the influence of heat input during the on-site welding process on the integrity of the aged materials. In addition, some new simulated on-site welds are produced by connecting aged materials with new ones and the microstructures of those are examined to compare with the aged on-site welds. Vickers hardness tests are also performed for these welds.

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Integrity Assessment of Aged and On-Site Welded Joints in LMFR, T Wakai et. al

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Samples Samples from JNC sodium test facilities

A number of component tests in liquid sodium environment have been performed mainly to demonstrate structural integrity of the components for the Japan prototype Fast Breeder Reactor (FBR) Monju at the O-arai Engineering Center (OEC) of Japan Nuclear Cycle development institute (JNC). For these tests, some sodium test facilities were constructed and have been operated. After the completion of the construction of Monju, these sodium test facilities have been dismantled. In this study, two aged samples made of type 304 austenitic stainless steel (304SS) were taken from the test facilities. The first sample was taken from a vessel in the test facility named “Structural Integrity Test for Monju Reactor vessel (SITR)”. The wall thickness of the vessel was 50 mm. SITR has been operated at 529oC for approximately 6,300 hours. The second was sampled from a pipe in the “Piping Bellows Test Loop (PBTL)”. The thickness and outer diameter of the pipe was 5 mm and 165.2 mm, respectively. The sample had been immersed in high temperature liquid sodium from 530 to 560oC for approximately 8,300 hours. Each sample contained a factory produced weld. Using the samples, simulated on-site welded joints were produced by connecting with new materials.

Samples from secondary coolant pipes of Phenix

An aged pipe including longitudinal and circumferential welded joints made of 304SS was delivered from the secondary coolant system of Phenix. The pipe had been used in Phenix at elevated temperature ranged from 526 to 545oC for approximately 88,000 hours. A simulated on-site welded joint was also supplied from Phenix. The aged pipe made of 304SS had been sampled from the secondary coolant pipe and a new pipe made of type 316L austenitic stainless steel (316LSS) was welded to the aged pipe. Figure 1 shows the shape and dimensions of the aged pipe and the simulated on-site welded pipe.

The chemical compositions, heat treatment conditions and mechanical properties of the base metal used in the aged and the simulated on-site welded joints are shown in Table 1, 2 and 3. The welding conditions of these welds are shown in Table 4, though most of them are not known. Table 5 shows the chemical compositions of the welding consumables for these welds.

Hardness tests

Vickers hardness tests were carried out for the aged and simulated on-site welds. The hardness distributions across the welds are shown in Fig.2. In these figures, the aged material locates on the right side and the new one locates on the left side for the simulated on-site welds. For the welds from SITR, which consist of 304SS with 50mm thickness, an uneven hardness distribution is observed and the hardness of HAZ on the aged material side of the simulated on-site weld is obviously greater than that of the aged one on the aged side. In contrast, for the aged weld and simulated on-site weld from PBTL which consist of 304SS with 5mm thickness, the hardness distribution is almost flat and there is little difference between the hardness of the aged weld and simulated on-site one. The original hardness of the materials had been 140 and 160VPN as shown in Table 3, respectively. It is certain that HAZ hardened in the on-site welding process. As the heat input during the welding


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process of the thick plate is supposed to be much larger than that of the thin wall pipe, the hardness of the weld from SITR increases with welding and that from PBTL doesn’t. However, such an uneven hardness distribution is commonly seen in a welded joint of an austenitic stainless steel. For the welds from the pipe of Phenix, the hardness of the simulated on-site weld is slightly larger than that of the aged one.

Metallurgical Examinations Observation using an optical microscope

Microstructures of HAZ of the aged material side of the simulated on-site weld have been compared to those of the aged weld to confirm the influence of heat input during the welding process on the microstructures. Figure 3 shows the micrographs obtained by an optical microscope. Three pictures on the left side of the figures (i.e. (a), (c) and (e)) are typical microstructures of HAZ of the aged material side of the simulated on-site welds, and the pictures on the right side (i.e. (b), (d) and (f)) are those of the aged welds. More carbides can be seen in HAZ of the aged welds than that of the aged material side in the simulated on-site welds. Strictly speaking, as the welding condition of the simulated on-site weld is not same as that of the aged one, it may be difficult to compare the HAZ of these welds simply. However, it is supposed that the microstructures of the aged material just prior to the on-site welding can be guessed by observing the microstructures of the base metal region sufficiently far from HAZ. A number of carbides are observed mainly on the grain boundaries of the aged base metal, but there are few carbides in HAZ of the aged material side in the simulated on-site welds. It is supposed that re-dissolution of the precipitated carbides into the matrix has occurred due to the heat input in the on-site welding process. Generally, the precipitation of carbides at the grain boundaries results in reduction of ductility and toughness. Therefore, disappearance of carbides due to on-site welding must not have any harmful effect on the structural integrity of the on-site welds. As far as this study observed, it was impossible to find a creep void in the weld metal, HAZ and base metal in all samples.

Observation and quantitative analyses using a scanning electron micro scope

For the simulated on-site weld and aged weld sampled from Phenix, observations using a Scanning Electron Microscope (SEM) and quantitative analyses using Energy Dispersive X-ray spectrometer (SEM-EDX) were performed to identify the compositions of the precipitates. Figure 4 shows SEM micrographs of the aged weld sampled from Phenix. In the weld metal, carbide and •-ferrite could be found very easily. Figure 4 (a) and (b) shows the typical carbides and •-ferrite, respectively. As the •-ferrite exists in weld metal, it is necessary to be careful with the loss of ductility in a welded joint as well as in a duplex stainless steel (Alexander et al [1]) (Hale et al [2]). Though minimum diameter of the SEM-EDX beam spot is not small enough to perform an accurate quantitative analysis, it is supposed that carbide is M23C6. In HAZ close to the weld metal, other types of precipitates could be detected. Analyzing by SEM-EDX, it was recognized that these precipitates included a lot of Cr, Ni and Si. Although a more detailed examination using a Transmission Electron Microscope (TEM), for example, must be carried out for identification purposes, it is supposed that this may be G-phase (Auger et al [3]).


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Figure 5 shows SEM micrographs of the aged material in the simulated on-site weld sampled from Phenix. The number of the precipitations is remarkably fewer than those observed in the aged weld. This is corresponding to the result of the optical microscope observations. The re-dissolution of carbides which had precipitated in service period might take place due to the heat input in the on-site welding process. Peculiar precipitations could not be detected in the simulated on-site weld, though a few precipitations which seemed to be G-phase could be observed in HAZ of the aged weld. It is difficult to guess whether a G-phase have precipitated before on-site welding or not.

Conclusions and Comments In order to contribute to the establishment of the structural integrity assessment methods for the on-site welds, some aged welds and some simulated on-site welds were examined to clarify the influence of heat input during the on-site welding process on the integrity of the aged materials. The following conclusions could be obtained:

1. A number of carbides could be observed mainly on the grain boundaries of the

heat affected zone (HAZ) of the aged welds. In contrast, there are few carbides in HAZ of the simulated on-site welds. It is supposed that the carbides which have precipitated in the service period may have redissolved the matrix due to the heat input in the on-site welding process.

2. No creep voids could be observed in any of the samples. 3. In HAZ close to the weld metal in the Phenix aged welds, peculiar precipitations

could be detected. Analyzing by SEM-EDX, it was recognized that the precipitation contained a lot of Cr, Ni and Si. Although a much detailed examination using TEM must be required, it is supposed that this may be G-phase. Such a precipitation could not be detected in the simulated on-site weld.

4. For the welds consisting of thick plates, uneven hardness distribution is observed and the hardness of HAZ of the aged material side of the simulated on-site weld is obviously greater than that of the aged one on the aged side.

Supposing precipitation and coarsening of carbides can be used as the index for

degradation of the steel, it can be said that there are little harmful effects of the heat input on the structural integrity of the on-site welds, because the carbides in HAZ of the on-site welds do not cohere and coarsen and redissolve in the matrix by the on-site welding, as far as this study observes. However, we have to pay attention to the behavior of re-nucleation of the carbides which dissolved once. Therefore, to investigate the microstructural evolution after the on-site welding, a series of creep tests of the simulated on-site welds are presently on-going. After the completion or interruption of the creep tests, the microstructures of the specimens will be re-examined and compared with those before testing.


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Acknowledgement The contribution of Mr. Takashi Kiriyama in Koryo Engineering ltd. to the optical microscope observation is acknowledged.

References 1. Alexiander, K.B., Miller, M.K., Alexander, D.J. and Nanstad, R.K. Mater. Sci.

Technol. 1990;

6, pp.314.

2. Hale, G.E. and Garwood S.J. Mater. Sci. Technol. 1990; 6, pp.230.

3. Auger, P., Dainoix, F., Menard, A., Bonnet, S., Bourgoin, J., and Guttmann, M., Mater. Sci. Technol. 1990; 6, pp.301.


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C Si Mn P S Cr Ni Mo Cu Co N B304SS (New) 0.05 0.51 0.9 0.033 0.004 18.05 8.08 ― ― ― ― ―304SS (Aged) 0.06 0.55 0.94 0.031 0.008 18.38 8.93 ― ― ― ― ―304SS (New) 0.02 0.44 1.04 0.031 0.004 18.13 9.16 ― ― ― ― ―304SS (Aged) 0.05 0.42 1.63 0.034 0.008 18.75 9.13 ― ― ― ― ―316LSS (New) 0.027 0.39 1.63 0.029 0.001 17.58 12.14 2.54 0.24 0.13 0.065 0.0005304SS (Aged) 0.062 0.68 1.93 0.027 0.014 18.42 10.11 ― ― ― ― ―

Phenix Simulated on-site weld/Aged weld

Simulated on-site weld/Aged weld


Sample

SITR

PBTL

Material

Notice）For 304SS, average of the analysis results for 8 batches.

304SS (New)304SS (Aged)304SS (New)304SS (Aged)316LSS (New)304SS (Aged)

PBTL Simulated on-site weld/Aged weld

Phenix

Sample

SITR Simulated on-site weld/Aged weld 1100℃×5h×W.Q.

1100℃×5min.×W.Q.1100℃×13min.×W.Q.1120℃±15℃×15minutes×W.Q.Unknown

Heat treatment conditions


Material1100℃×25.0min.×W.Q.

304SS (New)304SS (Aged)304SS (New)304SS (Aged)316LSS (New)304SS (Aged)

Phenix Simulated on-site weld/Aged weld

5948

577617

6671

626

PBTL

559SITR

Sample



0.2% proofstress

(N/mm2）

284256206256

Hardness

（HV）

Tensilestrength

(N/mm2）

Material Elongation(%)

UnknownUnknownUnknown

574UnknownUnknown

270 69Unknown

160140

Unknown

Table 1. Chemical compositions of the base metals (wt.%)

Table 2. Heat treatment conditions of the base metals

Table 3. Mechanical properties of base metals


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Material C Si Mn P S Ni Cr Mo

- -

Simulatedon-site weld 308 0.05

-

-

- - - -

-

0.03 0.003 9.58 19.6 0.01

- -

0.004 9.92 20.13

PBTL

Aged weld - - - -

0.01

- -

Aged weld

SITR

- -

0.04～0.08 1.5～2

1.5～2

7.5～9

7.5～9

15～17

15～17

0.01

<0.01

<0.5

<0.5

<2

<2

Simulatedon-site weld 308

16-8-2

16-8-2

1.76 0.0290.05 0.25

<0.01

0.27 1.65

Sample

Phenix

Simulatedon-site weld

Aged weld 0.04～0.08

TIG Unknown TIG Unknown TIG TIG

308 Unknown 308 Unknown 16-8-2 16-8-2

0.9 Unknown φ1.6、2.4 Unknown 2.5 2.5

Ar Unknown Ar Unknown Unknown Unknown

10～20 Unknown 5～15 Unknown Unknown 10

Ar Unknown Ar Unknown Unknown Unknown

Horizontal Unknown All Unknown Horizontal Unknown

120～240 Unknown 90～120 Unknown Unknown 140～160

8.3～8.5 Unknown 11 Unknown Unknown 28～29

6～8 Unknown 6～8 Unknown Unknown Unknown

Phenix

Simulatedon-site weld Aged weld Simulated

on-site weld

SITR PBTL

Aged weld Simulatedon-site weld Aged weld

Welding speed(cm/min)

Back shielding gas

Voltage(V)

Welding position

Current(A)

Rod diameter

Shielding gas

Gas flow rate（l/min）

Welding process

Welded consumables

Table 4. Welding conditions

Table 5. Chemical compositions of the welding consumables


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(a) Simulated on-site weld (b) Aged weld

Fig. 1. Shape and dimensions of the samples taken from “Phenix”

(a) SITR (304SS-304SS, Plate(t=50mm)) (b) PBTL (304SS-304SS, Pipe(t=5mm))

(c) Phenix simulated on-site weld (316LSS-304SS) and aged weld (304SS-304SS) (Pipe,

t=8mm)

Fig. 2. Results of Vickers hardness tests


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50µm50µm

50µm 50µm

50µm 50µm

Fig. 3 Comparison between HAZ microstructures on the aged material side of the simulated on-site welds and those of the aged welds

(a) HAZ on the aged material side of the simulated on-site weld from SITR

(b) HAZ of the aged weld from SITR

(c) HAZ on the aged material side of the simulated on-site weld from PBTL

(d) HAZ of the aged weld from PBTL

(e) HAZ on the aged material (304SS) side of the simulated on-site weld from Phenix

(f) HAZ of the aged weld from Phenix


10

50µ m

A B C DFe 47.24 86.64 46.30 33.24Cr 45.53 9.23 27.79 50.11Mn 2.17 0.96 2.04 1.80Ni 4.13 2.83 18.95 12.08Si 0.94 0.35 4.92 3.58

Phase Compositions (wt.%)

Fig. 4. SEM micrographs of the aged weld sampled from Phenix

5µ

m5

5µ

m

5µm

A C

DB

A

B

C

D

Weld metal HAZ Base metal

(a) Typical M23C6 observed near the fusion line

(b) Typical δ-ferrite observed near the fusion line

(c) G-phase observed in HAZ (probably)

(d) Typical M23C6 observed in the base metal


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50µ m

50µ m A1 A2 BFe 64.01 65.61 43.88Cr 22.90 21.57 26.76Mn 2.01 2.40 1.94Ni 7.85 8.19 8.64Si 0.78 0.75 1.36

(wt.％)

Fig. 5. SEM micrographs of aged material of the simulated on-site weld sampled from Phenix

5µ

m

5µ

m

(a) Typical δ-ferrite observed near the fusion line (b) Typical M23C6 observed in the base metal

Weld metal HAZ Base metal

B

B

A

A2

A1

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Integrity Assessment of Aged and On-Site Welded Joints … · Based on the mechanical test results expected to ... Keywords: Liquid metal fast reactor, welded joint ... Integrity