Technical Highlight Vol.3

DESIGNING HIGH QUALITY WELDING CONSUMABLES FOR NUCLEAR POWER REACTORS

Figure 1: Nuclear power supply systems with a boiling water reactor (left) and with a pressurized water reactor (top).

Figure 1: Nuclear power supply systems with a boiling water reactor (left) and with a pressurized water reactor (top).
[Source: Graphical Flip-chart of Nuclear & Energy Related Topics 2010 published by The Federation of Electric Power Companies of Japan].

Nuclear power, increasingly highlighted as a cleaner source of energy than fossil fuels, is seeing a brisk rise in power plant construction, particularly in Asian countries. This article introduces the special steels and welding consumables required for nuclear power plant construction.

Systems of Nuclear Power Generation

Table 1: Types of nuclear reactors
Reactor Fuel Moderator Coolant Note
Light water
reactor
Enriched
uranium
Light water Light water ▪ BWR
▪ ABWR
▪ PWR
Gas cooled
reactor
Natural or
enriched
uranium
Graphite CO2 ▪ AGR
▪ Calder
 Hall AGR
Heavy water
reactor
Natural or
enriched
uranium
Heavy water ▪ CO2
▪ Light water
▪ Heavy water
 
Hot gas
reactor
Enriched
uranium
Graphite Helium  
Fast breeder
reactor
Enriched
uranium or
Plutonium
None ▪ Sodium
▪ Na-K alloys
FBR

The electricity derived from nuclear power is a form of heat energy, generated by the fission chain reaction of enriched uranium in a reactor vessel, which is transferred to a coolant that produces the steam that rotates a turbine.

There are several types of nuclear reactors, utilizing different moderators and coolants, as shown in Table 1. Figure 1 (left) shows a typical diagram of boiling water reactor (BWR) and Figure 1 (top), a pressurized water reactor (PWR). Both are light water reactors (LWR), the most common types of nuclear reactors.


Nuclear Pressure Vessel Codes

Table 2: Criteria for evaluating RTNDT in ferritic materials such as Mn-Mo-Ni steels and weld metals
Testing method Evaluation criteria
Drop weight test Temperature, 5°C lower than the lowest
temperature where both of 2 drop weight test
pieces are judged as no-break, is defined as
TNDT.
Charpy impact test When all of 3 pieces in a Charpy impact test
at the temperature equal to or lower than
TNDT + 33°C satisfy the following conditions,
TNDT is defined as RTNDT:
(1) Absorbed energy is 68 J minimum.
(2) Lateral expansion is 0.90 mm minimum.

While each country defines its own regulations for its nuclear industries, the ASME codes of The American Society of Mechanical Engineering are widely adopted. ASME Sec. III Div. 1 (Nuclear Power Plant Components) and ASME Sec. XI (Rules for Inservice Inspection of Nuclear Power Plant Components); these codes specify in-depth requirements in terms of design, fabrication, test, inspection, and quality assurance. In particular, fracture toughness is one of the key requirements for materials because it governs the resistance to brittle fracture. For example, Table 2 shows the criteria for evaluating Reference Nil Ductility Transition Temperature (RTNDT) obtained through the fracture toughness tests for ferritic materials such as Mn-Mo-Ni steel and weld metal.

Specifications for Steels for Nuclear Reactors

Table 3: Chemical and mechanical properties of steels for reactor pressure vessel
ASME spec SA-533 SA-508
Type or grade Type B Gr. 2 Gr. 3
Class 1 2 1 1
C (%) ≤ 0.25 ≤ 0.25 ≤ 0.35 ≤ 0.75
Si 0.15-0.40 0.15-0.40 0.15-0.35 0.15-0.35
Mn 1.15-1.50 1.15-1.50 0.40-0.90 0.50-0.90
P ≤ 0.035 ≤ 0.035 ≤ 0.025 ≤ 0.025
S ≤ 0.04 ≤ 0.04 ≤ 0.025 ≤ 0.025
Ni 0.40-0.70 0.40-0.70 ≤ 0.4 0.50-1.00
Cr - - ≤ 0.25 0.25-0.45
Mo 0.45-0.60 0.45-0.60 ≤ 0.1 0.55-0.70
V - - ≤ 0.05 ≤ 0.05
0.2%YS
(MPa)
≥ 345 ≥ 485 ≥ 345 ≥ 345
TS (MPa) 550-690 620-795 550-725 550-725
El (%) ≥ 18 ≥ 16 ≥ 18 ≥ 18
RA (%) - - ≥ 38 ≥ 38
IV at
+4.4°C (J)
- - Each ≥ 34
Avg ≥ 41*1
Each ≥ 34
Avg ≥ 41*1
Relevant
JIS standard
JIS G 3120
SQV 2 A
JIS G 3120
SQV 2 B
JIS G 3204
SFVQ 2 A
JIS G 3120
SFVQ 1 B
*1: The computed average for three specimens.

Nuclear reactors consist of reactor pressure vessels (RPV); steam generator (SG) and pressurized used only in PWRs; the piping of the primary side cooling; and the containment structure. An RPV operates at high temperatures and high pressures; hence, its components are made of heat resistant steel, namely Mn-Mo-Ni steels as per ASME Sec. II Part A (Ferrous Material Specifications). SA-533 and SA-508 are commonly used for the RPV, as well as the pressurizer and SG in PWRs. Table 3 shows the chemical and mechanical properties and the relevant JIS standards for reference.

For the piping of the primary side cooling system, 304L type stainless steel and Ni-base alloys are mainly used, because of their anti-corrosion properties, high notch toughness and good weldability.


Specifications for Welding Consumables

Table 4: TRUSTARCTM welding consumables categorized by tensile strength level for Mn-Mo-Ni steels
Tensile strength class of welding consumable
  620 MPa class 690 MPa class
Applicable
steels
(ASME)
SA-533 Type B Cl.1
SA-508 Gr.2 Cl.1
SA-508 Gr.3 Cl.1
SA-533 Type B Cl.2
Welding
process
Trade
desig.
AWS
class.
Trade
desig.
AWS
class.
SMAW BL-96 A5.5
E9016-G
BL-106 A5.5
E10016-G
SAW MF-27X/
US-56B
A5.23
F9P4-EG-G
MF-29AX/
US-63S
A5.23
F10P2-EG-G
  PF-200/
US-56B
A5.23
F9P4-EG-G
PF-200/
US-63S
A5.23
F10P2-EG-G
GTAW TG-S56 A5.28
ER80S-G
TG-S63S A5.28
ER90S-G
Note: MF-27X is a fused flux, while PF-200 is a bonded flux.

When a nuclear power plant is constructed per ASME Sec. III, the welding consumables must be selected in compliance with ASME Sec. II Part C (Specifications for Welding Rods, Electrodes and Filler Metals), and the welding procedures must be qualified under ASME Sec. IX (Welding and Brazing Qualifications). Because all the welding consumables specified in ASME Sec. II Part C are identical to those in the AWS standard, this article will discuss welding consumables per the AWS standard.

Because safety is of paramount concern in nuclear power generation, the welding consumables must be reliable and have enough strength to withstand at elevated temperatures during operation, low temper embrittlement in case of emergency shutdown, high resistance to neutron irradiation brittleness, and good weldability.

Table 4 shows how welding consumables are matched to Mn-Mo-Ni steels. The welding consumables are divided into two tensile strength classes, 620 and 690 MPa, depending on the applicable steels. The typical chemical and mechanical properties of weld metals by 620 MPa and 690 MPa welding consumables can be seen in Tables 5 and 6, respectively.


Table 5: Typical chemical and mechanical properties of weld metals (620 MPa class welding consumables)
Welding
process
SMAW SAW GTAW
Trade
designation
BL-96 MF-27X/
US-56B
PF-200/
US-56B
TG-S56
Polarity AC*1 AC*1 AC*1 DCEN
C (%) 0.06 0.08 0.08 0.05
Si 0.54 0.28 0.11 0.41
Mn 1.30 1.05 1.23 1.54
P 0.005 0.009 0.007 0.008
S 0.004 0.004 0.003 0.006
Cu 0.02 0.08*2 0.08*2 0.15*2
Ni 0.37 0.87 0.83 0.66
Cr 0.02 0.06 0.02 0.03
Mo 0.53 0.50 0.43 0.52
Co 0.005 0.005 0.005 0.005
PWHT
(°C×hr)
620×
1
600×
16
595×
3
635×
26
590×
3
620×
11
620×
1
650×
15
0.2%YS
(MPa)
620 575 528 480 580 490 520 499
TS (MPa) 700 667 618 560 669 580 590 564
El (%) 26 25 33 32 28 30 31 33
IV at
0°C (J)
150 149 - - - - - -
IV at –
10°C (J)
- - - - - - - 171
IV at –
12°C (J)
- - 174 180 - - 290 -
IV at –
18°C (J)
- 89 - - - - - -
IV at –
20°C (J)
- - - - 189 210 - -
IV at –
40°C (J)
- - 137 - 142 - - 204
RTNDT
(°C)
- –35 –55 - - - - –70
*1 Only for AC. Not recommended for DC.
*2 Inclusive of Cu coating.
Table 6: Typical chemical and mechanical properties of weld metals (690 MPa class welding consumables)
Welding
process
SMAW SAW GTAW
Trade
designation
BL-106 MF-29AX/
US-63S
PF-200/
US-63S
TG-S63S
Polarity AC*1 AC*1 AC*1 DCEN
C (%) 0.10 0.10 0.08 0.09
Si 0.53 0.21 0.10 0.32
Mn 1.41 1.49 1.51 1.23
P 0.009 0.006 0.007 0.006
S 0.005 0.005 0.004 0.006
Cu 0.02 0.07*2 0.06*2 0.18*2
Ni 0.76 1.35 1.31 1.58
Cr 0.04 0.17 0.14 0.04
Mo 0.50 0.51 0.47 0.40
Co 0.005 0.005 0.005 0.003
PWHT
(°C×hr)
595×
3
613×
15
595×
3
612×
15
590×
3
600×
16
620×
1
635×
16
0.2%YS
(MPa)
670 561 640 589 620 552 570 563
TS (MPa) 770 657 740 691 700 641 620 636
El (%) 28 26 28 22 28 28 28 29
IV at
0°C (J)
110 170 - - - - - -
IV at –
10°C (J)
- - - - - - - 166
IV at –
12°C (J)
- - 120 105 - - - -
IV at –
15°C (J)
- - - - - 235 - -
IV at –
20°C (J)
- - - - 170 - - -
IV at –
30°C (J)
- 111 - 52 - - - -
IV at –
40°C (J)
- - 89 - 124 - - 195
IV at –
47°C (J)
- - - - - - 200 -
RTNDT
(°C)
- –45 - –45 - –18 - –70
*1 Only for AC. Not recommended for DC.
*2 Inclusive of Cu coating.

Several basic design concepts apply to welding consumables for Mn-Mo-Ni steel. One is to add Si, Mn, Ni and Mo to the weld metal in the same manner as the steel, in order to increase the quench-hardenability and to obtain the ferrite-bainite, bainite or bainite-martensite microstructure. Another is the addition of carbon. Carbon increases quench-hardenability and decreases the oxygen content in the weld metal, resulting in better notch toughness. But excessive carbon can also promote brittleness through carbide precipitation (e.g. cementite) during PWHT as well as reduce crack resistance. Therefore, the weld metal's carbon content is controlled to a slightly lower level as compared to the base metal. A third design concept is to minimize such impurities as P and Sn in order to avoid embrittlement of weld metal induced by PWHT. The increase of basicity, particularly on SAW flux, is yet another design concept, whereby the oxygen content in the weld metal is decreased, thereby obtaining high notch toughness. For example the use of TRUSTARCTM PF-200 (a bonded flux) in lieu of TRUSTARCTM MF-27X (a fused flux) obtains higher basicity and thus better notch toughness, as shown in Figure 3.

Increasing the crack resistance of the welding consumables is important to resist the residual stresses induced by welding in a thick pressure vessel. Controlling the S and C content will prevent hot cracks, and minimizing the diffusible hydrogen content will increase the resistance to cold cracks. In particular, the coverings of SMAW electrodes are designed to lessen the moisture absorption, one major source of diffusible hydrogen. As shown in Figure 4, the moisture resistant SMAW electrode offers slow moisture pickup, reducing diffusible hydrogen in the weld metal.

Figure 3: Comparison of notch toughness between fused flux and bonded flux.

Figure 3: Comparison of notch toughness between fused
flux and bonded flux.

Figure 4: Comparison of moisture absorption rates between conventional and moisture-resistant coverings.

Figure 4: Comparison of moisture absorption rates between
conventional and moisture-resistant coverings.


Another basic design concept is to consider neutron irradiation embrittlement and induced radioactivity resistance in relation to both weld metal and base metal. Because neutron irradiation embrittlement occurs in the belt line region of RPVs during operation, it is an important factor for not only steel but also weld metal. Cu and P, which enhance neutron irradiation embrittlement and such elements with high induced radioactivity as Co and Nb are reduced as low as possible. As a matter of fact, Non-Cu-coated SAW wires are now available.

The inner surfaces of an RPV, SG and the primary side piping constitute a severe corrosive environment due to the circulating cooling water contaminated with radioactive elements. The inner surface, in direct contact with the coolant, is overlay-welded with welding consumables for stainless steels or Ni-base alloys in order to protect it from corrosion.

On the shell and end plate inner surfaces of a large RPV, the efficient SAW or ESW mode overlay welding with strip electrode is applied. On the inner surfaces of pipes and nozzles, GTAW and GMAW are used. The concepts and the processes of overlay welding with strip electrode in the two modes are shown in Figures 5 and 6, respectively. The ESW mode is characterized by shallow penetration that reduces dilution by the base metal, thereby providing a low carbon weld with better corrosion resistance. The SAW mode offers low heat input due to faster welding speed; hence, it is a more favorable process for the base metal, which is susceptible to under-clad cracking (UCC).

Figure 5: Concepts of overlay welding processes (SAW and ESW) with strip electrodes.

Figure 5: Concepts of overlay welding processes
(SAW and ESW) with strip electrodes.

Figure 6: SAW process (left) and ESW process in operation on the inner surface of pressure vessels.

Figure 6: SAW process (left) and ESW process in
operation on the inner surface of pressure vessels.

Table 7 shows fluxes and strip electrodes for 304L weld metal by SAW and ESW mode overlay welding and the typical chemistries and ferrite numbers (FN per WRC diagram) of overlaid weld metals.

Table 8 shows SMAW and GTAW consumables for 304L overlay weld metal and the chemistries of the undiluted deposited metal. Table 9 shows Ni-base alloy welding consumables for SMAW and GTAW and the chemical and mechanical properties of undiluted deposited metal.

Table 7: SAW and ESW fluxes and strip electrodes for 304L weld metal and the chemistries and ferrite numbers of overlaid weld metals
Process SAW ESW
  Single layer*1 2nd layer Single layer*1 2nd layer
Trade
desig.*2
PF-B1/US-
BQN309L
PF-B1/US-
BQN308L
PF-B7FK/US-
BQN309L
PF-B7FK/US-
BQN308L
AWS
class.
A5.9
EQ309L
A5.9
EQ308L
A5.9
EQ309L
A5.9
EQ308L
Polarity DCEP DCEP DCEP DCEP
C (%) 0.030 0.028 0.018 0.015
Si 0.67 0.65 0.53 0.54
Mn 1.14 1.05 1.36 1.14
P 0.018 0.019 0.017 0.020
S 0.004 0.005 0.002 0.004
Cu 0.04 0.05 0.05 0.03
Ni 12.65 10.21 12.80 10.35
Cr 23.05 19.75 23.65 19.87
V 0.05 0.04 0.05 0.04
Co 0.04 0.04 0.04 0.04
N 0.041 0.019 0.048 0.020
FN*3 12 9 15 11
*1 For a single layer process or underlayer in a multilayer
process.
*2 Strip size available: 0.4 mm thick × 25, 50, and 75 mm wide.
*3 Per WRC diagram.
Table 8: SMAW and GTAW consumables for overlaying 304L weld metal and the chemistries and ferrite numbers of undiluted deposited metals
Process SMAW GTAW
Trade
desig.
NC-39L NC-38L TG-S309L TG-S308L
AWS
class.
A5.4
E309L-16
A5.4
E308L-16
A5.9
EQ309L
A5.9
EQ308L
Polarity DCEP
or AC
DCEP
or AC
DCEP DCEP
C (%) 0.023 0.029 0.012 0.007
Si 0.51 0.20 0.41 0.36
Mn 1.56 1.44 1.74 1.91
P 0.021 0.019 0.009 0.016
S 0.003 0.004 0.003 0.003
Cu 0.03 0.03 0.02 0.02
Ni 12.46 10.24 12.29 10.26
Cr 23.92 20.31 23.76 19.86
V 0.05 0.05 0.05 0.05
Co 0.04 0.04 0.05 0.02
N 0.053 0.050 0.048 0.043
FN*1 16 8 14 9
*1 Per WRC diagram.
Table 9: SMAW and GTAW consumables for Ni-base alloys and the chemical and mechanical properties of undiluted deposited metals in the as-welded condition
Process SMAW GTAW
Trade designation NI-C703D TG-S70NCb
AWS classification A5.11 ENiCrFe-3 A5.14 ERNiCr-3
Polarity DCEP DCEP
C (%) 0.06 0.02
Si 0.34 0.18
Mn 6.55 2.93
P 0.004 0.001
S 0.003 0.002
Ni 69.40 71.64
Cr 13.21 20.20
Nb+Ta 2.00 2.33
Fe 7.90 1.50
Ti 0.01 0.55
Co 0.03 0.02
0.2%YS (MPa) 360 370
TS (MPa) 620 680
El (%) 45 40
IV at –196°C (J) 110 150
Reactor pressure vessels require an integrated
manufacturing technique wherein base metals are matched with welding consumables of high and consistent quality.

Reactor pressure vessels require an integrated manufacturing
technique wherein base metals are matched with welding
consumables of high and consistent quality.

References:
[1] Kobe Steel: Welding Technical Report, Vol.49 2009-4.
[2] Kobe Steel: Welding of Nuclear Power Equipment, 1990.


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