Effect of Buffer-layered Buttering on Microstructure and Mechanical
Properties of Dissimilar Metal Weld Joints for Nuclear Plant Application
Dinesh W. Rathod1,*a, P. K. Singh2, Sunil Pandey1, S. Aravindan1
*
a
1
Corresponding author - email: [email protected], Tel.: +44 161 2751916
Present address: MTRL, School of MACE, University of Manchester, United Kingdom, M13 9PL
Deptt. of Mechanical Engg., Indian Institute of Technology Delhi, Hauz-khas, New Delhi-110016, India
2
Bhabha Atomic Research Centre, Mumbai-400085, India
Abstract
In this study, we present the metallurgical and mechanical investigation of four dissimilar welds
between SA508Gr.3Cl.1 and SS304LN. The welding processes for buttering deposition and fill-pass
welding were varied with ERNiCr-3/ENiCrFe-3 consumables. The Ni-Fe alloy buffer layer was
introduced as intermediate layer in buttering and then the joints (with and without buffer layer in
buttering) were fabricated. The effect of Ni-Fe buffer layered buttering and welding processes on the
resulting weld joints properties has been addressed. Metallurgical and mechanical properties,
fracture toughness were measured and various examinations were carried out for integrity
assessment on all the weld joints. Addition of a Ni-Fe buttering layer leads to the development of
more favourable properties than observed in welded joints made using the current practice without a
buffer layer. Control of carbon migration and its subsequent effect on metallurgical, mechanical
properties due to buffer layer has been justified in the study. Conventional procedure of DMW
fabrication has been proven to be the least favourable against the new technique suggested.
Modification in current integrity assessment procedure would be possible by considering the
properties at interfacial regions, introduction of yield strength ratio mismatch and the plastic
instability strength in the integrity assessment.
Keywords
Buffer layer; Dissimilar Welds; Structural Integrity; ERNiCr-3/ENiCrFe-3; Ni-Fe alloy;
SA508Gr.3Cl.1
1. Introduction
In nuclear power plants, the light water reactor pressure vessel is made of ferritic steel, typically
SA508Gr.3Cl.1, which must be joined to stainless steel pipelines, usually made of SA312 Type
304LN, using arc welding processes. Such dissimilar metal welds (DMW) are typically made with
Ni-base consumables as the filler metal in an attempt to mitigate the strong variation in physical,
chemical and mechanical properties across the weld [1, 2]. It is also common practice deposit a Ni1
base (ERNiCr-3) buttering layer onto the ferritic steel before making the joining weld to reduce
carbon migration [1-4]. The Ni-base consumables (ERNiCr-3/ENiCrFe-3) are extensively used for
DMW joint fabrication because of certain advantages of carbon migration. Despite the necessity of
DMW, and the improvement in properties conferred by the use of Ni-base consumables, the desired
design life has not been achieved [3-5] and many failures [3, 6-9] have occurred. The types of
failures and locations [3, 6-9] in DMW joints are still remain the big challenge to assess the causes of
failures. The performance of DMW joints is greatly affected by other often-associated problems [1-4,
10] like degradation of ferritic steel due to oxide notch (low oxidation resistance), metallurgical
deterioration at the interfaces, and residual stresses during buttering and welding. During any
welding procedure, thermal stresses develop which can be detrimental for structural integrity and
performance [11]. In case of DMWs the mismatch in coefficient of thermal expansion (CTE) of
austenitic and ferritic steel can cause different stress profile across the weld joint [1, 12-18]. The
chemical and microstructural variations across the joint can also be severe; the formation of a carbon
depleted soft zone and carbon enriched hard zone which forms due to carbon migration [1, 2, 7, 9,
12, 13, 15, 17, 19-22] has been implicated in failure. Controlling the carbon migration is therefore
crucial to minimising the likelihood of failure of the weld. DMW joints have varying metallurgical,
mechanical and fracture toughness properties across the weld joint, which also affect the integrity of
joints.
Although it has been shown in earlier study of Rathod et al. [7] that carbon migration can be
controlled through the use of Ni-Fe alloy buffer layer in buttering made with Gas Metal Arc Welding
(GMAW) process and ERNiCr-3. The implications of a Ni-Fe alloy buffer layer in buttering using
GMAW process on the mechanical properties of the joint have yet to be documented. Variations in
welding processes also have the potential to influence metallurgical and mechanical properties across
the joint. Standard practice is to deposit buttering layer of Inconel 82 (ERNiCr-3) using Gas
Tungsten Arc Welding (GTAW) process and a completion weld (fill-pass welding) with filler metal
Inconel 182 (ENiCrFe-3) using Shielded Metal Arc Welding (SMAW). This study compares the
microstructural and mechanical properties of joints produced using standard practice and those made
with an intermediate Ni-Fe alloy buffer layer in the buttering deposited with GMAW [7, 23]
technique. GMAW process is not commonly used for preparing DMWs due to mixing of O2 or CO2
in argon gas shielding to maintain the arc stability. In present study, we demonstrate the use of
GMAW with pure argon shielding for buttering and completion of welds. A comprehensive
assessment of the integrity of the welds was carried out by means of 100% radiographic inspection of
weld joints, all-weld tensile test of weldment zones, composite tensile test, Charpy V-notch test of
weldment zones, fracture toughness of weld, angular distortion, chemical analysis, microstructure
evolution and the micro-hardness measurement across the weld joints.
2
2. Materials and Experiments
2.1 Materials and Welding
The quenched and tempered SA508Gr.3Cl.1 and austenitic SS304LN steel supplied in solution
annealed condition of pipe form were machined into plate form (150x50x18mm) samples with single
‘V’ groove geometry with compound bevel joint design. Four DMW joints were fabricated with two
different buttering procedures and two different completion weld procedures. For two samples, four
layers of buttering, with total of sixteen passes of 2mm diameter ERNiCr-3 TIG rod were deposited
using GTAW process onto the machined surface of two SA508Gr.3Cl.1 plates. For remaining two
samples, an initial intermediate buffer layer of Ni-Fe alloy (ERNiFe-CI) was deposited with GTAW
process, the subsequent three buttering layers were deposited by GMAW process using 1.1mm
ERNiCr-3 MIG wires as described in earlier study [7] to give a total of thirteen passes for the four
layers. The thermal expansion and the tensile properties of consumables and base metals are same as
the properties reported in earlier study [8]. Chemical composition of base metals (BM) and filler
metals (FM) used in study is given in Table 1.
Table 1
Chemical composition of base metals and filler metals
Materials and Consumables
SA508Gr.3Cl.1(BM)
SS304LN(BM)
ERNiFe-CI (FM)
(TIG - 2.4mm)
ERNiCr-3(FM)
(TIG - 2mm)
ERNiCr-3(FM)
(MIG -1.1mm)
ENiCrFe- 3 (FM) (4mm)
Weight Percentage (wt %)
Cr
Fe
Mn
C
Ni
Nb
Ti
0.197
0.025
0.53
8.22
0.12
18.09
96.95
70.83
1.30
0.83
0.01
-
0.025
53.01
0.15
43.24
0.74
0.003
-
0.017
72.71
19.86
1.40
2.94
2.75
0.41
0.016
72.47
20.01
1.28
2.74
2.88
0.36
0.042
67.17
14.09
6.83
7.51
1.99
0.45
Fig. 1. Schematics of buttering deposits for (A) without buffer layer using GTAW process, and (B)
with Ni-Fe buffer layer using GMAW process
3
The schematics of buttering layers employed on ferritic steel plates after re-machining for groove
geometry are shown in Fig. 1A and B for the groove geometry of buttering deposits without and with
buffer layer respectively. GTAW buttering employed by 3mm diameter tungsten electrode with
straight polarity using ERNiCr-3 and ERNiFe-CI filler metals. Pure argon gas at 7 L/min was
provided during GTAW while, 14 L/min was employed during GMAW process. The contact tube to
work distance (CTWD) was maintained ~15-18 mm while the wire feed rate adopted was 4.57
m/min for GMAW process. The interpass temperature during buttering and welding was maintained
between 150-180oC. The process parameters employed for the buttering are given in Table 2.
Table 2
Process parameters for buttering deposition on ferritic steel plates of weld joints
Plates
Layer
Process
Consumables
Current
(amps)
Volts
(V)
Welding
speed
mm/sec
Heat
input
KJ/mm
Without
buffer layer
buttering
Layer
1-4
GTAW
ERNiCr3-ɸ2
mm-TIG rod
91-95
8.5-9.5
0.49
1.58
96-100
8.5-9.5
0.49
1.67
123-126
23
2.72
1.06
With buffer
layer
buttering
Buffer
layer 1
GTAW
Layer
2-4
GMAW
ERNiFe-CIɸ2.4 mm
TIG rod
ERNiCr3ɸ1.1 mmMIG wire
To attain lesser angular distortion, the compound bevel angle provided in the joint geometry
followed by preliminary investigation. The compound bevel geometry as shown in Fig. 2A and B has
been employed for the joints fabricated with SMAW and GMAW processes respectively.
Fig. 2. Schematics of compound bevel joint design for Joints (A) with SMAW process, and (B)
GMAW process
The buttering deposits were examined for defects using Dye Penetrant test as per the criteria of
ASME Sec-V, Article 6. After finding them to be defect free, the completion welds were carried out.
GTAW was used to produce two root passes using ERNiCr-3 TIG rods and back purging. The
subsequent completion welds were made with either SMAW or GMAW. The process parameters
during completion weld are given Table 3. The weld joints were classified as A-1, A-2, B-1 and B-2
according to the buttering and welding processes used and can be seen in Table 3.
4
Table 3
Process parameters during Weld Joints Fabrication /weld completion
Weld
Joint
A-1
A-2
B-1
B-2
Buttering
Without
buffer
layer
With NiFe buffer
layer
Pass
Process
Root GTAW
Fill SMAW
Root GTAW
Fill GMAW
Root GTAW
Fill SMAW
Root GTAW
Fill GMAW
Consumables
Current
(amps)
Volts
(V)
ERNiCr-3 (ɸ2 mm)
ENiCrFe-3 (ɸ4 mm)
ERNiCr-3 (ɸ2 mm)
ERNiCr-3 (ɸ1.1 mm)
ERNiCr-3 (ɸ2 mm)
ENiCrFe-3 (ɸ4 mm)
ERNiCr-3 (ɸ2 mm)
ERNiCr-3 (ɸ1.1 mm)
110-114
100-106
110-114
106-118
110-114
100-106
110-114
106-118
8.5-10
25-28
8.5-10
24-26
8.5-10
25-28
8.5-10
24-26
Welding
speed
mm/sec
0.86
0.82
0.86
1.04
0.86
0.86
0.86
1.14
Heat
input
KJ/mm
1.11
3.33
1.11
2.69
1.11
3.17
1.11
2.46
All welding activities (GTAW and SMAW, GMAW) were carried out in manual mode as per
requirement of ASME Sec-IX. Uniform dilution was attained by adopting the weaving bead
deposition and the run-in and run-out defects were to minimised by using dummy blocks. The four
as-welded joints after removal of dummy blocks are shown in Fig. 3.
Fig. 3. As welded weld joints after removal of dummy blocks
All four weld joints were subjected to 100% radiographic inspection as per the requirement of
ASME Sec-V, Article 2 and the joints were qualified according to the acceptance criteria of ASME
Sec-III.
2.2 Testing methods and procedure
2.2.1 Specimen Fabrication
The typical length of each weld joint was 150 mm out of which specimens were extracted for
mechanical and metallurgical test. All specimens were extracted and machined using wire cut
electric discharge machine (EDM). The location of specimens on the weld joints is shown in Fig. 4.
Three specimens for composite tensile test (CTT), five sub-size specimens for Charpy V-notch test
from each region of weldment, one specimen for fracture toughness of weld metal by single edge
bend specimen (SEBN), two specimens for metallurgical investigation and remaining length was
used to extract the four specimens from each weldment region for all-weld tensile test.
5
Fig. 4. Location of specimens extracted for the mechanical and metallurgical investigation
The extracted specimens and their respective positions on weld joints are shown in Fig. 5 for allweld tensile test. The position of extraction of Charpy V-notch test specimens for each weldment
region shown in Fig. 6. The specimen from HAZ ferritic steel, buttering, and weld metal regions are
shown in Fig. 6A, B and C respectively.
Fig. 5. Position of specimens extracted from each weldment region (HAZ ferritic steel, Buttering and
Weld metal) for all-weld tensile specimens
The extraction position of composite tensile test specimens and fracture toughness (SENB)
specimens are shown in Fig. 7A and B respectively.
6
Fig. 6. Position of extracted specimens for Charpy V-notch test (A) HAZ ferritic steel, (B) Buttering
and (C) Weld metal region
Fig. 7. Position of extracted specimens for (A) Composite tensile test, and (B) Fracture toughness
2.2.2 Mechanical testing
ASTM E8M standard was used for machining and testing the sub-size tensile (standard sheet type)
specimens at the ambient temperature (24oC) for composite tensile test and the all-weld tensile test.
The INSTRON 5582 machine was used with 2.5 mm/min strain rate for all tensile specimens under
this test. The standard sub-size specimens for Charpy V-notch impact test were machined according
to the dimensions specified in ASTM E23 standard by considering the notch position in desired
weldment region. The V-notch dimensions were confirmed with shadowgraph profiler. The testing
was conducted on conventional calibrated machine at the ambient temperature (24 oC). The specimen
fabrication and the analysis for fracture toughness (CTOD-Crack Tip Opening Displacement)
specimens of weld metal have been made as per the procedures given in ASTM E1290-02 and
E1820-09 standard. The notch and crack tip in standard single edge three point bend specimen
(SEBN) specimens were positioned in the centre of weld metal and in the direction of welding (Fig.
7B). The pre-cracking and the testing of SEBN specimens have been conducted on MTS 810
machine at ambient temperature (24oC) adopting the standard procedure given in standard. The
specimens for metallurgical test were used for angular measurement with digital image processing
and the results were confirmed by coordinate measuring machine (CMM).
2.2.3 Metallurgical testing
The microstructure of ferritic steel was revealed with 2%Nital for 3-4 seconds after the reaction
started. Stainless steel was etched electrolytically with 10% oxalic acid at 6V, ERNiCr-3/ENiCrFe-3
7
were observed with 10% ammonium persulphate electrolyte etch at 6V. The chemical analysis using
optical emission spectrometer (OES) has been conducted for weldment regions of weld joints. The
chemical compositions of Ni-base weld regions and stainless steel were obtained using the ARCMET 8000 (Oxford Instrument) Spectrometer and the SpectroMaxx spectrometer was used for the
ferritic steel. The average values from three locations on the specimens have been used in the
analysis. Similarly, the ASTM E415-99a and ASTM E2594-09 standards were used for measurement
of ferritic steel and Ni-base alloy respectively and the ASTM E1507-07 protocol was followed
during measurement. The hardness variations across the weldment zones have been measured using
Leica VMHT Auto instrument with 10gf and 100gf test loads. Lower load (10gf) used at the
interfaces of weld joints for more precise results and the 100gf load was used in the weldment
regions.
3. Results and Discussion
3.1 Angular distortion
Angular distortion in weld joints are shown in Fig. 8. The angular distortion [24, 25] is indicative of
presence of residual stresses. Residual stresses were not quantified but with angular distortion, it can
be qualitatively compared for the all four weld joints. The completion weld made with GMAW (A-2,
B-2) has shown same angular distortion as an effect of same heat input and associated solidification
rate.
Fig. 8. Angular distortion measured on weld joints
Higher angular distortion observed with the joint fabricated with current practice (A-1) and the lower
with buffer layered GMAW deposited joint completed with SMAW. The effect of buffer layer and
buttering process observed to be significant with completion weld by SMAW than the GMAW.
Variations in terms of heat input, dilution and cooling rate (welding conditions) are more significant
in joint B-1 and that might be sufficient for tempering of HAZ ferritic steel and buttering deposit by
multi-pass SMAW process (high dilution and heat input). Faster cooling rate attained with GMAW
processes was missed in joint A-1. Angular distortion could become the function of formed phases
and their subsequent tempering while rest of the welding parameters are remaining same. These
variations may cause to change the weld chemistry, which is diluted with buttering. The minor
variations in micro-alloying may affect solidification behaviour and resulting angular distortion. The
GTAW and GMAW have the marginal difference in heat input than the SMAW process and
8
therefore, angular distortion in joint A-2 and B-2 is same. Hence, favourable angular distortion (in
terms of lesser residual stresses) can be achieved by new procedure of buttering and welding
compared to the existing one (joint A-1).
3.2 Materials chemistry in weld regions
The chemical composition (OES) measured in different weld regions of the weld joints are given in
Table 4. It can be seen that despite dilution in the buttering deposits and weld metal due to thermal
input of the welding processes, the weld chemistry of buttering and weld metal meets the ASME
Section-II specified chemical composition for DMW joints with marginal variations owing to
dilution (increased Fe in buttering). The samples, which included a Ni-Fe alloy buffer layer, showed
a much lower amount of carbon in buttering region, although levels of carbon in the weld metal (fillpass) were similar, than samples without buffer layer. Carbon diffusion could be severe for buttering
region than weld metals because the carbon needs to travel (diffusion) through buttering region to
reach the weld metal, which is extremely long atomic distance for carbon to migrate in as-welded
condition.
Table 4
Chemical composition in the weld regions of all weld joints
Weld
Joint
A-1
A-2
B-1
B-2
Zone
Buttering
Weld Metal
Buttering
Weld Metal
Buttering
Weld Metal
Buttering
Weld Metal
C
0.050
0.051
0.041
0.048
0.022
0.049
0.021
0.052
Ni
68.11
65.46
68.24
65.47
62.14
64.86
62.41
65.88
Weight Percentage (wt %)
Cr
Fe
Mn
Mo
Nb
19.07 7.72 2.47 0.05 1.67
15.81 7.63 6.49 0.06 1.99
19.02 7.59 2.63 0.06 1.60
20.52 7.02 3.38 0.16 2.37
18.49 13.41 2.79 0.38 1.99
15.94 7.75 6.61 0.06 2.01
18.22 13.56 2.99 0.13 2.09
19.67 7.10 3.16 0.18 2.80
Ti
0.36
0.62
0.36
0.27
0.32
0.56
0.33
0.39
Si
0.22
0.98
0.19
0.30
0.23
1.19
0.21
0.25
Buffer layered samples also showed a higher percentage of iron (Fe) in buttering region which would
affect the solidification and partitioning behaviour of the resulting phases in buttering region [26,
27]. Niobium fraction is reduced in buttering and is significant in buttering without buffer layer than
buffer layer and might caused due to increased solubility of Fe in solution. In weld metal, no
considerable variation in Nb has observed. The increased Fe content in nickel (Ni) matrix could
reduce the CTE [28] and that would help in bridging the mismatch in CTEs of stainless and ferritic
steels [8]. This graded composition owing to buffer layer could provide a gradient in thermal
properties between two dissimilar metals whose CTE are significantly different and such graded
composition is not possible with ERNiCr-3 buttering alone. This variation could also affect the
residual stresses due to sharp differences in thermal properties of the dissimilar metals involved in
9
joints [29] and further quantification on residual stresses could also be possible and this is at present
is the limitation study.
3.3 Weldment Microstructure
The SA508Gr.3Cl.1 ferritic steel has been in bainitic structure in as received condition of quenched
and tempered pipe forging. Similarly, the SS304LN pipes in solution-annealed condition have the
equiaxed grain structure of austenite twins. The cross-section view of the weldment regions of weld
joints with the extent of HAZs, buttering region and weld metal are shown with macrograph in Fig.9.
The as-received microstructure of SA508Gr.3Cl.1 and SS304LN is shown in Fig. 10A and B. The
fusion interface between buttering and ferritic steel of the joints A-1 and B-1 fabricated with
buttering using without and with buffer layer of Ni-Fe alloy has been shown in Fig. 11A and B
respectively. The HAZ of ferritic steel microstructure of joint A-2 is similar to A-1 while that of B-2
is similar to B-1 due to identical buttering deposition in these joints.
Fig. 9. Macrograph of weld joints showing the weldment regions and base metals
The carbon denuded soft zone (the white phase field) owing to migration of carbon [2] is observed in
joint A-1 and A-2 and shown in Fig. 11A. The soft zone observed to be in width of ~50 micron. The
thin martensite at interface and the extent of decarburised region near the interface are shown by
arrows, which are formed due to sharp composition gradient [7, 30] and diffusion. The
microstructure is pearlite like bainite structure having the significantly fine grain size than parent
metal (Fig. 10A) owing to tempering caused by multi-passes.
The carbon-enriched zone (dark phase field) has been formed due to presence of Ni-Fe buffer layer
in the buttering of joint B-1 and B-2. The arrow in Fig. 11B of joint B-1 is showing the carbonenriched hard zone in the HAZ of ferritic steel. The pearlite-like structure with some fraction of
reformed martensite can be seen in the Fig. 11B. The grain structure is also fine compared to parent
metal owing to multi-pass tempering effect and the width of carbon-enriched zone is ~60 micron.
The formation of a carbon-denuded zone has been attributed to the affinity of carbon for chromium,
which is present in higher proportion in the ERNiCr-3/ENiCrFe-3. Carbon migrates from ferritic
steel towards the Ni-Fe alloy but builds up at the interface due to reduced diffusion rate of carbon on
Ni-Fe alloy [7]. In contrast to ERNiCr-3, which has chromium content of 18-21%, the Ni-Fe buffer
layer contains no chromium, thus carbon does not migrate from the ferritic steel to the Ni-Fe alloy
(buffer layered buttering)[7].
10
Fig. 10. As-received microstructure of base metals (A) SA508Gr.3Cl.1, and (B) SS304LN
Fig. 11. Microstructure of HAZ of ferritic steel for (A) joint A-1, and (B) Joint B-1
Hence, the Ni-Fe alloy buffer layer acts as graded composition [7, 21, 23, 31, 32] in the sandwich
pattern of DMW joints with buttering. Ni-Fe buffer layer significantly controls the carbon migration
and respective formation of carbon depleted / carbon-enriched zones in HAZ near the interface [7].
Fig. 12. Microstructure of HAZ of SS304LN for (A) joint B-1 with SMAW, and (B) Joint
B-2 with GMAW
Variations in buffer layer and buttering could not affect the HAZ of SS304LN as the SS304LN
comes in focus during completion weld. The considerable variations in HAZ of SS304LN have been
observed owing to heat inputs of welding processes. The joint A-1 and B-1 fabricated with SMAW
process while joint A-2 and B-2 with GMAW process. Hence, the HAZ of SS304LN of joint B-1 and
B-2 of SMAW and GMAW processes are shown in Fig. 12A and B respectively. A clear transition
11
region, known as partially mixed zone (PMZ), is visible in both HAZ as shown with arrows. The
SMAW welded joint has smaller equiaxed austenite grains than those in the same location with
respect to the fusion line in the GMAW weld. This is due to the higher heat input of SMAW process.
The austenite grains in Fig. 12A are smaller while, those in Fig. 12B are bigger than the respective
base metal (Fig. 10B).
The interior of buttering region microstructure of joint A-1 and B-1 (without and with buffer layer) is
shown in Fig. 13A and B respectively. The buttering and weld metal microstructure of ERNiCr-3
and ENiCrFe-3 is fully austenitic and indicating lack of allotropic transformation during
solidification [7, 26, 33]. The as-solidified columnar microstructure of joint A-1 shows much
orientational variation than that of B-1 and the different colour indicates the different
crystallographic orientations of austenite phase. In joint A-1, the buttering alloy (ERNiCr-3) is only
diluted with ferritic steel close to the fusion line, in B-1 however, the buttering alloy (ERNiCr-3) is
diluted with Ni-Fe buffer layer. Whereas, the Ni-Fe buffer layer itself diluted with ferritic steel at the
interface. Therefore, the buttering deposit with buffer layer would contain higher amount of Fe
compared to the buttering deposit without buffer layer. The increased amount of Fe in buttering
deposit leads to decrease in the Nb and Ti solubility in austenite phase and their ability to remain in
solution is getting limited [26].
Fig. 13. Interior of buttering microstructure for (A) joint A-1 without buffer layer, and (B)
Joint B-1 with buffer layer
Fig. 14. Interior of weld metal microstructure for (A) joint A-1 – SMAW , and (B) Joint
B-2 – GMAW process
12
The considerable content of Nb and Ti (3-4%) increases the bulk solidification temperature range
[27, 34] due to presence of more amount of Fe in buttering deposit. This increases the degree of
constitutional undercooling and causing the dendrite structure to widen [35]. Hence, the primary arm
spacing and dendrite width in Fig. 13B of joint B-1, with buffer layer, has been wider than the Fig.
13A of joint A-1, without buffer layer. Owing to more amount of Fe in buttering deposit with buffer
layer than without buffer layer, the solubility of Nb in solution is decreases. Therefore, the
partitioning of Nb and Ti to the interdendritic region is increased and it would result in formation of
laves (Fe2Nb) phases [7, 23]. Hence, the significantly more fraction of laves phases (dark spheroidal
indicated with arrows) can be seen in Fig. 13B of buttering with Ni-Fe buffer layer than without
buffer layer (Fig. 13A).
The weld metal microstructure of joint A-1 and B-2 with SMAW and GMAW process is shown in
Fig. 14A and B respectively. The similar microstructure is developed in each case due to use of the
same filler metals. The use of coated electrodes in SMAW process (joint A-1) leads to very fine slag
inclusions [8] as encircled can be seen clearly in micrograph of Fig. 14A. Such slag inclusion is not
present in Fig. 14B due to flux-free GMAW process. The chemistry of weld metal for Nb and Fe is
almost similar with marginal variation in each weld joint. Hence, due to fine slag inclusion [8],
different cooling rate and heat input during welding, some marginal variations have been observed in
joint A-1 and B-2. Fine secondary phase particles at grain boundary have been shown with arrows in
Fig. 14A. The migrated grain boundary (MGB) and solidification grain boundaries (SGB) are clearly
visible and arrows indicate them. The slightly courser grains were observed with GMAW process,
and the SGBs and MGBs are more prominent. The arrows in Fig. 14B indicate the shiny NbC phase
particles. Laves phases are not observed in weld metal of GMAW process perhaps due to the faster
solidification rate in GMAW process.
3.4 Micro-hardness evaluation
Micro-hardness variations across the weldment regions of all four weld joints are shown in Fig. 15.
Hardness in HAZ of ferritic steel has been increased due to reformed martensite fraction [36].
Martensite formation at the ferritic steel and buttering interface due to weld chemistry variation [7,
30] has been evidenced. The less heat input during GMAW than GTAW process has not caused the
tempering of martensite as an effect of multi-pass buttering deposition. This caused to have the more
hardness in HAZ of ferric steel for joints buttered with GMAW process. The effect of carbon
enriched / denuded zone within ~50 micron distance in HAZ ferritic steel from interface is
considerable.
13
Fig. 15. Micro-hardness variations across the weldment regions of four weld joints
The composition gradient in buttering layers and weld metals (ERNiCr-3/ENiCrFe-3) could remain
almost constant due to filler metal chemistry [1] hence the hardness in weld metal is not expected to
vary significantly. Minor variations could be possible in weld region because of different heat inputs.
Considerable variations in buttering near the interface are observed due to significant chemistry
variation and dilution. The hardness variations across the weldment regions of weld joints in present
study are in agreement with the earlier research [7-9, 12, 13, 17, 36, 37].
3.5 Composite Tensile Test
The engineering stress strain curves for the specimens of all four joints are shown in Fig. 16. All
specimens were fractured from the weaker portion of the base metal SS304LN except one specimen
from joint B-1. This specimen has been fractured from buttering region.
Fig. 16. Engineering stress strain curves for all weld joints
The yield strength (YS) and ultimate tensile strength (UTS) of specimens from joint with Ni-Fe
buffer layer observed to be more than without buffer layer joints. The obtained results from the
composite tensile test suggest the required strength and ductility for the DMW joint requirement
along with structural integrity.
14
3.6 All weld tensile properties of weldment regions
The tensile properties YS, UTS, uniform elongation (UE) and total elongation (TE) along with
associated properties, as yield strength ratio (YSR) and plastic instability strength (PIS) were
determined.
Fig. 17. Typical stress-strain curves of weldment regions for weld joints (A) A-1 and B-1,
and (B) A-2 and B-2
The typical stress strain curves for the specimens of joint A-1 and B-1 are given in Fig. 17A, while
those of joint A-2 and B-2 are shown in Fig. 17B. These curves are the example of the measurement
made for all samples and their results were used for presenting the tensile properties in this study.
The average YS observed in the weldment regions and base metals of the all weld joints is shown in
Fig. 18.
Fig. 18. Average Yield Strength (YS) for weld joints
The YS in HAZ of ferritic steel region is more than parent metal except joint A-1 (conventional) due
to carbon depleted zone and effective tempering of the reformed martensite. However, the joint A-2
has marginally more YS than parent metal due to completion weld by GMAW which was SMAW in
joint A-1. The YS of buffer-layered joints (B-1/B-2) is considerably more than parent metal and the
joint A-1/A-2. The formation of carbon-enriched zone and insufficient tempering of martensite due
to GMAW buttering have caused to increase the YS and the results are in agreement with the
hardness profiles. Yield Strength in buttering region of all joints is almost same.
15
Fig. 19. Average Ultimate Tensile Strength (UTS) for weld
joints
The weld chemistry variations are considerably significant (section 3.2) for the buttering deposits
due to dilution and buffer layer. This caused to have marginal compromise in chemical variations
suggested in ASME Sec-II, particularly for Fe content (more than specified). These variations have
not made any adverse effect on the YS of buttering regions. The YS in weld metal of GMAW
welded joints is more than SMAW joints due to presence of fine slag inclusions that affects the
dislocation mechanism. The UTS of all weld joints is shown in Fig. 19. The UTS is more than parent
metal (SA508) in HAZ region and is not seen with joint A-1 as seen with YS. This caused due to the
same reasons explained for YS, which seems to be almost consistent with rest of the joints with more
strength. The UTS in buttering and weld metal region is observed with same trend as YS. The lowest
UTS observed with joint A-1 (conventional) while the rest of joint showing considerably more
strength. The average UTS in joint A-1 is more than parent metal SS304LN but, the scattering in
weld metal of this joint cannot be truly considered safe for integrity assessment.
Fig. 20. Average Uniform Elongation (UE) for weld joints
The uniform and total elongation in the weld joints can be seen in Fig. 20 and Fig. 21 respectively.
The elongation is almost consistent with all joints for the HAZ ferritic steel region, which is
marginally less than the parent metal. The elongation (UE/TE) observed to be more with GMAW
16
process with buffer layer for buttering and weld metal. The UE and TE are not favourable and
desirable against the all weld joint in conventional procedure (joint A-1).
Fig. 21. Average Total Elongation (TE) for weld joints
The variation in UE/TE is in accordance with the variations observed in YS and UTS for the same
associated reasons. The tensile properties like UTS and UE influence the deformation and type of
fracture in the material. As in case of DMW joints, these properties significantly varied across the
weldment regions. PIS values in each material zone have been calculated using the equation1.
PIS = UTS x (%UE/100 + 1)
(1)
PIS is the true stress that reflects the influence of UTS and UE and the estimated PIS values for all
weld joints are given in Fig. 22. This strength is almost independent of stress concentration but
strongly depends on temperature [8, 9, 38]. It represents the resistance to local necking initiation and
used as local failure criteria for ductile materials [8, 9, 38]. Hence, the calculated values of PIS for all
joints in ductile (austenitic) region of buttering and weld metal have been examined.
Fig. 22. Average Plastic Instability Strength (PIS) for weld joints
PIS in HAZ ferritic steel of joint A-1 is considerably less than the rest of joint and is not desirable.
The favourable PIS profiles are noticed with the joints welded with the GMAW process due to the
better UE and UTS owing to process compared to SMAW. The effect due to buffer layer and
17
subsequent carbon migration can be notice with PIS variation in HAZ ferritic steel. Based on PIS, the
values with buffer layer and buttering deposition as well as welding with GMAW process is found to
be the favourable against conventional DMW joint manufacturing procedure for the ductile failure
criteria. Buffer layer and heat input variation (GMAW/SMAW) causes the favourable and desirable
effects especially in buttering and weld metal regions. This makes the YS, UTS, UTS, UE, TE, and
PIS better than without buffer layer joints. On the extent of identified properties, the excellent and
desirable properties are found in this sequence for joints B-2, A-2, B-1 and A-1. This suggests that
buffer layer introduction is adorable against the conventional procedure (DMW fabrication) while
the GMAW process suggest the better strength than SMAW process for the welds. Hence, using
ductile failure criteria, it can be suggested that the conventional procedure joint (A-1) is more
susceptible for failure compared to other joints in the severe plastic deformation regime.
The strength mismatch between the weldment regions and base metals could significantly affect the
crack driving force, crack growth resistance and the strain concentration location under fully plastic
condition [38]. The yield strength ratio (YSR) has been used [8, 9, 38] as a measure of weld strength
mismatch. Both base metals and consumables have the different tensile properties, which can lead to
vary strain concentration location in weldment regions accordingly [8, 9, 23]. The equation 2 has
been used to calculate the YSR for all joints for the ferritic and stainless steel side.
YSR = YSWM / YSBM
(2)
Where the YSWM and YSBM are yield strength of weld metal and base metal respectively. The YSR
more than one (overmatching) is generally desired, and if it is less than one (under-matching) then,
strain concentration can occur in weld region and fracture can be initiated from pre-existing defects
[8, 9, 23, 38] if any presents. The effect of mismatch is significant when the strength of mismatched
material exceed 10% [38].
Therefore, the estimation of YSR would be important for integrity assessment of DMW joints. The
calculated YSR for different locations like ferritic steel side (weld metal / base metal and buttering /
HAZ ferritic steel) and SS304LN are summarised in Table 5. It can be seen that for any process the
YSR is only overmatched on one side of the weld, the SS304LN. Current welding consumables do
not provide overmatching on the both sides and this is inherent problem for DMW integrity.
Table 5
Yield Strength Ratio (YSR) for all plate joints
Weld Joint
A-1
A-2
B-1
B-2
YSRWM-BM508
0.54
0.64
0.56
0.61
YSRBT-HZ508
0.70
0.60
0.56
0.58
YSRWM-BM304
1.39
1.65
1.46
1.58
The YSRWM-BM508 observed to be more favourable with joints welded with GMAW (Joint A-2, B-2)
process than SMAW (joint A-1, B-1) and is important for integrity assessment. The YSRBT-HZ508 for
18
joints with buffer layer observed to be less than the joints without buffer layer. This indicates that,
plastic strain concentration and fracture initiation do not occur initially in ferritic steel base metal,
HAZ ferritic steel, and weld metal but rather at SS304LN and/or in buttering region of these joints.
Considering the YSRBT-HZ508 for joints (Table 5), the results are found to be consistent with
composite tensile test during fully plastic deformation (Fig. 16). Hence, the one sample from joint B1 in CTT has been fractured from buttering region but satisfying the required strength as per DMW
requirement as per ASME [8]. The estimation of YSR could contribute the value addition in integrity
assessment [8, 9, 38] and same thing has been justified with this study. The under-match in weld
strength is not desirable but that is inherent problem in DMW fabrication. The worst mismatch in
terms of YSR needs to be included (quantification range) in the integrity assessment procedure and
codes to ascertain the location and type of failure mechanism.
3.7 Charpy V-notch impact toughness
The average impact toughness for all weldment regions of weld joints are presented in the Fig. 23.
The impact toughness of HAZ ferritic steel region has been observed to be more than base metal
ferritic steel due to fine grain structure of HAZ (FGHAZ) ahead of the crack tip (V-notch). The
coarse grains in HAZ (CGHAZ) are ~50 micron wider (Fig. 11A and B) hence, the exact positioning
of V-notch in CGHAZ is not possible. Therefore, the notches are positioned in the FGHAZ which
having the hardness 250-275 HV100 (Fig. 15). The FGHAZ and fraction of reformed martensite as
discussed earlier caused to increase the impact toughness marginally in the HAZ of joints than the
joint A-1. The weld chemistry in buttering and the resulting phase structure has shown the increased
impact toughness of buttering region due to buffer layer and GMAW buttering process. Same thing
is also revealed in weld metal region.
Fig. 23. Charpy V-notch impact toughness/energy for weldment regions of weld joints
The buffer layer buttering process / welding process variations suggest that the joints B-1, B-2 and
A-2 have the favourable profile of impact toughness in the spatial region of DMW and this is far
19
better than the joint fabricated with the existing procedure (joint A-1). Based on spatial variations in
DMW for impact toughness, the buffer layered joint are most desirable by integrity assessment.
Fig. 24. Fracture surfaces of Charpy specimens for buttering region of weld joints (A) A-1,
without buffer layer and (B) B-2, with buffer layer
The impact toughness in weldment regions is found to be consistent with microstructure and the
tensile properties in the study. The fracture surface of buttering region without buffer layer (joint A1) and with buffer layer (joint B-2) is shown in Fig. 24A and B respectively. Large number of small
size dimples shows evidence of ductile tearing, as cab be seen in Fig. 24A. Arrows indicate the
second phase particle nucleated micro-voids in buttering region. The multifaceted surface is
observed to be dominant than small size dimples in the Fig. 24B owing to variations caused by Ni-Fe
buffer layer. The multifaceted surface indicates ductile tearing along the grains. While, the ductile
tearing along the grains is significantly less in Fig. 24A (joint A-1) than the Fig. 24B (joint B-2) and
this is found to be consistent and in agreement with the observed results.
Fig. 25. Fracture surfaces of Charpy specimens for weld metal region of weld joints (A)
A-1, SMAW process and (B) B-2, GMAW process
The fracture surface of weld metal of SMAW (joint A-1) and GMAW (joint B-2) process are shown
in Fig. 25A and B respectively. The presence of fine slag inclusion due to coated electrodes in the
ductile dimples is clearly visible in SMAW (Fig. 25A) but absent in weld of GMAW (Fig. 25B)
process. The inclusion and second phase particle nucleated micro-voids are shown with arrows in
Fig. 25A and B. The dimple size in Fig. 25A is observed to be smaller than Fig. 25B (GMAW
20
process) and encircled in the figures. These observations are consistent with the recorded impact
toughness in the joints. The dimple orientation in buttering region shows the columnar structure
when compared with the fracture surface of weld metal.
3.8 Fracture toughness testing
Typical Load - crack mouth opening displacement (CMOD) diagram for weld metal of all weld
joints has been analysed on the specimens (Fig. 26A) and the Load-CMOD plots are shown in Fig.
26B. The CMOD values are found to increase steadily with load in the elastic-plastic condition.
Hence, the CTOD (  max ) corresponding to the maximum load point was employed for the evaluation
of CTOD (  Cmax ). For the computation of CTOD from the CMOD value, the total CMOD
corresponding to maximum load point was divided into two parts, first belongs to CMOD
corresponding to the elastic part (ve) and other one is CMOD corresponding to the plastic part (vp) of
crack opening. The elastic (δe) and plastic (δp) parts of CTOD were calculated [39] using equation 3
and 4 respectively.
e 
K I2 1   2 
2 E YS
(3)
where KI is the stress intensity factor corresponding to the critical load, E is the elastic modulus, σYS
is the yield strength and υ is the Poisson’s ratio.
p
Vp
a
1
r (W  a)
(4)
where Vp is the plastic component of CMOD corresponding to the critical load, a is the original
crack length, W is the width of the specimen and r is the rotation factor which may be taken as 0.4 as
per standard.
Fig. 26. (A)Test specimen after CTOD test, and (B) Typical Load-CMOD plots with
obtained results
21
Total CTOD (δc) was calculated using equation 5 as given below;
c  e  p
(5)
Stress intensity factor in case of SENB test was calculated using equation 6
KI 
PS
f
BW 3/2
where fα is the function of α= (a/W) and given as f 
(6)
3 0.5 [1.99   (1   )][2.15  3.93  2.7 2 ]
2(1  2 )(1   )1.5
where P is the maximum load, S is the span length and B is the thickness of the specimen. For CTOD
testing, according to ASTM 1290-02, the ratio of a/W is given as 0.45 ≤ a/W ≤ 0.70. Whereas, the
a/W ratio of specimen in this study is ranging from 0.51 – 0.56. Hence, all specimens have satisfied
the a/W requirement. The CTOD values for the joints welded with GMAW process have been
observed to be significantly higher than the welded by SMAW process owing to presence of fine slag
inclusion in the weld. As an effect of buttering deposition with buffer layer/buttering process, the
fracture toughness (CTOD) has been observed to be decreased marginally in buffer-layered joints
(B-1 and B-2) than joints A-1/A-2. The resulted weld chemistry in weld metal due to different
buttering deposits and dilution as shown in Table 4, the Fe and Nb content are marginally more in
buffer layered joints (B-1/B-2) than the joint A-1/A-2. The laves phase formation at grain boundary
would be more pronounced with joint B-1 and B-2 weld metal and that may cause to marginal
reduction in the CTOD fracture toughness. The better fracture toughness can be achieved by GMAW
process than the SMAW process welding.
4. Influence of investigated properties on integrity assessment
The reliable and precise methods for structural integrity assessment of DMW joints are not available
[9, 37]. The designing of DMW joints and its assessment is made at present on the figures/facts and
information obtained from similar metal welds. The structural integrity assessment at present is
depend on the outputs from several years of experience [9, 40]. According to the integrity assessment
described in R6, European method SINTAP, and FITNET FFS[41-43], the dissimilar weld is
considered as the sandwich combination of parent metals and different weldment region metals[9].
Variations in metallurgical and mechanical properties in HAZs and the regions at fusion interfaces
are significant due to dilution and chemistry mismatch. However, these variations and its resulting
effects are not considered in existing procedures and codes [9, 37] for integrity assessment. Undermatching and overmatching the weld strength with base metals is inherent property of DMWs
integrity and the defects or failures may appear anywhere in the weldment regions. The undermatching is serious concern and need to be quantified as the minimum required criteria for the
strength under-matching and ductile failures in severe plastic deformation. The variations weld
strength in properties of buttering, weld metal and even in HAZs could occur due to the weld
22
chemistry mismatch and that is apparent owing to composition gradient and the active diffusion
mechanism [7].
The fracture and failure mechanisms cannot be predicted for the notch positioned in any specific
weldment region or at interface owing to micro-segregation (chemistry mismatch) of the formed
phases and intermetallic compounds. In this condition, the initiation of crack and its propagation path
will be defined by the formed phase structure and the intensity of micro-segregation in very narrow
zones near the interfaces. This can suggest that, the existing procedure and codes cannot be really
used with the desired accuracy by compromising the failures in terms of nuclear hazards. The
variations in mechanical properties have significantly affected the fracture resistance and crack
growth path [37, 41]. The different metallurgical and mechanical properties with marginal disparities
in same material region could remain exist [37, 44, 45] in weld materials. Crack initiation resistance
and its propagation can greatly be affected by heterogeneity in mechanical properties across the
weldment regions [9, 46]. Therefore, the development and modifications in existing procedures and
codes of DMW integrity assessment can be made. This can be done by considering the local
mechanical properties with due considerations of HAZs and interfacial zones by analysing the microsegregation, weld chemistry mismatch and the metallurgical properties in terms of formed phase
structure.
5. Conclusion
In present study, welding processes, buttering processes and Ni-Fe buffer layer variations were
analysed for metallurgical, mechanical and fracture toughness properties. The derived conclusions
from the analysis are listed here.
1. Lesser angular distortion (can be in terms of measure for residual stresses) can be achieved by
introducing the buffer layer, varying the buttering process and welding process. This distortion
is significantly less than the existing procedure of DMW fabrication.
2. The Ni-Fe alloy buffer layer in buttering causes the certain variations in material chemistry of
buttering and this is desirable for the thermal, metallurgical and mechanical properties than the
buttering without buffer layer in the weld joints. Owing to Ni-Fe buffer layer, weld metal
chemistry in buttering changes and causes the effect on solubility of Nb and Ti due to increased
Fe content. This leads to more fraction of laves phases and wider dendrite arms compared to
buttering without Ni-Fe buffer layer.
3. Carbon depleted soft zone is formed (~50 micron) in HAZ of SA508Gr.3Cl.1 of the joints
without buffer-layered buttering while, the carbon enriched hard zone (~60 micron) is formed
with Ni-Fe buffer-layered joints. This variation justifies the control of carbon migration owing
to buffer layer in the DMW joints.
23
4. The tensile properties with buffer layer joints are more desirable and favourable. The most
favourable properties are recorded with buffer layered buttering joint welded with GMAW
process. While, the least favourable properties are found with conventional joint (joint A-1).
Charpy impact toughness in the weldment regions recommends the Ni-Fe buffer layer and the
GMAW process for welding of such DMWs. Weld strength and ductility of the weldment
(composite tensile test) is marginally favourable with Ni-Fe buffer layered joints than without
buffer-layered joints.
5. YSR and PIS suggests the conventional procedure of DMW (joint A-1) is least favourable
against the buffer layered and GMAW processes employed weld joints in study.
6. The fracture toughness (CTOD) of the GMAW weld joint is significantly more than SMAW
joints due to presence of fine slag inclusions from the coated electrode in the resulted weld.
7. Effective use of GMAW process for buttering and welding along with the buffer layer
application can be possible. While, the modification in integrity assessment procedure can be
possible by considering the interfacial region properties, chemistry mismatch and the YSR/PIS
measurement criteria.
Acknowledgement
The authors acknowledge the support given by Board of Research in Nuclear Sciences, Department
of Atomic Energy (India) for present work. In addition, author also thankfully acknowledging the
technical expertise offered by Joanna N. Walsh, School of Materials, University of Manchester for
the present work.
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