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Welding Underwater
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7/21/2019 Welding Underwater
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Journal of Materials Processing Technology 220 (2015) 76–86
Contents lists available at ScienceDirect
Journal of Materials Processing Technology
journal homepage: www.elsevier .com/ locate / jmatprotec
Microstructures and mechanical properties of dissimilar T91/347H
steel weldments
Rutash Mittal a,∗, Buta Singh Sidhu b
a Department of Mechanical Engineering, Malout Institute of Management& InformationTechnology, Malout, Dist. Sri Muktsar Sahib, 152107,
Punjab, Indiab Punjab TechnicalUniversity, Jalandhar-KapurthalaHighway, Kapurthala,Punjab, India
a r t i c l e i n f o
Article history:
Received 4 April 2014
Received in revised form 12 January 2015
Accepted 14 January 2015
Available online 23 January 2015
Keywords:
Dissimilar metal weldments
SMAW
GTAW
Mechanical characterization
Micro-hardness
Microstructures
a b s t r a c t
Shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW) and their combination are
processed by using austenitic and nickel based diverse welding electrodes/filler wires to prepare the
dissimilar weldments. The comparative evaluation of an appropriate welding process and welding con-
sumable is based on microstructural features, micro-hardness variation, tensile testing and fracture
morphology. The martensitic morphology is found responsible for the higher micro-hardness of HAZ
of T91 side of all the weldments. Higher tensile strength (638.2 MPa) of GTAW, ERNiCr3 combination is
observed than other weldments. The fractography corroborates the highest ductility of GTAW, ERNiCr3
combination with an elongation of 28.33% and the ductility of all the combinations of weldments except
GTAW, ERNiCr3 is observed to be less than their base metals. Hence, it can be concluded that GTAW
process, using nickel based weld metal offered the better results for the dissimilar joint between T91and
347H.
© 2015 Elsevier B.V. All rights reserved.
Introduction
Dissimilar metal weldments are widely used in various products
in chemical,petrochemical,nuclear andpower industries (Hanand
Sun, 1994). Satyanarayana et al. (2005) have favoured the adoption
of dissimilar metal joints, as it provides feasible solutions for the
flexible design of the products by using each material efficiently.
In power generating industry, components working at elevated
temperature are made of stainless steels and those used at lower
temperature aremade of ferritic steels. Dissimilarmetal weldments
with cheaper steels in place of highly alloyed steels make a con-
siderable saving on cost. Ferritic steels are having well considered
mechanical properties, good thermal conductivity, good ductility
and austenitic steels bear’s good corrosion resistance, better creep
strength and high temperature stability of microstructure during
the service. Sun (1996) emphasized the adoption of this combina-
tion, based on both technical and economical reasons, along with
satisfactory service performance as well as considerable savings.
The dissimilar metal joints are inclined to frequent failures and
these failures are by and large credited to one or a greater amount
of the accompanying reasons reported by Joseph et al. (2005).
∗ Corresponding author. Tel.: +91 9876333349; fax: +91 1637264511.
E-mail addresses: [email protected] (R. Mittal), [email protected]
(B.S. Sidhu).
(a) Difference in mechanical properties across the weld joint anddifference in thecoefficient of thermal expansionof twometals and
resultingcreepat theinterface,(b) general alloyingissues ofthe two
distinctive base metals, such as brittle phase formation and dilu-
tion, (c)carbon migrationfromferriticsteelinto austenitic steel,(d)
preferential oxidation at the interface, (e) residual stresses present
in the weld joints and (f) service conditions and other factors.
Jones (1974) stressed on the selection of proper welding process
and filler material for dissimilar metal weldments as an essential
factor in view of difference in physical, chemical and mechanical
properties of the base metals involved. Weld ability and dissim-
ilar joint features without using PWHT between Inconel 657 and
310SS were studied by Naffakh et al. (2009). Characterization anal-
ysis identified Inconel A to be the best among the four filler metals.
Jang et al. (2008) have examined the mechanical property variation
within the weld metal of Inconel 82/182 in the dissimilar joining
between low alloy steel and 316 stainless steel welded by SMAW
and GTAW processes. Sireesha et al. (2002) have used SMAW and
GTAW processes and Satyanarayana et al. (2005) have tried fric-
tion welding, whereas Sun (1996) has attempted dissimilar metal
weldments with laser beam welding for mechanical characteriza-
tion. Sudha et al. (2000) have identified the formation of soft and
hard zones in the dissimilar weldment of 9Cr-1Mo and2.25Cr-1Mo
steels. Bhaduri et al. (1994) and Raman and Tyagi (1994) have dis-
cussedabout the morphologyand distribution of precipitates in the
different sections of the weldment and concluded their difference
http://dx.doi.org/10.1016/j.jmatprotec.2015.01.008
0924-0136/© 2015 Elsevier B.V. All rights reserved.
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R. Mittal, B.S. Sidhu / Journal of Materials Processing Technology 220 (2015) 76–86 77
in the heat affected zone (HAZ) than those of base metal (BM)
and weld metal (WM). The precipitates revealed in the HAZ are
either parallelepiped or rod shaped which are typical shapes of the
M7C3 or M23C6 types of carbides respectively. Sudha et al. (2006)
reported the precipitation sequence and micro-chemistry of main
secondary phases of M23C6, M6C in the hard and of M2C inthesoft
zone in the weldments of 9Cr-1Mo steel and 2.25Cr-1Mo ferritic
steel that was post weld heat treated. The formation of hard HAZ
by virtue of presence of martensite phases in the weldment is an
important phenomenon, which needed to be studied from every
aspect. Foroozmehret al. (2011) havequoted theformation of mar-
tensite and austenite phases in ball milled powders through XRD
peaks. Detailed literature reviewrelated to formation of martensite
and its different crystal structures with respect to varying carbon
content has been published by Sherby Oleg et al. (2008). White
William (1992) has conveyed the corrosion and mechanical prop-
erty disintegration of dissimilar weldments and discussed about
the sensitization in the HAZ leading to inter-granular attack, weld
decayand preferential corrosion of secondary phases in multiphase
weld metal. The strength of the dissimilar weldments is generally
inferior to its base metals. Sireesha et al. (2000a) and Arivazhagan
et al. (2011) have reported the in-service failures of DMWs from
weld metal and HAZ respectively. Mohandas et al. (1999) have dis-
cussed thehigherstrength andductility of GTAW welds apparently
due to equiaxed fusion zone and protective nature of shielding
gas than SMAW welds for AISI430 ferritic steels. The formation of
martensitic layer leads to microstructural and mechanical prop-
erty variation across the weld interface with HAZ and is partly
responsible for the premature failure of the dissimilar welds at ele-
vated temperature (Dupont and Kusko, 2007). Viswanathan et al.
(1982) have reported about the failure of austenitic weld metal
at elevated temperatures for DMWs of martensitic and austenitic
steel. Shah Hosseini et al. (2011) also observed the better perfor-
mance of Inconel82 (Ni based filler material)than 310SS (austenitic
filler material) based on mechanical characterization features. The
superior performance of Inconel filler materials in DMW of 316LN
and alloy 800 is also corroborated by Sireesha et al. (2000a). The
authors have demonstrated the superiority of Ni based welds byvirtue of absence of precipitates at elevated service temperature.
The transition joints between ferritic/austenitic steels suffer from a
mismatch in coefficient of thermal expansion, which causes reduc-
tion in weldment integrity, to overcome these issues Ni based
weldment have been suggested (Sireesha et al., 2000b). Bhaduri
et al. (1994) and Das et al. (2009) also reported the longer service
life of nickel based weld metals for steam generator applications.
Detailed review of microstructural evolution and high tempera-
ture failure of ferritic to austenitic dissimilar welds have been done
in detailed manner by Dupont (2012). Steep microstructural and
microstructural gradients, large variation of coefficient of expan-
sion, formation of carbides and preferred oxidation of ferritic steel
were reported to be the main reasons of failure.
However, no systematic work on characterization studies hasbeen conducted on the dissimilar joint of T91/347H by SMAW and
GTAW processes. As better results are being listed in the litera-
ture, regarding the nickel based filler metal, the work on dissimilar
metal weldments of T91 and 347H by using austenitic as well as
nickelbased filler material has been attempted. Foreach weldment
combination, detailed studies are conducted on microstructure
characterization, micro-hardness variation, tensile strength and
fracture morphology using X-ray diffraction (XRD), scanning elec-
tron microscopy (SEM)/energy dispersive spectroscopy (EDS).
Base materials andwelding processes
Two types of materials in the pipe form, ferritic boiler steel,
namely SA213T91 (T91) of dimensions 50.5mm (O.D.)×
5.59mm
(thickness) and austenitic steel AISI347H (347H) of dimensions
50.8mm (O.D.)×5.59mm (thickness) were used as base materi-
als. The pipes were machined to make V joint geometry having a
root gap as 2mm and the included angle of 75◦ for welding joint.
SA213T91 belongs to 9Cr-1Mo ferritic steel category and AISI347H
is from austenitic steel family. Then the test samples in the form
of flat strips by keeping the weld metal approximately in the mid-
dle were prepared from the dissimilar metal weldments. The test
specimens of the base metals of SA213T91 and AISI347H were
also additionally prepared of the same dimensions with the pur-
pose of comparison. The chemical composition of base metals and
welding electrodes/filler wires are presented in Table 1. In the
present investigation two welding electrodes, namely Rutox-B and
Rutox-Ast by SMAW process are used just to resemble the actual
weldments used in the boilers of G.N.D.T.P. (one of the power plant
in North India), Bathinda, Punjab, India. These first two combina-
tionsare experimented to simulate the actual industrialconditions.
The other two proposed combinations are used in accordance with
the suggestions of welding experts as well as literature. White
William, 1992, Wang et al.(2011) and Arivazhagan et al. (2011) and
other authors have quoted in favour of GTAW process to weld the
metals. Daset al.(2009) and Dupont (2012) along with other author
have concluded benefits of Ni based weld metal. Some authors
like Jang et al. (2008), Han and Sun (1994) and Sireesha et al.
(2002) have tried GTAW+ SMAW combination and shown better
output. These welding procedures along with welding electrodes
already used in the power plant of G.N.D.T.P. were selected along
with the proposed combinations to simulate the actual industrial
conditions. The purpose was improvement in the design of weld-
ment which is directly applicable and can be recommended with
comparison to the working industry. The present study is a part
of high temperature corrosion study of dissimilar weldments of
T91 and 347H, showing difference in the oxidation behaviour in
micro structurally varied regions. The weldments have not expe-
rienced any preheating or post weld heat treatment (PWHT), to
simulate the characterization of actual industrial weldments. The
details of welding processes, electrodes, filler wires and welding
specifications are listed in Table 2.
Characterization of weldments
The dissimilar metal weldment combinations are portrayed for
optical microscopy to focus the optical microstructure of different
zones at diverse magnifications by utilizing an optical microscope
of Leica DM4000M at IIT Ropar, India. The standard metallographic
procedure has been adopted for obtaining the microstructures of
base metals, HAZs, weld metals and their interfaces. The sam-
ple preparation comprises polishing on different grades of emery
papers starting from 80 to 2000 grade and finally cloth polishing
with alumina paste on a rotating disc. Iteratively different combi-
nations of etching agents have been tediously applied to see themicrostructures as the evolution of structures is difficult in dis-
similar metal joints. The microstructure of 347H and its HAZ is
examined by Marble’s reagent {CuSo4 (4g) + HCl (20 ml) + distilled
water (20ml)} and of the T91, its HAZ and weld metals are treated
with fry’s reagent comprising {CuCl2 (5 g)+ HCl (40 m l) +ethyl
alcohol (25 ml) +distilled water (30ml)}. The Microhardness of
different sections of the weldment is carried out using Vickers’s
Microhardness tester of Wilson Instrument (an Instron company,
Model No. 402MVD) make at the regular interval of 0.5 mm across
the width of the sample covering all the regions of the weldment.
The tensile testing of the weldments has been done on the H25K-
S make of Hounsfield company tensile testing machine of 25kN
capacity fitted with a digital extensometer at Material testing lab-
oratory of Metallurgical and Materials Engineering Department of
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Table 1
Chemical composition (wt.%) of the base metals and welding electrodes/wires.
Base material/welding electrode/wire C Cr Mn Mo Ni Si V Nb Cb Fe
SA213T91 0.0964 8.76 0.478 1.03 – 0.37 0.26 0.09 – Balance
AISI347H 0.0681 20.06 2.03 0.27 9.39 0.62 – 0.44 Balance
Rutox-B 0.03 19.8 1.40 – 10 0.40 – – – Balance
Rutox-Ast 0.03 19 1.20 10 0.45 – – 0.5 Balance
ERNiCr3 0.035 20 3.0 – 72.6 0.30 – – – 3.0
Table 2
Specifications of welding process, combinations and welding electrodes/wires.
S. no. Welding process used Specification of welding
electrode/filler wire used
Applied v oltage a nd c urrent Polarity a nd t he t otal n umber
of passes
1 SMAW (shielded metal arc
welding)
Rutox-B (D&H secheron)
AWS/SFA-5.4:E308L-16
3.15 mm dia. Electrode
Root Run:
V =15V, I =100A
Subsequent passes:
V = 15V, I =130A
DCEN, three
2 SMAW (shielded metal arc
welding)
Rutox-Ast (D&H secheron)
AWS/SFA-5.4:E347-16
3.15 mm dia. Electrode
Root Run:
V = 15V, I =100A
Subsequent passes:
V = 15V, I =130A
DCEN, three
3 GTAW (gas tungsten arc
welding), Shielding gas= argon,
flowrate= 10L/min. Purging
gas= argon,flow rate= 5 L/min.
ERNiCr3
AWS/SFA-5.11:ENiCrFe3
2.5mm dia. Filler wire
Root Run:
V =15V, I = 70A
Subsequent passes:
V =15V, I =100A
DCEN, three
4 GTAW*+ SMAW, *{shielding
gas= argon,flow
rate =10 L/min. purging
gas= argon,flow rate= 5L/min}
ERNiCr3 , 2.5mm dia. Filler
wire+ Rutox-Ast (D&H
Secheron), 3.15mm dia.
Electrode
Root Run:
V =15V, I = 70A
Subsequent passes:
V =15V, I =130A
DCEN, three (Root runwith
GTAW, further with SMAW)
Fig. 1. Dimensions of the tensile specimen (ASTM E8M-04).
I.I.T. Roorkee, India. For each type of reading, three test specimens
have been evaluated and average tensile strength is obtained, to
ensure repeatability. The basic dimensions of the tensile testing
specimen have been taken in accordance with the ASTM E8 stan-
dards as outlined in Fig. 1. Scanning electron microscope (SEM) of
JEOL Japan make having model JSM-6610LV equipped with EDS of
Oxford instrument facility having model number 51-ADD0013 is
used to study the microscopic and fracture modes of the samples.
X-ray diffraction facility using Instrument of PAN Alytical Model
X’Pert PROMPD made in the Netherlands is also used to have XRD
patterns ofthe specimen.Insteadof a singlescan ofthe whole weld-
ment as done by Arivazhagan et al. (2012), particular XRD scans of
the specific section i.e. HAZ, WM, etc. are performedfor the specific
information about the phases.
Results and discussion
Visual observations andmetallographic studies
The welded samples of all the four types of the dissimilar weld-
ments are demonstrated in Fig. 2, indicating that all the weldments
have a properpenetrationof filler electrode/wires. Thewidthof the
spread of the weld metal in the case of GTAW, ERNiCr3 is lesser in
comparison to other weldments.
Fig. 2. Different combinations of the dissimilar metal weldments.
White William (1992) enlisted that the characteristics of the
microstructures through weld zones, its size and extent of heat
affected zones of weldments will depend on the types of metals
being joined, whether or notthey areheat treatable, andthe classes
of welding or joining processes. The optical microstructures of
weldment of SMAW, Rutox-B combination are shown in Fig. 3a–l.The XRD patterns of different sections of all the combination of
weldments are displayed in Fig. 4a–f.
The microstructure of T91 BM shown in Fig. 3a comprises of
tempered lath martensite. The prior austenite grain boundaries, as
well as lath boundaries are decorated with precipitates (Laha et al.,
1995). The micro mechanism responsible for the formation of mar-
tensite structure on HAZ of T91 depends on the peak temperature
attained andcooling rate of that specified region. Depending on the
temperature of HAZto be above AC3or above AC1(i.e.betweenAC1
and AC3), austenitic or a mixture of ferritic and austenitic structure
gets formed which on cooling gets transformed into the marten-
sitic structure (Kou, 2003). The microstructures of the HAZ of T91
are segregated into two zones, namely fine grained heat affected
zone (FGHAZ) and coarse grained heat affected zone (CGHAZ) as
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Fig. 3. Optical microstructures at different sections of SMAW, Rutox-B weldmentsof T91 and 347H.
shown in Fig.3b–f.The CGHAZ(Fig.3d) hasbeen just adjacent tothe
weld metal having large grain size, where as FGHAZ (Fig. 3c), just
after theT91 BM with lessergrainsize,having welding temperature
range between AC1 and AC3. The T91 CGHAZ depicts a martensitic
structurein thepresenceof some delta ferrite indicatedin Fig.3d–f,
just similar to the observations of Bhaduri et al. (1994). Pilling andRidley (1982) have also reported the formation of delta ferrite in
2.25Cr-1Mo steel. Delta ferrite is a chromium rich phase of Fe–Cr
alloys and may have formed due to depletion of Cr from the alloy
matrix. Depending on thepeak temperature andits cooling charac-
teristics of the thermal cycle, the phase of delta ferrite of T91 steel
can be as high as 35%. The presence of delta ferrite in addition to
martensite structure of HAZof T91 can facilitate the depletion of Crthrough precipitation and ultimatelyaffect the oxidationresistance
Fig.4. X-raydiffraction patternsfor differentcombinations of weldmentsof differentsectionsalong the weldments. (a) HAZof 347H,SMAW, Rutox-B (b)HAZ ofT91, SMAW,
Rutox-B (c)HAZ of T91, SMAW, Rutox-Ast(d) HAZ of T91, GTAW+ SMAW, weldment (e)HAZ of T91, GTAW, ERNiCr3 (f) WM of GTAW, ERNiCr3 weldment.
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Fig. 5. SEMmicrographsat differentsectionsof SMAW, Rutox-B weldments of T91 and 347H.
Table 3
Variation of elements along theHAZ of T91 and weld metal interface forSMAW, Rutox B weldment.
T91 HAZ WM (Rutox-B)
Elements 1 2 3 4 5 6 7
Cr 9.05 11.07 12.17 13.39 13.5 18.5 23.10
Mo 1.97 2.06 7.57 1.06 3.97 0.14 –
Fe 89.04 86.86 80.26 83.5 73.47 72.06 65.5
Ni – – – 1.97 9.01 9.25 11.74
Fig. 6. Optical microstructures at different sections of SMAW, Rutox-Astweldments of T91 and 347H.
Fig. 7. SEMmicrographsat differentsectionsof SMAW, Rutox-Ast weldments of T91 and 347H.
Table 4
Variation of elements along theHAZ of T91 and weld metal interface forSMAW, Rutox Ast weldment.
T91 HAZ WM (Rutox Ast)
Elements 1 2 3 4 5 6 7
Cr 9.25 12.03 12.5 16.06 11.15 18.48 16.48
Mo 3.08 2.45 1.10 5.00 – – –
Fe 87.67 85.5 86.03 78.6 8.5 73.06 76.34
Ni – – – – 3.85 8.46 7.88
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Fig. 8. Optical microstructuresat differentsectionsof GTAW +SMAW,weldments of T91 and 347H.
Fig. 9. SEMmicrographsat differentsectionsof GTAW+ SMAW, weldmentsof T91 and 347H.
Table 5
Variationof elements along theHAZ of T91 and weld metal interface for GTAW +SMAW,weldment.
T91 HAZ WM
Elements 1 2 3 4 5 6 7
Cr 9.94 11.02 10.3 11.45 23.13 18.8 23.49
Mo 2.24 0.91 2.42 0.87 – – –
Fe 87.64 88.3 87.2 71.1 53.8 65.6 53.81
Ni – – – 13.06 23.06 15.5 13.63
Fig. 10. Optical microstructuresat differentsectionsof GTAW, ERNiCr3 weldments of T91 and 347H.
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Fig. 11. SEMmicrographsat differentsectionsof GTAW ERNiCr3,weldments of T91 and 347H.
Table 6
Variation of elements along theHAZ of T91 and weld metal interface forGTAW, ERNiCr3 weldment.
T91 HAZ WM
Elements 1 2 3 4 5 6 7 8 9
Cr 9.82 10.8 12.59 14.51 15.71 11.94 17.46 18.95 16.25
Mo 1.84 2.38 2.49 1.21 1.78 – 1.12 1.39 1.72
Fe 85.3 85.4 82.4 77.92 78.15 10.29 6.25 7.2 5.91
Ni – 1.26 3.97 3.44 68.5 70.5 71.8 71.5
Fig. 12. Micro-hardness variation of combinations of all the weldment of (a) SMAW, Rutox-B (b) SMAW, Rutox-Ast (c) GTAW +SMAW, ERNiCr3+ Rutox Ast (d) GTAW,
ERNiCr3.
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Fig. 13. Photograph of thetensile tested specimensof all thecombinations of weldment (a)SMAW, Rutox-B, (b) SMAW, Rutox-Ast, (c) GTAW+ SMAW, ERNiCr3 + Rutox Ast,
(d) GTAW, ERNiCr3.
The average value of micro-hardness of the weld metal is around
204 Hv and distribution of micro-hardness seems to be uniform
throughout the weld metal region. Similarly the micro-hardness
pattern in HAZ of 347H and 347H BM seems tobe uniform with an
average value to be around 175 Hv.
The micro-hardness profile of the weldment produced by the
combination of GTAW and SMAW is presented in Fig. 12c. An
increase in the micro-hardness value can be seen while mov-ing from weld metal to HAZ of T91 side. The sharp increase in
micro-hardness in the HAZ of the T91 has been correlated to
the martensitic morphology (Fig. 8b–d) having average micro-
hardness to be around 420 Hv. The morphology of HAZ of T91 side
seems to be needle like martensitic structure, depicted in the SEM
micrograph of Fig. 9b andc. The presence of carbides i.e. (Cr,Fe)7C3,
Cr7C3 along with Ni-Cr-Fe and martensite phases in the HAZ of
T91 validated in XRD analysis (Fig. 4d), can also observed to be the
cause of higher micro-hardness. Yajiang et al. (2002) and Tavares
et al.(2002) havealso observed thepresence of martensite phase in
weld metal of 9Cr-1Mo steel. The average value of micro-hardness
of the weld metal is 170 Hv having uniform distribution through-
out the weld metal area, but a gradual increase in its value being
observed in theHAZ andBM of the347H. This increase in themicro-
hardness of HAZ of 347H side may be attributed to the presence of
carbides, as indicated in the optical micrographs in Fig. 8g and h.The micro-hardness profile of the weldment of GTAW, ERNiCr3
given in Fig. 12d presentsa sharpincrease in itsvalue, while moving
from weld metal to HAZ of T91 side. The average micro-hardness
of the HAZ is around 440Hv. An increase in the micro-hardness,
while moving from weld metal towards T91 BM may be attributed
to the needle shaped morphology of martensite and delta ferrite
of HAZ in Fig. 10c and d and SEM micrograph in Fig. 11b and c.
(Cr,Fe)7C3, Cr7C3, Ni-Cr-Fe along with martensite phases observed
Table 7
Tensile strength, micro-hardness variation and fractography of different combinations of the weldment.
Sample/joint
specifications of
T91-347H type
Yield strength
(MPa)
Ultimate
tensile strength
(MPa)
Percentage
elongation
Fracture
location
Maximum
micro-hardness on
weldment (Vickers), Hv
Tensile fractography
SMAW, Rutox-B No yielding
–
480 15 Weld metal 421 Parabolic shape dimple formation,
small tearing ridge, quasi-cleavage
fracture
SMAW, Rutox-Ast 404.3 580 22.27 BM of T91 450 Layered tearing ridge, quasi cleavage
fracture
GTAW+ SMAW,
ERNiCr3 +Rutox-Ast
427.6 508 17.63 Weld metal 450 Ductile fracture with micro-void
coalescence, small sized dimple
formation
GTAW, ERNiCr3 376 638.20 28.33 BM of T91 476 Fine and uniform dimples, some large
dimples surrounded by small dimples,
ductile fracture
Base Metal T91 573.6 624 25.71 – – Layered tearing ridge, quasi cleavage
fracture
Base Metal 347H 340.7 628 54.24 – – Ductile Fracture, small shallow
dimples indicative of high ductility and
tensile strength
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Fig. 14. SEMfractographyof thetensile tested specimensof base metals anddissimilar weldments, (a) T91 BM, (b) 347H BM, (c) SMAW, Rutox-B, (d) SMAW, Rutox-Ast, (e)
GTAW +SMAW, ERNiCr3 + Rutox-Ast, (f) GTAW, ERNiCr3.
in theparticular XRDpattern of Fig.4e can alsobe a factorforhigher
micro-hardness. The formation of various phases as identified by
the specific XRD analysis (Fig. 4f) might have contributed to the
observed increase in micro-hardness of the weld metal with an
average value to be around 214Hv. A gradual increase of micro-
hardness observed in the HAZ and 347H BM, might be by virtue
of re-crystallization of the austenitic structure of the 347H side
(Fig. 10 j).
Tensile testing and fracture analysis
Thetensile testing of allthe base metalsand jointsformulatedby
using different welding techniques and filler electrodes/wires have
been made and presented in Fig. 13. The ultimate tensile strength,
yield strength and percentage elongation along with features of
fractography has been presentedin tabular formin Table 7. The ten-
sile strength so obtained show that maximum tensile strength of
638.2MPa,possessed bythe specimen produced byGTAW,ERNiCr3
combination followed by 580MPa of SMAW, Rutox-Ast combina-
tion. The lower heat evolved in the GTAW, ERNiCr3 (I = 7 0 & 100 A )
weldment than other welded combinations have produced better
joint integrity. Kumar andShahi (2011) and Mohandas et al. (1999)
have recommended lower heat input process to attain good tensilestrength, ductility and joint integrity as higher heat input can pro-
duce coarsening in HAZ and weld metal. Han and Sun (1994) have
quoted that for dissimilar metal joints, it is common practice that
mechanical properties of the joints should not be worse than those
of the inferior base metal. The tensile properties (tensile strength
and elongation), micro-hardness of the joint depends on chemical
composition and microstructure of the weld metal. Formation of
fully austenitic vermicular shaped microstructure of weld metal of
GTAW, ERNiCr3 due to thepresence of higher percentage of Ni than
the weld metals of other weldments led to enhancement of ductil-
ityand tensile strength, which ensures its safe application in steam
generator circuits. The joints have broken from the T91BM in case
of GTAW, ERNiCr3 and SMAW, Rutox-Ast combination, whereas
weldments of SMAW, Rutox-B and GTAW+ SMAW combination,
have fractured from the weld metal. It has been observed that
the percentage elongation of all the dissimilar weldments except
GTAW, ERNiCr3 is less than the percentage elongation of their
respective base metals; hence the weld metals produced by differ-
ent combinations, except GTAW, ERNiCr3 are less ductile than the
base metals i.e. T91 and 347H. The greater ductility and strength of
GTAW, ERNiCr3 weldment as compared to other combinations of
weldments, can be attributed to the equi-axed and austenitic mor-
phology ofthe weld metalin thegas tungstenarc welds,alsoto inertgas shielding. Less inclusion content has been observed in welds
produced withGTAW process thanSMAW process, whichpromotes
better joint strength and integrity (Dehmolaei et al., 2008). The
general low ductility of the welds of all the combinations except
GTAW, ERNiCr3 compared to that of initial base metals may be by
virtueof thecastmicrostructure ofthe fusionzone(Mohandaset al.,
1999).
The fractured surfaces of the tensile tested specimens were
analyzed using scanning electron microscopy and presented in
Fig. 14a–f. Dimples of varying size and shape were observed in all
the fractured surfaces, which indicate the major fracturing mecha-
nism to be ductile. Blach et al.(2011) have reported theinitiation of
the formation of dimples at secondaryphase particles, which even-
tually resulted in different morphology of each studied dimplesfracture, according to its own particular microstructural character-
istics. Thefractureof weldmentof SMAW, Rutox-B fromweld metal
reveals a parabolic dimple formation with the river line pattern of
quasi-cleavage fracture along with a small area of tearing ridge for-
mation (Fig. 14c). The fracture of weldment of SMAW, Rutox-Ast is
from the T91 BM just close to hard HAZ of T91 side, which reveals a
mixed mode of cleavage fracture having layered facets of fracture
as can be seen in Fig. 14d. In the fractography of GTAW+ SMAW
combination, ductile fracture with micro-void coalesces with rela-
tively smaller sizeddimple formationis observed in Fig. 14e. Finally
the GTAW, ERNiCr3 weldment fracture is from T91 BM, which
comprises fine and uniform dimples in which larger dimples are
surrounded by the cluster of fine dimples having a ductile fracture
as demonstrated in Fig. 14f. The fractography of the base metal T91
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