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CHAPTER ONE
BACKGROUND STUDIES
1.1 Introduction
Till date modem industries mainly focused on light and high performance materials
for static and dynamic machine structures by single step fabrication technique [1]. But
due to complex shape, size, heterogeneous materials, fabrication has become complex
and unable to fabricate machine structures by one step casting [2]. Machine parts are
cast separately and joined to form their final shape. Joining of the part / materials is
one of the vast area which includes permanent joints such as welding [3], brazing [4]
and soldering [5] and temporary joints such as mechanical fasteners [6], adhesive
bonding [7], and riveting [8]. Latter methods (temporary joints) are used for only light
and frequently loading structures. In the former methods welding is especially used
for rigid and permanent structures which are exposed to dynamic / variable loading
environment [9]. On the other hand brazing and soldering are generally used for
electrical equipments [10]. As industrial machine structures are huge and complex in
nature, casting of these machine parts need more attention and special tailored made
fixtures are necessary and become more expensive [11]. Casting parts need secondary
operations such as removal of extra materials and unwanted materials using
machining centers. Hence majority of machine parts are fabricated by joining of
different cast structures. Some of the dynamic structures require relative motion with
respect to rigid structures. The type of joint is dictated by the applications of the final
assembly, strength requirement, type of the materials used, type of interaction of
forces involved among the subassemblies, geometry of the components and cost
issues [12]. Presently welding is one of the most important joining techniques in
industries. More than 50% of joining of structures depends on different welding
techniques [13].
1.2 Purpose of Welding
Variety of joining processes are available for engineering applications such as
welding [14], solid state welding [15], brazing [16], soldering [17], mechanical
fastening [18] and adhesive bonding [19]. Since many of the industrial applications
involve use of metals and alloys, welding as a joining process has gained popularity
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over other methods, as it is efficient, economical, reliable, rigid, repairable, and can
be performed quickly [20]. Other methods of joining processes such as solid state
welding though may yield good strength among metals or alloys but it is expensive
and demand highly cleaned surfaces [21]. It is limited to specialized applications such
as in aerospace and automotive industries. On the other hand, brazing, soldering and
adhesive bonding though are comparatively cheaper, the joint strength is low [22].
Joints formed by mechanical fastening are a result of friction or mechanical
interlocking. These joints are best suited for components which involve frequent
disassembly for maintenance purposes. Creating a mechanical joint involves
additional manufacturing process like hole-drilling, reaming, bolting, washer / rivet
placing, tightening / hammering. These methods are used only in sheet metal working
where minimum loading conditions apply [23]. Among all joining processes, the arc
welding is the most preferred method for heavy static and dynamic structures due to
their rigid and good bond between the structures.
1.3 Novel Welding Techniques
Every day industries develop new manufacturing designs or concepts to replace older
version to improve their functional performance and also to increase specific strength
of the structure [24], Presently, conventional welding such as arc welding is used in
more than 75 % of the times in all type of industries due to simplicity and cost
effectiveness [25]. The new design of machine structure with complex joints, use of
dissimilar alloys for diverse environment has made conventional welding techniques
obsolete and hence is unable to fulfill the service requirements [26].
1.3.1 Underwater Welding
Several researchers suggested Laser Beam welding (LBW) technique to repair
underwater offshore structures, cross continental pipelines by any of the two
techniques such as dry [27-29] and wet underwater welding [30] technique. In wet
underwater welding LBW is preferred due to low heat input, high cooling rate, small
heat affected zone and lower residual stresses [31]. Xudong Zhang et al have
suggested using LBW to repair underwater structures as the input power is transmitted
through optical fiber and hence welding is flexible and location of weld is precise
inspite of problems like effect of water on weld metallurgy and absorption of water by
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laser beam [32]. Other researchers [33-35] endorsed wet water welding using self
shielding flux cored arc welding (FCAW) due to its low cost compared to other
welding techniques wherein extra engineering vessels and shielding gas has to be
installed at the time of welding [36]. On the other hand Nixon et al [37] have
developed dry hyperbaric chamber to overcome the difficulties such as influences of
arc behavior, metal transfer, welding metallurgy and mechanical properties. [38].
1.3.2 Welding in Space
Welding in space presents unique challenges such as microgravity and vacuum. Paton
et al[F39] through their studies have concluded that vacuum condition in space are
favorable for welding process since there are no problems such as base metal
oxidation and absence of other contaminants in the atmosphere, hence base metal
covering by flux and inert gas are also not required. Though Milo Nance et al[40]
proposed both electron beam welding (EBW) technique and laser beam welding
(LBW) technique for space welding applications both do not require filler metal (low
payload for the space flight) but the laser beams are inherently dangerous as laser
pointing some other direction can harm other structures or welder himself. Also,
power requirements for LBW process is more than EBW. McKannan and Monroe et
al [41-43] studied microstructure of weld joint of three materials such as tantalum,
SS304, and 2219 T87 aluminum after welding by EBM technique in space and
reported that all the weldments showed significant grain refinement compared to the
same metals being welded on earth. Also in pure metals like tantalum, no solid phase
transformation was noticed. Block-Bolten et al [44] have suggested gas tungsten arc
welding or tungsten inert gas welding (GTAW/TIG) technique for space
environments but maintaining constant gas supply through compressed gas cylinders
(excess payload) for arc stability will be a problem.
1.3.3 Friction Stir Welding
The inherent problems in fusion welding of metals such as resolidification, formation
of secondary phases, porosity, hydrogen embrittlement and liquation cracking may
not be fully eliminated in spite of taking great care [45]. Hence, friction stir welding
(FSW) is preferred as the alloy is not melted during the process, joints with lower
distortion and lower residual stresses can be obtained. Hence the mechanical
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properties of the joint are superior to other joining processes. Thomas and Dawes [46-
48] have successfully demonstrated welding of similar and dissimilar metals by
means of FSW. A non-consumable rotating tool with pin and shoulder is used for
joining in FSW. Various researchers [49, 50] have shown applications of FSW in
many fields of engineering such as aerospace, automotive and defense applications.
1.3.4 Electron Beam Welding
Electron beam welding (EBM) is used for fabricating structures which have to pass
stringent quality, strength and joint reliability tests [51, 52]. Several researchers [53-
58] have reported good weldability between similar and dissimilar metal joints due to
its highly dense and sharply focused beam of electrons (100-kV, 10-mA). Cam et
al[53] have showed that joining rate is higher in EBM welding technique compared to
other fusion welding techniques [54]. Its narrow heat affected zone (HAZ) [55],
minimum distortion[56,57] and welding in vacuum [58] eliminates the problem of
oxidation in the weld joint.
1.3.5 Laser Beam Welding Technique
Several studies [59-61] have reported use of laser beam welding technology in joining
new materials in the field of automobile and aerospace. Weichiat et al[62] in their
study on laser welding of martensitic steel have reported the fine quality weld seam
deep penetration, high speed and small heat affected zone attributed to its low heat
input per unit volume, high density power, sharply focused beam and delivery of
beam at the precise position by optic fiber.
1.4 Welding in Closed Place
Several researchers have discussed the different types welding used to weld or repair
structures within enclosed space such as interiors of boilers, ships and submarines.
Welding techniques such as gas metal arc welding/metal inert gas welding
GMAW/MIG [63, 64], laser beam welding [65, 66], submerged arc welding
(SAW)[67], flux cored arc welding (FCAW) [68], shielded metal arc welding
(SMAW)[69] and others are used. Lesnewich et al [70] have reported good quality
weld joints in primary tubing of power plant made of Cr-Mo steel by MIG welding
technique with gas mixtures of He, Ar, and COj. The productivity of the welding is
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also high in MIG welding. Missori et al [71] in their findings have reported the use of
transition joint Ni-Cr-Fe filler metal to obtain good quality weld between austenitic
stainless steels and ferritic low alloy steels by means of pulsed laser beam welding in
the main steam lines of thermal power plant. Kabir et al [72] have proved that good
quality welds free from porosity were obtained when welding Ti alloy by laser
welding technique. Kanjilal et al [73] have reported that in SAW welding of low
carbon steel, mechanical properties of the weld primarily depended on the welding
parameters and quality of the flux used to submerge the electrode. Cary et al [74] have
reported FCAW as one of the popular welding process among fusion welding
techniques due to its high productivity, good quality welds, low fume generation,
effective utilization of electrode materials, low distortions and residual stresses. Raja
et al [75] have reported the applications of FCAW welding in steel fabrication, public
works (e.g. bridges), naval works, boiler making, tube/pipe welding, heterogeneous
assemblies, and others. Cary et al[76] have reported SMAW as simplest in terms of
equipment requirements among all the welding techniques and is used for all types of
manufacturing, construction and repair/maintenance type of applications. Technical
guide from Hobart Brothers Co [77] reports though primarily the welding in SMAW
is dependent on the skill of the welder, by careful control of welding parameters, good
quality welds can be achieved and was demonstrated in the construction of submarine
pressure hull sections and high-pressure oil/gas pipe lines.
Several researchers [78-79] have reported scope of closed welding techniques, quality
of the weld joints, its effect on the mechanical properties and ongoing effort to further
improve the quality of the weld but very few researchers have made attempts to
quantify and document the various types of weld fumes and its adverse effects on the
welders and the people in the welding area vicinity. As per the estimates of Bureau of
Labor Statistics (US) [80], worldwide, there are more than two million welders
working in hazardous conditions. Zimmer and Biswas et al [78] reported the
formation of gaseous and aerosols byproducts generated from the electrode or flux
materials at the time of welding which react with air and become particles of
respirable size. These airborne particles, when inhaled cause respiratory disorders and
affect the health of the welders by causing asthma and in some conditions even lung
cancer. Occupational Safety and Health Administration (OSHA) in its annual report
[81] have suggested use of safety equipments such as gloves and mask during
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welding, but these equipments affects the welder’s productivity. Other solution
proposed by OSHA is by providing exhaust systems. Though exhaust systems protect
the welders from harmful fumes, such solutions become expensive and impractical
when the welding is done in closed or interiors of a ship or a power plant. Wittczak et
al [82,301] have reported the presence of heavy metals such as Cr, Mn, Ni, Fe in the
welding fumes and they are responsible for causing respiratory disorders such as
metal fume fever, bronchial asthma, chronic obstructive pulmonary disease,
pneumoconiosis and lung cancer.
Inspite of the advances in welding, there are equal number of problems to be
addressed in both open and closed environment welding processes. Technical notes of
Miyachi Unitech Ltd (US) [83] reports five prominent sources of problems in
welding. They are
1. Weld materials
2. Electrode materials
3. Welding technique
4. Power input
5. Fumes.
Wrong welding technique [84], wrong selection of electrodes and base materials [85]
leads to weld defects such as cracks, porosity, oxidation, distortion of the weld joint,
brittle welds, residual stress accumulation along the weld joint, harmful fumes
generation etc. In case of harmful fumes generation, installation of exhaust systems at
the place of welding does not provide 100% removal of poisons gases. Hence in the
present research work effort to reduce the adverse effects of weld fumes has been
addressed specifically carcinogenic hexavalent chromium fumes generation during the
welding of stainless steel is attempted.
1.5 Different Types of Welding Electrode
1.5.1 Coated Electrodes
As per American Society of Materials (ASM) handbook vol 6 [86] welding electrodes
are classified broadly as non consumable and consumable electrodes. Technical notes
by ESAB [87] have defined non consumable electrode as an electrode which does not
become part of the finished weld but is used to strike an arc and melt the filler and
base metal such as in case of TIG welding technique, whereas [87] consumable
electrode is defined as an electrode which becomes part of finished weld such as in
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case of SMAW, MIG, etc. Consumable electrode can be further subdivided into
coated electrode and bare or wire electrode. Based on the coating, electrodes (for low
and mild alloy steel) can be classified into four types.
1) Cellulosic electrodes
2) Rutile electrodes
3) Iron oxide silicate
4) Basic electrodes.
As per welding guide of Bohler [88] it is reported that, 1) Gas shielding cellulose
electrodes contain 30 per cent of organic material such as alpha flock, wood flour or
other cellulose. At the time of welding, voluminous gas shield of H2, CO and CO2
gives good protection for the molten weld metal, hence good weld metal properties,
excellent penetration, suitable for all welding positions and requires direct current
(DC). As per the welding electrode product information from Sandvik materials
technology centre, it is reported that, 2) Rutile electrodes [89] contain 50% Titania or
Ti02, and is known to give good arc stability, low operating voltage is low, and can
be used with alternating current (AC). At the time of welding, hydrogen, oxides of
carbon, nitrogen, together with an acidic slag protect the weld from the air
contamination. Other advantages include easy slag control, low spatter, medium
penetration and high deposition rate, good mechanical properties, good weld
appearance but low ductility. As per the training materials information from Lincoln
electric company it is reported that, 3) Iron oxide silicate coated electrodes [90] apart
from welding give low gas shielding but high supply of acidic slag which results in
intense slag metal reaction. Due to high oxygen availability and low carbon in the
weld region, the strength of the weld is good and ductile. The electrodes can operate
for both AC and DC. Fluidity of the slag being more, the welding in vertical position
is difficult but nevertheless the weld profile obtained at the time of weld is concave
and good in appearance especially for fillet weld. Other advantages are high
deposition rate, good penetration and low spattering, but low notch ductility. 4) Basic
electrodes [91] also called as low hydrogen or lime-ferritic or lime-fiuorspar
electrodes have high limestone (CaCOs), fluorspar (Cap2) contents. Also present are
clays, asbestos and other minerals which are mixed with very minimum contents of
water to ensure very low hydrogen contents in the weld deposit. Some of the
advantages of using such type of welding electrodes are good ductility, high notch
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toughness, good weld soundness is good, high resistance of the weld to hot and cold
cracking and hence suitable for welding higher strength steels such as high carbon or
high sulphur containing steels. It is less sensitive to plate quality.
1.5.2 Wire or Bare Electrodes
As per the technical information brochure of CMW metallic company [92] and hand
book of welding from ESAB [87] it is reported that many or several non ferrous
materials welding is usually done by TIG welding and electrodes used are bare or
wire electrodes. Various manufacturers around the world manufacture bare wire or
welding electrode alloy material suitable to weld that particular alloy, such as copper
wires with alloying elements Si to weld bronze, copper-silicon, copper-phosphorous
and copper-aluminum alloys. As per ESAB hand book [87] on welding it is reported
that aluminium wire with presence of alloying elements like Fe, Cu, Mn, Si, Zn, Cr,
Ni, Ti, Zr, Li, Pb is used to weld their respective aluminum alloys grades ranging
from Ixxx to 7xxx . The weld strength and ductility are found to be good. Varying
diameter of wires from 1-5 mm welding wire or rods are used to weld aluminum
alloys. As per the welding alloy group product catalogue [93] nickel based welding
electrodes such as ENi6182, ENi6082, ENi6625 are used to weld Nickel based alloys
such as Udimet, Inconel 738L, Hastelloy A2, Nimonic alloys, waspoly etc. As per the
author’s opinion, the weld strength depends on various factors like alloy elements
addition, pre and post heat treatment of the weld plates and electrodes wires. The sizes
of the wire to be used for specific application are available in the product catalogues
of the different manufacturers. The welding strength obtained is high, ductility of the
weld joint and resistance to stress corrosion cracking is also high.
1.5.3 Tubular Electrodes
Many manufacturers produce tubular electrodes primarily for hard facing purpose. As
per the catalogue of Inter group [94] of companies, tubular electrode come in different
sizes starting from 6 - 1 2 mm dia. Specific type of tubular electrodes are
recommended for specific substrate. As per the authors [94, 95] claim, such tubular
electrodes can deposit at high rate carbide particle on any given substrate such as mild
steel, cast iron, stainless steel, etc. The mix of matrix (austenite, martensite matrix)
and carbide particles gives the hard facing wear resistant and toughness for external
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Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steel
impact loads. Other advantages reported by the authors [94, 95] are low dilution rate,
high alloy concentrations, little or no slag formation and no moisture pickup during its
storage.
1.6 Welding Rod Manufacturing
Several welding electrode manufacturing firms such as ESAB[96], AdorFontech [97],
L&T [98], Lincoln [99] have proposed almost similar methods to manufacture
different types of welding electrodes.
1.6.1 Tubular Welding Electrode Manufacturing
Lincoln Global Inc in its patent No “US7807948 B2” [100] has proposed a
methodology of manufacturing tubular welding electrodes. As per the patent
document [100] the manufacturing process involves a sheet of metal of appropriate
width so as to obtain a tube of desired diameter which is first straightened and
flattened. The sheet is then bent into a V-shape or U-shape with the help of roller
channels. Fill materials are poured on the V/U bent sheet. The sheet is then closed to
form a circular cross section and finally is tube drawn. The drawing process not only
welds the sheet together but also helps in adhering of the fill material permanently
within the tube. Different fill materials can be poured to obtain different electrodes.
Figure 1.2, shows flow chart of manufacturing process of the same. For the ease of
drawing, lubricants are applied intermittently.
1.6.2 Wire Electrode Manufacturing
Some of the welding electrode manufacturing firms such as ESAB[96], AdorFontech
[97], L&T [98], and Lincoln [99] have developed similar techniques to manufacture
bare wire or wire electrode. As per the manufacturing manuals [96-99] of these
companies, the wire electrode of required alloy is usually taken in the form of a coil
of 6-8 mm diameter and then straightened. Later, the wires are pulled through series
of dies to gradually reduce the wire cross-section by 1mm. After achieving the
required diameter, required lengths of wires are cut and quality inspection is
performed. Wires are checked for its diameter, straightness and for defects formed
during wire drawing. The defective products are recycled based on the type of defects
R V Center for Cognitive Technologies, R V College of Engineering, Bangalore-59.
detected and good quality electrodes are sent for packaging. Figure 1.3 shows the
flow chart of wire manufacturing process.
Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steelfor Naval applications
Figure 1.1: Flow chart of the manufacturing of Tubular welding electrode
1.6.3 Flux Cored Electrode Manufacturing
Eureka Systems and Electrodes Pvt Ltd (ESEPL) [101], a Chennai based firm has
proposed flux cored electrode manufacturing methodology. As per the project report
[101], manufacturing of flux cored welding electrode can be divided into three stages.
First stage involves wire extrusion and straightening process, second stage involves
flux preparation process and third stage involves flux coating on the electrode and
simultaneously extrusion (for adherence of flux to welding rod) followed by baking
and air drying. The flow chart of flux cored electrode making is shown in Figure 1.1.
Intermittent quality checks in between various stages of manufacturing are performed
to achieve high quality welding electrodes.
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1.6.4 Wire Electrode Manufacturing at Research and Development Stage
Some researchers [102] have suggested using wire Electric Discharge Machine
(EDM) for manufacturing of novel welding electrode developed in the laboratory or at
research and development (R&D) stage. As per Li Li et al [103] wire electrode
development at R&D stages usually involves small quantities of welding electrodes to
be produced and hence wire EDM manufacturing is suitable. Though the
manufacturing process is time consuming and expensive, it is free of defects.
Different diameter wires and lengths can also be achieved with good accuracy. Based
on the weldability studies of new electrode and its materials properties, a suitable wire
fabrication method can be evolved. In the present work, wire EDM machine was used
to manufacture novel wire electrodes.
Wire Coil (Quality control (QC) Approved) (5.5,6, 6.5 & 7/8 mm Dia)
Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steelfor Naval applications
Figure 1.2: Flow chart of manufacturing of wire electrode
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Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steel_________________________ for Naval applications_________________________
W ire Coil (QC Approved)Raw Materials (QC Approved)
Ferro Alloys & Ceramic material/ Powders
Figure 1.3: Flow chart of Flux cored electrode manufacturing
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1.7 Welding Electrode Materials Characterization Studies
Various researchers [104, 123, 135, 143] suggested different methods to evaluate
material properties such as X-ray NDT inspection of electrode material, materials
property prediction by simulation techniques and experimental determination of phase
transformation using Differential Scanning Calorimeter (DSC). Destructive
techniques to determine materials properties have also been suggested such as tension
test, hardness tests, and impact tests. Some authors [143] have investigated
microstructure using optical / scanning electron microscope (SEM) / transmission
electron microscope (TEM) techniques to study grain boundaries, primary and
secondary phases of welding materials. Energy dispersion spectroscopy (EDS),
electron probe micro analysis (EPMA) and X-ray diffraction (XRD) are most
prominent methods [144] to determine the chemical composition of micro
constituents in the weld alloys.
1.7.1 Weld Alloy Defects Inspection Methods
Several researchers [104-106] suggested using non-destructive methods (NDT) such
as ultrasonic (UT) and radiography (RT) to check for casting defects of the electrode
material developed by casting technique. Tian Yuan et al [104] suggested X-ray
techniques as one of the most popular techniques used to identify casting defects such
as shrinkage porosity, voids, cracks, blow holes and sand inclusions. Damn et al [105]
used X-ray techniques to identify defects in metal casting / plates of maximum
thickness of 200 mm. Wang et al [106] reported use of x-ray to identify the micro and
macro pores in aluminum castings. Yi Sun et al [107] reported use of fuzzy logic for
pattern recognition to identify weld defects on real time in case of digital real time
radiography technique. Hayens et al [108] have suggested use of X-ray to identify
surface defects (surface cracks) which otherwise was done by other techniques such
as dye penetrate test (DPT), magnetic particle inspection (MPI) and eddy current
inspection technique (ECIT). Antoni et al [109] have developed a new pulse echo
technique based ultrasonic inspection procedure to identify defects in powder
metallurgy based materials. Young-Sang et al [110, 111] reported use of ultrasonic
based immersion and waveguide based sensor for visual inspection of reactor core and
internal components of sodium faced fast reactor. Several researchers [112, 113] have
recommended variety of NDT methods such as DPT, MPI, ECIT, RT, UT for weld
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quality inspection to ensure that the structures meet the design requirements. Among
variety of techniques Gang Wang et al [113] recommended UT and RT tests for
identification of subsurface defects such as crack, porosity and slag inclusion for the
weld joints. Further they [113] developed, via image processing technique, an
automated system for identification of different types of welding defects using fuzzy
k-nearest neighbor and multi-layer perceptron neural networks classifiers. Liao et al
[114, 115] suggested fuzzy reasoning based expert system for identification of weld
defects in radiographic images. Several researchers have suggested using machine
vision for automatic inspection [116, 117] of welding defects, weld defect analysis
[118,119], detection of welding defects [120] and analysis of radiography [121-122].
In the present work. X-ray technique is used to identify the defects in new materials
and weld joint due to its wide acceptability, ease and low cost.
1.7.2 Materials Properties Simulation Studies
Several researchers have opinions that alloy development is expensive and time
consuming [123-127] process. Newer materials are constantly developed to meet
aggressive service conditions such as high temperature, highly corrosive environment,
and to withstand various forces on the materials. Modem alloys [124] have as many
as 15 elements and understanding the interaction among themselves, is too complex to
understand. Alloy designers [125] use thermodynamic based tools such as Thermo-
Calc, FACTSage, MTDATA, Pandat and JMatPro to construct phase diagram,
simulate thermo-physical, mechanical and other materials properties. Fu et al [126]
studied the role of Al contents on formation of y' precipitates and compared it with
experimental studies. Guo et al [127] used JMAT PRO to modeled multi component
material like cast iron, Ni-based super alloy 713 and Aluminum alloy ADC 12 for
properties such as density and volume change with respect to temperature and other
thermo physical properties which are critical for casting simulations. Guo and
Saunders et al [128] have calculated high temperature strength and generated stress-
strain curves for Al A319 using JMATPRO to study the deformation simulation.
Different authors have reported, predicting materials properties through different
softwares, such as Chen et al. [129] reported breakthroughs in thermodynamic
calculations using PANDAT software, Kao et al. [130] simulated thermal properties
of electronic materials. Nataraj et al [131] generated phase diagram and simulated
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mechanical properties of novel Cr-free nickel based material for welding applications.
Other authors [132-134], reported breakthrough in research work due to reliable and
correct prediction of materials properties such as melting temperature, strength and in
many cases the solidification behavior of the alloy. In the present work, JMAT Pro
materials property simulation software is used to predict the properties. The wide
usage of the software in the scientific community for research and development
purpose and availability of nickel database are the reasons for the selection of this
software in the current work.
1.7.3 Experimental Method of Determinination of Phase Transformation Studies
Many researchers [135-142] have suggested several experimental techniques to
determine phase transformations such as XRD, energy dispersion X-ray analysis
spectroscopy (EDXA), EDS, X-ray flourosence spectroscopy (XRP) and DSC. Meng
et al [135] reported the significance of solidification or cooling rate of alloy or weld
joint, as it affects the grain size of the microstructure which in turn has a profound
effect on the mechanical properties. Dobrzanski et al [136] have suggested use of
differential scanning calorimeter (DSC) technique among various thermal analysis
techniques, as the most popular due to its accurate method of determination of phase
change, solidification range and cooling curve during the alloy solidification. Rojas et
al [137] reported identification of different primary and secondary phases in 9% Cr
heat resistant steel, based on the alloy composition through DSC techniques. Young-
Sun Kim et al [138] through DSC studies concluded that addition of some alloying
elements (Bi & C) has an effect on the onset of solidification and range of
solidification on Sn-Zn-Bi alloy system. Seo et al [139] studied the solidification
sequence of different phases in IN792 + Hf using DSC technique. Zhai et al [140]
performed studies on liquidus and enthalpy of fusion in Cu-Sn alloys across all the
composition range. Naffakh et al [141] performed studies on solidification path,
micro-segregation of alloying elements in the interdendritic regions, solidification
temperature ranges, to predict the formation of secondary structures and the
castability behavior of four newly developed as-cast nickel based superalloys using
DSC. Nurveren et al [142] studied the changes in the phase transformation
characteristics at different heating / cooling rates by means of DSC measurements for
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four NiTi alloys. In the present work, DSC method of determining phase
transformation is done due to its accuracy and availability of the equipment.
1.7.4 Microstructure Characterization of Electrode Materials
Several researchers [143-145] have reported microstructure characterization as an
NDT technique to study the relationship between microstructure of an alloy and its
mechanical properties. The study of relationship between the microstructure and
mechanical properties has attracted great interest over several decades [143], owing to
improvement in microscopy technology and demand for tailor made materials. As per
“ASM vol 10: Materials Characterization”, microstructure characterization [144] of
nickel based alloys involve 1) Study of morphology, volume fraction, location and
distribution of primary phases (grain), secondary phases and precipitates by means of
optical micoscopy/SEM/TEM. 2) Identification of chemical composition of phases by
means of EDS and XRD. Various researchers have characterized the alloys for
determining size and volume fraction of primary phases, secondary phases, carbide
phases and deleterious phases using optical microscopy (OM), SEM and TEM.
Several researchers [145-151] characterized the alloys using XRD and EDS to
determine chemical and elemental composition of each microconstituent in an alloy
system. Many researchers [152-158] have characterized the alloy, using SEM, to
study the effect of heat treatment, mechanical loading on the morphology, volume
fraction of different phases and on phase transformation in different alloys. Xuebin et
al [159] and Stevens et al [160] have studied the effect of solution treatment on the
dissolution of y' gamma prime in IN738LC nickel alloy using SEM.
1.7.5 Mechanical Properties Characterization of Alloy Material
Many researchers [161-170] suggested destructive methods to determine the
mechanical properties of electrode material with tests such as tension, compression,
hardness, fatigue, creep, wear and impact. Balikci et al [161] reported the importance
of determining mechanical properties for design of structures which can operate
satisfactorily in severe working environments. Some of the desirable material
properties include high temperature stability and strength, resistance to corrosion,
good impact toughness and wear resistance, good fatigue and creep strength.
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1.7.6 Hardness Testing Studies
WAN Wen-juan [FI62] characterized newly developed Ni-Co AEREX350 for
hardness using for Rockwell test. Nader El-Bagoury reported the effect of aging in
nickel based super alloy IN738LC on the hardness using Vickers hardness testing.
Gerald et al reported mechanical characterization methodology of ATI Allvac 718Plus
a commercial by available nickel based alloy using microhardness tester, by applying
a load of 500g, dwell time 20s, specimen size 10 mm x 10 mm xlO mm, as per
standard EN ISO 10002 using hardness testing machine EMCO-Test, M4C-025-G3M.
Sushanta et al [166] characterized Al-M g-Si alloy for micro hardness measurements
on samples 10 mm x 10 mm x 10 mm using a load of 100 g with indenter dwell time
of 15s.
1.7.7 Toughness Measurement Studies
Shuangqun Zhao et al [163] reported measurement of toughness of inconel alloy for
aged specimens with V-notch at room temperature. Edward et al [168] of special
metals corporation reported impact toughness test methodology for nickel alloys 686,
925, 725 and 725HS using American Society of Testing materials (ASTM) Test
Method E992.
1.7.8 Tension Measurement Technique
Gupta et al [165] reported the methodology of tension tests for Al-Li alloy samples
treated under various heat treatment conditions, with strain rate of 1 mm/min,
specimen size of 120 mm, gauge length 32 mm and gauge diameter 12 using ASTM
E8 standard and INSTRON universal testing machine (UTM). Sushanta et al [166]
characterized Al-Mg-Si alloy by tensile strength, yield strength and ductility at strain
rate of 5X10“ /s using ASTM E8 standard and specimen gauge diameter 12 mm.
Sheldon Winkler [167] et al studied the effect of heat treatment on the mechanical
properties of Nickel Chrome base alloy using Instron UTM at strain rate of
0.5 mm/min. Also determined were material stress at 0.2% and 0.02% offsets in a
stress strain curves, yield strength. Ultimate Tensile strength (UTS) and % elongation.
These experiments were performed for various heat treatment conditions which
mimicked annealing, homogenizing and solution treatment conditions. The test
specimens used were as per ASTM standards A370 with a gauge length of 1 cm.
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1.7.9 Fracture Studies
Several researchers [163, 165, 170] have also reported the fracture studies of the new
alloy developed to understand the mode of failure on tensile tested or impact tested
specimens. Shuangqun Zhao et al [163] reported fracture studies of Inconel alloy 740
using SEM. Fractography studies to determine the presence of secondary precipitates
in the voids of the fractured surface and analysis of the surface texture to determine
the failure mode were also presented. Gupta et al [165] reported the fracture studies of
Al-Li alloy using SEM by means of crack initiation spot, nature of crack propagation
and crack length. Guo-hua et al [170] have reported fractographic characteristics and
mechanisms of the fracture in the near eutectic Al-Si piston alloys. Post failure
analysis was conducted on the fracture surface using a SEM with EDS attachment. In
the present work mechanical characterization of nickel alloy welding materials for
tensile strength, impact and hardness tests and failure mode were investigated.
1.8 Characterization of Weld Joints
Vander Voort et al [171] have reported microstructure studies of weld interface for
dissimilar welding using optical microscope / SEM / TEM. Naffakh et al [172]
reported that dissimilar weld interface microstructure consists of four regions 1. Base
metal (BM), 2. Heat affected Zone (HAZ), 3. Fusion zone (FZ) and 4. Weld metal
(WM). In order to view this microstructure in optical microscope and SEM, ASM
Volume 9 - Metallography and Microstructures [171] has reported detailed procedures
to prepare specimens for microstructure studies. As per Vander Voort [173], specimen
preparation method is classified into three steps 1.Grinding, 2.Polishing, 3.Etching.
Several authors [174-181] have suggested different reagents such as Marbles reagent,
70 ml H3PO4 + 30 ml H2O, Villela’s reagent, 10% perchloric acid and 90% methanol
etc, as etchants to selectively etch specific type of micro constituent of an alloy. Some
of the authors [182-185] have also suggested electrolytical etching, as the process is
quick and not much specimen preparation is required (such as grinding and
polishing). Vander Voort has [171] reported specimen sizes of the weld joint required
for microstructure characterization to be about 25 mm x 25 mm x 6 mm.
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1.8.1 Effect of Welding on Base Metal Characterization
Many researchers [186-188] have reported characterization of base metal portion of
the weld joint using optical microscope, SEM, TEM and focussed ion beam (FIB)
using SEM [186]. They have also reported the chemical analysis of the alloy such as
primary phases, secondary phases and carbide precipitates using XRD, EDS, X-ray
elemental mapping, and electron probe micro analysis (EPMA) techniques [187], Lee
et al [188] have reported the characterization of base metal stainless steel SS304L
using optical microscope/SEM for grain morphology, presence of secondary
precipitates (carbides) in grain boundaries, annealed twin and presence of ferritic
phase. Bala Srinivasan et al [189] have reported the characterization of duplex
stainless steel (of grade IINS 31803) for determining amounts of austenitic phase and
ferrite phase. Also characterization of carbon steel (of grade IS 2062) for amounts of
ferrite and pearlite phases and grain morphology using optical microscope was
undertaken. Ravi Shankar et al [190] have reported characterization of two base metal
Zircaloy-4 and SS304L for grain morphology and presence of secondary precipitates.
Some researchers have also reported the characterization of heat effected zone (HAZ)
using optical microscope and SEM. Naffakh et al [172] have reported the changes of
grain morphology of the HAZ due to heat transfer from weld pool at the time of
welding using optical microscope. Lee et al [188] have reported the changes in the
grain boundary chemical composition due to diffusion of some elements at the time of
welding using EDS. In conclusion, it was clear that base metal characterization is very
essential to use OM and SEM for morphological studies and XRD for chemical
analysis of micro constituents. Hence in this research work all base metals are
characterized for both microstructure and chemical anysis done using OM, SEM,
XRD and EDS.
1.8.2 Fusion Metal Characterization
Several [172, 187, 188] researchers have suggested characterization of the weld
region as important part of weldability studies as it corelates the structure and
mechanical property of the weld joint. In this direction Belloni et al [191] studied the
a-chromium principal precipitates in HAZ and FZ of dissimilar weld joint between
Inconel 657 and stainless steel 310 using EDS and SEM. Caironi et al [192] reported
segregation studies of elements like Nb in interdendritic regions of FZ of dissimilar
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weld using SEM and EDS. Dupont et al [193] have reported calculation procedure of
determining dilution levels of electrode material in base metal using optical and SEM
with the expression as shown in Eq. 1.1. This is done by measuring individual
geometric cross-sectional areas of the deposited filler metal and melted base metal.
Dilution of the weld joint is given by the ratio of the melted base metal (Abm) to the
total melted cross sectional area of the filler metal and base metal (Abm+ Afm)
combined
A .
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D = bm
fin
Various authors [172, 187, 188, 190] have reported weld microstructure for base
metal, FZ, and WM in case of dissimilar welding using optical microscope / SEM and
TEM. Several researchers [194-197] have reported carbon migration studies from
lower Cr base metal to higher Cr base metal in a dissimilar welding between carbon
steel pipe and SS304 using EDS attached to SEM equipment. Changheui et al [198]
reported dendrite structures, their location, direction and spacing in the FZ of
dissimilar welding between low alloy steel and SS 316 stainless steel using optical
microscope. Sireesha et al [199] studied recrystallization with extensive grain
boundary migration behavior of FZ in dissimilar welding between 316LN austenitic
stainless steel and alloy 800 using optical microscope. Devendranath et al [200] have
also reported the microstructure studies of dissimilar welding as a NDT technique to
identify welding defects such as porosity, cracks, segregation, formation of secondary
precipitates, dilutions using SEM, OM studies. From the above literature survey, it is
evident that microstructure study of the weld interface provides valuable information
about the structure which can be correlated to mechanical properties. Hence in the
present work optical microscopy and SEM are used in the characterization of weld
interface.
1.8.3 Weld Joint Preparation for Mechanical Studies
Many researchers [199] have reported mechanical testing of dissimilar weld joints as
an important characterization technique to study the weldability of dissimilar weld
joints. Sireesha et al [199] have reported tension tests, micro hardness tests across the
weld joints, impact tests, longitudinal varstraint testing and three point bend tests as
some of the important tests to evaluate mechanical properties of dissimilar weld
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joints. Sireesha et al [199] further reported that nickel based electrodes 316, 16-8-2,
Inconel 182, and Inconel 82 were used to weld with two base plates (SS316LN and
alloy 800) of 12 mm thick with double V groove edge preparation and included-angle
of 70°. Bala Srinivasan et al [189] have reported using Duplex stainless steel (DSS)
electrode E2209 and austenitic SS electrode E309 to weld 5 mm thick plates of DSS
of UNS31803 grade and boiler grade plain CS IS 2062. They further reported that two
base plates of dimensions 200 mm x 80 mm x 5 mm were used for producing the
joints with a 70° single V groove edge preparation followed with a root face/gap of 2
mm. Lee et al [188] reported using several grades of Inconel Welding Electrode of
152 series with gradual increase of Ti percentage to weld two base plates (SUS 304L
and alloy 690) of dimensions 80 mm x 70 mm x 6 mm. The beveled test plates to be
welded together were arranged to form an 80° V groove with a 3.2 mm root opening
gap and a 1 mm root face. To achieve good quality welds, many researchers [188,
189, 199] have reported solution annealed treatment for base plates before welding.
Sireesha et al [198] have reported the welding parameters as current = 100 A, voltage
= 11.5 V and travel speed = 4.2 mm/s. Naffakh et al have reported the welding
parameters in the range of 95A-135A, 12-26 volts, 0.77-3 mm/s welding speed.
0.83-2.3KJ/mm heat input for various combinations of weld electrode and base metal,
also heat input calculations were done using the equation E W , where E= welding
voltage, I is the current, V is the welding speed. Ravi Shankar et al [190] have
suggested to wire cut the specimen required for various tests from the main welded
plates. In the present work weld plate dimensions and welding parameters were
selected as per respective ASTM standards.
1.8.4 Tensile Studies
Several researchers have reported tension test as one of the important mechanical tests
to characterize mechanical properties such as UTS [172], yield strength [188] and
ductility [178] of the weld joint. Specimen sizes generally preferred for tensile testing
are 140 mm x 25 mm x 1.5 mm with a weld bead width ranging from 2.4 to 3.3mm
[202], button-head type cylindrical all-weld and transverse tensile specimens (28.6
mm gauge length and 4 mm gauge diameter) [201], test specimens machined and
prepared as per ASTM E-8 standards with gauge diameter of 7.5 mm and gauge
length of 72 mm [190], a nominal specimen width of 30 mm and mandrel diameter of
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91 mm [203] and also with specimen dimensions of length 140 mm, gauge length 60
mm, plate width 20 mm and plate thickness 6 mm, root gap opening 3.2 mm [188]
were also reported. The tests were generally conducted at room temperature at various
strain rates of 5.4 x lO s [198], 1.2 mm/min [172], 2.4 x lO /s [201], Taban et al
[203] have also reported tensile tests carried on longitudinal section of the weld joints.
Ravi Shankar et al [190] have reported three point bend test on specimens machined
and prepared as per ASTM standard with 8 mm diameter and 120 mm long, load
applied exactly at the interface supported at a distance of 60 mm apart from the
center. Naffakh et al [172] reported conduction of varstraint tests to determine hot
cracking susceptibility of dissimilar weld at various strain rates of 1%, 2% and 4%. In
the present work, tensile test procedures as per ASTM E-8 standard were followed.
1.8.5 Hardness Studies
Several researchers [204,302,303] have suggested hardness tests across the dissimilar
weld joints to report microstructural heterogeneities. Xiu-Bo Liu et [204] have
reported microhardness test as one of the important property for wear and abrasive
dynamic structures. Magnabosco et al [205] have reported the variation of hardness
across weld joint due to the effect of dilution of electrode materials with base metal
and effect of several passes of welding. Bala Srinivasan et al [189] have reported the
presence of harder microconstituents, such as carbide precipitates and martensite
through hardness measurement in FZ of a dissimilar welding between Unified
numbering system (USN) 31803 and (International systems) IS 2062 steels. Several
researchers [201, 199,304] have reported Vickers hardness testing as popular hardness
measurement techniques and suggested specimen sizes of 30 mm x 10 mm x 6 mm
[201], load of 500 g [201], lOOOg [205], 2000g [189] and dwell time of 20s [201], 30s
[205] as procedures for hardness testing. In the present work, hardness of the weld
zone was determined by applying a load of 500 g and 30s dwell time was considered.
1.8.6 Impact Studies
Several researchers [172,189, 198, 206] have reported impact studies as one of the
characterization techniques for dissimilar weld joint. Sireesha et al [198] have
reported room temperature toughness for a dissimilar weld joint between 316LN
austenitic stainless steel and Alloy 800 using impact studies. Bala Srinivasan et al
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[189] and Muthupandi et al [206] have studied the low temperature (-40°C) impact
toughness and room temperature impact toughness on dissimilar weld joint between
IS 2062 and UNS 31803. Naffakh et al [172] have studied the room temperature
impact toughness of dissimilar joint between AISI 310 austenitic stainless steel to
nickel-based alloy Inconel 657 using Charpy impact tester. Taban et al [203] studied
the effect of notch (2 mm deep) position, such as on the weld metal, HAZ, face and
root side of the weld joint at -20°C and at room temperature for a dissimilar weld joint
between 12% Cr modified Ferritic Stainless Steel and Carbon Steel S355. For impact
tests several researchers have used standard specimens sizes of 55 mm x 10 mm x 10
mm [178, 198, 203], and sub-size specimens (3 x 10 x 55 mm^) [189] using charpy
impact tester machine. In the present work standard specimen sizes at room
temperature using charpy impact testing machine was used for studying the impact
toughness of various weld joints.
1.8.7 Fracture Studies
Several researchers [207-209] have reported fracture studies of the weld joints to
study the mode of failure using SEM. Li and Congleton [207] have reported the
fracture in the dissimilar welding joint between low plain carbon steel and austenitic
stainless steel due to carbon migration from low carbon steel to austenitic stainless
steel in a simulated corrosive environment using SEM. Joseph et al [208] have
reported a case study of an in-service failure of a dissimilar weld joint in a steam
generator using SEM. Bala Srinivasan et al [189] have studied crack propagation by
impact testing machine on a dissimilar weld joint through optical microscope and
SEM. Liang et al [209,305] studied the fracture surfaces of Ni-Cu and Ni-Cu-Ru
electrodes welding with SS304L using SEM and OM. The authors reported ductile
modes of fractures with its characteristic dimples formation in the HAZ region of the
base metal which were exposed to air, but weld samples exposed to corrosive fluids
like NaCl showed low ductile mode failures or underwent stress corrosion cracking
due to embrittlement of the base metal in the corrosive fluids. Lee et al [188] reported
the fracture studies of tensile tested specimens of a dissimilar welded joint SS304L
and alloy 690 using SEM. They further reported ductile mode of failure at the fusion
zone of the weld joint. Changheui et al [198] reported ductile mode of failure with
presence of dimples on fractured surface from the weld specimens taken from the
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bottom portion of a 40 mm thick weld joint where as top portion of the weld joint
specimens showed shear-stretch mode of failure with high toughness values. The
dissimilar weld joint between low alloy steel and stainless steel was welded by
Inconel 82/182 filler metal and fracture surface was studied under SEM at different
magnifications such as lOOX and 500X [198]. In the present work fractured surfaces
of the dissimilar weld joint was analysed by SEM at magnification of lOOX, 500X etc.
1.9 FEM Welding simulation Studies
Various authors [210-222] have reported Finite Element methods (FEM) based
welding simulation techniques which are in agreement with experimental studies.
Various authors [210-222] worked FEM based weldability studies on the following
parameters
1) Temperature distribution around the weld joint,
2) Type and magnitude of longitudinal and transverse residual stress distribution
around weld joints and
3) Magnitude of the distortion of the weld joint.
Many investigators have compared analytical with and experimental methods to study
and predict welding residual stresses. With the advancement in computer technology
and numerous improvements in numerical techniques, analyzing residual stress in
welded structures using FEM has gained popularity. Earlier, Norton and Rosenthal
[210, 211] measured residual stresses by the X-ray diffraction technique. Also Pange
and Pukas [213] presented hole-drilling and strain gauges to determine residual
stresses. Cheng et al, [212] investigated the residual stresses due to surface treatment
using the compliance method which can measure a rapidly varying compressive stress
under the interface which even the X-ray technique would fail to detect. Also, Muraki
et al, [214] developed FEM based elasto-plastic model to predict thermal stresses and
heat source movement to simulate welding. Kuang and Atluri [215] used a moving-
mesh finite element technique to study the effect on temperature field due to a moving
heat source. Shim et al, [216] developed an analytical method for predicting
distribution of residual stresses due to multi pass welding across the thickness for a
thick plate using FEM. Chidiac et al, [217] presented the iterative procedure to
determine the thermal cycle required for different types of welding simulation through
non-linear heat transfer analysis. Josefson [218] estimated residual stresses in a multi
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pass weld and in a spot-welded box beam with SOL VIA and ABAQUS commercially
available FE-codes through non-linear analyses. Yang and Xiao [219] proposed an
analytical model to examine the residual stress distribution across the weld of panels
welded with mechanical constraints. Furthermore, Ueda and coworkers [220-222]
presented a novel measuring method of three-dimensional residual stresses (based on
the principle which is simplified by utilizing the characteristics of the distribution of
inherent strains induced in a long welded joint). Residual stresses during welding are
unavoidable and their effects on welded structures cannot be disregarded. Design and
fabrication conditions, such as the structure thickness, joint design, welding
conditions and welding sequence, must be altered so that the adverse effects of
residual stresses can be reduced to acceptable levels. In this work, the residual stresses
are determined during one-pass arc welding in a steel plate using ansys finite element
techniques.
1.10 Experimental Determination of Residual Stress Using XRD
Many researchers [223] have reported residual stresses in welded structures promote
brittle fracture, fatigue, stress corrosion cracking and reduction in buckling strength
[224] of base plate. Several researchers [223, 224, 225, 226] have reported destructive
and non-destructive methods of measurement of residual stresses. Destructive method
of residual stress determination was done by Pang and Pukas et al [224] by a
technique called stress-relaxation methods using strain gauge techniques. NDT
approach followed by Chang and Teng, et al [225], calculated residual stresses using
finite element methods. Chandra et al [226] in the weld bead measured residual stress
using XRD techniques. They [225, 226] reported XRD method as an accurate method
to measure the residual stress in a thin surface layer. As per Robbins [227], residual
stress changes the atomic spacing of the weld joints. Such stressed materials change
the angular position of the diffracted X-ray beam due to change in atomic spacing
which in turn is the measure of residual stresses. Joseph et al [228] determined
residual stress using X-ray diffraction technique for a dissimilar weld joint between
2.25Cr-lMo ferritic steel and (American Iron and Steel Institute) AISI type 316
stainless steel with and without Inconel-82 buttering on the ferritic steel side. Spiess et
al [229] have reported the calculation method of determination of residual stress using
X-ray diffraction technique using sin v(/ -method. B. Chen et al [230] compared
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experimental results (XRD technique) with numerical algorithm (FEA simulation) of
residual stress distribution in the welding slot area of a Cr21Ni6Mo9 stainless steel
pipe. Omar Hatamleh et al [231] applied XRD technique to determine residual stress
distribution on a surface of FSW sample of aluminum alloy 2195 and 7075 after it had
undergone surface treatment. They further reported that small sample size of 10 mm x
10 mm X 10 mm is required for this investigation. In the present work X-ray
diffraction technique was used to determine the residual stress in different type of
weld joints as this technique is reported to give accurate measurements.
1.11 Applications of Dissimilar Welding
1.11.1 Power Plants
Modem power plant boilers operate at 1050°C and make use of two or more materials
/alloys. Outer pipes made of comparatively low cost stainless steel 310 are joined to
pipes made of high cost Inconel 657 [232]. Though pipes made of Inconel 657 are
expensive, they are known to be stable at high temperatures, resistant to highly
oxidizing and carburizing environments.
1.11.2 Nuclear Power Plants
In fast breeder nuclear reactor ferrite and austenite steel tubing is joined by a
transition joint such as nickel based alloy 800. Ferrite steel P91/T91 is a precipitate
strengthened steel used for modules where high temperature and superior creep
properties are the requirements and SS310 is an austenitic structural steel suitable for
sodium environment [233-237]. Also, the transition joint alloy 800 provides a
gradation of Co-efficient of Thermal Expansion (CTE) between two base plates
thereby providing a better distribution of thermal stresses. Zircaloy-4 is dissimilar
welded with AISI type 304L by friction welding [238]. Zirconium and its alloys
exhibited outstanding corrosion resistance in nitric acid environment. In fast reactor
reprocessing facilities, zircaloy-4 is used as a candidate material for dissolvers and
evaporators [239-244].
1.11.3 Earth Moving Excavators
The buckets of the excavators are fabricated with low alloy steel due to its low cost,
better toughness and strength but have low resistance to corrosion. During excavation
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R V Center for Cognitive Technologies, R V College of Engineering, Bangalore-59. 26
of the earth the teeth of the excavator buckets are exposed to moisture and mineral
salts in the earth and hence prone to corrosion and abrasion. By weld overlaying
(cladding) the teeth with nickel based alloy, life of the teeth is not only prolonged but
rendered corrosion resistant [245].
1.11.4 Cryogenic Applications
Normally 8-9% Nickel based steel such as SS308L and SS316L are used for
cryogenic applications due to their balanced austenitic structure. For cryogenic
applications, weld joint with 156 MPa fracture toughness is the requirement. Joints
obtained with special electrodes containing high Nickel (25%) and high nitrogen
(0.18%) contents are free of delta ferrite and non metallic inclusions such as carbides
and nitrides [246].
1.11.5 Other Dissimilar Welding Applications
Sun et al [247] have reported that industries adopt dissimilar welding as it is
advantageous both economically and technically. They also reported that dissimilar
welding provided possibility of flexibility in design as each material can be used
efficiently and benefit in functional way due to their specific property. They further
added that, materials possessing large variation in properties create problems such as
residual stress, cracks etc, in weld joint but efforts are on to mitigate this problem.
Several researchers have studied dissimilar joints as it finds applications as a
transition joint [248] which accommodates the co-efficient of thermal expansion
between two materials with widely varying CTE properties. Bhaduri et al [249] have
done a comparative study between two, nickel based filler materials alloy 800 and
Inconel 182 welded between ferrtite steel and 9Cr-lMo steel. The study was done to
select the best filler material among the two as weld joint failed in service due to
thermal stresses along the weld fusion line. Several researchers [250-255] have
recommended use of nickel based filler materials to weld ferritic steel and austenitic
steel as they prevent carbon migration from ferrtic steel side towards austenitic steel
thus avoiding degradation of the properties. Due to its wide application and major
benefits, in the present research work, it was decided to use nickel based Cr-free
welding material as filler alloy to weld stainless steel.
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1.12 Role of Alloying Elements in Welding Electrode
Many researchers [256-258] have studied the role of alloying element on the weld
alloy as it affects the weld quality in terms of mechanical strength, ductility and
corrosion reistance. Several authors [256,265] have reported that the selection of
dissimilar welding electrode is based on the base metals to be welded and the alloying
element present in the welding electrode. Choi et al [260] have reported that alloying
elements have a direct effect on the microstructure and thereby on mechanical
properties. Many researchers [256-257,261] have reported nickel-based weld filler
metal majorly consists of elements such as Fe, Mn, Mo, Co, Nb, C, B, Al, Si, Cu, and
minor content of S, P, Pb. Investigations of Scrimgeour et al and Shoemaker et al
[257, 268] have shown that adding of C and Al has a certain graphitizing effect on the
partially fusion zone (PFZ) of arc welded ferritic ductile iron. The strength and
elongation of weld metal decreases with more addition of carbon content (0.2 to
2.91%) and Al promotes precipitation strengthening. Further, they also reported that
width of the carbide layer decreases with the increase of Al content in the weld.
Addition of Ti content helps to improve the mechanical properties of weld metal.
Various authors have reported [256-257, 260, 262,264] the effect of Ti. They reported
that Ti forms carbides (TiC) which improves the strength and elongation of the weld
metal in stainless steel. Many researchers [257-259,261,263-265, 268-269] have
suggested use of Fe with Ni to reduce overall alloy cost. They also reported that Fe
additions in Ni not only provides improved resistance to H2SO4 (sulphuric acid) when
concentrations are greater than 50% but also increases the solubility of carbon, which
improves resistance to high-temperature carburization. Daniel et al [266,271] reported
that though increase in copper content increases corrosion resistance of nickel alloy,
its segregation in interdendritic region promotes solidification cracking. Numerous
authors [257, 258-259,261, 263-265,268-269] have reported that presence of iron
increases the weld metal strength due to solid solution strengthening. They further
stated that, if Fe content is less than 20%, fusion of the weld metal to base metal will
be weak due to segregation of Fe to inter-dendritic cores and thereby increases the
hardness of the transition zone. Several authors [257,262,268] have recommended
addition of manganese to nickel alloy benefits the weld metal significantly, such as
improvement in ductility of the weld joint, improved resitance to weld cracking,
formation of much stable compound MnS compared to FeS thereby mitigating the
Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steelfor Naval applications __________
R V Center for Cognitive TeiJinologies, R V College of Engineering, Bangaiore-S9. 28
harmful effect of sulfur in stainless steel welding. Adachi et al and Shoemaker et
al[257, 264] have investigated and reported the effect of various levels of Titanium
addition on the weld microstructure, fusion zone (FZ) microstructure and mechanical
properties of dissimilar nickel-based alloy 690 and SUS 304L SS weldaments. They
further reported the beneficial use of addition of Ti on the increase in hardness of the
FZ, Ti-Fe compound rather than pure Ti-powder additive. Also elongation increased
while the tensile strength remained constant. Addition of Ti promotes formation of
TiC and TiNi inter-metallics. As per William Smith, addition of Mo and Co [270,302,
303] enhances the solid solution strengthening in the weld region. Futher they
reported that Co discourages the solubility of elements such as Al and Ti at low
temperature enabling the formation of greater amount of precipitates strengthening
phases for a given amount of Al and Ti in the weld region. Different authors
[261,263,265,268-269] have reported that Cr not only increases the resistance to stress
corrosion cracking but also enhances the weld hardness in the transition region of the
weld joint.
1.13 Regulations on Hexavalent Chromium Emission
Environmental and occupational safety awareness is increasing day by day. Various
findings are reported by researchers around the world about ill effects of inhaling of
the welding fumes by the welders. Occupational Safety and Health Administration
(OSHA) in United states of America (US) in its latest rulings has drastically reduced
the permissible exposure limit of harmful hexavalent Cr" fumes by the welders to
5|ig/m^ from 52|xg/m^ [78, 81]. This ruling is a direct result of findings that welders
are at high risk of developing respiratory disorders such as asthma, nasal and skin
damage and in certain conditions may even lead to lung cancer. Regulations like these
already exist in European Union, Canada and Australia. Similar regulations are also
expected to come into effect in third world countries such as India and China. This
regulation clearly presents an engineering challenge especially for stainless steel
welding industries. Hence OSHA has recommended use of Cr-free welding electrodes
for stainless steel.
Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steelfor Naval applications
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Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steelfor Naval applications
1.14 Research Gap
After an extensive literature survey, it was found that few researchers have worked
towards the development of Cr-free filler material for welding stainless steel.
Research gaps in welding of stainless steel by Cr-free welding electrodes are listed in
this section.
1. No research work focused to develop Cr-free welding electrode based on
multiple alloy elements (Ni, Mo, Fe, Cu, Co, Al, Ti, B, P, S, Si, and Mn) as
they can substitute Cr based properties such as good mechanical strength and'v
corrosion resistance.
2. Only few research works utilized simulation tools (JMAT Pro, Thermocalc
and MTDATA) for development of novel alloys. Developing novel alloys in
conventional methods are tedious, time consuming and high cost processes.
3. Effect of solutionzing treatment on the electrode materials properties are yet to
be investigated as solutionzing treatment improves the properties of electrode
such as ductility of the materials (and reduction of brittleness).
4. No research work has involved investigation of effect of precipitation
hardening on the electrode materials and its effect on weld properties. Though
precipitation hardening enhances the hardness or mechanical properties of
stainless steels and nickel based super alloys, its effect on welding needs to
explored.
5. Although the residual stresses degrades mechanical strength but No research
work focused on residual stresses induced by Cr-free electrode during stainless
steel welding.
6. Generally, weld joint suffers due to environmental corrosion (galvanic
corrosion). Hence there is scope for study on weld joint corrosion and methods
of protection in reducing and oxidizing environments are limited.
7. All mechanical structures generally are subjected to fatigue or cyclic load
Hence there is scope for fatigue studies of the weld joints.
8. In industries, they follow pre heat treatment of base plates and post heat
treatment of the weld joint to avoid welding defects but none of the works till
date have been investigated and effect of the same on the welding joints.
9. Presently, the simulation of welding is one of the significant research areas to
do state of art research using commercial or tailor made softwares for
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Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steel
investigating the effect of welding sequence, joint type and welding
parameters.
Although research in welding has grown enormously, still there is lot of scope for
doing research in welding. The present research work addresses points 1, 2, 3 and 5
as mentioned above.
1.15 Research Objectives
After extensive literature survey and discussions on dissimilar welding in chapter 1, it
is clear that reduction of hexavalent chromium fumes in the welding of stainless steel
is difficult without the use of Cr-free filler rod, so the order of the day is to develop
new Cr-free filler material.
The main objectives of the research work are,
> Development of new Cr-free nickel based weld alloy filler material.
> Prediction of phase transformations of weld alloys prepared, using JMATPro
material simulation software.
> Characterization of newly developed weld alloy for its microstructure and
mechanical properties.
> Evaluation of mechanical properties and microstructure of welded joints.
> Determination of residual stresses and distortion in the weld joints using
elemental birth and death technique.
> Experimental determination of residual stresses using XRD techniques.
1.16 Research Methodology
Present research work methodology can be divided into two stages
1. Weld alloy synthesis and its characterization
2. Weld joint characterization.
1.16.1 Nickel Alloy Synthesis and Characterization
> Prediction of various properties of electrode materials such as metallurgical
and mechanical properties, using JMATPro-material property simulation
software.
> Electrode alloy synthesis using an induction furnace
R V Center for Cognitive Technologies, R \/ College of Engineering, Bangaiore-S9. 31
> Identification of casting defects such as shrinkage porosity, cracks, blow holes
etc by exposing to X-rays as per ASME standards
Evaluation of mechanical properties of as-cast and solution treated material
such as Yield Strength (YS), Ultimate Tensile Strength (UTS) and hardness as
per ASTM standards and fracture studies using micrographs generated by
SEM
> Microstructure characterization of as-cast and solution treated materials such
as grain size, morphology, secondary phase distribution etc, using optical
microscopy and SEM.
> Identification of various phases using XRD techniques
1.16.2 Weldability studies
> Base metal and filler rod preparation using wire EDM. Specimen preparation
for welding , deciding on welding parameters based on weld trial runs
> Weld quality inspection by exposing weld joints to X-rays as per ASME
standards.
> Evaluation of mechanical properties of the weld joint such as tensile tests,
UTS, YS, ductility, impact and hardness profile across the weld joint.
> Determination of mode of failure or fracture studies on tensile tested
specimens using SEM
> Microstructure characterization of the weld joints such as Heat Affected Zone
(HAZ), Fusion Zone (FZ), Partially Mixed Zone (PMZ) and Unmixed Zone
(UZ) using SEM and Optical microscope
> Determination of residual stresses and distortion in a weld joint based on the
temperature distribution using FEM and XRD techniques.
Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steelfor Naval applications ____________________
R V Center for Cognitive Technologies, R V College of Engineering, Bangalore-59. 32
Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steel_________________________ for Naval applications_________________________
1.16.3 Flow Chart of Methodology of the Research Work
Nickel Based Alloy Characterization andits Weldability Studies
Alloy Synthesis
Alloy Characterization
Microstructural Mechanical Properties
SEM XRD Ootical JMATPro Simulation UTS YS % Elongation Hardness Fracture
Filler Material Development
Welding of Base Plates
Microstructural Studies Mechanical Prooerties FEM Simulation
SEM
UTS YS % Elongation Hardness Fracture
Ootical EDS Stress Distortion
Result and Discussion
Conclusion & Future Scooe
Figure 1.4: Shows the flow chart of the methodology of the research work
R V Center for Cognitive Technologies, R V College of Engineering, Bangalore-S9. 33
1.17 Oi^anization of Thesis
In the present dissertation work, the main focus is weldability studies between newly
developed nickel based Cr-free electrodes and stainless steel. The weldability study
includes weld joint strength, ductility, hardness and impact toughness. Chapter one
gives detailed description of concepts of welding, types of welding, elsewhere studies
on weldability of stainless steel, background of the problem and it concludes with
objectives and methodology of the research work. Chapter two describes in detail,
the simulation procedures to predict phase diagram, cooling curves and phase stability
of the newly developed electrode material, experimental methods and procedures
employed to evaluate mechanical and microstructural properties of various versions of
nickel weld materials and dissimilar weld joints with stainless steel. Chapter three
presents the discussion of alloy characterization and Chapter four involves
weldability studies of weld joints which include results of weld joint microstructure,
mechanical properties of weld joint and fracture studies. Chapter five presents FEM
simulation results to predict the residual stress, distortion in a weld joint and
comparison of the results with experimental methods and finally, Chapter six
consists of summary of the key findings of the research work and proposes the scope
of the future work followed by references, publications and appendix.
Development and Characterization of Novel Cr-Free Electrode for Welding Stainless Steel__________________________ for Naval applications__________________________
R V Center for Cognitive Technologies, R V College of Engineering, Bangalore-59. 34