effect of different metalurgical phases on the welding residual stresses of base metal
TRANSCRIPT
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EFFECT OF DIFFERENT METALLURGICAL PHASES
ON THE WELDING RESIDUAL STRESSES OF BASE
METAL S355
GROUP NO. 09 Batch: 2009-2010
Name Seat No.
Salman Zafar MY-015
Sadia Abro MY-025
Mohammad Tehmas Khan MY-053
Iqbal Ahmed Alvi MY-054
Supervisor: Engr. Bilal Ahmed
Co-Supervisor: Engr. M. Ali Siddiqui
DEPARTMENT OF METALLURGICAL ENGINEERING
NED UNIVERSITY OF ENGINEERING & TECHNOLOGY
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CERTIFICATE
Submitted in partial fulfilment of the requirement of the degree of
Bachelors of Engineering (Metallurgical Engineering).
Effect of Different Metallurgical Phases on the Welding
Residual Stresses of Base Metal S355
Group No. 09 Batch: 2009-2010
Name Seat No.
Salman Zafar (G.L) MY-015
Sadia Abro MY-025
Mohammad Tehmas Khan MY-053
Iqbal Ahmed Alvi MY-054
__________________ __________________
Supervisor Co-Supervisor
__________________ __________________
Examiner-1 Examiner-2
DEPARTMENT OF METALLURGICAL ENGINEERING
NED UNIVERSITY OF ENGINEERING & TECHNOLOGY
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ACKNOWLEDGEMENT
All Praise is for Almighty Allah, Who granted us the will and ability to work on this
project and bring it to completion.
We would like to express the highest gratitude towards our project supervisor Engr.
Bilal Ahmed, for his kind supervision and encouragement without which we would
not have accomplished our objectives.
We would like to warmly thank our co-supervisor, Engr. Muhammad Ali Siddiqui, for
his immense support and encouragement. His helpful nature and constant help in this
project deserves great appreciation. We would also like to express our appreciation
towards Engr. Faisal Nadeem for his cooperation and guidance.
We would extend a thank you to Engr. Kashif Iqbal, who helped us with his
intellectual suggestions. We would also like to thank all the faculty members and
technical staff of Metallurgical Engineering Department who helped us throughout
our project, sparing time for us from their busy schedules, listening to our problems
and suggesting solutions thus making this project a success for us.
Our thanks and appreciations also go to our colleagues in developing the project and
people who have willingly helped us out with their abilities.
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TABLE OF CONTENTS
Chapter 1: Introduction
1.1
Welding Technology 1
1.2Factors affecting Residual Stresses during Welding 1
1.3Research Objectives 2
Chapter 2: Literature Review
2.1 Welding 3
2.2 Fusion Welding Process 3
2.3 Welding processes and materials 3
2.4 Types of joints and welding positions 4
2.5 Heat flow in welding 6
2.6 Analysis of heat flow in welding 7
2.7 Effect of Welding Parameters 8
2.7.1 Pool Shape 8
2.7.2 Cooling rate and temperature gradient 11
2.8 Heat sink effect of work piece 112.9 Residual stresses 11
2.9.1 Residual stresses during welding 12
2.9.2 Basic Mechanism 12
2.9.3 Types of residual stress 14
2.9.4 Sources of residual stress 14
2.9.4.1 Residual Stresses Due To Shrinkage Process 14
2.9.4.2 Residual Stresses Due To Rapid Cooling Of the
Surface 14
2.9.4.3 Residual Stresses Due To Phase Transformation 14
2.10 Phase Transformation in Weldments 15
2.10.1 Special Factors Affecting Transformation Behaviour in
weldment 16
2.10.2 HAZ of a Single-Pass Weld 17
2.10.2.1 Peak Temperature-Cooling Time Diagrams 17
2.11 Volumetric Changes Due To Phase Transformation 20
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2.12 Prediction of Welding Residual Stresses 21
2.12.1 Basic Principles of FEM 22
2.13 Tungsten Inert Gas Welding 28
2.13.1 Polarity 29
2.13.1.1 Direct-Current Electrode Negative (DCEN) 29
2.13.1.2 Direct-Current Electrode Positive (DCEP) 30
2.13.1.3 Alternating Current 30
2.13.2 Electrodes 30
2.13.3 Advantages of TIG 30
2.13.4 Disadvantages of TIG 31
Chapter 3: Experimental Work
3.1 Visual Mesh Environment 32
3.1.1 Benefits 33
3.1.2 Steps for Creating Mesh 33
3.2 SYSWELD 37
3.3 Applications of SYSWELD 37
3.3.1 Evaluate Residual stresses 37
3.3.2 Minimize Residual Stresses 37
3.3.3 Study the Sensitivity of Geometry, Material and Process
Parameters 38
3.3.4 Optimize the Welding Process 38
3.4 Procedure 38
3.4.1 Defining Material Properties 39
3.4.2 Developing a Mesh of Geometry Model 40
3.4.3 Defining and Setting Boundary Conditions 42
3.4.4 Modelling Heat Input 43
3.4.5 Performing the Analysis 44
3.4.6 Visualizing and Interpreting the Result 47
Chapter 4: Results and Discussion
4.1 Analysis of Welding Residual Stresses without Using any Filler
Material 49
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LIST OF FIGURES
Figure 2.1: Five basic types of weld joint designs 5
Figure 2.2: Typical weld joint variations 5
Figure 2.3: Four welding positions 6
Figure 2.4: Heat Source efficiencies of several welding processes 6
Figure 2.5: Transverse cross section of weld showing areas representing
contributions from base metal and filler metal 7
Figure 2.6: Coordinate system (x, y, z) moving with heat source 7
Figure 2.7: Computer simulation of GTAW of 3.2-mm-thick 6061 aluminum,
110A, 10V, and 4.23mm/s: (a) fusion boundaries and isotherms;
(b) Thermal cycles 9
Figure 2.8: Weld pool shapes in GTAW of 304 stainless steel sheets 9
Figure 2.9: Sharp pool end in GTAW of 309 Stainless Steel preserved by ice
quenching during welding 10
Figure 2.10: Variation in cooling rates with heat input per unit length of weld 10
Figure 2.11: Thermal cycles of electro-slag and arc welds 11
Figure 2.12: Changes in temperature and stresses during welding 13
Figure 2.13: Conventional CCT diagram for AISI 1541 15Figure 2.14: Graphs to show differences in thermal cycles (a) Thermal cycles
used to generate a conventional CCT diagram. (b) Weld thermal
cycles 17
Figure 2.15: Schematic showing various subzones 18
Figure 2.16: Effect of a change in the peak temperature of the weld thermal
Cycle 19
Figure 2.17: Typical peak temperatures versus cooling time diagram 19
Figure 2.18: The atomic arrangement of ferrite, austenite and martensite 20
Figure 2.19: Schematic diagram of volume change due to phase transformation 21
Figure 2.20: Goldak's double ellipsoidal model 24
Figure 2.21: Schematic diagram of tungsten arc welding 29
Figure 3.1: Placing of nodes 34
Figure 3.2: 2D Mesh 34
Figure 3.3: Elements of the 2D Mesh 35
Figure 3.4: 3D extruded mesh 35
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Figure 3.5: Weld line and Reference Line 36
Figure 3.6: Collector groups 36
Figure 3.7: Flowchart of SYSWELD procedure 39
Figure 3.8: Defining material properties from Material Database 40
Figure 3.9: Dimension for 2D mesh 41
Figure 3.10: Dimensions for 3D mesh 41
Figure 3.11: 3D mesh of T-joint 42
Figure 3.12: Defining Boundary Conditions 42
Figure 3.13: Heat Input Fitting 44
Figure 3.14: Welding Wizard 45
Figure 3.15: Selections of Material Properties 45
Figure 3.16: Welding Operation Description 46
Figure 3.17: Solving Parameters 46
Figure 3.18: Post processing results 48
Figure 4.1: Phase distribution of Martensite 49
Figure 4.2: Phase distribution of Austenite 50
Figure 4.3: Phase distribution of Ferrite 50
Figure 4.4:Temperature Distribution Curve at 5mm, 8mm, 20mm and
50mm Respectively 51
Figure 4.5: Residual Stress Distributions along the Welding Direction 51
Figure 4.6: Residual Stresses at 5mm, 8mm, 20mm and 50mm 52
Figure 4.7: Phase Distribution of Austenite 54
Figure 4.8: Phase Distribution of Martensite 54
Figure 4.9: Phase Distribution of Ferrite 55
Figure 4.10:Temperature Distribution Curve at 5mm, 8mm, 20 mm
And 50mm Away From the Weld Line 55
Figure 4.11: Residual Stress Distributions along the Welding Direction 56
Figure 4.12:Welding Residual Stresses at 5mm, 8mm, 20mm and 50mm
Away From the Weld Line 56
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LIST OF TABLES
Table 1: An overview of welding processes 4
Table 2: Thermal Properties for Several Materials 8
Table 3: Parameters Used During Simulation 32
Table 4: Composition of material usedS355 40
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CHAPTER 1
INTRODUCTION
1.1 WELDING TECHNOLOGY
In heavy fabrication industries such as ship building and construction, welding
technology and design have come to play an important role in productivity and
quality. Despite its advantages of being an economical way to join metal parts,
welding can lead to structural problems such as distortion and residual stresses in the
weldments as a result of rapid heating and cooling. Reworking is necessary to remove
residual stresses and distortion, this reworking result in additional costs in man power
and materials and leads to project time delays. On the other hand, allowing the
distortion and residual stresses to remain can decrease structural integrity.
These residual stresses are self-balancing internal system of stresses arising from non-
uniform mechanical or thermal straining with some measure of plastic flow. Residual
stresses that developed in and around the welding zone are detrimental to the integrity
and the service behaviour of the welded structures. The welding residual stresses may
promote brittle fracture, reduce the buckling strength and the fatigue life and promote
stress corrosion cracking during service. Residual stresses also promote cold cracking
associated with hydrogen in certain steels before the welded part is put into service.
1.2 FACTORS AFFECTING RESIDUAL STRESSES DURING WELDING
Several factors may contribute to the formation of residual stresses and deformation.
The plastic deformation produced in the base metal and weld metal is a function
design (structure), material and fabrication parameters. The design parameters include
the joint type and the stresses of plates. The material parameter reflects the
metallurgical conditions of base metal and the weld metal. Fabrication parameters
include welding method, heat input, preheating, welding sequence and the restraint
condition.
In certain steel welded parts, the solid state austenite-martensite transformation during
cooling has a significant influence on the residual stresses and distortion. The
martensitic transformation is diffusionless solid state shear deformation. Therefore,
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when the martensite is formed, the volume of metal is increased and the
transformation plasticity is also produced. During the welding process, the magnitude
of the volumetric expansion in the heat affected zone (HAZ) and fusion zone (FZ)
depends on the volume fraction of the martensite that in formed. Therefore, accurate
prediction and reduction of welding residual stresses are critical in improving the
quality of the weldments.
1.3 RESEARCH OBJECTIVES
The faying surfaces and circumstances of the welds undergo metallurgical changes,
termed metallurgical phase transformation during heating and cooling. To evaluate
these residual stresses accurately, metallurgical phase transformation must be
considered. In this study, a finite element computation procedure solid state
transformation is developed based on the existing researches and the effectiveness of
the proposed numerical method for analyzing the residual stresses in carbon steels
specific to tungsten inert gas (TIG) single pass welding is demonstrated. The finite
element analysis package, SYSWELD is used in this study. This method is the most
popular method because numerous researchers have come up with various finite
element methods to model and analyze the welding process, including thermo-elastic
plastic approach. This method takes into account transient temperature history and in
some cases, material properties. One of the most important components in FEM
method is the modelling of the heat input.
Most of the reviewed publications focused on multi-passed welding. Therefore, the
purpose of this study is to analyze weld induced residual stresses when using single-
passed TIG welding on butt joints with varying composition of filler metal, based on
the already established software, that is, SYSWELD 2010. SYSWELD have beenwidely used to stimulate simple welding geometry such as butt weld and has been
shown to provide good results in simulating the single welding process.
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CHAPTER 2
LITERATURE REVIEW
2.1 WELDING
Welding is a fabrication process that joins material by causing coalescence. This is
often done by melting the work pieces and adding the filler material to form a pool of
molten material that cools to become strong joint, with pressure sometimes used in
conjunction with heat.
2.2 FUSION WELDING PROCESSES
Fusion welding is a joining process that uses fusion of the base metal to make the
weld. The three major types of fusion welding processes are as follows:
1.
Oxyacetylene Welding
2. Shielded Metal Arc Welding
3. GasTungsten Arc Welding
4.
Plasma Arc Welding
5.
GasMetal Arc Welding
6. Flux-Core Arc Welding
7.
Submerged Arc Welding
8. Electro-slag Welding
9. Electron Beam Welding
10.
Laser Beam Welding
2.3 WELDING PROCESSES AND MATERIALS
Table 1 summarizes the fusion welding processes recommended for carbon steels,
low-alloy steels, stainless steels, cast irons, nickel-base alloys, and aluminum alloys
[3]. For one example, GMAW can be used for all the materials of almost all thickness
ranges while GTAW is mostly for thinner work pieces. For another example, any arc
welding process that requires the use of a flux, such as SMAW, SAW, FCAW, and
ESW, is not applicable to aluminum alloys.
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Table 1: An overview of welding processes
2.4 TYPES OF JOINTS AND WELDING POSITIONS
Figure 2.1 shows the basic weld joint designs in fusion welding: the butt, lap, T-,
edge, and corner joints. Figure 2.2 shows the transverse cross section of some typical
weld joint variations. The surface of the weld is called the face, the two junctions
between the face and the work piece surface are called the toes, and the portion of the
weld beyond the work piece surface is called the reinforcement. Figure 2.3 shows four
welding positions.
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Figure 2.1: Five basic types of weld joint designs
Figure 2.2: Typical weld joint variations
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Figure 2.3: Four welding positions
2.5 HEAT FLOW IN WELDING
Heat flow during welding, can strongly affect phase transformations during welding
and thus the resultant microstructure and properties of the weld. It is also responsible
for weld residual stresses and distortion.
Figure 2.4: Heat Source efficiencies of several welding processes
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Table 2: Thermal Properties for Several Materials
2.7 EFFECT OF WELDING PARAMETERS
2.7.1Pool Shape
As the heat input Q and welding speed Vboth increase, the weld pool becomes more
elongated, shifting from elliptical to teardrop shaped. Figure 2.8 shows the weld pools
traced from photos taken during autogenously GTAW of 304 stainless steel sheets 1.6
mm thick (4). Since the pools were photographed from the side at an inclined angle
(rather than vertically), the scale bar applies only to lengths in the welding direction.In each pool the cross indicates the position of the electrode tip relative to the pool.
The higher the welding speeds, the greater the lengthwidth ratio becomes and the
more the geometric centre of the pool lags behind the electrode tip.
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Kou and Le (5) quenched the weld pool during autogenous GTAW of 1.6mm 309
stainless steel sheets and observed the sharp pool end shown in Figure 2.9. The
welding current was 85A, voltage 10V, and speed 4.2mm/s [10in./min (ipm)].The
sharp end characteristic of a teardrop-shaped weld pool is evident. The effect of the
welding parameters on the pool shape is more significant in stainless steel sheets than
in aluminum sheets. The much lower thermal conductivity of stainless steels makes it
more difficult for the weld pool to dissipate heat and solidify.
Figure 2.9: Sharp pool end in GTAW of 309 stainless steel preserved by
ice quenching during welding [5].
Figure 2.10: Variation in cooling rates with heat input per unit length of weld (EI/V)
[6].
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2.7.2. Cooling Rate and Temperature Gradient
The ratioEI/V represents the amount of heat input per unit length of weld. Lee et al.
[6] measured the cooling rate in GTAW of 2024 aluminum by sticking a thermocouple
into the weld pool. Figure 2.23 show that increasing EI/V decreases the cooling rate
(the slope). Kihara et al. [7] showed that the cooling rate decreases with increasing
EI/V and preheating. Figure 2.11 shows that the cooling rate in ESW, which is known
to have a very high Q/V, is much smaller than that in arc welding [6].
Figure 2.11: Thermal cycles of electro-slag and arc welds [8].
2.8 HEAT SINK EFFECT OF WORKPIECE
Kihara et al. [7] showed that the cooling rate increases with the thickness of the work
piece. This is because a thicker work piece acts as a better heat sink to cool the weld
down. Inagaki and Sekiguchi [9] showed that, under the same heat input and platethickness, the cooling time is shorter for fillet welding (a T-joint between two plates)
than for bead-on-plate welding because of the greater heat sink effect in the former.
2.9 RESIDUAL STRESSES
Residual stresses are stresses that would exist in a body if all external loads were
removed. They are sometimes called internal stresses. Residual stresses that exist in a
body that has previously been subjected to non-uniform temperature changes, such asthose during welding, are often called thermal stresses [12].
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2.9.1 Residual Stresses during Welding
The base metal and weld metal experience complex temperature changes and volume
changes during a welding process which results in the temporary thermal strains and
non-uniform distribution of elastic strains. These thermal strains cause both residual
stresses and distortion in welded parts.
2.9.2 Basic Mechanism
The expansion and contraction of the weld metal and the adjacent base metal are
restrained by the areas farther away from the weld metal. Consequently, after cooling
to the room temperature, residual tensile stresses exist in the weld metal and the
adjacent base metal, while residual compressive stresses exist in the areas farther
away from the weld metal. [12]
Figure 2.12 is a schematic representation of the temperature change and stress in the
welding direction (x) during welding. The crosshatched area MM is the region
where plastic deformation occurs. Section AA is ahead of the heat source and is not
yet significantly affected by the heat input; the temperature change due to welding, is
essentially zero. Along section BB intersecting the heat source, the temperature
distribution is rather steep. Along section CC at some distance behind the heat
source, the temperature distribution becomes less steep and is eventually uniform
along section DD far away behind the heat source [10].
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2.9.3 Types of Residual Stresses
Residual stresses are commonly classified into two groups as either macro or micro.
Macro residual stresses are those of engineering nature and which are measured over
a gauge length that encompasses several grains. Micro residual stresses are relate to
stress systems set up by microstructural in homogeneities which can either be
confined within a single grain or particular set of grains of the same preferred
orientation [12].
2.9.4 Sources of Residual Stresses
Welding residual stresses arise due to the variation in shrinkage of differently heated
areas, surface quenching effect and also due to phase transformation [13].
2.9.4.1 Residual Stresses due to Shrinkage Process
Shrinkage process is an important source of residual stresses. It happens due to the
difference in temperature of weld zone and base metal. The weld metal subjected to
highest temperature due to which it tends to contract more than all other areas but this
contraction is hindered by the cooler part of the joint. Thus the weld metal is
subjected to tensile stresses in the longitudinal direction and it increases with
increasing yield strength of material as a result of decrease in temperature [13].
2.9.4.2 Residual Stresses due to Rapid Cooling of the Surface
During the cooling process, the weld metal cools more rapidly because it is directly
linked with the cooler areas of base metal, even with air cooling. this rapid cooling of
the surface is called a quenching effect and this results in the development of residual
stresses. [13]
2.9.4.3 Residual Stresses due to Phase Transformation
During cooling, phase transformation from austenite to ferrite, bainite or martensite
will occur either at a certain temperature or over a temperature range. Due to this
phase transformation, there is an increase in specific volume and so the material
which is being transformed, tends to expand. but the expansion is hindered by the
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cooler material not being transferred. Thus the transformed area is subjected to the
development of residual stresses [13].
2.10 PHASE TRANSFORMATION IN WELDMENTS
Solid-State phase transformations occurring in a weld are highly non-equilibrium in
nature and differ distinctly from those experienced during casting, thermo-mechanical
processing, and heat treatment [14].
A concise method of describing the transformation behaviour of steel is by a
continuous cooling transformation diagram (Fig. 2.13). However, a conventional CCT
diagram such as the one shown in Fig. 2.13 cannot be used to accurately describe the
transformation behaviour in a weldment of same material because weld thermal
cycles are very different from those used for generating conventional CCT diagrams
[14].
Figure 2.13: Conventional CCT diagram for AISI 1541
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2.10.1 Special Factors Affecting Transformation Behaviour in a Weldment
Several aspects of the weld thermal cycle and weld segregation should be considered
because of their effect on the transformation upon cooling:
Peak temperatures reached in the heat-affected zone (HAZ) can be very much
higher than the ac3 temperature (that is, the temperature at which transformation
of ferrite to austenite is completed during heating). The heating rates are very
high, and the times spent at high temperature are only of the order of a few
seconds [14].
The temperature gradient in the HAZ is very steep, and this complicates the
problem of studying insitu transformations in the HAZ during welding [14]. During solidification of the weld metal, alloying and impurity elements tend to
segregate extensively to the inter-dendritic or intercellular regions under the
conditions of rapid cooling. Also, the pickup of elements like oxygen by the
molten weld pool leads to the entrapment of oxide inclusions in the solidified
weld. These inclusions then serve as heterogeneous nucleation sites and can
substantially influence the kinetics of subsequent solid state transformations.
Accordingly, the weld metal transformation behaviour is quite different from that
of the base metal, even though the nominal chemical composition has not been
significantly changed by the welding process [14].
Welding may be carried out in several passes, and this may result in the
superposition of several different heating and cooling cycles at one point [14].
Solidification of the weld metal is accompanied by shrinkage, and the isothermal
conditions already emphasized cause deformation. The thermal cycles are
therefore acting on metal that is subjected to mechanical stresses at the same time
[14].
The essential differences between weld thermal cycles and then thermal cycles used
for generating a conventional CCT diagram are summarized in Fig. 2.14. Figure
2.14(a) shows thermal cycles which involve a slow heating rate, soak at a temperature
just above the Ac3 temperature, and various constant cooling rates. The weld thermal
cycles shown in Fig. 2.14(b) are very different, and this is why a conventional CCT
diagram can give only an approximate idea of the transformation behaviour in theHAZ of weldments [14].
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Figure 2.14: Graphs to show differences in thermal cycles. (a) Thermal cycles used
to generate a conventional CCT diagram. (b) Weld thermal cycles.
2.10.2 HAZ of a Single-Pass Weld
HAZ comprises of the following factors:
2.10.2.1 Peak Temperature-Cooling Time Diagrams
The gradient in microstructure than can be obtained in a single pass weld is shown in
Fig.2.4.High peak temperatures in the HAZ just adjacent to the fusion line cause
coarsening of the austenite() grains, and this in turn increases the harden ability of
this region compared to other regions. Because each of the subzones shown in Fig.
2.15 occurs in a small volume, it is difficult to study the transformation behaviour of
individual regions by in situ methods [14].
Fig. 2.16 shows how a change in peak temperature of the thermal cycle affects the
CCT characteristics of steel. The well known effect of a larger grain size (due to a
higher peak temperature) in increasing the harden ability of the steel is seen. Topresent the information about CCT behaviour for a number of peak temperature (see
Fig. 2.3b and 2.15), it is more convenient to adopt the scheme shown in Fig. 2.17
[14].
In this peak temperature-cooling time (PTCT), each point represents a weld thermal
cycle with a peak temperature, Tp, given by the ordinate and the cooling time, t8-5
(that is, required for cooling from 800 to 500 C). A microstructural constituent or a
combination of two or more constituents is shown to occur over an area in the
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diagram. The upward slope in the boundary between the two areas is consistent with
the information presented earlier in Fig. 2.15 that the harden ability increases with an
increase in the peak temperature of the thermal cycle. Hardness and C V transition
temperature are also shown in the diagram corresponding to different thermal cycles.
[12]
Figure 2.15: Schematic showing various subzones that can form in the HAZ of a
Carbon steel containing 0.15% C
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Figure 2.16: Effect of a change in the peak temperature of the weld thermal cycle on
CCT characteristics
Figure 2.17: Typical peak temperatures versus cooling time diagram
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2.11 VOLUMETRIC CHANGES DUE TO PHASE TRANSFORMATION
The main factor for the generation of residual stresses during welding process is thevolume changes due to the phase transformation.
Depending on composition and temperature, an alloy may contain one or several
equilibrium phases. In each phase the atoms are arranged in a repeated three
dimensional crystal structure. Since each phase has its own specific volume, all phase
transformations occurring affect the total volume of the alloy, giving rise to
distortions. In steel, most of the atoms are Fe-atoms and consequently the phases and
phase transformations of pure iron play an important role in steels [18].
Normally, at room temperature, steels contain two different phases i.e. ferrite and
pearlite. When the original structure is heated aoe C, pearlite becomes unstable
and transforms into austenite. The microstructure then consists of a mixture of ferrite
and austenite. During continued heating, the amount of ferrite will decrease until it
completely disappears. Since formation of pearlite requires nucleation and growth by
diffusion, it is a time-dependent process. Then, if a specimen is rapidly quenched
from the austenite phase field, there is no time for pearlite to form. Instead the
austenite becomes under-cooled transforming very rapidly to a phase known as
martensite when a characteristic, so-called MS, temperature is reached. The
transformation is completed when the temperature has reached another characteristic
value, known as the MF temperature [18].
The atomic arrangements of ferrite, austenite and martensite are illustrated in Figure
2.18. Since the atoms are more closely packed in austenite, it has a lower specific
volume than ferrite and martensite [18].
Figure 2.18: The atomic arrangement of ferrite, austenite and martensite
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Metallurgical phase transformation in the welding process affects thermal stress
because it induces volume changes in the base material. As shown in Fig. 2.19, the
volume decreases when the steel transforms from pearlite ferrite to austenite on
heating, and volume increases when it transforms from austenite to pearlite ferrite
upon cooling. If the cooling rate is very high, the austenite transforms to martensite,
which has greater volume. When such metallurgical transformations occur, density
and yield stress change in addition to the volume change of the base material.
Figure 1.19: Schematic diagram of volume change due to phase transformation
2.12 PREDICTION OF WELDING RESIDUAL STRESSES
The measurement and prediction of welding residual stresses can be done by various
ways. Hole Drilling Technique, X-ray diffraction and Neutron diffraction are some
methods used to measure the welding residual stresses experimentally. But there is an
alternative to the experimental measuring techniques to evaluate the welding residual
stresses is to perform virtual simulation of welding process by using numerical
modelling schemes such as FEM.
Finite element method (FEM) simulation is known as a complementary tool withrespect to experimental techniques applied to determine the behaviour and
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interactions between complex physical phenomena in the welding process. However,
simulation of the welding process is not an easy task since it involves the interaction
of thermal, mechanical and metallurgical phenomena [16].
2.12.1 Basic Principles of FEM
Heat energy can be transferred from one system to another as a result of temperature
differences. The total heat input (Q in W/m3) in arc welding is the product of arc
power (VI in W) and process efficiency () [16].
eqn (1)
For GTAW, the heat source efficiency ranged from 70% to 80%. The heat input fromthe welding source (heat source) in the weld pool transferred to the base metal by
means of conduction and to surrounding surfaces by convection and radiation. Heat
diffusion by conduction is based on Fourier's Law, where heat flux (q in W/m2) flows
from hot to cooler regions and are linearly dependent on the temperature gradient,
where k is thermal conductivity (in W/m K), which is the ability of a material to
conduct heat and can depend on temperature or represent a tensor in anisotropic cases
[16].
eqn (2)For the unit surface with unit vector n, the rate at which heat is conducted across the
surface per unit area in the direction of n is formulated as [16]:
eqn (3)
When heat diffusion is treated with an enthalpy based formula to solve the problem in
liquid and solid domains, and if k is inserted in the energy conservation equation, the
heat equation in the transient case can be written as follows [14]:
eqn (4)
eqn(5)
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In this formula , cpand Q represent density ( ink g/m3), specific heat (in J/kg K), and
the internal heat source (in W/m3), respectiely. The product of . cp reflects the
capacity of the material to store energy. The rate of heat transfer by convection is
observed to be proportional to the temperature difference, and is given by Newton's
law of cooling where qconvis convective heat flux (in W/m2K), hconvis the convection
heat transfer coefficient and (Ts- T) [16]:
eqn (6)The heat transfer by radiation is given by the Stefan-Boltzmann law where andhrad are Stefan Bokltzmanns constant and thermal emissivity and radiation heat
transfer coefficients, respectively [16]:
eqn (7)The thermal boundary conditions are summarized as follows, where n represents the
external normal to the side wall:
(a)
Heat flux density q, imposed on the wall:
eqn (8)
(b) Imposed coefficient of thermal change:
eqn (9)
Likewise, using energy balance, the radiation boundary condition on the surface can
be expressed as:
eqn (10)
The overall thermal boundary condition can therefore be defined as:
eqn (11)
For many arc welds, good approximation of heat input (Q) is achieved by using
double ellipsoidal shape as proposed by Goldak and Akhlaghi using the following
equation [5]:
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eqn (12)
In this model, ff and fr are the fractions of the heat deposited in the front and rear
quadrant respectively, where ff+ fr= 2, and a, b, c are the dimension parameters of theheat source, v is the welding velocity, t is time, and W is lag factor of the heat
deposited at t = 0.Figure 2 shows the proposed Goldak's Double Ellipsoid model. [16]
Figure 2.20: Goldak's double ellipsoidal model
The thermal calculation is based on the resolution of the modified heat equation,
taking into account the latent heat of fusion and solidification and the phase
transformation heat in the solid state. Metallurgical and thermal calculations are fully
coupled at each temperature. There are three types of interaction between thermal and
metallurgical analyses, which are metallurgical transformations depending directly on
the thermal history of the part, metallurgical transformations accompanied by latent
heat effects which modify temperature distribution, and phase-dependent thermo-physical properties. The latent heat affect due to metallurgical transformation is given
by the following equation where H is enthalpy, P1 is the initial phase, P2 is the final
phase, and T is the temperature [16]:
eqn (12)While the specific heat at constant pressure is equal to a change in enthalpy in a
temperature range, treating the heat diffusion with an enthalpy-based formulation tosolve the problem in liquid and solid domains gives the following equation [16]:
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eqn (13)
Where,
eqn (14)
Thermo-metallurgical calculation provides thermal cycles, rates, flows and changes in
the phase proportions and the austenitic grain size from the thermal properties of the
materials (conductivity k, specific heat cpor enthalpy H), welding process parameters
and the metallurgical transformation diagram, which is formalized mathematically. In
practice, a number of transformation models can be described in a material. It can be
characterized by proportions pi of its various constitutive phases. In the case of steel, a
distinction is generally made between the diffusion type phase and the martensitic
type transformation [16].
The diffusion type transformation is described most frequently by Johnson-Mehl-
Avrami under the isothermal conditions given as follows, where p represents the
phase proportion obtained after an infinite time at temperature T,is the delay time,and n is an exponent associated with the reaction speed [16]:
eqn (15)For an isothermal condition, the kinetic transformation proposed by Leblond is more
popular due to its simplicity and can be used to represent any type of transformation,
whether by heating or cooling. The basic equation is expressed as follows [16]:
eqn (16)
Where P is the metallurgical phase proportion, P is the phase proportion at the
equilibrium and is delaytime. For this transformation law, the required parametersare obtained from the Continuous Cooling Diagram (CCT) [16].
The other type of transformation is the martensitic transformation which depends on
temperature alone and is described by the Koistinen-Marburger law as [16]:
eqn (17)
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In this case, p represents the proportion otained at an infinitely low temperature
which is frequently assimilated to 1. Ms and b characterize the initial transformation
temperature and the evolution of the transformation process according to temperature,
respectively [16].
The welding process induces stress due to uniform temperature changes, which can
result in deformation. Stresses are forces acting on materials that tend to change the
dimensions of those materials (deformation). When a material is distorted by stresses
it is said to be strained. A strain is the ratio of an elongation or a deflection to an
original dimension [16].
The calculations are based on thermal and metallurgical history. The influence of the
thermal history on the mechanical history results simultaneously from variations of
the mechanical properties (Young's Modulus, yield strength) in regard to the
temperature and from thermal expansions or contractions, whilst the metallurgy is
involved in the mechanical analysis principally through volume changes caused by
modifications to the crystalline structure of material during metallurgical
transformations. These changes are added to conventional thermal strain and are
modelled by means of the thermo metallurgical strain [16].
eqn (18)
In this equation represents the temperature-related thermal strain of metallurgicalphase i. the thermal strain of each phase not only differs in terms of its gradient
representing the coefficients of expansion, but also by the origin of the ordinate
reflecting the change of volume during phase transformation, which leads to a major
contribution to the generation of residual stresses and strain [16].
In addition, metallurgy also becomes a consideration in mechanical analysis through
special behaviours linked to the multi-phase aspect of material. The material law for
the calculation of mechanical behaviour, which depends on temperature, does take the
combination of phases into consideration and also includes the transformation
plasticity phenomenon. The material behaviour during the transformation of phases is
assumed to be elastic-plastic. In this material model, the total strain is broken down
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into elastic strain, plastic strain, and thermal strain and written in incremental form as:
[16]
eqn (19)
The plastic strain rate () is expressed as the sum of stress variation (),temperature variation (T) and phase proportion variations (p). The first two termsrepresent the conventional plastic strain rate while the third represents transformation
induced plastic strain. In a wide class of material behaviour, the plastic strain rate can
be modelled using plastic potential which is generally written as [16]:
eqn (20)
Here, g is the scalar function differentiated with respect to stress, while expressesplastic strain. is a consistency parameter representing the plastic strain. When theplastic potential is equal to yield function or plasticity criterion (F), the equation 20
then becomes [16]:
eqn (21)The general form of Eqn. 21 is also known as the associated flow rule due to its
association with a particular yield criterion. The selected plasticity criterion is based
on Von Mises criterion, which is commonly used particularly because of its suitability
in analyzing metals behaviour. The mechanical analysis was performed using a
thermo-elastic-plastic material formulation with Von Mises yield criterion as shown
below: where , and are the principal stresses coupled to strain hardening rule[16].
eqn (22)
In this simulation study, isotropic strain hardening was selected due to the non
cycling loading, whereas kinematic strain hardening is recommended for cyclic
applications. For isotropic hardening materials, the mechanical calculations based on
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the metallurgical history principally follow the constitutive equation proposed by
Leblond, whereby the plastic transformation strain represents the transformationplastic flow which occurs during the phase transformation and is computed from
evolution law [16].
eqn (23)
K represents the coefficient of transformation plasticity, the ferrite proportion,the Von Mises equivalent stress, Sifthe stress deviator components and hthe corrector
function. The yield stress () is computed using a non-linear law for an austenitic-ferritic mixture using the following equation [16]:
eqn (24)
Therefore, the total strain rate can be defined as a sum of the elastic strainrate , plastic strain rate , transformation plastic strain rate andthermo-metallurgical strain [16].
eqn (25)2.13 TUNGSTEN INERT GAS WELDING
Gastungsten arc welding (GTAW) is a process that melts and joins metals by heating
them with an arc established between a non-consumable tungsten electrode and the
metals,. The torch hold the tungsten electrode which is connected to a shielding gas
cylinder as well as one terminal of the power source, the tungsten electrode is usually
in contact with a water-cooled copper tube, called the contact tube, which is
connected to the welding cable (cable 1) from the terminal. This allows both the
welding current from the power source to enter the electrode and the electrode to be
cooled to prevent overheating. The work piece is connected to the other terminal of
the power source through a different cable (cable 2). The shielding gas goes through
the torch body and is directed by a nozzle toward the weld pool to protect it from the
air. Protection from the air is much better in GTAW than in SMAW because an inert
gas such as argon or helium is usually used as the shielding gas and because the
shielding gas is directed toward the weld pool.
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For this reason, GTAW is also called tungsteninert gas (TIG) welding. However, in
special occasions a non inert gas (Chapter 3) can be added in a small quantity to the
shielding gas. Therefore, GTAW seems a more appropriate name for this welding
process. When a filler rod is needed, for instance, for joining thicker materials, it can
be fed either manually or automatically into the arc.
Figure 2.21: Schematic diagram of tungsten arc welding
2.13.1 Polarity
In TIG welding there are three polarities
Direct-Current Electrode Negative (DCEN) Direct-Current Electrode Positive (DCEP)
Alternating Current (AC)
2.13.1.1 Direct- Current Electrode Negative (DCEN):
This, also called the straight polarity, is the most common polarity in GTAW. The
electrode is connected to the negative terminal of the power supply. Electrons are
emitted from the tungsten electrode and accelerated while travelling through the arc.
A significant amount of energy, called the work function, is required for an electron to
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base metal and the fusion of the filler metal. Therefore, the control of dilution
and energy input to the weld can be achieved without changing the size of the
weld. It can also be used to weld butt joints of thin sheets by fusion alone, that
is, without the addition of filler metals or autogenous welding.
The GTAW process is a very clean welding process, it can be used to weld
reactive metals, such as titanium and zirconium, aluminum and magnesium.
2.13.4 Disadvantages of TIG:
The deposition rate in GTAW is low.
Excessive welding currents can cause melting of the tungsten electrode and
results in brittle tungsten inclusions in the weld metal.
However, by using preheated filler metals, the deposition rate can be improved. In the
hot-wire GTAW process, the wire is fed into and in contact with the weld pool so that
resistance heating can be obtained by passing an electric current through the wire.
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CHAPTER 3
EXPERIMENTAL WORK
The experimental work is based on simulations which are carried out on two software:
1. Visual Mesh Environment
2.
SYSWELD
Following are the input parameters which are used during the simulations:
Table 3: Parameters Used During Simulation
Dimensions of Butt Joint 100x100x3Power Efficiency 2000-2500
Speed of Torch 5mm/sec
Imposed Temperature 25C
Height of Bead 4.6mm
Length of Bead 14.4mm
Width of Bead 8.2mm
3.1 VISUAL MESH ENVIRONMENT:
Visual-Mesh is meshing software which provides guided surfaces coalescence,
application of generation of specific mesh and intuitive post mesh editing features.
Focus on building high quality digital models for all commonly used solvers and all
popular CAD and solver data formats.
Visual-Mesh generates simulation specific meshes for Manufacturing, ComputationalFluid Dynamics (CFD) and Welding joints applications in order to achieve the best
possible quality result in combination with the shortest possible simulation time.
3.1.1 Benefits
Automated surface clean up,
Increased productivity thanks to mid-surface creation and meshing
Reduced learning curve and training overhead with the intuitive guided mesh
process
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The use of a single mesh for multiple solver application.
3.1.2 Steps for Creating a Mesh:
1.
First nodes are selected and placed for the meshing of the weld joint2. After nodes are placed, the surface is stitched together using the blend (spline)
tool which makes a 2D surface of the mesh.
3.
Now the 2D surface of the mesh is divided into many small elements (square
shaped boxes) and different parts of the mesh can be divided into desired
number of elements.
4. After completing 2D meshing, the mesh is extruded into a 3D mesh.
5. Weld line and reference line are sketched onto the mesh.
6.
Clamping conditions are provided by the user.
7. All the aoe steps are saed into different groups which are named as parts.
For example, Part 1, Part 2 etc
8.
These parts are then made into collector groups which represent the respective
parts of the mesh in the simulation software.
9. This mesh is then loaded into the simulation software where all the processing
takes place.
Following pictorial steps are of t-joint weld, but can be applied for the meshing of butt
weld joints.
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Figure 3.1: Placing Of Nodes
Figure 3.2: 2D Mesh
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Figure 3.3: Elements of the 2D Mesh
Figure 3.4: 3D Extruded Mesh
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Figure 3.5 Weld Line and Reference Line
Figure 3.6 Collector Groups
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3.2 SYSWELD
SYSWELD is the leading tool for the simulation of heat treatment, welding and
welding assembly processes, taking into account all aspects of material behaviour,
design and process.
Key success factors in the welding industry focus one eliminating as much as possible
the distortions of structural assemblies and component repair, as well as addressing
durability problems related to welding processes. Engineers involved in welding try to
find the optimum between distortions, residual stresses and plastic strains by fully
optimizing the process type and the process parameters, bringing understanding of
their influence of the part shape and the resultant material behaviour.
It is a powerful tool that guides to find out the optimum process parameters with
respect to distortions, residual stresses and plastic strains.
3.3 APPLICATIONS OF SYSWELD
Following are the applications of SYSWELD:
3.3.1 Evaluate residual distortions:
Assembling a structure requires sequential continuous and/or spot welding joints.
Therefore, defining the welding sequence and the places where the parts will be
welded is crucial for the correct completion of the welding assembly process.
Simulation allows prediction and minimization of distortions which generate an
increase of the overall product quality as well as drastic cost saving.
3.3.2 Minimize residual stresses:
Simulating the welding process aims to control the process in a way that minimizes
the stress gradient and tensile surface stresses. As a result, lifetime of a part increases
as fewer cracks appear after load cycles. Compressive stresses can also be detected on
the surface of the component, therefore improving part quality and avoiding corrosion
risks due to tensile stresses.
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3.3.3 Study the sensitivity of geometry, material and process parameters:
Used in the design phases, SYSWELD decreases costly design errors. At each step of
the development cycle, the cost of corrections gradually increases. SYSWELD helps
to optimize part geometry, materials and process parameters during the early stages of
a new design cycle avoiding expensive engineering changes that could occur later.
3.3.4 Optimize the welding process:
SYSWELD allows user-defined weld sequencing and control of the weld
manufacturing parameters such as velocity, energy input and many others.
3.4 PROCEDURE
SYSWELD is capable of simulating both single and multi-passed welding processes.
The standard welding simulation methods ( moving heat source) only can be applied
to the single passed welding but not the multi passed welding process due to
computation files that need to be managed and considerable disk space requirement.
The simulation of the single passed welding process involves the following steps:
Defining material properties
Developing a mesh of Geometry model
Defining and setting boundary conditions
Modelling heat input
Performing the Analysis
Visualizing and Interpreting the Result
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Figure 3.7: Flowchart of SYSWELD procedure
3.4.1 Defining Material Properties
The material used for the project is High strength low alloy steel that is S355 have the
following composition.
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Table 4: Composition of material usedS355
Grade C% Mn% Si% P% S%
S355 0.23 max 1.60 max 0.05 max 0.05 max 0.05 max
SYSWELD enables to use predefined properties data or to develop thermo-
metallurgical and mechanical behaviour data for the simulation. SYSWELD provides
a material data base.
In welding computations, thermal material properties are strongly non-linear and
depend on temperature and phases.
Figure 3.8: Defining Material Properties from Material Database
3.4.2 Developing a Mesh of Geometry Model
The geometry model is developed using Visual Mesh which is powerful geometry and
meshing tool used to develop a customized mesh model. A file
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(Filename_DATA1000.ASC) created in visual mesh is imported into SYSWELD to
be used as input data in the simulation.
Figure 3.9: Dimensions for 2D Mesh
Figure 3.10: Dimensions for 3D Mesh
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Figure 3.11: 3D Mesh of T-Joint
3.4.3 Defining and Setting Boundary Conditions
Two steps of boundary conditions (Thermal and Mechanical) are to be defined.
Thermal boundary conditions are caused by convection and radiation loss, the
mechanical boundary conditions are defined by clamping. The clamping conditionsare modelled as rigid clamping.
Figure 3.12: Defining Boundary Conditions
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3.4.4 Modelling Heat Input
In this simulation method, the correct heat input selection and modelling were
instrumental to the outcome.
It is very important to understand the concept of the heat input that was employed.
The heat source was modelled using Goldak's double ellipsoidal model to describe the
heat input of the welding process.
The heat source function enables users to calibrate the heat source parameters and
perform a steady-state thermal analysis of the welding process. The resulting analysis
provides the user with a temperature contour plot showing the predicted weld fusion
zone. Based on the result, it is possible for the user to calibrate the heat source by
comparing the predicted weld fusion zone with the actual macrograph from the tested
specimen. The heat model is calibrated by adjusting the Gaussian parameter until it
generates a fusion zone as per users requirement. Howeer, efore this heat source
could be used to simulate the multi-passed welding process, the average thermal cycle
had to be extracted and integrated into the function. It is done to reduce the
computation load by using the average thermal input. In order to obtain the thermal
cycle for the multi-pass simulation, a 2-D transient analysis had to be performed.
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Figure 3.13: Heat Input Fitting
3.4.5 Performing the Analysis
The analysis is performed through the computer simulation. The input data is stored
and saved under the name "welding wizard". Then the project is solved using "multi-
passed advisor". The 2-D and 3-D multi-passed analyses are performed separately
using similar steps. The running simulation of the 2-D multi-passed analysis takes
several minutes while the 3-D multi-passed analysis takes several hours due to the
massive number of elements and nodes.
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Figure 3.14: Welding Wizard
Figure 3.15: Selections of Material Properties
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Figure 3.16: Welding Operation Description
Figure 3.17: Solving Parameters
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3.4.6 Visualizing and Interpreting the Result
This is the final step of the simulation where the result is visualized and interpreted.
This is done using the "post-processing" menu available under "welding advisor".
Several methods are available to be used to interpret data, such as contour and curve
plots.
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Figure 3.18: Post Processing Results
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CHAPTER 4
RESULTS AND DISCUSSION
This chapter includes the result of the simulations performed for this study and the
discussion conducted on the basis of these results including the investigation of the
influence of phase proportion of martensite and austenite on the welding residual
stresses of base metal (S355).
4.1 ANALYSIS OF WELDING RESIDUAL STRESSES WITHOUT USING ANY
FILLER MATERIAL
4.1.1 Simulation Results
Following are the results of the simulations performed without any filler material so
as to analyze the generated welding residual stresses of base metal (S355). Figure 4.1
shows the simulated phase distribution of martensite. The figure consists of several
different regions, represented by distinct colours. Each colour represents a separate
phase. The red region in Figure 4.1 signifies the proportion of Martensite. The
maximum proportion of martensite in this case is 31%.
Figure 4.1: Phase distribution of Martensite
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The red region in Figure 4.2 indicates the proportion of austenite obtained in the same
simulation. It can be observed that the maximum proportion of austenite in the weld
region is only about 0.1%.
Figure 4.2: Phase distribution of Austenite
Figure 4.3 given below specifies the proportion of ferrite phase in the base metal
S355.
Figure 4.3: Phase distribution of Ferrite
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Figure 4.4 shows the temperature distribution at 5mm, 8mm, 20mm and 50mm away
from weld line.
Figure 4.4: Temperature distribution curve at 5mm, 8mm, 20mm and 50mm
respectively
Red curve shows the temperature distribution at weld line which shows that the peak
temperature at weld line is about 2800and as we go away from the weld line, thepeak temperature decreases.Figure 4.5 shows the residual stress distribution in the
welding direction along the weld line.
Figure 4.5: Residual Stress distribution along the welding direction
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Figure 4.6 shows the curve of the magnitude of residual stress at 5mm, 8mm, 20 mm,
50 mm away from the centre line.
Figure 4.6: Residual stresses at 5mm, 8mm, 20mm and 50mm
It can be observed form Figure 4.6 that the residual stresses are tensile in nature near
weld line but small in magnitude. As we go away from the weld line the magnitude of
stresses increases up to 460MPa (tension) and after this magnitude start decreases and
becomes compressive in nature up to 230MPa (compression).
4.1.2 Discussion
In this case the absence of a filler material supports diffusionless transformation. As
evident from the results, the proportion of martensite is greater as compared to
austenite and ferrite. In principle, all metals and alloys can be made to undergo
diffusionless transformations provided the cooling rate or heating rate is rapid enough
to prevent transformation by an alternative mechanism involving the diffusional
movement of atoms. In the case of martensite in steels, the cooling rate is such that
the majority of the carbon atoms in solution in the FCC -Fe remain in the solution in
the -Fe phase.
Martensitic transformation leads to the crystallographic transformation of face
centered cubic (FCC) austenite in to body centered tetragonal (BCT) martensite. Thecarbon atoms that are randomly distributed on the interstitial sites in FCC do not have
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time to migrate to the BCC in a random manner and hence move in a coordinated
motion. This increases the tetragonality of the BCC lattice and thus the carbon
containing martensite is of body centered tetragonal (BCT) structure.
In addition to the increased tetragonality, the increase in carbon content also leads to a
volume expansion, i.e. dilatation. Moreover, the solid state nature of the
transformation, that takes place by a cooperative movement of atoms, requires the two
phases to be highly coherent and hence gives rise to a large amount of internal
stresses inside the material.
Since the martensitic proportion is greater in this case, it is only logical to interpret
that the stresses generated by this phase will be greater as well. Keeping in mind the
welding practice, the weld bead will transform into martensite and hence generate
increased tensile forces. On the other hand, the region in the vicinity of the bead
which did not undergo any transformation will continue to apply compressive stresses
on the weld bead. These stresses are not practically feasible and are unwanted in a
weldment.
4.2 ANALYSIS OF WELDING RESIDUAL STRESSES BY USING 316L AS A
FILLER MATERIAL
4.2.1 Simulation Results
In this case we used 316L as filler material in order to analyze the effect of austenite
on the welding residual stresses of base metal (S355). Figure 4.7 shows the simulated
distribution of austenite.
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Figure 4.9: Phase distribution of Ferrite
Fig 4.10 shows the temperature distribution at 5mm, 8mm, 20mm and 50mm away
from weld line. Blue curve shows the temperature distribution at weld line which
shows that the peak temperature at weld line is about 1300and as we go away fromthe weld line, the peak temperature decreases.
Figure 4.10: Temperature distribution curve at 5mm, 8mm, 20mm and 50mm away
from the weld line
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Figure 4.11 shows the residual stress distribution in the welding direction along the
weld line.
Figure 4.11: Residual stress distribution along the welding direction
Figure 4.12 shows the curve of the magnitude of residual stress at 5mm, 8mm, 20 mm
and 50mm away from the centre line.
Figure 4.12: Welding residual stresses at 5mm, 8mm, 20mm and 50mm away from
the weld line
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It is shown from Figure 4.12 that the residual stresses are compressive in nature
(90MPa) near weld line but small in magnitude. As we go away from the weld line the
magnitude of stresses increases up to 480MPa (tension) and after this magnitude start
decreases and becomes compressive in nature up to 230MPa (compression).
4.2.2 Discussion
In this case, 316L is used as the filler material. It contains nickel which is an austenite
stabilizer and thus leads to the formation of a greater proportion of austenite as
compared to martensite and ferrite. Metallurgical phase transformation in the welding
process affects thermal stress because it induces volume changes in the base material.
Although the volume increases when the material transforms from austenite to
pearlite ferrite upon cooling, still the specific volume of austenite is extremely low
in comparison to martensitic structure.
The weld bead formed in this case does not involve volumetric expansion (dilation)
and hence sufficient tensile stresses will not be generated. However, the base metal
will generate compressive stresses which will be greater and will hence play a vital
role in enhancing the strength of the material. This phase transformation is thus
preferred and is far more feasible.
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CHAPTER 5
CONCLUSION AND RECOMMENDATIONS
5.1 CONCLUSION
An investigation of the effect of different metallurgical phases on the welding residual
stresses of base metal S355 was conducted. After interpreting the results of this study
on the basis of residual stresses and phase proportion of martensite and austenite, the
chief findings in this work are summarized as follows:
1. As in first case, welding is done without using any filler metal, the proportion of
martensite phase is much higher than the austenite i.e. 31% which results in the
formation of tensile residual stresses on the weld area and compressive residual
stresses away from the weld area. This is because of the fact that martensitic
transformation in the weld area results in the volume expansion.
2.
In second case, 316L is used as filler metal in order to get the maximum
proportion of austenite in weld area. In this case, the proportion of austenite is
about 88%. Since, the specific volume of austenite is much less than that of
martensite. Therefore, in this case, compressive residual stresses form in the
weld area and tensile residual stresses form away from the weld area.
5.2 RECOMMENDATIONS
In order to get the high strength weld joint, it is recommended to minimize the
proportion of martensite in the weld area as it results in the formation of tensile
stresses in the weld area and lowers the weld joint strength.
As an extension to this study, it is also recommended that the further study should be
performed to study the effects of other phases like ferrite, bainite and pearlite on
welding residual stresses.
It is also recommended that the students of Metallurgical Engineering must be taught
the basics of FEM, so that they can get accustomed to it and utilize this innovative
technique to evaluate the evaluate the behaviour of a variety of materials operating
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under different conditions. The study of FEM offers a futuristic outlook which opens
doors to new advancements in the field of metallurgy.
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