Effect of Different Metalurgical Phases on the Welding Residual Stresses of Base Metal

8/10/2019 Effect of Different Metalurgical Phases on the Welding Residual Stresses of Base Metal

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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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REFERENCES

[1] Lu, M., and Kou, S., Weld. J., 67: 29s, 1988.

[2] Kou, S., and Le,Y., Metall. Trans. A, 14A: 2245, 1983.

[3] Rosenthal, D., Weld. J., 20: 220s, 1941.

[4] Smolik, G., private communication, Idaho National Engineering

Laboratories,Idaho Falls, Idaho, 1984.

[5] Kou, S., and Le,Y., Metall. Trans.A, 13A: 1141, 1982.

[6] Lee, J.Y., Park, J. M., Lee, C. H., and Yoon, E. P., in Synthesis/Processing of

Lightweight Metallic Materials II, Eds. C. M.Ward-Close, F. H. Froes, S. S.

Cho, and D. J. Chellman,The Minerals, Metals and Materials

Society,Warrendale, PA, 1996, p. 49.

[9] Kihara, H., Suzuki, H., and Tamura, H., Researches on Weldable High-Strength

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Japan,Tokyo, 1957.

[10] Liu, S., Brandi, S. D., and Thomas, R. D., in ASM Handbook, Vol. 6, ASM

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[11] Inagaki, M and Sekiguchi, H., Trans. Nat. Res. Inst. Metals, Tokyo, Japan, 2(2):

102(1960)[12] Kou, Sindo, Welding Metallurgy, Second Edition, Wiley-Interscience

Publication, United States of America, 2002.

[14] Ravi Vishnu P. Solid-State Transformations in Weldments. ASM handbook,

1994, Vol. 6

[15] Deng, Dean, FEM prediction of welding residual stresses and distortion in

carbon steel considering phase transformation.

[16] Lidam RN, Yupiter HPM, Redza MR, Rahim MR, Sulaiman MS, Zakaria MY,

Tham G, Abasa SK, Haruman E, Chau CY, Simulation Study on Multipassed

Welding Distortion of Combined Joint Types using Thermo-Elastic-Plastic

FEM, The Journal of Engineering Research, 2012, Vol. 9

[17] Cho SH, Kim JW, Analysis of welding residual stress in carbon steel weldment

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[18] Tehler Matilda, Jonsson Stefan, A material model for simulating volume

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Effect of Different Metalurgical Phases on the Welding Residual Stresses of Base Metal