DEPARTMENT OF MECHANICAL ENGINEERING PROJECT REPORT (ME-719) “EFFECT OF HEAT INPUT ON THE MICROSTRUCTURE OF THE WELD BEAD IN SAW WELDING USING THE RECYCLED SLAG.” SUBMITTED BY RAHUL VIKRAM (04231169) GWT/7709/04 DEEPAK KUMAR CHOUDHARY (04231151) GWT/7703/04 SUBHASH CHANDER (04231177) GWT/7770/04 KOUSTOV MONDOL (04231158) GWT/7771/04 UNDER GUIDANCE Mr. KULWANT SINGH (Assistance Professor Mech. Engg. Deptt.) Submitted to Department of Mechanical Engineering in the partial fulfillment for the Degree programme in MECHANICAL ENGINEERING (Specialization in Welding Technology) SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY (Estd. by Govt. of India) LONGOWAL- 148106 DECEMBER 2006 SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 1
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DEPARTMENT OF MECHANICAL ENGINEERING
PROJECT REPORT (ME-719)
“EFFECT OF HEAT INPUT ON THE MICROSTRUCTURE OF THE WELD BEAD IN SAW WELDING USING THE RECYCLED SLAG.”
Mr. KULWANT SINGH (Assistance Professor Mech. Engg. Deptt.)
Submitted to Department of Mechanical Engineering in the partial fulfillment for the
Degree programme in
MECHANICAL ENGINEERING (Specialization in Welding Technology)
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY (Estd. by Govt. of India) LONGOWAL- 148106
DECEMBER 2006
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 1
DEPARTMENT OF MECHANICAL ENGINEERING
CERTIFICATE
This is to certify that the project entitled “EFFECT OF HEAT INPUT ON THE
MICROSTRUCTURE OF THE WELD BEAD (IN SAW WELDING USING THE
RECYCLED SLAG.” being submitted by:
RAHUL VIKRAM (04231169) GWT/7709/04
DEEPAK KUMAR CHOUDHARY (04231151) GWT/7703/04
SUBHASH CHANDER (04231177) GWT/7770/04
KOUSTOV MONDOL (04231158) GWT/7771/04
To Sant Longowal Institute of Engineering & Technology towards the partial fulfillment
of the requirements for the award of B.Tech degree is the record of bonafide work
carried out by students under my supervision and guidance.
(Signature)
Mr. KULWANT SINGH
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 2
(Assistance Professor Mech. Engg. Deptt.)
DEPARTMENT OF MECHANICAL ENGINEERING
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 3
SR. NO
CONTENTS
PAGE NO.
1
Acknowledgement
i
2
Abstract
ii
3
List of figures
iii
4
INTRODUCTION 4.1 Introduction to SAW process 4.2 Equipments 4.3 Advantages and Major Uses 4.4 Limitations of the Process 4.5 Principles of Operation 4.6 Applications
1 2 2 4 4 5 10
5
METALLURGY OF WELD METAL 5.1 Iron –iron carbide Equilibrium Diagram 5.2 Transformation and Microstructure 5.3 Continuous Cooling Transformation Diagrams 5.4 Properties of weld metal 5.5 Microstructural products in weldments
11 11 17 17 20 23
6
WELDING: SOLIDIFICATION AND MICROSTRUCTURE
26
7
EXPERIMENTATION 7.1 Agglomerated Fluxes 7.2 Processing of Slag 7.3 Process variables 7.4 Bead on plate using Recycled slag 7.5 Specimen preparation
30 30 31 33 35 36
DEPARTMENT OF MECHANICAL ENGINEERING
8 METALLURGICAL INVESTIGATION 8.1 Grain size determination 8.2 Specimen preparation for optical microscopy 8.3 Microstructures 8.4 Properties of weld zone of weld bead (B3, B4, B5, B6) 8.5 Properties of base metal of weld bead of weld bead (B3, B4, B5, B6) 8.6 Properties of heat affected zone of weld bead (B3, B4, B5, B6) 8.7 Work piece samples
40 42 43 44 53 57 57 58
9
RESULT AND DISCUSSSION 9.1 Calculation for finding heat input for four different weld beads
59 60
10
CONCLUSION
62
11
Reference
64
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 4
DEPARTMENT OF MECHANICAL ENGINEERING
ACKNOWLEDGEMENT
We deem it as a proud privilege to express our sincere regard and
gratitude to Mr. KULWANT SINGH (Assistance Professor Mech. Engg. Deptt.),
who as a project guide provided this valuable opportunity to pursue this work with him.
His sincere support and continuous guidance helped us in overcoming all the hurdles
that came during the progress of this project.
We hereby express our deep gratitude to DR. ANAND J. VAZ HOD
(Mechanical Engg.) for extending a hand of support whenever needed in the practical
work.
We would like to show our deep regards to SRI. A.S. SHAHI Project co-
coordinator, Deptt. Of Mech. Engg. SLIET and his valuable assistance regarding project
submission.
And last but not least, we are grateful our worthy Director, for her deep concern
about formulating the syllabus of degree module.
RAHUL VIKRAM
DEEPAK KUMAR CHOUDHARY
SUBHASH CHANDER
KOUSTOV MONDOL
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 5
DEPARTMENT OF MECHANICAL ENGINEERING
ABSTRACT
The metallurgical and mechanical properties of weld metal depend upon its
microstructure which is governed by the rate of heat input. The rate of heat input is
further dependant on the welding parameters. Weld metal generally constitutes of grain
boundary ferrite, polygonal ferrite and acicular ferrite. In order to have maximum
toughness and good strength it is desirable to obtain maximum amount of acicular ferrite
in the weld metal.
So, investigations were carried out to study the microstructure of weld metal.
Beads on plates were deposited at various heat input by changing the welding parameters
accordingly. The microstructure of the base metal, heat affected zone and weld metal was
investigated with metallurgical microscope. The microstructures were examined and
analyses were done. It was found out that the weld metal which was welded with highest
heat input has maximum concentration of acicular ferrite in its weld zone. As we know
that the weld metal that contains maximum concentration of acicular ferrite has better
mechanical properties and thus this type of a microstructure is desirable.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 6
DEPARTMENT OF MECHANICAL ENGINEERING
List of Figures
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 7
Fig No. Explanation
1 Block diagram of Submerged Arc Welding Process
2 The 3D diagram of the SAW process
3 Process diagram – Submerged Arc Welding
4 Iron –iron carbide Equilibrium Diagram
5 Three distinct regions in the weldment are the fusion zone, the heat
affected zone, and the base metal.
6 The fusion line dividing the base metal with the welded part.
7 (a) CCT and (b) TTT diagrams.
8 A Submerged Arc Welding machine
9 An Automatic Polishing Machine with Two Rotating Discs
10 Optical Microscope Fitted with 35mm Camera and Digital Camera
11 SAMPLE NO: B3
12 SAMPLE NO: B4
13 SAMPLE NO: B5
14 SAMPLE NO: B6
15 WELD ZONE OF SAMPLE B3
16 WELD ZONE OF SAMPLE B4
17 WELD BEAD ZONE OF SAMPLE B5
18 WELD ZONE OF SAMPLE B6
19 The welded metal specimens over which the microstructures are being
analyzed.
20 Effect of Heat Input on amount of Acicular Ferrite
21 A schematic CCT Diagram for a weld deposit showing the relationship
of the acicular ferrite phase field to those of other constituents.
DEPARTMENT OF MECHANICAL ENGINEERING
EFFECT OF HEAT INPUT ON THE MICROSTRUCTURE
OF THE WELD BEAD IN SUBMERGED ARC WELDING
USING THE RECYCLED SLAG
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 8
DEPARTMENT OF MECHANICAL ENGINEERING
INTRODUCTION
“EFFECT OF HEAT INPUT ON THE MICROSTRUCTURE OF THE WELD
BEAD IN SAW WELDING USING THE RECYCLED SLAG.”
Figure 1. Block diagram of Submerged Arc Welding Process
In submerged arc welding (SAW), selecting appropriate values for Process
variables are essential in order to control weld bead and heat-affected zone (HAZ)
dimensions and get the required bead size and quality. Also, conditions must be selected
that will ensure a predictable and reproducible weld bead, which is critical for obtaining
high quality. In this investigation, mathematical models were developed to study the
effects of heat input on the microstructure of the weld bead (M.S.) in SAW welding using
the recycled flux.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 9
DEPARTMENT OF MECHANICAL ENGINEERING
INTRODUCTION TO SAW PROCESS
Figure 2. The 3D diagram of the SAW process
Submerged arc welding is a process in which the joining of metals is produced by
heating with an arc or arcs between a bare metal electrode or electrodes and the work.
The arc is shielded by a blanket of granular fusible material on the work. Pressure is not
used. Filler metal is obtained from the electrode or from a supplementary welding rod.
Equipment.
(1) The equipment components required for submerged arc welding are shown by figure.
Equipment consists of a welding machine or power source, the wire feeder and control
system, the welding torch for automatic welding or the welding gun and cable assembly
for semiautomatic welding, the flux hopper and feeding mechanism, usually a flux
recovery system, and a travel mechanism for automatic welding.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 10
(2) The power source for submerged arc welding must be rated for a 100 percent duty
cycle, since the submerged arc welding operations are continuous and the length of time
for making a weld may exceed 10 minutes. If a 60 percent duty cycle power source is
used, it must be derated according to the duty cycle curve for 100 percent operation.
DEPARTMENT OF MECHANICAL ENGINEERING
(3) When constant current is used, either ac or dc, the voltage sensing electrode wire
feeder system must be used. When constant voltage is used, the simpler fixed speed wire
feeder system is used. The CV system is only used with direct current.
(4) Both generator and transformer-rectifier power sources are used, but the rectifier
machines are more popular. Welding machines for submerged arc welding range in size
from 300 amperes to 1500 amperes. They may be connected in parallel to provide extra
power for high-current applications. Direct current power is used for semiautomatic
applications, but alternating current power is used primarily with the machine or the
automatic method. Multiple electrode systems require specialized types of circuits,
especially when ac is employed.
(5) For semiautomatic application, a welding gun and cable assembly are used to carry
the electrode and current and to provide the flux at the arc. A small flux hopper is
attached to the end of the cable assembly. The electrode wire is fed through the bottom of
this flux hopper through a current pickup tip to the arc. The flux is fed from the hopper to
the welding area by means of gravity. The amount of flux fed depends on how high the
gun is held above the work. The hopper gun may include a start switch to initiate the
weld or it may utilize a "hot" electrode so that when the electrode is touched to the work,
feeding will begin automatically.
(6) For automatic welding, the torch is attached to the wire feed motor and includes
current pickup tips for transmitting the welding current to the electrode wire. The flux
hopper is normally attached to the torch, and may have magnetically operated valves
which can be opened or closed by the control system.
(7) Other pieces of equipment sometimes used may include a travel carriage, which can
be a simple tractor or a complex moving specialized fixture. A flux recovery unit is
normally provided to collect the unused submerged arc flux and return it to the supply
hopper.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 11
(8) Submerged arc welding system can become quite complex by incorporating
additional devices such as seam followers, weavers, and work rovers.
DEPARTMENT OF MECHANICAL ENGINEERING
Advantages and Major Uses.
(1) The major advantages of the submerged arc welding process are:
(a) High quality of the weld metal.
(b) Extremely high deposition rate and speed.
(c) Smooth, uniform finished weld with no spatter.
(d) Little or no smoke.
(e) No arc flash, thus minimal need for protective clothing.
(f) High utilization of electrode wire.
(g) Easy automation for high-operator factor.
(h) Normally, no involvement of manipulative skills.
(2) The submerged arc process is widely used in heavy steel plate fabrication work. This
includes the welding of structural shapes, the longitudinal seam of larger diameter pipe,
the manufacture of machine components for all types of heavy industry, and the
manufacture of vessels and tanks for pressure and storage use. It is widely used in the
shipbuilding industry for splicing and fabricating subassemblies, and by many other
industries where steels are used in medium to heavy thicknesses. It is also used for
surfacing and buildup work, maintenance, and repair.
Limitations of the Process.
(1) A major limitation of submerged arc welding is its limitation of welding positions.
The other limitation is that it is primarily used only to weld mild and low-alloy high-
strength steels.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 12
(2) The high-heat input, slow-cooling cycle can be a problem when welding quenched
and tempered steels. The heat input limitation of the steel in question must be strictly
adhered to when using submerged arc welding. This may require the making of multipass
welds where a single pass weld would be acceptable in mild steel. In some cases, the
economic advantages may be reduced to the point where flux-cored arc welding or some
other process should be considered.
DEPARTMENT OF MECHANICAL ENGINEERING
(3) In semiautomatic submerged arc welding, the inability to see the arc and puddle can
be a disadvantage in reaching the root of a groove weld and properly filling or sizing.
Principles of Operation.
(1) The submerged arc welding process is shown by figure.
It utilizes the heat of an arc between a continuously fed electrode and the work.
Figure 3. Process diagram – Submerged Arc Welding
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 13
The heat of the arc melts the surface of the base metal and the end of the
electrode. The metal melted off the electrode is transferred through the arc to the work
piece, where it becomes the deposited weld metal. Shielding is obtained from a blanket of
granular flux, which is laid directly over the weld area. The flux close to the arc melts
and intermixes with the molten weld metal, helping to purify and fortify it. The flux
forms a glass-like slag that is lighter in weight than the deposited weld metal and floats
on the surface as a protective cover. The weld is submerged under this layer of flux and
slag, hence the name submerged arc welding. The flux and slag normally cover the arc so
that it is not visible. The Unmelted portion of the flux can be reused. The electrode is fed
into the arc automatically from a coil. The arc is maintained automatically. Travel can be
manual or by machine. The arc is initiated by a fuse type start or by a reversing or retracts
system.
DEPARTMENT OF MECHANICAL ENGINEERING
(2) Normal method of application and position capabilities.
The most popular method of application is the machine method, where the
operator monitors the welding operation. Second in popularity is the automatic method,
where welding is a pushbutton operation. The process can be applied semi automatically;
however, this method of application is not too popular. The process cannot be applied
manually because it is impossible for a welder to control an arc that is not visible. The
submerged arc welding process is a limited-position welding process. The welding
positions are limited because the large pool of molten metal and the slag are very fluid
and will tend to run out of the joint. Welding can be done in the flat position and in the
horizontal fillet position with ease. Under special controlled procedures, it is possible to
weld in the horizontal position, sometimes called 3 o'clock welding. This requires special
devices to hold the flux up so that the molten slag and weld metal cannot run away. The
process cannot be used in the vertical or overhead position.
(3) Metals weldable and thickness range.
Submerged arc welding is used to weld low- and medium-carbon steels, low-alloy
high-strength steels, quenched and tempered steels, and many stainless steels.
Experimentally, it has been used to weld certain copper alloys, nickel alloys, and even
uranium. Metal thicknesses from 1/16 to 1/2 in. (1.6 to 12.7 mm) can be welded with no
edge preparation. With edge preparation, welds can be made with a single pass on
material from 1/4 to 1 in. (6.4 to 25.4 mm). When multipass technique is used, the
maximum thickness is practically unlimited. This information is summarized in table 10-
22. Horizontal fillet welds can be made up to 3/8 in. (9.5 mm) in a single pass and in the
flat position, fillet welds can be made up to 1 in. (25 mm) size.
(4) Joint design.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 14
Although the submerged arc welding process can utilize the same joint design details as
the shielded metal arc welding process, different joint details are suggested for maximum
utilization and efficiency of submerged arc welding. For groove welds, the square groove
design can be used up to 5/8 in. (16 mm) thickness. Beyond this thickness, bevels are
DEPARTMENT OF MECHANICAL ENGINEERING
required. Open roots are used but backing bars are necessary since the molten metal will
run through the joint. When welding thicker metal, if a sufficiently large root face is used,
the backing bar may be eliminate. However, to assure full penetration when welding from
one side, backing bars are recommended. Where both sides are accessible, a backing
weld can be made which will fuse into the original weld to provide full penetration.
(5) Welding circuit and current.
(a) The welding circuit employed for single electrode submerged arc welding is
shown by figure. This requires a wire feeder system and a power supply.
(b) The submerged arc welding process uses either direct or alternating current for
welding power. Direct current is used for most applications which use a single arc. Both
direct current electrode positive (DCEP) and electrode negative (DCEN) are used.
(c) The constant voltage type of direct current power is more popular for
submerged arc welding with 1/8 in. (3.2 mm) and smaller diameter electrode wires.
(d) The constant current power system is normally used for welding with 5/3 2 in.
(4 mm) and larger-diameter electrode wires. The control circuit for CC power is more
complex since it attempts to duplicate the actions of the welder to retain a specific arc
length. The wire feed system must sense the voltage across the arc and feed the electrode
wire into the arc to maintain this voltage. As conditions change, the wire feed must slow
down or speed up to maintain the prefixed voltage across the arc. This adds complexity to
the control system. The system cannot react instantaneously. Arc starting is more
complicated with the constant current system since it requires the use of a reversing
system to strike the arc, retract, and then maintain the preset arc voltage.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 15
(e) For ac welding, the constant current power is always used. When multiple
electrode wire systems are used with both ac and dc arcs, the constant current power
system is utilized. The constant voltage system, however, can be applied when two wires
are fed into the arc supplied by a single power source. Welding current for submerged arc
DEPARTMENT OF MECHANICAL ENGINEERING
welding can vary from as low as 50 amperes to as high as 2000 amperes. Most
submerged arc welding is done in the range of 200 to 1200 amperes.
(6) Deposition rates and weld quality.
(a) The deposition rates of the submerged arc welding process are higher than any
other arc welding process. Deposition rates for single electrodes are shown by figure 10-
62. There are at least four related factors that control the deposition rate of submerged arc
welding: polarity, long stickout, additives in the flux, and additional electrodes. The
deposition rate is the highest for direct current electrode negative (DCEN). The
deposition rate for alternating current is between DCEP and DCEN. The polarity of
maximum heat is the negative pole.
(b) The deposition rate with any welding current can be increased by extending
the "stickout." This is the distance from the point where current is introduced into the
electrode to the arc. When using "long stickout" the amount of penetration is reduced.
The deposition rates can be increased by metal additives in the submerged arc flux.
Additional electrodes can be used to increase the overall deposition rate.
(c) The quality of the weld metal deposited by the submerged arc welding process
is high. The weld metal strength and ductility exceeds that of the mild steel or low-alloy
base material when the correct combination of electrode wire and submerged arc flux is
used. When submerged arc welds are made by machine or automatically, the human
factor inherent to the manual welding processes is eliminated. The weld will be more
uniform and free from inconsistencies. In general, the weld bead size per pass is much
greater with submerged arc welding than with any of the other arc welding processes.
The heat input is higher and cooling rates are slower. For this reason, gases are allowed
more time to escape. Additionally, since the submerged arc slag is lower in density than
the weld metal, it will float out to the top of the weld. Uniformity and consistency are
advantages of this process when applied automatically.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 16
(d) Several problems may occur when using the semiautomatic application
method. The electrode wire may be curved when it leaves the nozzle of the welding gun.
DEPARTMENT OF MECHANICAL ENGINEERING
This curvature can cause the arc to be struck in a location not expected by the welder.
When welding in fairly deep grooves, the curvature may cause the arc to be against one
side of the weld joint rather than at the root. This will cause incomplete root fusion. Flux
will be trapped at the root of the weld. Another problem with semiautomatic welding is
that of completely filling the weld groove or maintaining exact size, since the weld is
hidden and cannot be observed while it is being made. This requires making an extra
pass. In some cases, too much weld is deposited. Variations in root opening affect the
travel speed. If travel speed is uniform, the weld may be under- or overfilled in different
areas. High operator skill will overcome this problem.
(e) There is another quality problem associated with extremely large single-pass
weld deposits. When these large welds solidify, the impurities in the melted base metal
and in the weld metal all collect at the last point to freeze, which is the centerline of the
weld. If there is sufficient restraint and enough impurities are collected at this point,
centerline cracking may occur. This can happen when making large single-pass flat fillet
welds if the base metal plates are 45º from flat. A simple solution is to avoid placing the
parts at a true 45º angle. It should be varied approximately 10º so that the root of the joint
is not in line with the centerline of the fillet weld. Another solution is to make multiple
passes rather than attempting to make a large weld in a single pass.
(f) Another quality problem has to do with the hardness of the deposited weld
metal. Excessively hard weld deposits contribute to cracking of the weld during
fabrication or during service. A maximum hardness level of 225 Brinell is recommended.
The reason for the hard weld in carbon and low-alloy steels is too rapid cooling,
inadequate postweld treatment, or excessive alloy pickup in the weld metal. Excessive
alloy pickup is due to selecting an electrode that has too much alloy, selecting a flux that
introduces too much alloy into the weld, or the use of excessively high welding voltages.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 17
(g) In automatic and machine welding, defects may occur at the start or at the end
of the weld. The best solution is to use runout tabs so that starts and stops will be on the
tabs rather than on the product.
DEPARTMENT OF MECHANICAL ENGINEERING
(7) Weld schedules.
The submerged arc welding process applied by machine or fully automatically
should be done in accordance with welding procedure schedules. The table can be used
for welding other ferrous materials, but was developed for mild steel. All of the welds
made by this procedure should pass qualification, tests, assuming that the correct
electrode and flux have been selected. If the schedules are varied more than 10 percent,
qualification tests should be performed to determine the weld quality.
Applications
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 18
SAW is ideally suited for longitudinal and circumferential butt and fillet welds.
However, because of high fluidity of the weld pool, molten slag and loose flux layer,
welding is generally carried out on butt joints in the flat position and fillet joints in both
the flat and horizontal-vertical positions. For circumferential joints, the workpiece is
rotated under a fixed welding head with welding taking place in the flat position.
Depending on material thickness, either single-pass, two-pass or multipass weld
procedures can be carried out. There is virtually no restriction on the material thickness,
provided a suitable joint preparation is adopted. Most commonly welded materials are
carbon-manganese steels, low alloy steels and stainless steels, although the process is
capable of welding some non-ferrous materials with judicious choice of electrode filler
wire and flux combinations.
DEPARTMENT OF MECHANICAL ENGINEERING
METALLURGY OF WELD METAL
Iron –iron carbide Equilibrium Diagram
A study of the constitution and structure of all steels and irons must first start with
the iron-carbon equilibrium diagram. Many of the basic features of this system (Fig. 1)
influence the behavior of even the most complex alloy steels. For example, the phases
found in the simple binary Fe-C system persist in complex steels, but it is necessary to
examine the effects alloying elements have on the formation and properties of these
phases. The iron-carbon diagram provides a valuable foundation on which to build
knowledge of both plain carbon and alloy steels in their immense variety.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 19
Figure4. Iron –iron carbide Equilibrium Diagram
DEPARTMENT OF MECHANICAL ENGINEERING
The Iron–Iron Carbide (Fe–Fe3C) Phase Diagram
This is one of the most important alloys for structural applications. The diagram
Fe—C is simplified at low carbon concentrations by assuming it is the Fe—Fe3C
diagram. Concentrations are usually given in weight percent. The possible phases are:
• α-ferrite (BCC) Fe-C solution
• γ-austenite (FCC) Fe-C solution
• δ-ferrite (BCC) Fe-C solution
• liquid Fe-C solution
• Fe3C (iron carbide) or cementite. An intermetallic compound.
The maximum solubility of C in α- ferrite is 0.022 wt%. δ−ferrite is only stable at
high temperatures. It is not important in practice. Austenite has a maximum C
concentration of 2.14 wt %. It is not stable below the eutectic temperature (727 C) unless
cooled rapidly (Chapter 10). Cementite is in reality metastable, decomposing into α-Fe
and C when heated for several years between 650 and 770 C.
For their role in mechanical properties of the alloy, it is important to note that:
• Ferrite is soft and ductile
• Cementite is hard and brittle
Thus, combining these two phases in solution an alloy can be obtained with
intermediate properties. (Mechanical properties also depend on the microstructure, that is,
how ferrite and cementite are mixed.)
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 20
DEPARTMENT OF MECHANICAL ENGINEERING
It should first be pointed out that the normal equilibrium diagram really represents
the metastable equilibrium between iron and iron carbide (cementite). Cementite is
metastable, and the true equilibrium should be between iron and graphite. Although
graphite occurs extensively in cast irons (2-4 wt % C), it is usually difficult to obtain this
equilibrium phase in steels (0.03-1.5 wt %C). Therefore, the metastable equilibrium
between iron and iron carbide should be considered, because it is relevant to the behavior
of most steels in practice.
The much larger phase field of γ-iron (austenite) compared with that of α-iron
(ferrite) reflects the much greater solubility of carbon in γ-iron, with a maximum value of
just over 2 wt % at 1147°C (E, Fig.1). This high solubility of carbon in γ-iron is of
extreme importance in heat treatment, when solution treatment in the γ-region followed
by rapid quenching to room temperature allows a supersaturated solid solution of carbon
in iron to be formed.
The α-iron phase field is severely restricted, with a maximum carbon solubility of
0.02 wt% at 723°C (P), so over the carbon range encountered in steels from 0.05 to 1.5
wt%, α-iron is normally associated with iron carbide in one form or another. Similarly,
the δ-phase field is very restricted between 1390 and 1534°C and disappears completely
when the carbon content reaches 0.5 wt% (B).
There are several temperatures or critical points in the diagram, which are
important, both from the basic and from the practical point of view.
Firstly, there is the A1, temperature at which the eutectoid reaction occurs (P-S-
K), which is 723°C in the binary diagram.
Secondly, there is the A3, temperature when α-iron transforms to γ-iron. For pure
iron this occurs at 910°C, but the transformation temperature is progressively lowered
along the line GS by the addition of carbon.
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 21
The third point is A4 at which γ-iron transforms to δ-iron, 1390°C in pure iron,
hut this is raised as carbon is added. The A2, point is the Curie point when iron changes
DEPARTMENT OF MECHANICAL ENGINEERING
from the ferro- to the paramagnetic condition. This temperature is 769°C for pure iron,
but no change in crystal structure is involved. The A1, A3 and A4 points are easily
detected by thermal analysis or dilatometry during cooling or heating cycles, and some
hysteresis is observed. Consequently, three values for each point can be obtained. Ac for
heating, Ar for cooling and Ae (equilibrium}, but it should be emphasized that the Ac and
Ar values will be sensitive to the rates of heating and cooling, as well as to the presence
of alloying elements.
The great difference in carbon solubility between γ- and α-iron leads normally to
the rejection of carbon as iron carbide at the boundaries of the γ phase field. The
transformation of γ to α - iron occurs via a eutectoid reaction, which plays a dominant
role in heat treatment.
The eutectoid temperature is 723°C while the eutectoid composition is 0.80%
C(s). On cooling alloys containing less than 0,80% C slowly, hypo-eutectoid ferrite is
formed from austenite in the range 910-723°C with enrichment of the residual austenite
in carbon, until at 723°C the remaining austenite, now containing 0.8% carbon transforms
to pearlite, a lamellar mixture of ferrite and iron carbide (cementite). In austenite with
0,80 to 2,06% carbon, on cooling slowly in the temperature interval 1147°C to 723°C,
cementite first forms progressively depleting the austenite in carbon, until at 723°C, the
austenite contains 0.8% carbon and transforms to pearlite.
Steels with less than about 0.8% carbon are thus hypo-eutectoid alloys with ferrite
and pearlite as the prime constituents, the relative volume fractions being determined by
the lever rule which states that as the carbon content is increased, the volume percentage
of pearlite increases, until it is 100% at the eutectoid composition. Above 0.8% C,
cementite becomes the hyper-eutectoid phase, and a similar variation in volume fraction
of cementite and pearlite occurs on this side of the eutectoid composition.
The three phases, ferrite, cementite and pearlite are thus the principle constituents
of the infrastructure of plain carbon steels, provided they have been subjected to
relatively slow cooling rates to avoid the formation of metastable phases.
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DEPARTMENT OF MECHANICAL ENGINEERING
The austenite- ferrite transformation
Under equilibrium conditions, pro-eutectoid ferrite will form in iron-carbon alloys
containing up to 0.8 % carbon. The reaction occurs at 910°C in pure iron, but takes place
between 910°C and 723°C in iron-carbon alloys.
However, by quenching from the austenitic state to temperatures below the
eutectoid temperature Ae1, ferrite can be formed down to temperatures as low as 600°C.
There are pronounced morphological changes as the transformation temperature is
lowered, which it should be emphasized apply in general to hypo-and hyper-eutectoid
phases, although in each case there will be variations due to the precise crystallography
of the phases involved. For example, the same principles apply to the formation of
cementite from austenite, but it is not difficult to distinguish ferrite from cementite
morphologically.
The austenite-cementite transformation
The Dube classification applies equally well to the various morphologies of
cementite formed at progressively lower transformation temperatures. The initial
development of grain boundary allotriomorphs is very similar to that of ferrite, and the
growth of side plates or Widmanstaten cementite follows the same pattern. The cementite
plates are more rigorously crystallographic in form, despite the fact that the orientation
relationship with austenite is a more complex one.
As in the case of ferrite, most of the side plates originate from grain boundary
allotriomorphs, but in the cementite reaction more side plates nucleate at twin boundaries
in austenite.
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DEPARTMENT OF MECHANICAL ENGINEERING
The austenite-pearlite reaction
Pearlite is probably the most familiar micro structural feature in the whole science
of metallography. It was discovered by Sorby over 100 years ago, who correctly assumed
it to be a lamellar mixture of iron and iron carbide.
Pearlite is a very common constituent of a wide variety of steels, where it
provides a substantial contribution to strength. Lamellar eutectoid structures of this type
are widespread in metallurgy, and frequently pearlite is used as a generic term to describe
them.
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These structures have much in common with the cellular precipitation reactions.
Both types of reaction occur by nucleation and growth, and are, therefore, diffusion
controlled. Pearlite nuclei occur on austenite grain boundaries, but it is clear that they can
also be associated with both pro-eutectoid ferrite and cementite. In commercial steels,
pearlite nodules can nucleate on inclusions.
DEPARTMENT OF MECHANICAL ENGINEERING
Transformation and Microstructure
Introduction
The goal is to obtain specific microstructures that will improve the mechanical
properties of a metal, in addition to grain-size refinement, solid-solution strengthening,
and strain-hardening.
Basic Concepts
Phase transformations that involve a change in the microstructure can occur through:
• Diffusion
• Maintaining the type and number of phases (e.g., solidification of a pure metal,
(Fe2O3), and diamond compound. With the exception of diamond compound these
abrasive are normally used in a distilled water suspension, but if the metal to be polished
is not compatible with water, other suspensions, such as ethylene glycol, alcohol,
kerosene or glycerin, may be required. The diamond compounds should be extended only
with the carrier recommended by the manufacture.
Aluminum oxide (alumina) is the polishing abrasive most widely used for general metal.
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DEPARTMENT OF MECHANICAL ENGINEERING
METALLURGICAL INVESTIGATION
Metallography is the study of metals by optical and electron microscopes.
Structures which are coarse enough to be discernible by the naked eye or under low
magnifications are termed macrostructures. Useful information can often be gained by
examination with the naked eye of the surface of metal objects or polished and etched
sections. Those which require high magnification to be visible are termed
microstructures. Microscopes are required for the examination of the microstructure of
the metals. Optical microscopes are used for resolutions down to roughly the wavelength
of light (about half a micron) and electron microscope are used for detail below this level,
down to atomic resolution. The most commonly used microscope is the conventional
light microscope. In principle, optical microscopes may be used to look through
specimens (‘in transmission’) as well as at them (‘in reflection’). Many materials,
however, do not transmit light and so we are restricted to looking at the surface of the
specimens with an optical microscope. Electron microscope can be used in the
transmission e.g. Transmission Electron Microscope (TEM) and to look at the surfaces
e.g. Scanning Electron Microscope (SEM) Microscopy can give information concerning a
material’s composition, previous treatment and properties. Particular features of interest
are
(I) Grain size
(II) Phases present
(III) Chemical homogeneity
(IV) Distribution of phases
(V) Elongated structures formed by plastic deformation
Optical Microscopy
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With optical microscopy, the light microscope is used to study the microstructure;
optical illumination systems are its basic elements. For materials that are opaque to
visible light (all metals, many ceramics and polymers), only the surface is subject to
DEPARTMENT OF MECHANICAL ENGINEERING
observation, and the light microscope must be used in a reflective mode. Contrasts in the
image produced result from differences in reflectivity of the various regions of the
microstructure. Careful and meticulous surface preparations are necessary to reveal the
important details of the microstructure. The specimen surface must first be ground and
polished to a smooth and mirror like finish. This is accomplished by using successively
finer abrasive papers and powders. The microstructure is revealed by a surface treatment
using an appropriate chemical reagent in a procedure termed etching. The etching
reagents depend on the material used and after etching the specimen must be washed with
alcohol and ether to remove the grease. The atoms at the grain boundaries are chemically
more active, and consequently dissolve more readily than those within the grains forming
small grooves. These grooves become discernible when viewed under a microscope
because they reflect light at an angle different from that of the grains themselves.
When the microstructure of a two phase alloy is to be examined, an enchant is
chosen that produces a different texture for each phase so that the different phases may be
distinguished from each other. The maximum possible magnification with an electron
microscope is approximately 2000 diameters.
Electron Microscopy:
Transmission Electron Microscopy (TEM):
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 49
The image seen with a TEM is formed by an electron beam that passes through
the specimen. Details of the internal microstructural features are accessible to
observation; contrasts in the image are produced by differences in beam scattering or
diffraction produced between various elements of the microstructure or defect. In the
TEM, electrons are focused on an extremely thin foil of the material; the beam of
electrons interacts with imperfections in the material, causing differences in the fraction
of electrons that are transmitted. The transmitted beam is projected onto a fluorescent
screen or a photographic film so that the image may be viewed. Magnifications
approaching 1000000x are possible with TEM. The TEM is used to observe dislocations.
DEPARTMENT OF MECHANICAL ENGINEERING
Scanning Electron Microscopy (SEM):
The surface to be examined is scanned with an electron beam, and the reflected
beam of electrons is collected, then displayed at the same scanning rate on a cathode ray
tube. The image that appears on the screen, which may be photographed, represents the
surface features of the specimen. The surface may or may not be polished and etched, but
it must be electrically conductive; a very thin metallic coating must be applied to non
conductive materials. Magnifications ranging from 10 to in excess of 50 000 diameters
and also very great depths of field are possible.
GRAIN SIZE DETERMINATION:
The grain size of metals is usually expressed as the American Society for Testing
and Materials (ASTM) grain size number. The ASTM has prepared 10 standards
comparison charts, all having different average grain sizes. To each is assigned a grain
size number, n, ranging from 1to 10, the larger the number, the smaller the grains. The
designation is based on the equation, N = 2n-1
where, N is the number of grains in an area of 1 sq. in at 100x magnification.
A specimen must be properly prepared to reveal the grain structure, which is
photographed at a magnification of 100x. Thus a steel with N=6 has, on average, 32
grains in an area of 1 sq. in. at 100x. Grain size is expressed as the grain size number of
the chart that most nearly matches the grains in the micrograph. Grain size may also be
determined using an intercept method described below. Straight lines all of the same
length are drawn through several photomicrographs that show the grain structure. The
grains intersected by each line segment are counted; the line length is then divided by an
average of the number of grains intersected, taken over all the line segments. The average
grain diameter is found by dividing this result by the linear magnification of the
micrographs.
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DEPARTMENT OF MECHANICAL ENGINEERING
SPECIMEN PREPARATION FOR OPTICAL MICROSCOPY
The examination of materials by optical microscopy is essential in order to
understand the relationship between properties and microstructure. Metallography is the
study of metals by optical examination. This is most commonly done using a
conventional light microscope. However useful information can be gained by
examination with the naked eye of the surface of metal objects or of polished and etched
sections. Structures which are coarse enough to be discernible be the naked eye is termed
macrostructures. Those which require magnification to be visible are termed
microstructures.
Figure 10: Optical Microscope Fitted with 35mm Camera and Digital Camera
SANT LONGOWAL INSTITUTE OF ENGINEERING AND TECHNOLOGY 51
DEPARTMENT OF MECHANICAL ENGINEERING
MICROSTRUCTURES
The preparation of a specimen to reveal its microstructure involves.
• Sawing the section to be examined
• Mounting in resins (if sample is too small)
• Coarse grinding
• Grinding on progressively finer emery paper
• Polishing using alumina powder or diamond paste on rotating wheel
• Etching in dilute acid (2% Nital for steel)
• Washing in Alcohol and drying
• Typical magnifications used are between 50x and 1000x
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DEPARTMENT OF MECHANICAL ENGINEERING
Figure 11. SAMPLE NO: B3
(A). WELD AND BASE METAL
(B). BASE METAL
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DEPARTMENT OF MECHANICAL ENGINEERING
(C). WELD ZONE
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DEPARTMENT OF MECHANICAL ENGINEERING
Figure 12. SAMPLE NO: B4
(A). WELD AND BASE METAL
(B). BASE METAL
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DEPARTMENT OF MECHANICAL ENGINEERING
(C). WELD ZONE
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DEPARTMENT OF MECHANICAL ENGINEERING
Figure 13. SAMPLE NO: B5
(A). WELD AND BASE METAL
(B). BASE METAL
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DEPARTMENT OF MECHANICAL ENGINEERING
(C). WELD ZONE
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DEPARTMENT OF MECHANICAL ENGINEERING
Figure 14. SAMPLE NO: B6
(A). WELD AND BASE METAL
(B). BASE METAL
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DEPARTMENT OF MECHANICAL ENGINEERING
(C). WELD ZONE
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DEPARTMENT OF MECHANICAL ENGINEERING
PROPERTIES OF WELD ZONE OF WELD BEAD (B3, B4, B5, B6) 1. PROPERTIES OF WELD ZONE OF WELD BEAD (B3)
A) Grain size is coarse B) Width of columns increased due to less heat input C) Grain boundary ferrite increased due to less heat input D) Acicular ferrite decreased due to less heat input E) Polygonal ferrite more F) Cast structure of weld zone
Figure 15. WELD ZONE OF SAMPLE B3
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DEPARTMENT OF MECHANICAL ENGINEERING
2. PROPERTIES OF WELD ZONE OF WELD BEAD (B4)
A) Grain size is smaller than the B3 (WELD BEAD) B) Width of columns smaller than the B3 (WELD BEAD)
C) Grain boundary ferrite something less than the B3 (WELD BEAD)
D) Polygonal ferrite less than the B3 (WELD BEAD) E) Cast structure of weld zone
Figure 16 WELD ZONE OF SAMPLE B4
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DEPARTMENT OF MECHANICAL ENGINEERING
3. PROPERTIES OF WELD ZONE OF WELD BEAD (B5)
A) Grain size is smaller then the B4 (WELD BEAD)
B) Width of columns smaller than the B4 (WELD BEAD) due to increase heat input
C) Grain boundary ferrite decreased due to increased heat input
D) Polygonal ferrite less than the B4 (WELD BEAD) due to increased heat input.
E) Acicular ferrite increased due to increased heat input.
F) Cast structure of Weld Zone B5.
Figure 17. WELD BEAD ZONE OF SAMPLE B5
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DEPARTMENT OF MECHANICAL ENGINEERING
4. PROPERTIES OF WELD ZONE OF WELD BEAD B6
A) Width of column decreases due to higher heat input. B) Grain boundary ferrite decreased due to increased heat input
C) Grain structure fine
D) Polygonal ferrite less than the B3, B4, B5.
E) Acicular ferrite increased due to increased heat input, it is desirable it has more
toughness and more ductility and more strength than the B3, B4 & B5
F) Cast structure of Weld Zone same as B5.
Figure 18. WELD ZONE OF SAMPLE B6
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DEPARTMENT OF MECHANICAL ENGINEERING
PROPERTIES OF BASE METAL OF WELD BEAD (B3, B4, AND B5 & B6)
Outside the heat affected zone is the parent metal that was not heated sufficiently to change its microstructure. From the microstructure of base metal of weld bead (B3, B4, B5 & B6) we get uniformly distributed ferrite pearlite grain structure.
PROPERTIES OF HEAT EFFECTED ZONE OF WELD BEAD (B3, B4, B5 & B6)
(1) Adjacent to the weld metal zone is the heat affected zone that is composed of parent metal that is composed of parent metal that did not melt but was heated to a high enough temperature for sufficient period that grain growth occurred. (2) heat- affected zone is the portion of the portion of the base metal whose mechanical properties and microstructure have been altered by the heat of welding (3) The width of HAZ varies according to the welding process and technique: in arc welds it extends only a few mm from the fusion boundary, but in oxy- acetylene and electro slag welds it is some- what wider. NOTE: - The microstructure of HAZ of weld bead B3, B4, B5, & B6 at the given heat input approximately same which is shown in figure
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DEPARTMENT OF MECHANICAL ENGINEERING
WORKPIECE SAMPLES
B3 B4
B5 B6
Polished side of the workpieces Top view of the workpieces
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Figure 19. The welded metal specimens over which the microstructures are being analysed.
DEPARTMENT OF MECHANICAL ENGINEERING
RESULTS AND DISCUSSION
Table 2.VARIABLE PARAMETERS FOR WELDING
Value
Weld Current (W)
Voltage (V)
Speed (S)
(Higher Value)
500 amp (A)
40 Volts (V)
0.5 meter/min
Table3. WELDING PARAMETERS FOR THE FOUR WORKPIECES
S.No
W(Weld Current)
V (Voltage)
S(Speed)
B3
300 amp
32 Volts
0.5 meter/min
B4
500 amp
32 Volts
0.5 meter/min
B5
300 amp
40 Volts
0.25 meter/min
B6 500 amp
40 Volts 0.25 meter/min
Formula Used For Finding Heat Input (H) H = {(V*W/S)* 60}/ 106
* η Kilo joule/mm Where η = Efficiency of SAW Welding Process (η is 0.9) V = Voltage W = Weld Current (in Amperes) S = Speed (in meter/min)
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(Lower Value
300 amp (A)
32 Volts (V)
0.25 meter/min
DEPARTMENT OF MECHANICAL ENGINEERING
CALCULATIONS FOR FINDING HEAT INPUT OF FOUR
DIFFERENT WELD BEADS
1) FOR B3 WELD BEAD W=300amp, V= 32 volt, S=0.5 meter/min For using heat input formula
H = {(V*W/S)* 60}/ 106 * η Kilo joule/mm
Where η = Efficiency of SAW Welding Process (η is 0.9)
2) FOR B4 WELD BEAD W=500amp, V= 32 volt, S=0.5 meter/min For using heat input formula
H = {(V*W/S)* 60}/ 106 * η Kilo joule /mm
Where η = Efficiency of SAW Welding Process (η is 0.9)
H= {(500*32*60)/ 106*0.5}*0.9 = 1.728 Kilo joule/mm
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DEPARTMENT OF MECHANICAL ENGINEERING
3) FOR B5 WELD BEAD
W=300amp, V= 40 volt, S=0.25 meter/min For using heat input formula
H = {(V*W/S)* 60}/ 106 * η Kilo joule/mm
Where η = Efficiency of SAW Welding Process (η is 0.9)
H= {(300*40*60)/ 106*0.25}*0.9 = 2.592 Kilo joule /mm
4) FOR B6 WELD BEAD W=500amp , V= 40 volt , S=0.25 meter/min For using heat input formula
H = {(V*W/S)* 60}/ 106 * η Kilo joule/mm
Where η = Efficiency of SAW Welding Process (η is 0.9)
H= {(500*40*60)/ 106*0.25}*0.9 = 4.32 Kilo joule/mm ( Maximum heat Input )
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DEPARTMENT OF MECHANICAL ENGINEERING
CONCLUSION
VARIATION OF ACICULAR FERRITE CONTENT WITH CURRENT UNDER ISO HEAT INPUE CONDITIONS
From Figure 20.,we get that when we increase the value of current, the percentage of acicular ferrite content increases. When the percentage of acicular ferrite increases, the toughness as well as ductility also increases, this is desirable.
Figure 20. Effect of Heat Input on amount of Acicular Ferrite
From calculation of four different weld beads, B3, B4, B5 & B6, we get
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The percentage of acicular ferrite content for B6 weld bead at 500 ampere and
4.32 kilojoules /mm is between 65% to 70%, which is maximum among the four weld
beads.
DEPARTMENT OF MECHANICAL ENGINEERING
Figure 21. A schematic CCT Diagram for a weld deposit showing the relationship of
the acicular ferrite phase field to those of other constituents.
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After studying the cooling curve transformation diagram, it is concluded that if
heat input is increased the cooling rate of the metal decreases and thus according to the
above curve, the concentration of the acicular ferrite increases. If we have higher
concentration of acicular ferrite in the metal structure, then the ductility and toughness of
the metal increases i.e., the mechanical properties of the metal is increased which is
desirable.
DEPARTMENT OF MECHANICAL ENGINEERING
Reference
1:- Kulwant Singh, 2006. “Some studies in recycling of submerged arc welding slag as a flux.” Thesis of Doctor of Philosophy 2006. 2:- Prashanta Kanjilal, Sujit Kumar Majumdar and Tapan Kumar Pal, 2005. “Predication of acicular ferrite from flux ingredients in submerged arc weld metal of C-Mn steel.” Isij international, vol.45. 3:- B.Basu and R.Raman, 2002. “Micro structural variation in a high strength structural steel weld under isoheat input conditions.” Welding journal November 2002. 4:- A.Joarder, S.C.Saha and A.K.Ghose,1991. “ Study of submerged arc weld metal and heat affected zone microstructure of a plain carbon steel.” Welding research supplement june 1991. 5:- C.B. Dallam, S.Liu, and D.L. Olson, 1985. “Flux composition depends of microstructure and toughness of submerged arc HSLA weldments.” Welding research supplement may 1985. 6:- R. Kohno, T. Takami, N. Nagano, 1982. “New fluxes of improved weld metal toughness for HSLA steels.” Welding research supplement December 1982. 7:- G.M. Evans, 1983. “ The effect of carbon on the microstructure and properties of C-Mn all weld metals deposits.” Welding research supplement November 1983. 8:- E.S. Surian and L.A.de Vedia, 1999. “ All weld metal design for AWS E 10018M, E11018M, and E12018M type electrodes.” Welding research supplement june 1999. 9:- http:/www.recycleflux.com 10:- http:/www.titussteel.com 11:- http:/www.paton.kiew.ua 12:- http://www.metallographic.com 13:- “Modern arc welding” by S.V. Nadkarni. 14:- “Welding process and technology” by R.S.Parmar. 15:- Metallurgy of welding by J.F. LANCASTER.
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