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We will start with the welding processes. Welding processes has used in ship building
industry. Primary we look into that. So, there as you can see, generally all the welding
techniques involved. They are fusion welding processes. Here, I have listed some of them,
which are more relevant as far as the ship building industry is concerned.
Among this fusion welding, fusion welding means, where fusion takes place by virtue
of fusion. We would do the joining of two components. So, S M A W, that is shielded
metal arc welding. Then, you have G M A W, gas metal arc welding. G T A W, gas tungsten
arc welding. We will take them up little more in detail. Subsequently, just now highlighting
what are the methods. S A W, submerged arc welding, then you have
something called electro slag welding, electro gas welding. So, these are as you can see
6 such methods, which are based on fusion welding process. Then, we have another welding
process which can be referred to as solid state welding. By solid state, we mean that
there is no fusion involved. It is no melting involved. That is how the name solid state.
So, under that, a welding technique which is referred to as friction stir welding, that
is also slowly gaining so-called popularity and acceptance in that ship building industry.
In this, we do not go for melting of the plates to be welded. So, that is how the name solid
state welding. We will look into that later little more in detail. So, today we will look
into this shielded metal arc welding little more in detail.
So, you can see this shielded metal arc welding. Why it is called shielded metal arc welding?
Essentially, it is the arc is shielded from the environment. As you can see in this schematic
diagram, these dotted lines, they are actually shielding the arc as well as the primarily
the arc from the environment, from the atmosphere, such that contamination, oxygen and nitrogen
contamination does not take place, right.
So, essentially these are fusion welding process. It required the, required heat is produced
from an electric arc. Will this fusion welding processes, which I have just now mentioned
of them, these first 4, the heat is generated from electric arc. So, there are, they are
some form of electric arc welding. There are some forms of electric arc welding. Whereas,
it is actually electrical resistance, the heat generated from the electrical resistance
of molten slag. That is how the name electro slag welding because all these are fusion
welding process. So, there you need to have some heat generated, which will melt the metal
as well as the electrode. So, in the first 4, the heat is generated
from electric arc. In this electro slag welding, it is generated from the molten slag. Again,
electro gas is from electric arc. It is also from electric arc here. Essentially, a gas
shielding is there. We will see how it is different from gas metal arc welding later.
So, again going back to this shielded metal arc welding, as we can see that here the required
heat is produced from the electric arc between the tip of the flux, coated electrode and
the surface of a base metal. The electric arc we have between that obviously. So, here
we can see that, the electrode as well as the filler metal is the, I mean that is one
and the same. So, this is schematically shown. You must have done it in your first year workshop
practice. So, here what do you see is, that this is our electrode, here we have the covering,
the coating of the electrode, and the central part is the core wire, which carries the current
and access the filler metal. Here, you have the so-called welding arc,
electric arc where schematically you have tried to show that the metal transfer is taking
place. These black spots are as if what happens, the metal gets melted, and it gets transferred
through the arc, through the arc column. So, here your molten metal from the electrode
is getting transferred to the base, metal part of the base metal is getting melted.
Also, there by you have the weld metal deposit over which you have a solidified slag. That
slag is nothing, but the burnt flux. The flux, the coating what it has over the electrode,
that burned flux is termed as slag, and the slag under that heat remains in a molten state.
So, which floats over the weld metal and thereby, it provides a shielding to a molten metal.
Whereas, in that heat, the flux will burn and generate some gases. Those gases protect
the arc column, the plasma column. Plasma column also needs to be protected from atmospheric
contamination. Why? It is because through the plasma column, you have the metal droplets
getting transferred. Electrode tip is also there at that high temperature. So, if it
is not shielded from that atmospheric environment, the metal getting transferred as well as the
electrode tip will get oxidized.
So, what you see in the shielded metal arc welding is, it is essentially a manual welding
process. Also at times, it is referred to as MMAW, manual metal arc welding. Also, it
is referred to like that. So, here what we have is the, one of the important constituent
is the welding electrode depending on the different types of electrode, the welding
quality, I mean end usage of the welding process also changes varies.
So, we will take a look at what are the kinds of welding electrodes, the broad classification.
So, electrodes are nothing, but it has a metal core with a flux cover. Electrodes have certain
standard length. Electrodes are available, so they have the required metal core. That
metal composition will depend on, well depend on your usage. That means, of course if you
are going to weld certain particular steel material or some metal object, you are going
to weld. So, you will have to choose the suitable electrode
for that purpose. Choosing suitable electrode means, we will have to look into the weld
metal composition or the parent metal composition, which is to be welded, such that it matches
with the welding electrode, and then, other aspects which concerns the flux, so that we
will see. So, here what do you see is, that the metal core acts as the electrode as well
as the filler metal. As we have seen because for electric arc welding, you need to establish
the arc. So, to establish the arc, you need an electrode. So, here the electrode is a
consumable electrode. It generates the arc, generates the heat and in that heat, the electrode
itself gets melted and gets deposited, thereby we do the welding.
So, primary function of the flux cover. Why the flux cover is there? The meat core, we
can understand the flux. It provides shielding to the welding pool. Welding pool means, when
you strike an arc under that action of that heat, the part of the best metal just below
arc melts and forms like a molten pool of metal. That molten pool is referred to as
the weld pool. So, the function of flux cover is one of the function is shielding the weld
pool, and during metal transfer from the electrode tip to the weld pool from atmosphere, that
means that molten pool has to be shielded as well as the arc column has to be shielded
because metal transfer is taking place as I have already said.
So, that shielding is provided by that arc column, shielding is provided with gas is
created from, where it is created? By the coating decomposes under the arc heat. The
flux coating that burns and decomposes under the arc heat, and produce the necessary shielding
environment. That is what is referred to as welding fume. When you do a metal manual arc
or shielded metal arc welding, we will find lot of fumes getting generated. They are purposely
generated, means the flux composition is such that those fumes are generated because those
fumes are needed to give a shielding, to give coverage to the arc column as well as the
electrode tip, whereas the molten metal is given a shielding by the molten slag.
So, that is what has been said here, the gas is not enough for proper shielding, means
the fumes, welding fumes which are getting generated or the gas which is getting generated,
that does not give the entire shielding. So, what happens is, the flux coating burns and
produces a protective slag. The flux after getting burnt, we refer it to a slag that
keeps the molten metal, weld metal shielded from atmospheric contamination because atmospheric
contamination is primary contamination is oxygen. It will get oxidized and also nitrogen,
there is a nitrogen contamination also takes place.
So, to protect it from that, this protective slag or the molten slag acts as a protective
coating. This molten slag has lesser density, there by it floats above the molten metal
because when I am welding it, your metal is getting melted, flux is getting burnt forming
slag, and that is also a molten state. So, both are mixed, but the molten slag being
lighter, it will float up the molten metal. So, that is also important because it should
have sufficient fluidity, the molten slag such that, it can float up because the molten
metal, the weld pool in the molten state is not remaining for a very long time because
you are moving the electric arc, you are doing the welding. Doing welding means, one of the
motions is you are taking away the heat source, electrode is moving, so as you are taking
away the heat source, means what the molten pool is also moving or in other words, the
molten pool is getting solidified. So, the slag should be able to float up before it
gets solidified. They are in a mixed condition because under
the action of the arc, there will be quite a substantial turbulence in the molten metal
pool. In that turbulent environment, your molten metal as well as the molten slag, they
are in the mixed condition. So, it should have the molten slag should have such property,
such viscosity and density, that it can float up very fast and gives the protection and
because if it cannot float up, then it remain in trapped within the molten solidified metal,
which is then a defect in the welded joint.
So, the layer of slag thus forms not only prevents the deposited metal from atmosphere
contamination, it also does another work. As you can see, also slows down the cooling
rate because what is happening, if you go back to here in this schematic figure, this
is the solidified metal, say this is my solidified layer. Over that it is the solidified slag.
So, when I am doing the welding, as I am progressing I will find there is a layer of slag remaining
over the weld bead. What is the weld bead? That is nothing, but the solidified metal.
That means, there where the metal deposition has taken place.
So, it does that additional work, that it slows down the cooling rate because as the
solidified metal or the weld bead is covered with a layer and obviously, this slag which
is formed, that we will have, that is a bad conductor of heat and it is covered with that.
So, that leads to reduction of the cooling rate, and we know if the cooling rate is slower
than it produces a more ductile material because steel as you know, if the cooling rate is
very fast, that is what is referred to as quenching. You heat up a metal and dip it
in water, it becomes hard, it becomes brittle, but you heat it up and allow it to cool very
slowly, it is aniline. Some what that effect of aniline takes place.
So, there by the slag functions not only in terms of protecting it from oxidation, from
atmospheric contamination, also it helps in positive sense, that it make the well deposit
ductile because always is preferable to be to have ductile property than an arc in such
fabrications.
It is not solidified electrode. You mean to say, the electrode is something like this, a rod
over which above the top coating is the flux. Then, we call it flux, like you can see here.
The electrode covering what we have region, this shaded part. Electrode covering here
it is a section taken. So, this covering is the flux, that is a solid flux, a solid coating
over the metal wire. Suppose, you have a metal wire inside 3 millimetre dia wire, which is
actually the electrode over which you have a coating. Another thick coating of few millimetre
diameter, few millimetre thicknesses, so that is what the flux coating is. We will see what
the flux is made up of. So, now when the arc is struck in that heat,
the core electrode, this core wire that is the electrode. That core wire melts. At the
same time, the coating which is the flux, it burns. Coating is not melting, that is
burning because it is in the atmosphere environment with oxygen burning and after burning like
this, inside core wire is melting and forming a molten deposit, flux is burning and forming
a slag flux burnt flux is refer to as slag. Yes, the slag is forming and that slag is
in molten state, and also because of the burning spot of the flux gets decomposed and forms
fumes gases. Yeah, not actually molten slag is not forming any gas. This flux is forming
molten slag plus gas, like you burn a piece of coal. What you get? You get some gases
as well as some ash, some residue. So, here the residue is the slag and some part is the
gas. So, the gas what we are getting here, that
is to our benefit. When the coal piece of coal is burnt, the gas it goes in the atmosphere.
It does not benefit us in anyway, but here it benefits us because that gas, it protects
this arc column because through the arc column, when you have the electrode, it is not touching
the metal. There is a gap. Through this gap, the molten metal is getting transferred, electrode
tip is melting and getting transferred. So, while getting transferred, it will get oxidized
because there it is not protected by the slag, it is only on the above the molten pool.
So, this is exposed as well as the tip is exposed. So, the fumes coming out that is
protected in it. That is how it is said that, this gas gives the shielding for the arc and
then, here we are talking about, that there is not enough, so flux slag also provides
the necessary shielding anyway.
So, what do you see? That the chemical composition of the flux, we have seen that there is some
flux coating on the electrode. Now, that chemical composition of the flux coating that has a
great influence on the performance of the welding, on the end quality of the weld deposit,
and also on the process, on the performance, like it influence the arc stability. Arc stability
means, what that means? The arc is nothing, but it is a plasma column. That gas in between
that gap, between the metal plate, or the job plate which is being welded and the electrode
tip. There what happened, gas is there to start
with its only air atmosphere. Now, unless until you have the circuit closed, current
will not flow. So, if I just hold the electrode like this, nothing will happen. You may have
observed, what you do, you just touch it, and then lift it up by touching and lifting
up. What essentially I am doing by touching? I am completing the circuit. It is a high
ampere is power source. So, momentarily that touching will generate an enormous heat here
and by lifting it what happens. That heat itself will as well as the arc created because
of just breaking the circuit by lifting it. You may have observed when you switch off
a switch. There is a spark kind of a thing. That means, when the electrical connect, when
it gets disconnected, there is an arc. I mean a kind of an arc jumps between the two poles.
So, when I am touching it, lifting it, two things are happening. The momentarily touching,
huge current is flowing because there is a momentary short circuit in the entire welding
circuit. That a current will heat up with the electrode as well as the best metal at
that location. Second thing, the moment I am lifting it up, that there will be an arc
between the electrode and the plate. So, both the action of the arc and heat will ionize
the air column there to start with. It will ionize those things. It will happen in a millisecond,
nanosecond time. It will ionize the air column, and once the air column is ionized, it is
no more non-conducting. It becomes a conducting column.
So, the circuit is completed and current flows. Current flows means, what flow of electron
takes place or bombardment of electron takes place in the process. That heat is generated
in that plasma column. That heat then keeps on melting the metal, as well as the electrode.
Now, that will keep on melting, provided the arc is stable. If the arc gets extinguished,
then there is no heat. Again you will have to strike the thing, create the arc.
So, that is what is the arc stability. That means a stable arc will give you a stable
heat now and then. If the arc is getting extinguished or arc is getting deflected, arc power is
reducing, increasing, fluctuating. Then, what happens, your heat generated is fluctuating
because at the end, we are interested in the amount of heat generated. We are not truly
interested what is happening to the arc or interested in heat generated. Why? Because
that heat will determine how much metal is getting melted. Heat will determine how much
the best metal is getting melted, what is the rate of deposition. All those things will
depend on heat. That is number 1. So, to get that heat uniformly all through,
I need a stable arc. To get a stable arc, I need a suitable composition of the flux
coating. Why? Because initial staring, I said the plasma column is formed by the ionization
of the atmospheric air, but subsequently, that atmospheric air is replaced by those
gases which are being generated by burning the flux. So, the flux composition is becoming
important. That means, what gas is getting released, their ionization potential becomes
important and that leads to the question of arc stability.
So, you see that the chemical composition of the flux coating, it influences the arc
stability. Next, it influences the depth of penetration. Depth of penetration means you
are striking the arc, that heat. How much it is penetrating in the metal plate? How
much it is penetrating in the metal plate, means till what extent it is melting the plate.
How much depth? That of course, also will depend on the thermal conductivity of the
plate material, but also will depend on the arc characteristics. Arc characteristics means,
what whether that arc has a certain kind of kinetic energy force, the jet, the arc jet
into the metal plate, whether it creates turbulence, where by the conduction process is enhanced
kind of metal movement. You assume a molten material zone, where you have some molten
material and the molecules are moving in this direction. That means, the metal is coming
and going down, again going through the side up and going down or in other word like this.
Suppose, the fusion zone is this, and what I was saying is the movement is something
of this order, of this nature, then what happens, it as a kind of a penetrating force, conduction
of heat will be more. So, your depth of penetration increases, whereas
if the movement of the molten particles are like this, just the opposite as if it is going
from bottom and taking an outward motion, then I have the heat is conducted more on
the sides. The wall fusion is more. It is becoming wider, whereas in this, it is becoming
deeper, narrower and deeper. So, all those again will depend also on the chemical composition
of the flux because this will depend on what kind of forces is generated.
It will also affect the metal deposition rate. Obviously, the heat, how much heat is getting
generated? How much heat is getting generated is not only from the point of view of how
much power is being spent, but also ionization potential of gas. That also determines how
much heat will be generated like in helium environment, some amount of heat organ environment,
some another amount of heat will be generated. So, that will then decide on the, will have
the effect on the metal deposition rate because metal deposition rate is nothing, but the
melting rate of the electrode which is getting deposited. Then, positional welding capacity,
positional welding capability means, you know welding. I mean if we see welding situations,
the best welding situation is like this. That means down hand. Down hand welding metal transfer
is taking place in the direction of gravity. This is called down hand welding, say this
is my welding electrode, this is what is down hand welding. There can be a situation, say
this is a vertical wall, and you are welding in this direction, say the vertical wall and
you are welding on the vertical wall.
So, this is a horizontal welding. It is in horizontal welding. You may have such the
same thing, another vertical wall, and you are going up vertically. So, that is a vertical
welding, and the worst is the overhead and you are doing the welding. This is overhead.
That means taking the electrode, and doing the welding overhead. So, this is the worst
metal transfer is taking place against the force of gravity. In the next, worst is vertical
welding metal transfer is taking place against the force of gravity. So, there is a tendency
of metal dripping little better than that. When I am doing a horizontal welding also
metal transfer against gravity, but uniformly all along best is down hand.
So, these are called positional welding. Taken together, it is referred to as positional
welding. So, what it says that the capability, positional welding capability also will depend
on the flux coating composition. These we are talking for manual metal arc welding for
shielded metal arc welding. So, that means a particular type of electrode may be suitable
for down hand welding, may not be suitable for overhead welding. If I have to do a positional
welding, means where generally by positional welding, it means any welding position, other
than down hand. Other than down hand, that means where your
metal transfer will be acting somewhat against the gravitational force. So, these are the
aspects. Arc stability, depth of penetration, metal deposition, positional welding capability,
they depend; they are influenced by the chemical composition. So, the flux composition of what
it serves, what are the purpose, it induces easier arc, starting induces easier arc. Starting
means, what it is easy with the suitable flux composition. You will have favourable gas
is generated, which gets ionized more easily, such that the electric arc is initiated very
easily. If you recall your work shop practice days,
you may recall that many a time you had difficulty to initiate the arc. It is not, it just changed
the electrode take a suitable electrode arc is initiated very easily, means with the same
hand, and you take some other electrode. You may have difficulty in initiating the arc.
Well, that may have other advantages that coating; this coating may have other disadvantages,
but one of the things is that a suitable composition may help you in easier arc starting. This
is nothing, but depends on the ionization potential of the gases stabilize the arc.
That also is there, so that you get a fairly stable arc.
Then, improve weld bead appearance and penetration weld bead appearance means nothing, but when
you do a welding, this is what is the appearance bead appearance. That means, the bead profile
it has to be smooth. If it is very rough, this is not a good bead. This is a better
bead have a smooth surface. Why it is important to have a smooth surface? Because you should
remember that at the end, you will have to give a coating of paint to protect it from
corrosion. When a ship hall, I mean or the offshore platform,
we working in a marine environment, you will have to give protection. If the surface is
rough, and the protection will not be proper, the paint fill life of the paint fill will
not be, will be less. So, it is preferable, it is desirable to have a smooth bead surface.
That is what is referred to as bead appearance and penetration. Well, such that proper fusion
takes place along the thickness.
Reduce spatter, what is spatter? Spatter is essentially when you are doing welding. A
molten pool has formed is not under the action of the arc. A molten metal pool has formed
and the metal is getting transferred little in an enlarged few. Suppose schematically
it is like this. Here, you have the molten metal molten pool, you have the arc and the
metal droplets are getting transferred. So, as it drops here, there can be a case of spattering.
It is not exactly spatter. That is a different phenomenon. Just it is something like when
on a molten in a cup you have water, and you drop something, some water gets splashed,
something similar. The molten metal is getting splashed out.
So, that is how what is referred to as spatter. Obviously, this spatter is not a good aspect
to happen. Why? Because if the moment it is getting splashed, means that metal is getting
wasted. It is getting out of the molten pool, and getting deposited on the side which has
to be eventually removed. It cannot leave them as there because they will form otherwise
small beads that to be removed. So, this spattering is not a good thing. So, a suitable composition
will reduce the spatter well.
So, that is what it does and what purpose it serves about the flux composition. So,
we see what the basic different types of flux covering are. So, the different types of electrodes,
they are either referred to as cellulosic electrode or rutile or basic electrode. They
are broadly under 3 heads. The electrodes are classified based on their flux coating,
composition of the flux coating. So cellulosic electrode. What is that the
flux is reaching cellulose? The flux composition is reaching cellulose; it burns to produce
hydrogen and carbon monoxide which provide shielding to the arc. So, you can see the
welding fume which is generated. The person who is welding is not very healthy for him
because there is a carbon monoxide revolution is there. So, that is why whenever the manual
welding is there, you have a proper means of extracting the fumes, such that the welder
does not get much expose to the fume, such that there is a section system because as
the welding arc is moving, the gas is generated there giving the necessary shielding.
Then, it is getting dispersed in the environment and in that environment, the welder is sitting
and that environment is not very healthy. It is a carbon monoxide, so it has to be extracted.
That is another issue. Anyway, we find cellulosic electrode. Because of its organic compound
primarily based on carbon, so it produces a carbon monoxide as well as the hydrocarbons.
They decompose and forms hydrogen which provides a shielding to the arc.
So, these are suitable with DC power, and electrode positive polarity. So, a cellulosic
electrode if you are using, they are suitable for using with DC power. DC power means, when
you are doing the welding electric power source is needed, so it can be the DC or can be AC.
So, that is what it is suitable with DC, and with a configuration of electrode positive
polarity because as we change the polarity, the performance changes. If I am using DC,
keep the electrode negative, the performance will be different. We will see those aspects
later.
So, presence of these gases in the arc with high ionization potential, this hydrogen and
carbon monoxide, they have a high ionizing potential. It results in high arc voltage.
So, you can see how it is effecting the heats generation. The hydrogen and carbon monoxide,
they have a higher ionization potential which results in high arc voltage, and therefore,
high arc energy is increasing. That results in a deeply penetrating arc, means a cellulosic
electrode where you need to have a higher penetration depending on the plate material
to be welded, depending on the situation because you know how much will be the weld fusion.
The penetration means this depth of fusion.
This depth of fusion to increase that primary driving force is the current. You increase
the current; more heat is generated because heat is directionally propositional to the
square of the current. More heat is generated, so more mean penetration should be achieved,
but these are other aspects which also help in improving penetrations. That means to achieve
a final control on the deposition pattern. On the fusion pattern, you will have to take
a cumulative effect of all the welding variables. So, one of the welding variables is, what
is the flux? What is the shielding medium? What kind of shielding medium? What properties
it has? So, that also affects the deposition rate, the deposition pattern, the fusion pattern.
So, that is what you see here in cellulosic electrode. It results in deeply penetrating
arc, and a rapid burn off rate. Rapid burn off rate means, what it is essentially there
is a term used burn off. It is not burning off; it is melting the electrode calling for
higher welding speeds, means what higher heat is generated because of high ionization potential
higher energy we have, I am getting arc energy. That means higher heat. If higher heat, then
it gives me a deeply penetrating arc as well as higher melting of the electrode, thereby
I can increase the welding speed. Also, other aspect is what as much of the
coating is of carbon origin, so little slag is left on the weld deposit. That means not
much of slag is left there. So, that has other effects. One of the good effects of the little
slag left means, there is a less chance of slag inclusion. What it was telling that the
slag should get floated up. It should float up, such that it does not get trap in size.
So, less possibility of such thing happening. That is number 1. Again at the same time,
a thinner layer of slag coating, so that annealing effect will not be, will be little less. It
will be somewhat little less. So, on one side, you have advantage, other
side you may have a little disadvantage. It is not a pure disadvantage because you may
not need that level of annealing. Whatever it is shielding, it is giving from heat dissipation
good enough, but the idea is to know that a little slag is left from the weld deposit.
Why? Basically, it gets burnt out, and it forms carbon monoxide. Well, with strong plasma
jet, it produces a forceful arc, fine. So, mix the electrode suitable for all position
welding, so there by we see, since it results in a deeply penetrating arc, a forceful arc,
deeply penetrating arc. Forceful arc means, it will fuse. The fusion power will be more
along the thickness direction having strong plasma jet means, throwing up the molten metal
also will have with a force. So, it can overcome the gravitational pull, and the metal can
be deposited even in overhead condition. The molten droplet should be able to travel against
the gravitational force. So, that is happening because of this strong
plasma jet. So, that is how it makes suitable for all position of welding. So, if I have
to do over head welding, then it will look for a cellulosic electrode one of the solution.
The basic features of this cellulosic electrode, just to summarize deep penetration in all
position suitable for vertical down welding. Vertical down welding means, when you are
coming down vertically from the top, coming down vertically because going up is easier
in positional welding. When you weld from bottom and going up, it is easier. Why? Because
continuously the solidified metal is giving you support, you understand.
Suppose, a vertical up welding you are doing means, you have suppose two plates. They are
sending vertically, and you are doing the welding up. So, what is happening is gradually,
I am progressing like this. So, this solidified metal itself is providing me support. The
metal is getting transferred. It is getting a support, the solidified metal below, but
when I am coming down from top, that is a different thing. The molten metal will tend
to fall off because there is no support below.
So, here it says, that it is also suitable for vertical down welding because there can
be situation, where I cannot go from bottom up. I have to come from top to bottom. So,
it is not only has a good penetrating power in all position welding, also suitable for
vertical down welding, reasonably good mechanical properties. That means, well the weld property,
what we get I mean, these are all you know qualitative reasonable good, we are saying
high level.
Well, here another aspect as you can see that the gases it was generating is hydrogen and
carbon monoxide. Now, hydrogen is one aspect which causes a phenomenon called hydrogen
embrittlement. Hydrogen embrittlement means that means, as if hydrogen is causing is rendering
the material brittle. Suppose, steel you are welding, and what actually happens is the
hydrogen embrittlement effect of that is after the welding is over. Then, you will find some
portion, it is cracked. Now, why cracked as if the metal become brittle?
So, it cracked under the stress, thermal stress. So, it has been found that what happens is
during welding, this hydrogen in the atomic state, it gets defused in the micro structure.
I would not say, it is getting trapped in the molten metal. No, that is a different
issue. It is going in the micro structure in the atomic state and then, as the temperature
is coming down, those atomic hydrogen’s, they are coming to form the hydrogen molecules.
It is not 2 molecules combining, it is to form helium. It is hydrogen atoms combing
to form hydrogen molecule. Now, what happens, 2 atoms and 1 molecule,
there is a difference of volume. So, that exerts a tremendous force that exerts a tremendous
force. So, schematically, it could be seen something like this. Suppose, these are your
crystals, or you see under high magnification the micro structure, these are the grains.
Now, these locations if we think, they are the kind of voids where in your hydrogen has
gone in. So, they join together to from hydrogen molecules.
So, thereby they exerted tremendous force in all direction, and that force, if the micro
structure of the metal cannot sustain the crack, will form a crack, will form that means,
a crack initiation will take place. Then, under the action of the external forces, all
forces due to thermal stresses because we have welded. It got heated up and getting
cool down, so kind of thermal stresses are forming.
So, under the action of that, that crack further gets propagates and a failure takes place.
That means, a long crack forms. So, this phenomenon is referred to as hydrogen embrittlement.
So, the culprit is hydrogen. So, to avoid hydrogen embrittlement, you will have to avoid
hydrogen in the welding source, in the welding place, in the weld metal, but unfortunately,
here the flux cover is that it evolves hydrogen.
So, this is one of the bad aspects of cellulose electrode. That means high level of hydrogen
generated risk. High level of hydrogen is generated. That risk of cracking in the heat
affected zone increases. Well, heat affected zone means that is the weaker zone. So, possibility
of cracking developing there it increases. So, that is one of the bad aspects of cellulosic
electrode.
Then, you come to the rutile electrode. What are the rutile electrodes? This contains the
high proposition of titanium dioxide. The titanium dioxide is another name of rutile.
That is how the name rutile electrode. So, the flux contains a high level of titanium
dioxide. This titanium oxide promotes, well this promotes
easy ignition, smooth arc operation, and low spatter, all that we saw in cellulosic electrode
also. Similar things we find that titanium dioxide is actually a titanium dioxide. Anyway,
it does not matter. Titanium oxide or titanium dioxide, that promotes easy arc ignition by
virtue of platinum oxide being there in that flux coating. Arc ignition is there. It leads
to smooth arc operation means, stable arc and also low spatter.
So, classified as general purpose electrode with good welding properties. So, this is
a rutile electrode are the so-called general purpose electrodes means, in a wider range
of field, wider range of requirement purpose, it can be used. Because of the rutile and
the ionizers in the coating, some ionizers are added in the coating. This electrode can
be used with either polarity, and in all position. There you talked about cellulosic electrode,
positive polarity, and preferably DC power source. There was a limitation DC power source
with electrode positive. Here, you find that it can be used with either
polarity, and in all positions and the moment, it becomes either polarity, means I can even
use AC source. That means, for general purpose welding, you can use that, but definitely
for ship building or where you want a superior quality of a weld property etcetera, naturally
you have to, well I mean you will have to look for whether this gives you the required
mechanical properties or not. So, these electrodes are specially suitable
for fillet welding in horizontal and vertical positions. So, the basic features of this
to summarize, it has the moderate weld metal mechanical properties. As you can see, these
are more generalized kind of electrode which can be used over a general wider range in
all positions, everywhere, but it gives moderate kind of mechanical properties. As you can
note of high order good bead shape produces because of viscous slag. Bead shape is, bead
profile is good because you have the slag viscous, means it forms a viscous covering
over the molten metal. There by you get a good bead profile, positional welding possible
with fluid slag like substantially fluid enough. All position welding is possible. Slag removal
is also easy. This also is an important aspect. After the welding, the slag has to be removed.
It should be able to remove easily. If it cannot remove easily, there will be a possibility
of small pieces of slag remaining on the weld bead, and if you have to do multi-run welding,
that remains in trapped because that will not float up a solidified slag. So, slag removal
is also another, which should be looking to while designing the coating.
Then, basic electrodes, these contains high proportion of calcium carbonate and calcium
fluoride. These are referred to as low hydrogen electrodes. We had just now talked about hydrogen
embrittlement. So, these are low hydrogen electrodes, means here you can achieve an
environment where hydrogen evolution is minimal. So, it makes the slag more fluid than that
in case of rutile coating. It is more fluidic slag. Slag is a fast freezing type, suitable
for vertical and overhead position. Why? This slag gets frozen and that provides support
to the molten metal also, so it is more suitable for vertical overhead position.
The coating composition is produced with a very low moisture content. Why? Because they
are low hydrogen electrodes, so will have to control the moisture content also, otherwise
it may generate the hydrogen in cellulosic. The composition is such, that the hydrogen
evolution was there, but here the question of hydrogen is not there. It comes from the
atmosphere moisture if the coating has observed. So, we will have to see the hydrogen content
of the deposited metal is usually less than with other type of electrodes. Weld deposits
have hydrogen content. It makes them suitable for welding low-alloy steels susceptible to
hydrogen heat affective zone cracking. So, these basic electrodes are suitable for
welding of low-alloy steels, means higher tensile strength steels. If you have to weld,
you will have to use basic electrodes for high tensile steels because low alloy steels
are generally high tensile steels. It has a high resistance to hot cracking. Hot cracking
means because of the thermal stress, the crack takes place. Steel, when the weld metal is
in hot condition, suitable for welding of thicker steels, and steels with higher carbon
content, the moment it has higher carbon content, the possibility of harder phase not in side
phase formation increases. We have talked about this.
So, weld metal has excellent mechanical property in the rutile. We have seen moderate in cellulosic.
We have said good here. We say excellent mechanical property, particularly impact property. So,
in ship building, that says that in ship building applications, it is preferable to use wherever
manual welding use a basic electrode.
So, these electrodes are used for high quality applications, which calls for low hydrogen
content to keep, prevent electrode coating from moisture absorption. They are to be properly
stored and dried. Like for example, when you are welding HSLA steel, HSLA is high strength
low alloy steels. They are additional baking immediately before welding is needed. Immediately
before welding, you bake it. So, what is done is, they directly use from
a drier. The welder carries a portable drier, a kind of a flux as if which is electrically
heated. You keep the electrodes. Inside is something like, if have seen Ramayana and
all that, Rama used to carry the arrow, the container of the arrow something similar,
exactly similar. That kind of a thing, instead of an arrow, you keep the electrodes. They
are that is the electrically connected, such that you continuously keep them at an elevated
temperature and just take out the hot electrode, and start welding, such that you minimize
the possibility of the moisture absorption. Baking is that, baking is heating of a certain
item at a certain temperature level for certain time, that is baking. You hold that thing
at a certain temperature level, so it is to drive out the moisture.
So, the basic feature are the weld deposit with good mechanical properties, low hydrogen
content in weld deposit, relatively fluid slag, but you see everything is good, but
this is bad poor bead profile. Generally, the bead profile becomes somewhat more rough.
Slag removal is difficult. They are two negative aspects. So, that that is all about the shielded
metal arc welding. Next, we will look into gas metal arc welding.
Well, there is something about metal powder electrode. That also may be, we will talk
it about in the next class.