Tip:
Highlight text to annotate it
X
Today, we will start with another welding process which is referred to as electrogas
welding. As we have seen, the electro slag welding, there the heat of the molten slag
was used, that means the joule heating of the molten slag was used to generate the necessary
heat for melting of the electrode as well as the parent metal.
Here, in the electrogas welding, this is a kind of a, one can say a one-step, well, a
kind of a development further from electro slag welding, but at the same time, somewhat
similar to that of gas metal arc welding. In gas metal arc welding, we have seen the
welding arc and the molten pool are shielded by an inert gas media. Here also, in electrogas
welding, it is shielded by inert gas medium. So, this is also an arc welding essentially,
it is not like, unlike, electro slag welding where there is no arc; it is the joule heating
- the resistance heating - of the molten slag, that is what it is used, but in electrogas
welding, it is again an arc welding; it is a fusion welding process, the heat source
bearing the electric arc. And it is shielded by the gas, inert gas.
So, the difference, what is the difference then between gas metal arc welding and this?
The difference is in gas metal arc welding, the gas continuously keeps flowing, and that
is used in all positional welding, but this is again a vertical welding process as that
of as like electro slag welding, and here, the gas is fed; the gas remains in that position
which is covered by that similar kind of shoes, what you have seen in case of electro slag
welding.
That means a molten pool is formed, and the sliding shoes, they slide up the plate surfaces,
vertical plate surfaces and the shielding medium is the inert gas which is put there,
right. So, that is how we see that it is a very useful welding method, in which, one
can weld very thick plates like in a electro slag welding, here also one can go up to thicknesses
of around 100 millimeter thick and the welding can be done at one run at a single run. So,
obviously then compared to a conventional gas metal arc welding. It leads to a very
substantial savings cost savings. Because, if you have to weld a plate of say
100 millimeter thick and that you will have to, if you have to do with a conventional
gas metal arc welding, you will have to do several runs, because the deposition rate
is much less. Whereas, if I use electrogas welding, then one can go that welding deposition
rate can be made much higher and welding can be done at one run.
In this process, these are the some of the features; it uses solid or cored wire. Cored
wire means, in the core at times flux is used that is called flux cored wire, like, just
the opposite of the electrode, welding electrode for manual welding. In manual welding, you
have the flux coating outside, outside the metal electrode. Imagine that is just the
reverse of it; you have the electrode with a flux inside.
That means -it is an electrode having a fine hole at the center which is filled up with
a flux. So, that is what is referred to as flux cored wire; it produces smaller heat
affected zone; it produces a weld joints with better notch toughness. Notch toughness means
impact strength compared to electro slag welding. That means, here, so or in other words, if
you want, I mean, a superior mechanical properties can be achieved with electrogas welding as
compared to that of electro slag welding because these are the two welding methods, where in
you can do welding of very thick plates in a single run.
So, in a single run, so that, that is how both these methods are very useful for welding
of vertical joints, and especially in ship building or offshore industry, you may have
such long vertical joints of very thick plates. So, these two methods are very much suitable
for this purpose of these two electrogas is a superior method one can say because it gives
me a better mechanical property; it gives me a better notch toughness that means the
impact strength of the joint achieved is better.
It gives an advantage of extra melting of electrode wire leading to high deposition
rate and produces less molten base material; there by resulting lesser heat affected zone.
What does it mean means? It with electrogas, we can achieve a higher deposition rate. It
is a high deposition rate welding, but at the same time, it produces lesser heat affected
zone. The heat affected zone in electro slag welding is more; heat affected zone means,
if you recall once again, say a conventional butt welding, if it is done, suppose they
are the two plates butt welding has been done, and the fusion zone looks like this; this
is my fusion zone. So, the heat affected zone would be the zone
just adjacent to this. This is what it is referred to as heat affected zone that means
it is between the fusion boundary and about 1000 degree isotherm because, this particular,
this particular line, which refers to the fusion boundary; that refers to what? It is
the melting temperature isotherm essentially is not it that means melting temperature of
steel is around 1500 degree centigrade. Because this shaded zone is the fusion zone;
this is my fusion zone, that means there the total fusion took place; fusion took place
means, what that means, this boundary line refers to the solid liquid interface line,
or in other words, this is the isotherm of 1500 degree centigrade, means along this line
maximum temperature attended was 1500. So, similarly, if I can locate a line where 1000
degree was attend, so that zone in between is the, is the zone which is called heat affected
zone, this part is called the heat affected zone; what happens here, your microstructure
re-crystallization takes place. So, either the grains becomes finer or become
coarse; either it turns fine or coarse or both essentially what happens, immediately
closer to the boundary of the fusion zone. You have a fine grain structure; little away
from that, we have a coarse grain structure. Anyway, in other words, the heat affected
zone is the, one can say location in within the welded joint, which has a different material
property compared to the parent material property. Why? Because the microstructure has changed,
and within that zone, a smaller part would be of fine grain structure - fine means finer
than the parent metal - and rest would be coarse grain structure. So, it will be coarser
than the parent metal, and as you know, fine grain structure gives you a superior mechanical
property.
And coarse grain structure gives inferior mechanical properties, or in other words,
a good welding procedure would be one which produces minimum width of heat affected zone.
So, that is why it says here that electrode gas welding produces smaller heat affected
zone, whereas, smaller means what? Smaller as compared to that of electro slag welding.
In electro slag welding, a larger heat affected zone occurs, why? Because the heat resident
time of the heat at a given location is much higher, because there the molten slag is floating
there on top of the molten metal which is at a much higher temperature, what temperature?
More than 1500 degree centigrade, so, the quantum of heat at one place is much more,
so it gets enough time for the heat to get conducted along the plate.
Because more the heat get getting conducted, you have this isotherms shifted away, is not
it? If I hold the heat for a longer time, then this isotherm of 1500 also will shift
to some extend and the isotherm of thousand also will shift further, means what? That
means more heat is getting conducted; more zone is getting a temperature rise to above
1000 degree centigrade.
So, you have a wider heat affected zone. So, that is what in electrogas that does not happen
means, here you have a lesser heat affected zone, so, it is a better welding process.
That is why it gives a better notch toughness compared to electro slag welding.
Well, if we summarize the salient features - this electrode gas welding is a automatic
welding process like electro slag welding and you can tell I am in the class It is an automatic
process; it welds only in the vertical direction. The weld pool is bound either by two sliding
copper cooled water cooled copper shoes or it has a water cooled copper shoe in the front
and a ceramic backing strip at the back. Because say two vertical plates are there,
it is being welded. So, it has to be covered from both the sides. If I say this side is
the fore front portion, this is the back portion; so, either there are two copper shoes which
slides up or a copper shoe as well as a ceramic shoe.
The filler wire electrode is fed through a copper nozzle; electrode is melted by a shielded
arc, that means, melted by a shielded arc means what? It is nothing but the entire mountain
zone as well as the arc is shielded by a, I mean a gas shielding is only provided. It
starts on a run up plate and finishes on a run off plate.
Constant current DC power supply is generally used and electrode is generally kept positive,
that means DC e p power supply is used. Electrode is kept positive because of the simple reason
I want to melt more of electrode at a time that means higher melting rate is desirable.
So, the electrode is kept positive. What is the process? Once the arc is initiated, in
the arc heat electrode and plate edges along the vertical groove melts forming a weld pool
covered sideways with the covered copper shoes. That means a weld pool is formed and that
weld pool is continuously maintained. With further feeding of electrode in the weld pool
the molten volume increases. In relation with the ascending rate of the weld pool volume,
the welding nozzle and the copper shoes are pulled upwards. Like same, in that case of
the electro slag welding, here also you will have to synchronize; the lifting up of the
shoes as well as lifting up of the welding nozzles which is feeding the electrode.
In the process, the molten material at the lower end of the groove solidifies and the
process continues till it reaches the upper end. Starts on a run up plate, run up plate
means essentially it is starting on a best plate and which finally makes the bottom part
of the weld zone of the weld pool which holds the molten metal. Continuous to a run of plate
that means it goes out because the welding should continue beyond the plate edge; if
it does not continue, then the plate edge will remain un-welded, then again you will
have to weld there. So, it will continue beyond that, and that
extra, that is what is called run off plate; then run off plate will be cutout. Synchronization
of the feeding of electrode and moving up of the shoes is, obviously, extremely important.
Any mismatch in this synchronization may lead to arc instability or stubbing of electrode,
both ways. If it is going up faster, electrode is not moving at the right speed, then it
will stub means, it will actually touch the molten metal pool and touching the molten
metal pool means that arc will get extinguished instantaneously, there will be certain drop
in the current. So, the process will stop. So, if that happens, then you will have to
restart the arc and that may give rise to inadequate fusion causing welding defects
locally. That means these processes like same in the electro slag welding also that you
cannot afford to as such, you cannot afford to have a stoppage in between the process,
because the moment it stops, to restart it becomes difficult; not only difficult, it
leads to local defects and those defects will be difficult to rectify, because, here, we
are talking about welding of very thick plates.
Well, this electrode gas welding it is being termed as single electrode that means there
is a possibility of multi electrode welding also; it only signifies that. If I say single
electrode electrogas welding, that means there can be a multi electrode also. The whole idea
is single electrode and double or multi electrode is, if I need, if I have to weld a much thicker
material, thicker plate, then I need higher still higher deposition.
So, for that one may have to use multiple electrodes. So, we will see what this single
electrode is. This is to reduce the heat input a fine diameter wire is used with lower welding
current and narrow groove. Narrow groove means you make the weld, the joint geometry; the
weld joint geometry having a like one is this. This is what is referred to as we have said
square butt. This is a v group. Now, this angle, if I make
it much smaller, this is what is referred to as narrow groove, that means, this groove
make it narrow. There is aspect that means, one of the extreme cases of this is the square
butt, and the edges become parallel. Other extreme is the very wide groove. So, narrow
groove welding means you have to deposit less amount of material again. So, obviously, it
is better best would be square butt.
Butt, but in square butt, you may have difficulty in getting fusion over the entire zone. That
is why this v grooving is done, and now, if I make it wider, then there is a chance of
much higher heat input, because much higher volume of molten metal will be there. So,
that is how it says that it for reducing heat input, one goes for a narrow groove welding.
So, there is another corollary from this, that means to have a, where distortion is
of very important issue, that means the dimensional deformation because as you know, when you
do welding, it generates stresses and leads to some dimensional deformation, structural
deformations, which are referred to as distortions. So, if distortion of is of great important,
then one should go for a welding process which leads to lesser distortion, one of that is
in narrow groove welding.
Why? Because, here, the heat input required is less, and once the heat input required
is less, your resulting distortion will be less. So, in narrow groove welding, results
in lesser distortion also. To attain a sound weld bead formation, the welding wire is oscillated
in the plate thickness direction. So, this is another aspect you can see, here the electrode
is oscillated, somewhat schematically it is shown like this.
This is the view I mean, in the elevation view, this is the plan view; this is what
is the, these are the two plates, this white part. These are the two plates being welded.
Here, you have the ceramic backing strip, here is the water cooled copper shoe and this
is that narrow groove, what has been formed, narrow groove. - What I was talking about
here? The narrow groove - so, this is what is the narrow groove and the electrode is
fed like this and the electrode is oscillated along the thickness, as if you are depositing
the material over the thickness.
So, that is what it is oscillated or the entire plate thickness. This enables, this oscillation
process enables proper welding with adequate reinforcement on both sides because you need
to have the reinforcement on both the sides.
Also reduces the heat input and the amount of weld material. How much weld material is
going that that reduces as well as the heat input. Heat affected zones toughness and the
welding efficiency substantially increases by which by oscillating the nozzle. The oscillating
the electrode, if we oscillate the electrode, it leads to a higher welding efficiency. Results
in a narrow weld bead and narrow heat affected zone. It can be gainfully used to weld steel
plates up to even 50 millimeter thick.
This is what schematic description of the thing is, that means, even of the order of
50 millimeter thick plates can be welded. So, here you can see this is a wider zone
where you have the, well, this, the fused metal, the fused metal here is supported by
the backing strips one at the back and one at the front. Those strips has a profiled
surface to accommodate further reinforcement, such that, you get that weld bead you get
that reinforcement on the front surface as well as on the back surface.
And the electrode is oscillated to and fro along the thickness directions such that one
can have a proper inform deposition all along the thickness. Through this nozzle, the inert
gas is fed in, this is schematic representation
Well, for steel plates exceeding 50 millimeter welding defects, I mean with single electrode
welding if we do, we have said that up to 50 millimeter it can do suitable welding.
If the plate thickness exceeds 50 millimeter, then welding defects are generated when using
the oscillating single electrode process; essentially, lack of fusion takes place.
These are actually, well, these are actually, there is, I mean one can these are the upper
limit of the process. That means - if I want to do implement so called electrogas welding
for plates of, say 70 millimeter thickness, it will not do a good job. That means, there
will be a possibility of defects coming in, or in other words, electrogas welding using
single electrode, it has a limitation; it can weld up to a plate of thickness of maximum
50 millimeter, beyond that you will have to look for some other method that is what it
says.
To handle these increased plate thicknesses, necessary dwelling time for an arc to fully
fuse a groove face is insufficient when only using a single electrode, even if arc oscillation
width is increased. What does that mean? That means, if I have a thicker plate, even a thicker
plate, then one can try this oscillating width to increase the oscillating width, such that,
I deposit sufficient metal, but the trouble is what happens, the heat available, the arc
heat available becomes insufficient. The dwelling time of the arc head, that means - the arc
at this level should be present for the sufficient time, such that it fuses the parent metal
both sides, I mean both sides, here, both sides the parent metals are fused along the
entire thickness as well as necessary electrode is also melted and gets deposited.
But if it is too wide, it is moving up and down the arc, right, you will have to do it
at a certain speed. If you do it very slow, then the metal at the front will get solidified
or by the time it travels there metal at the back get solidified. If you do it too fast,
then enough heat is not going.
So, there is a limitation up to 50 millimeter, it can be done with desirable qualities; beyond
that a single electrode is not a feasible solution. So, comes to the necessity of double
electrode, electrogas welding. So, what is done here? Welding of such thick plates and
oscillating 2 electrode system is used.
Here also direct current power sources are used. One of the electrodes is made positive
and the other is kept negative, because there is again other difficulty. This provides a
stable arc because when you are doing a double electrodes, say, I mean the schematic is similar,
only thing in case of only difference is, instead of a single nozzle, you have two nozzles
and both of them oscillating simultaneously. Now the gap between the two nozzles means
the gap between the two electrodes that again plays a role.
Because if both are positive, then there will be repulsion in the arc column. So, what has
been found is that they are kept one is kept positive another negative; that gives us a
stable arc. If both are of same polarity, then arc becomes unstable and leads to high
amount of spatter possibly because of continuous repulsion between the arcs, there will be
a high amount of spatter as well as the arc tends to be become unstable.
And also with electrode positive, we get a wider bead formation and less penetration.
Because, this again is, as you know as the electrode is positive, more heat is generated
electrode. So, it causes a less penetration, but a wider bead formation means more melting
of the electrode leading to wider bead. A narrow polarity, a negative polarity will
give a narrow bead and deeper penetration.
So, you get the advantage of both because you need a wider bead also you need, because
this is thicker material, and you need proper penetration. For a wide single v groove, the
electrode on the wider grooved surface is made positive and the electrode on the rear
is made negative, that is another aspect. That means, here, suppose, well, say schematically
it is something like this; so along the wider groove, you have the electrode is kept positive
and the electrode closer to the narrower end is kept negative.
The electrode on the wider side is positive; on the narrower side is negative, because
you see, on the, here it is kept positive means what? It is a wider side, you need more
deposition of the metal more electrode melting is required. So, you kept the electrode positive.
So, firstly we have say, we have seen that we will not keep both of them of same polarity.
We will have to keep one of them positive; another negative. So then automatic question
comes which one positive, which one negative. So, this is the logic of keeping the front,
keeping the one which is on the wider side of the opening positive. The logic is that
you need more melting of electrode. So, that is how that becomes positive; the other automatically
becomes then negative.
So this is what is schematically it is represented. You have two electrodes in tandem which are
oscillated together; rest other aspects are same. So, this is a typical geometry of that,
suppose you have a 80 millimeter wide plate, then the group as you can see is quite narrow
because where 80 millimeter deep. The phase opening is only 25 millimeter, and the root
gap at the narrower end is 10 millimeter. So, the difference you can see all the other
butt welding processes as I was telling you, the minimum root gap, the root gap is this
particular distance. The distance between the 2 plates are at the
narrower end. In case of a square butt, of course, the root gap both at the bottom and
at the top are same, because it is a square butt; there is no edge preparation, but if
it is a v groove, then the gap at the bottom, or the minimum gap between the 2 plates are
referred to as root gap. So, in all butt welding cases, the root gaps are of the order of it
varies from 0 to, well, around say 5 millimeter maximum.
But here, you see the so called root gap is equivalent in this case, here, it is about
10 millimeter in electrogas welding. So, that is what is the difference, that means, because
here the aspect is somewhat different not in case, as we do, in case of butt welding.
Well, these are some of the typical welding parameters, as you can see; say a welding
of a 70 millimeter thick plate. The first electrode is fed with 420 amperes, whereas,
the second electrode is with 460 ampere, that means there is a little bit of difference
in the front and I mean the ampere is in the two electrodes.
So, these are some of the sample weld parameters. The welding voltage one interesting thing,
one can see in electrogas welding is the welding voltage is generally very high is of the order
of 40 volts. Whereas, in gas metal arc welding, if we look into the welding voltage, would
be of the order of 20 to 30 volts maximum. And the welding current, if it is a manual
gas metal arc welding, well, the welding current also will be less; it can be of the order
of 200 250 ampere. Here, the welding current, you see, if I, in case of submerged arc welding,
one can go up to welding current of 1000 ampere, if not even 1500 ampere depending on plate
thicknesses and at what, how many runs you are doing, it will be like that. Like, for
example, if you weld a plate of say 12 millimeter thick at a single run, then one can go up
to a current of the order of 700 ampere. But here, you see a 70 millimeter thick plate
is being welded, the current is only 400 in one electrode again another around 400 in
another electrode. So, that way one can say total current is only 800 ampere approximately
total current, but the plate being welded is 70 millimeter thick and that too at one
run. So, that is how these processes are very effective, very efficient process, but of
course, the voltage level is much higher here. The shielding gas use is generally carbon
dioxide in electrogas welding; the 100 percent carbon dioxide shielding is done. Obviously,
carbon dioxide will be used when we do welding of steel; here, we are talking about welding
of carbon manganese steels not stainless steels or any other non-ferrous material. If you
weld non-ferrous material, obviously carbon dioxide, you cannot use you; go for argon
or argon helium combination.
So, welding consumable and backing strip. So, these are the important aspect that backing
strip means this backing strip, which is supporting the molten metal both sides, these at the
back; these at the forward. So, design of those strips also determines the weld quality.
The configuration of the backing strip and the welding consumables play an important
role in the amount of slag generated during electrogas welding and the subsequent quality
of the weld deposit. So, groove in the backing strip accommodates
the slag that is generated from the use of the flux core welding wire, where from the
slag is coming for the flux cored, in the core, there is a flux. So, that is forming
the slag, why it is used? Because, well, it improves the, improves the fluidity of the
material; improves the arc characteristics; helps in initiating the electric arc; helps
in stabilizing the electric arc, so, and so forth.
So, having a flux core wire is good, but if you have that, you will have to have a mechanism
of getting the flux out of the molten metal. So, where it gets out? It gets out at the
backing strips. Obviously, it should float out. So, whatever, so, you will have to have
a proper groove which can accommodate apart from the reinforcement, the amount of flux
which may get generated if I use a flux core welding wire.
To keep the slag generation to a minimum using flux core wire, the first electrode near the
face is of flux cored type, and the second electrode is a solid wire, because ideal would
have been if we use solid wire electrode, because shielding here is being done by the
gas, the inert gas or CO2, whatever is being used, but to take advantage of or to get the
additional benefits of flux, one can go for a flux cored wire welding.
So, now, then it brings in a problem that means the slag forms. So, you will have to
accommodate the slag. To minimize that difficulty of handling the slag as well as get the benefit
of the flux core. So, what is done? Again a compromise is done, that means the one of
the electrode is used with a flux core type; another one used as a solid one, like in submerged
arc welding. We do not go for flux cored wire; there is no necessity, because you are using
solid flux, the whole shielding is being done by flux.
So, having a flux core does not make any sense. So, there a solid wire is used, solid wire
electrode. Similarly, here, we are saying that the first electrode near the face means
on the wider side, the first electrode used is a flux core type and the second one that
means this is a flux cored type, and the one which is at the rear, is a solid one, is a
solid wire electrode. So, thereby, one so called make a compromise
on the usage of flux cored wire reduces the slag generation. At the same time gets the
benefit whatever is there from a flux cored wire.
So, this is the typical geometry of the ceramic backing strip for a cycle electrode, single
electrode electrogas welding. So, here you can see that a typical groove is made for
accommodating the flux, the slag and as well as the molten metal such that it forms the
reinforcement.
The shielding gas used is generally CO2 carbon dioxide is used. Using double electrode, the
amount of shielding gas consumed per welding length is approximately halved compared to
that of the single electrode welding. So, what you see is that when you go for a
double electrode welding, then, in fact, the consumption of shielding gas becomes half
becomes less as compared to the amount of metal getting deposited. Because, to deposit
metal from a single electrode, the amount of shielding needed remains more or less the
same if I use 2 electrodes, but by using 2 electrodes, I increase, I double the deposition
rate, but the shielding requirement remains the same. This is the, suppose for a plate
thickness, say a plate thickness of 40 millimeter plate is being welded and I go for a double
electrode welding, I will have less, I mean the cost of production will become less because
I will be using the more or less the same amount of welding, the shielding gas, but
my deposition will become double; deposition becoming double means what? Deposition rate
becoming double means I finish the weld much faster. So, if I finish the weld faster means
I consume less shielding gas. So, that is how over all costing becomes less. So, that
is what it says that the amount of shielding gas consumed per welding length is approximately
halved compared to that of the single electrode welding.
So, per welding length whatever is the cost involved, in case of a single electrode welding,
the cost involved will be more compared to that of a double electrode welding. The method
works out to be more economical.
What are the applications of these and how are the welding performances double wired
electrogas welding with appropriate welding consumables and backing strips is a highly
efficient process for single pass welding of 50 to 70 millimeter thick plates.
Double wire electrode is a good metal for such kind of plates produces adequate top
and bottom reinforcement, by top and bottom means both the sides. Because this term top
and bottom reinforce they are rather relative, they relates to a horizontal plate where the
top surface and the bottom surface, but here the plates are vertical. So, in any case,
the top means the wider surface; the wider opening is the top; the narrower opening is
the bottom or the back surface. So, produces adequate reinforcements, it gives
proper penetration. Proper penetration means total fusion of the entire thickness and it
produces weld joints without defects such as lack of fusion. Why it is talked about
because if such thickness plates are being welded using a single electrode, there can
be a defect of lack of fusion. Lack of fusion is a defect means not proper penetration of
the in the metal thickness. Deposition rate is almost twice with set up time being more
or less the same this is another aspect. If we go for a double electrode welding, obviously,
the deposition rate becomes roughly doubled, but the entire setting up time for the welding,
because you see, the welding is such a process that actual welding operation takes very less
time. What time it takes, it depends on the what
is the welding speed. Sa,y the welding speed is half a meter per minute, what does that
mean? That means a 10 meter long plate to weld, it will take 20 minutes, just 20 minutes,
a 10 meter long plate, but in reality, if you weld a 10 meter plate, you will find you
have you have spent about 2 hours. So, where the time has gone? It is a setting up time.
First you have to align the plates; set the welding instrument; set the electrode; check
the alignment; all those things and then switch on the power and it start welding; Say a automatic
submerged arc welding. That means, once every alignment is done, you provide the power,
then electrode will be automatically fed and will deposit the required metal along the
weld length, and you will find from that point, twenty minutes job is done, but before that
you have spent probably one and half hours setting up the thing aligning the plate, etcetera.
So, that is what it says that the setting up time being more or less the same. So, whether
we are setting up for a single electrode or for a double electrode, the setting up time
is more or less the same. So, thereby, process efficiency is nearly double to that of single
electrode process. Here, we are talking about double electrode process; the application
and the performance of double electrode process.
So, we see that the process efficiency is nearly double to that of the single electrode
process. Amount of, at the same time amount of shielding gas consumed per unit weld length
becomes nearly half, as I was saying, because the weld pool volume is not changing, neither
the weld joint geometry is changing. So, the requirement of your shielding gas is remaining,
is totally remaining the same. So, that is how the remaining same. So, if I compare that
with that of the length of the weld, then one can say that it has become nearly half
because the deposition rate has become doubled, so, my unit weld length has increased; gas
consumption has remained the same. So, thereby, effective gas consumption becomes half. Double
electrode process works out to be more economical, that is one of the conclusions, that means,
wherever if one will have to do a electrogas welding, then one should try to look into
whether a double electrode process can be implemented. If that is so, then we have a
more economical process operation. It is a highly efficient automatic welding
method for vertical position welding of steel plates having thickness of 50 to 70 mm stable
weld metal penetration in single pass. So, it can be gainfully applied in shipbuilding
applications for ultra large vessels where plate thicknesses are 50 millimeter beyond.
That means very those big vessels, essentially as I was telling that application of electro
slag welding, same way for connecting the blocks, the vertical side shell plates, those
welding arc done using this electrogas or electro slag welding.
Well, next, so, this is all about the some of the welding processes which are widely
used in shipbuilding. So, what we have talked about are all essentially thermal processes
involving fusion, or can be referred to as fusion welding whether it is a shielded metal
arc welding. So, once again just to summarize shielded
metal arc welding. Then we had gas metal arc welding, gas tungsten arc welding, submerged
arc, then electro slag, and then electrogas, so, what happens? All of them are fusion welding
process. Only in case of electro slag, welding the heat generated is from not from the electric
arc but from the resistance heating rest all where from electric arc.
So, all are essentially electric welding process as well as a fusion welding process. Now the
moment a welding process becomes a fusion welding process, then you have along with
it, some of the advantages and disadvantages built in the system. One of the primary disadvantage
is the moment metal is fusing; there is a chance of gas entrapment; there is a chance
of slag entrap entrapment; solidified the molten metal is getting solidified, so, a
very high thermal stresses work; so, chance of solidification cracking; so, all this defects
are probable or possible. I do not say that you do fusion welding process
so you will have all those defects, no, but they are more likely to take place. So, if
I avoid that entire process of fusion, then all those defects are or difficulties are
avoided, that means, if I can develop a welding method where in I do not melt the material,
so it becomes a process where in I do not have all those difficulties related to fusion.
So, one such method, so where we do not melt the material is referred to as solid state
welding, because in the fusion welding, the material was going in the liquid state, in
the liquid phase - here we are not going in the liquid phase, so it can be referred to
as a solid state welding. One of the methods is referred to as friction stir welding friction stir welding.
So, we will see what does this method. Here, the primarily, as you can see it is a solid
state joining technology, the joining method, and as you can see, it is not a very old technique;
it is patented as sort of late as 1991 only sort of came into being very new technique,
whereas all the electric arc welding methods, they are all I mean many of those methods
were in full operation and use in the time of, I mean at the time of second world war.
Well, so, that is what it is fairly a new technique. It involves joining of metals without
fusion or filler material as I was saying there is no fusion taking place and once no
fusion taking place means there is no need of filler metal also; in all the other fusion
welding processes we have seen there is a filler metal generally always there is a filler
metal. The filler metal can be a can be a separate
filler metal or can be the electrode itself acting as the filler metal that means it gets
melted and gets deposited, but here the question of filler metal is not there that means in
this process, the question of any square butt, v groove, wide groove, narrow groove nothing
occurs, so, it has to be that root phase, root gap should be 0, because there is no,
since there is no filler metal means, you cannot afford to have any gap between the
plates or gap between the edges which are being joined, welded.
Well, how about this is also a thermal process, like, we said: the welding we talked about
are the thermal processes involving fusion, it was there all the previous welding techniques.
In this solid state process, it is also a kind of a thermal process that means some
amount of heat is obviously generated, but it is not enough or we do not heat that much
heat that it leads to fusion. So, here what do we see is that about 80 percent
of the melting temperature is attained that means, in this process, friction stir process,
the whatever heat is generated that is about 80 percent of the melting temperature.
So, it is most, this technique is most suitable for welding of long flat panels for long longer
welding, it is good. Obviously, this is not be a convenient process for very small and
short welding. For long panels, like, say a deck panel will be fabricating or the bulkhead
panels will be fabricating, wherein, you have to do though do the butt welding of 10 meter
15 meter length. So, this solid, this friction stir welding is a good method for that.
Well, so, this is the so called the basic features of the process, and now, let us see
what the process is. Here, it works under the combined action of the frictional heating
and mechanical deformation. What happens is the whatever heat is generated that is generated
by the frictional forces acting between the welding tool, here instead of a welding electrode
or a welding torch, you have a welding tool which rotates at a given r p m. So, that tools
when it comes in touch in contact with the plate material to be welded, that friction
generates the heat, and the tool, we will see some of the typical tools which has a
small nib kind of a thing which penetrates along the joint, penetrates along the thickness
and keeps rotating.
So, in the process what happens, that rotation, it generates enough friction and that friction
generates sufficient heat to plastify the material because as we know, the if you look
into the material properties of say steel, if we draw the temperature verses yield stress
variation of the yield stress with temperature we would find a variation somewhat like this,
what this temperature would be? This is say room temperature. What this temperature here
should be? Tell me, what do you think at this point?
The yield stress is 0. This is the melting temperature. Melting temperature means all
also it is referred to as refer to as liquidious temperature; at this point that metal is liquid,
so, it does not sustain any stress. Anyway, so, what we said? Sometime back that it attains
a around 0.8 the melting temperature, 80 percent of melting temperature. So, say it is here
that means my I attain a temperature level of this.
So, there the stress is so much or in other words much less than what is there at the
room temperature, or in other words, the metal is at a straight that means, the stress level
needed is much less to take it to the yield point stress, because you will have to attain
only this much. Well, so, in the next class, we will see further how this works, this solid
state welding process.