Metal forming process: Cold working and hot working process


Metal forming process

  Once you increase the temperature, the
flow stress required basically is 
decreased. Now, the thing is that this
is the advantage in case of increase in temperature, but then there are
associated disadvantages also with the hot working. So, hot and cold working
when we talk about basically hot and cold means certainly it will refer to one standard
temperature and that temperature is basically the recrystallization
temperature. So, when we do the working of the metal at large temperature, so
that is known as hot working. And when we do at the smaller temperature then it
is known as cold working. Now, there is no you know you cannot say what is high
and what is low value of working temperature, so that depends upon basically
the melting temperature of the material. Now, the melting temperature of
certain metal may be very small like tin or lead or so, and for some metals it
is quite high let us say for steel or you can say for tungsten ,so it is quite
high. So, basically for certain metals, the hot working of others may be the cold
working in such cases like if you take the 1100 degree centigrade temperature
it is hot working for a steels, but it is cold work condition for the tungsten.

Similarly, if you take the tin or lead
or so, such alloys, when the room temperature is the I mean you can say at that
that is itself a case of hot working temperature. So, basically normally 5.5 to
6 times the melting temperature in Kelvin is defined as the recrystallization
temperature. 


Hot working process
Hot working process


So, what is the hot working process. So,
by definition it is defined as the deformation under the condition of
temperature and strain rate such that recovery takes place simultaneously with
the deformation. So, what happens that as we know when we apply the stresses on
the body, then because of the when the yield strength value I mean when the
stresses which are basically generated inside, and when that basically is
increased above the yield strength of the material, then there is plastic
deformation of the material.
So, what is the hot working process. So,
by definition it is defined as the deformation under the condition of
temperature and strain rate such that recovery takes place simultaneously with
the deformation. So, what happens that as we know when we apply the stresses on
the body, then because of the when the yield strength value I mean when the
stresses which are basically generated inside, and when that basically is
increased above the yield strength of the material, then there is plastic
deformation of the material.
 So,
how that is how the plastic deformation starts. Now, this stress which is
required to deform it plastically that is known as the flow stress. And we have
already discussed about the flow curves, then the stress true stress and true
strain terms which are used with such analysis. Now, in the case of hot working
as it tells that recovery takes place, so what happens when we apply the
pressure then the grains are strained, because the grains which are there they
will be under the action of the stresses and then they will be deformed. So,
strains are or these will be strain fields generated, but then because of the high
temperature, the recovery takes place in that and there is no strain field
remaining, so that is what the difference between these two.

So, in this process, basically in the
case of cold working that does not take place. The recovery is not taking place
the grain which is strained when we apply the grain is deformed and the strain
energy which is stored so that is there. So, basically it is under so lot of energy
is going into the material and that is not and in that case that is relieved whatever
though in the strain field is there so that is basically relieved so, but in
this case that is not relieved. So, that is why in the case of cold working
when we do the cold working of the material, the grains are quite elongated
they get distorted you have the, and in that case basically you are putting
that, so strain hardening is taking place and 
because of this strain hardening and the
strength basically is increased.
So, in the case of cold working, the
strength will increase, but then the ductility will decrease. Whereas, in the
case of hot working what happens that since it is at higher temperatures, after
once it is deformed then at that particular height temperature there will be
recrystallization of grains, a small stress free grains basically
recrystallize. So, normally what happens because of that and since the
temperature is high, so that way the recovery process is active in those cases
so that is the main basic difference between hot working and cold working.


Now, we will discuss about the traits of
hot working. Now, in the case of hot working as we see the point is that there
is no strain hardening. So, what happens that because the it is high
temperature and grains are distorted, and then again rapidly there is formation
of new grains. So, because of this high temperature, this strain hardening does
not take place. So, because of the recrystallization process or because of the
formation of new grains, you have always a formation of new grains, so that way
you do not have the strain hardening taking place. Second trait of this hot
working processes that you can allow for large deformations. So, because the
recovery process is active, you do not have the condition where the flow stress
basically is increasing with time. So, basically flow stress is normally
constant which is the third point occurs generally at constant flow stress. So,
because of this constant flow stress value, because the temperature is higher,
so because of that you can go for larger degree of reduction. So, large
deformation is possible and flow stress is 
normally constant because we are doing
at a temperature or a higher value of temperature. So, normally you have the
constant value of flow stress and the value of flow stress is normally quite
smaller as compared to that in the case of cold working because in the cold
working as we do the straining, so the flow stress goes on increasing. So, as
we go on doing the reduction, so with the degree of reduction more and more
this flow stress value will be more and more, so that way that is the
difference between hot working and
the cold working.

So, again recrystallization temperature
is a function of melting temperature of materials, so that we have already
discussed that. It is basically a function of the melting temperature of
material; normally 0.5 to 0.6 times the melting temperature and the maximum
temperature can be the temperature of melting. So, normally is kept below 50 degree
c so that will be the higher range and there will be lower range certainly
defined I mean that will be defined by this process of recrystallization, so
that way we define the lower limit of the recrystallization.

Now, so again the working temperature for
hot working as we discussed earlier that it depends upon material to material.
So, many material, if they are deformed when at room temperature it is a
condition of hot working basically, so because the melting temperature is quite
slow, so tin and lead come into that varieties. Whereas, for certain materials,
even a very high temperature, where I mean like even steels are considered to
be in hot working range at 1100 degree centigrade, and that is the cold working
range for a materials like tungsten. So, that again depends upon the type of or
different melting point of the material which is being basically you know hot
worked.

Now, basically what happens that when
you do the metalworking, then there will be temperature which will be generated
in the workpiece, and this temperature generated basically will be depending
upon the initial temperature of tool and material. Then you have there will be
heat generated because of the plastic deformation. You have basically heat
transfer will be there between material and the dies and the surrounding environment,
so there will be heat transfer that also occurring. So, depending upon that also
you will have the increase in temperature in that case.

And also you have the friction of the at
the material and die metal interface. So, because of that also there may be
change in the temperature of during that process of deformation. So, I mean it
is seen that when you do the plastic deformation, there will be increase in the
temperature. And maximum increase in the temperature during the deformation
that can be found out, so basically that can be found out by formula like you have
rho c into delta t. So, that will be the heat you know what is going for increasing
the temperature. So, that way you have rho as the density of the material, c
will be specific heat of the material that is workpiece, and delta t will be
the change in. So, that should be 
equal to what work plastic deformation
what work it does. So, basically that will be mean value of a stress into mean
value of strain, and then that can be multiplied by with you know one fraction
so that fraction basically will tell about the how much of work will be
converted into heat. So, this way you can say that what will be the increase in
the temperature during the plastic deformation. So, basically it is seen that
if you say the increase in temperature during plastic deformation, and you take
frictionless condition. So, basically you are doing the work, so that is why if
you take the increase during the deformation T d. So, this will be increased that
will be basically taken as sigma mean value of a stress mean value of a strain,
and then there is a parameter that is beta which basically talks about what
percentage of the work will be converted to heat.

And then you will have to convert it
multiply this with rho and c. So, basically this is you know equivalent rho is
a density of the work piece, c is the specific heat of the work piece, this is
a temperature rise, this is the mean flow value of flow stress mean value of strain.
And this beta is the factor which talks about that how much part of the work is
converted into heat. So, this way you can say that how much will be converted
So, normally this value is taken as 0.95 or so. So, this has been given like
that. Similarly, you will have also that temperature increase because of
friction. So, this is when you do not take the friction into case, so it is
case of frictionless deformation. Now, if you take the frictionless deformation
in that case again similar you know expression will come. So, in that case, it
will be again like rho c into v and then you are going to multiply that with t
f so that it will be increase in the temperature because of the friction, 
and that will further be you know equal
to mu times you know p into a, so that will be force. And then further you have
into multiplied by velocity and then multiplied by again time interval of
consideration so that will be equated. So, that is why if you look at T f that
increase in temperature, it will be mu into p a into V again in to delta t
divided by rho c into v that is volume. So, this way you can find these values.

So, because of friction, you will have T
f as so T f will be multiplied with rho c and v and then at the top you will
have. So, rho is again the density, c is the you know specific heat, v is the
volume. And then here you will have mu pressure then you have multiplied by area
then you have multiplied by velocity. So, this will be force then velocity and
so that will talk about this force, here then you have velocity, and then you
have the time duration for which it is working. So, this is how you can find
the increase in temperatures when you go for deformation you need to know some
of the things that basically is quite you know simple.

So, p will be basically the stress
normal to the interface, so p is that. And then further you have a as surface
area or the interface. So, this way you have these terminologies which are very
much common. And v will be the volume of the material which is subjected to temperature
rise, so that is why you have a this is v. So, this will be mass into c or so.
So, you see this expression will talk about the increase in temperature because
of the friction, so that is how you calculate it. Now, again coming to the
different aspects of hot working what you see is that as we discussed that
because of the rapid diffusion, so you know in the case of this hot working, so
that basically aids into decreasing the chemical in homogeneity. This point this
is chemical in homogeneity. So, what happens that since the hot working is done
at higher temperature, so there is diffusion at a higher rate at that
temperatures, because diffusion will increase rate of diffusion will be more at
higher temperatures. So, in that case, the chemical in homogeneity which is
there inside the material that is basically minimized. So, the material will be
more and more homogeneous chemically. Another thing is that because of the
deformation whatever you have the blowholes and 
porosities they are basically welded up.
So, blowholes and porosities they are minimized. So, this is another advantage
of the hot working processes. So, you have when you cast the ingot, basically
you have larger chances of larger probability of having these you know defects
like blowholes or you know segregation and also the porosities. So, basically
once you go for hot forming initially, and you require to go for that and then during
that process basically this is minimized. Another advantage is that you have
the formation of small equiaxed recrystallized grains. So, what happens that
you know because of the pressure you know these you have the growth of the
normally in the cost structure, you have the growth of columnar grains. Now,
when you plastically deform these columnar grains are broken. And because of
the recrystallization, you have the formation of new grains; and at that time,
the formation of the grains or the shape of the grains
will be normally be equiaxed. So, you will have a small fine equiaxed grains.

So, basically the structure which will
be changed from the columnar to equiaxed and that basically enhances the
property of the material, so that gives you more ductility and more toughness
over the caster state. Because in the caster state, you will have the you know columnar
structure that is we have already discussed in the lectures of casting that in
the normal case you will have initially the formation of equiaxed grains, but
then later on you will have the columnar grains because of the growth. So, that
basically breaks up and you will have a refined structure fine equiaxed
structure which is giving you a better 
property. So, this is about this is the
advantages associated with this hot working processes.
Now, let us discuss something even some
disadvantages of also the hot working 
processes.

What are the typical
disadvantages? 
The disadvantages that you have the probability of surface
reaction between metal and furnace atmosphere. So, furnace has atmosphere and
then you have metal, so you may have the reaction, and that may lead to you
know the undesirable you know products. So, you have surface reaction, so
reactionproducts will be or the surface, so that is undesirable. Further, you
have loss of metal due to oxidation. So, because you have air normally in the
furnace, if the furnace is not completely you know free of this air or so, or
you do not have that medium like vacuum or so. The normal cases in normal
circumstances the furnace has air and the oxygen reacts and the reactive metals
react with the oxygen and make oxides, so that is how you lose metal up to
certain extent. So, you have loss of oxides, loss of the metal in terms of
oxides. So, many a times especially for reactive metals and many highly
reactive metals, you require the inert environment because otherwise there will
be loss of the metal to a larger extent and that will affect the productivity
of the process. So, this is the one thing which has to be kept in mind in the case
of hot working. These are the challenges that you will have to you know pass.

So, basically then next is the surface
decarburization. So, there may be the cases of surface decarburization. And in
that case, you may have to remove certain layers from the top, so that you get
adequate you know surface of requisite quality, so that basically enhances the
further you know operating cost for the making it you know and the machine to
work satisfactorily for the intended purposes.
Again, the next point is the rolled in
oxides may harm surface finish. So, as we discussed that if the control
atmosphere is not there, then there may be formation of oxides. Now, these
oxides need to be removed if they are formed and if you are going for rolling
in subsequent passes, because you are there it is not the rolling is done in
only one go and  you get the desired
shape or size, you have to go in many and you know stages that is known as
passes. So, when the material is under one case set of rolls or rolling you can
say forming or working. So, when it is going for one pass in that case, in the
second pass where it is going you have to ensure that if there has been surface
reactions or there has been formation of oxides. Then these oxides must be
removed.

Otherwise, these oxides which are there
at the surface they may be rolled in they may go into in between the die and
the metal itself or the surface of the metal itself and then that may dent that
may have a dent mark on the surface of the material, so that may decrease the
surface finish of the material. So, this way it harms the surface finish. And
you require to have a better surface for that you have to again further
machine. So, again you have to increase the machining allowances and tolerances
and all that. So, that is the one of the point which needs to be taken care. Then
you have a structure and property may not be uniform over the cross section.
So, what does it mean, it means that what happens that at the surface, you have
larger heat extraction rate; whereas in the interior you will have the slower
cooling rate higher temperature sources, you will have lower temperature at the
surface. So, when they come, the die and metal surface come into contact you
have larger heat extraction rate. Whereas in the inside because the heat which
is generated which goes inside, so that is another factor then the more heat
content is inside. So, because of that the cooling rate in inside is basically
smaller, temperature is higher.

Now, what happens because of that that
on the surface you will have the larger heat transfer inside the surface and
inside the core portion you will have slower cool heat transfer? So, basically
that leads to the grain growth maybe inside. So, basically you will have the
change in structure over the cross section of the whole you know product which is
basically metal work. So, that is why you see that you have the possibility of
grain growth in the interior of the casting because of higher temperature and
slower cooling rate in the interior part. Now, few things which are to be kept
in mind when we go for hot working is that you 
must be basically careful about what
should be the lower limit of the hot working. So, it should not go below that,
below that recrystallization temperature, because then it will be coming under
the cold working cases. So, you will have to work in that zone. And then you
should not go also too high and approach towards the melting temperature. So,
what happens in that reason if there are basically the you know low temperature
melting point phases that in that case also there may be problems like hot
shortness. So, they may you know fail or you know they may collapse
or sort. So, that basically is the condition of hot shortness of burning. So,
you will have to have a proper control of the working temperature range. Another
important consideration which has to be kept in mind that these operations are not
the operation used only in one go you are getting the final shape, so you have
many intermediate passes. Now, intermediate passes are normally done at a
higher temperature. So, when you do at higher temperature, certainly flow
stress requirement is less. So, you require less forces to do the deformation,
but if you leave from there in that case the chances of grain growth will be more.
So, you will have the may coarse grain structures or so which is not desirable.
So, what is the normal practice is that in those cases when you go for final
pass after which the material will be simply going to the dispatch section or
it will be told as formed material, in that case the temperature at which it
should be worked it is should be very close to the workable range. You should
not have, so if suppose you have a temperature range like this and this, this
is a temperature range. And then this is a lower temperature lower range and
this is the upper range. So, normally what we do is normally we do in this
temperature ranges. And then the final pass should be done not in towards this
side, but towards this side. So, certainly above this, but in this range
because at that time finally, it has to go out, so the chances of this grain
growth will not be there, so that kind of you know I mean care you will have to have in the case of hot
working. Also the curves are their processing maps are there which talk that
how you know and the pressure will vary I mean what will be the maximum degree
of reduction you know at any temperature. So, with certain pressure what should
be that or similarly you know the strain rate, if you change how the pressure
will change or how the degree of reduction is affected. So, there may be the
maps, so these processing maps will talk about it. Then we come to cold
working. Now, cold working as we know that the cold working is again the
condition of deformation and under the pressure and strain rate in such a way that
your recovery is not active in that case. So, that is why it results into
increase in the strength and decrease in ductility in case of cold working. So,
what happens that many a times if you do the excessive cold working, it results
into the fracture before it reaches the final shape. So, normally the cold
working is done in such a way that it is followed
by annealing operation. So, there also
you have multiple passes. And you do the cold working and then you go for
annealing operation.


The advantage of cold working is that in
the case of hot working we saw that because of high temperature you have the
chances of oxide formation reaction these problems are not there in case of
cold working. But certainly in the annealing so what we saw is that you have a
cold work anneal cycle. So, normally because when you go for cold working as
you go on deforming because of strain hardening, the flow stress requirement
goes on 
increasing. So, basically if you try to
deform more in that case there may be a situation, where you know that the
material that inside the stress generator is more than the yield strength, so their
material may fail or it may fracture. So, for avoiding that we do the annealing
operation in between. So, although it is increasing the forming cost, but it increases
also the versatility of the process. In the sense that you can have a degree of
reduction, also you can see that degree of a strain hardening possible. And
normally the result the practices that I mean you can control the hardness of
the material you know in this cold work and anneal cycle. So, what we do is
that whatever type of hardness you have to have depending upon that normally
you see that this annealing if you want to have a hardness to be high then the this
cold working should be the last operation. Or, if you want the material to be
in soft state, then the annealing, it has to be ended with annealing. So, this
way you can control the hardness of softness of the materials. Similarly, in
the case of wire drawing or. So, you can control the temper. So, based on based
on this cold work annealed cycle, you can have the different kinds of hardness
or the you know properties of the wire itself like you know spring temper or
how much hard, water hard, half hard or so, so this is all based on this cold
work annealed cycle. So, this is how we see that the temperature is how much
important in the cases of working. And we will study about a different
processes and then see that how and you have based on these temperature itself
you have the cold working and hot working. Apart from that, also you have warm
working process. So, if you see warm working process, it is basically in
between it is take the advantage of both cold working as well as hot working.
So, it goes below the workable range that in which the recrystallization the hot
working and then comes up to room temperature. So, it has the advantage of both
hot and cold. So, sometimes less load or and less chances of you know more I
mean this one’s less load then less wastage because of the oxidation or so, so
this takes the advantage of both cold working as well as hot working and it is
in between that temperatures. So, this is about these temperature effects in
the case of metal working.

Thank you very much.

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