Tip:
Highlight text to annotate it
X
In the next few lectures, I will be discussing on solidification and casting of steel. Under
this, we will be discussing principles of solidification, then, casting processes and
then, final, finishing operations.
So, as such, first, we start with the principles of solidification. Liquid steel is tapped
from primary steel making vessel. Then, various operations have been done in secondary processing;
inclusion, modification, temperature control, whatever, has been done, that are the principles
of solidification.First, what is solidification? In fact, solidification is, liquid to solid
transformation; liquid to solid transformation, on cooling; the reverse, solid to liquid on
heating.So, solidification is liquid to solid transformation, which occurs, when super heat and latent heat
of solidification areremoved. Now, here, pure metals, they solidify at constant temperature;
they solidify at constant temperature, as all of you know. What about alloys? They solidify
over a range of temperature, over a range of temperature; what does it mean? This I can illustrate. For example, I take
an isomorphous system, say, I take an isomorphous phase diagram, consists of A and B; here it
is, say,100 percent A; here it is 100 percent B, and I am adding here, weight percent B,
then typically, an isomorphous system, it looks,for example, this one. This is an alloy
of A and B. So, if I want to consider the cooling of this particular concentration of
alloy, then, typically it cools, if this is the liquid; this is the liquid plus solid,
and this is the solid. So, this particular alloy of this composition, this is somewhere
here, temperature, then, it cools along this particular path. So, accordingly, this particular
line is known as liquidus, and this particular line is known as solidus.
Here is the temperature. So, when I say, alloy solidify over a range of temperature, then,
a super-heated liquid, which is just above the melting point, say,for example, here,we
will remove its super heat; it will touch this particular point; it will not solidify
immediately, on reaching the liquidus temperature; but, it will take some time, before it is
completely solidified. So, this particular region you are seeing, this means, the solidification
occurs over a range of temperature, and the range of temperature consists of, say, if
this is the T L, which is the liquidus temperature;so, this one is the solidus temperature. So, that
is what is meant by, solidification over a range of temperature.
Now, on touching the liquidus line, as we cool further, what will happen, a solid will
form. For example, if I consider a point at this particular region, which is the liquid
and solid, that means, I have cooled this alloy, from this temperature to this temperature,then,
a solid will precipitate; a solid solution will precipitate. And so, if this is the solid
and this is the liquid, then, this hasbecome a solid liquid interface. So, how the solidification
will proceed? It will proceed through the advancement of solid liquid interface, and
as a result of this, more and more solid will form.
So, it is the movement of solid liquid interface. Now, we can have either a plane front solidification,
either a plane front solidification; this plane front solidification can occur, when
temperature, the actual temperature is greater than liquidus temperature. Now, thissituation can occur, when no segregation
is involved; due to segregation of solute, the liquid adjacent to the solid liquid interface
is super cooled. How does it happen? As the liquid cools, solid is formed. Then, there
is an solubility of the impurity, in the solid, as well as in the liquid So, depending upon
the solubility of the impurity in the solid, the excess solute will be rejected into the
liquid. So,therefore, the liquid which is adjacent to the solid liquid interface will
get enriched with the solute element, and this is called the phenomenon of segregation.
So, this type of, say, solidification, it occurs, which is called a dendritic solidification,
called dendritic solidification. As a result of rejection of excess solute, which gets
accumulated near the solid liquid interface, a condition arise, where T actual becomes
less than Tliquidus. If I want to show over a diagram,for example, I take this one; this
is, here is the temperature and this is here, distance in the liquid, distancein the liquid
ahead of solidification front;ahead of solidification front; that means, if it is zero,so, this
particular thing, is the location of solid liquid interface. So, as I move ahead of the
solidification front in the liquid, then, I get this condition; that means, this is
the T actual and this is Tliquidus. So, you are seeing, upto this region,upto this region,
due to rejection of solute ahead, solid liquid interface,upto this region,T actual is less
than Tliquidus; that is,Tliquidus is greater than T actual.
So, that means, the liquid within this region, which is a segregated one, is said to be constitutionally
super cooled; constitutionally super cooled. And, this constitutionally super cooled phenomena
occurs, because of segregation of the solute element; and this segregation occurs, because
of the solubility difference betweenthe solid and the liquid. So, the excess solute will
be rejected by the solid, and the impurity will concentrate near the solid liquid interface,
in the liquid. So, on account of that, in that particular region,upto which the segregated
portion is there, the liquid is said to be super cooled, and this particular zone is
also called, mushy zone. So, the solidification occurs; that means, due to super cooling,
due to super cooling, what has happened, formation of cells at the interface takes place; formation
of cells at the interface and these cells grow rapidly; these cells grow rapidly, normal
to the interface, in some preferred crystallographic direction, at high rates of solidification.
So, on account of this, the formation of primary dendrites will occur; primary dendrites will
form, and the lateral growth of primary dendrite, that is, the lateral growthof primary dendrite,
will give us,so called secondary and tertiary dendrites, and tertiary dendrites. And, that
is how, the dendritic type of solidification, it proceeds in this particular fashion; this
is contrary to the plane front. So, that means, a mushy zone consists of a mixture of solid
dendrites, plus inter-dendritic liquid, because, when the dendrites have been formed, some
amount of liquid which will be entrapped in the dendritic region, that is called inter-dendritic
liquid; that is, liquid which is entrapped between the two dendrites.
Imagine, the dendritic structure is just like a tree-likestructure.Now, with this background,
let us see the solidification of liquid steel. Let us see, solidification of liquid steel,
of liquid steel. Now, steel, as all of you know, is an alloy of iron, carbon, silicon,sulphur,
manganese, oxygen, hydrogen, nitrogen and inclusions. From a steel making, you have
seen that, the liquid steel which you have, we have produced, it contains all theimpurities,
which I have mentioned just now. So, that means, one thing is clear. It will solidify
over a range oftemperature; it does not solidify at a constant temperature, but, it will solidify
over a range of temperature. When this happens, what will happen to the
impurity? On solidification, whatever solid will form, the impurity will partition between
solid and liquid. So, at solid liquid equilibrium, at solid liquidequilibrium, it define a equilibrium
partition coefficient which is K e, that is equal to, concentration of impurity in the
solid, on concentration of impurity in the liquid So, thisequilibrium partition coefficient,
and its value, will determine the extent of segregation. Now, let us see some of the values,for
example. If I take, say, element and value of K e in delta iron and in gamma iron. So,
I take here, carbon, sulphur, phosphorus, oxygen, nitrogen and hydrogen. Here it is
0.13, 0.02, 0.13, 0.02, 0.28 and 0.32.The K e value in gamma iron is 0.36, 0.02,0.06,
0.02, 0.54 and 0.45. So, particularly, you note the equilibrium partition ratio of sulphur
and phosphorous. So,it is very low, followed by oxygen also.
But, so, what will happen, during solidification, because of a very low solubility of sulphur
and phosphorous, the excess sulphur and phosphorous will be rejected into solidifying liquid.
And, as a result, the last liquid which will solidify, it will have a higher concentration
of sulphur and phosphorous, which will lead to formation of FeS. As I have said, in a
steel making, when I was mentioning that, it is very important to control the impurity
like sulphur and phosphorous. Now, you can correlate it, why it is so important. Now,for
example, if I take, say, for sulphur, if we take,for example, the K e value is equal to
0.02. So,CS, that is equal to 0.02 L;so, you can,0.02CL. So, you can imagine, the concentration,
or the solubility of sulphur in solid steel, is very low.
Now,so, that means, excess sulphur will be rejected adjacent to the solid. Now, say,
one can obtain the equilibrium solidification condition; if equilibrium is reached, then,
the extent of segregation will be minimized. In order to reach equilibrium condition, one
has to give very long time, because the diffusion processes in the solid, is very extremely
low, as compared to in the liquid. So, considering the equilibrium, say, equilibrium solidification,
it requires mixing and homogenization of composition, in both liquid and solid state. Now, say,
if we assume that, there is a complete mixing in the solid state; that is, we make, if we
assume that, complete mixing, complete mixing in liquid stage, and no diffusion in solid,that
is a very ideal condition, no diffusion in solid, because, when solid will form, the
diffusion processes will take place in the solid, to equalize the concentration, and
that requires a very long time. So, based on this assumption, complete mixing,
no diffusion in the solid, one can arrive at an equation, the concentration of C L that
is equal to C 0, 1 minus f s, raised to the power K e minus 1, and this equation is known
as Scheil’s equation. So, very famous equation, Scheil’s equation, where f s is the fraction
of solid, f s is fraction of solid; C L and C 0, I have already defined. So, with this
equation, one can analyze, what is the concentration of an impurity in liquid. We know f s, and
f s value can be determined from the phase diagram, by applying lever rule. And, accordingly,
the value of K e can be obtained, from what I have given over here, or from any standard
book. So, one can analyze, what will be theextent of segregation and so on.
During solidification, we have solid steel shell near the mold, followed by liquid, rich
in solute element, which is called mushy zoneand liquid steel, when we observe solidification,
normal to mold surface. Now, this is what, in this particular diagram, I am going to
show, temperature versus distance profile, normal to mold surfaceduring ingot solidification.
Now, let me, first of all, write down, this particular is the mold, and this particular
region, this is the air gap, and this is the air gap; and the air gap forms, due to thermal
expansion of mold and solidification shrinkage. That means, air gap forms, due to thermal
expansion of mold, thermal expansion of moldand solidification shrinkage.
So, this is the reason for formation of air gap, which is a very important phenomena in
solidification, because of the differential rates of thermal expansion and solidification
shrinkage, the air gap forms. Then, next to air gap, we have, this is the solid shell;
this is the solid shell, that is, from here to here is the solid shell. Then, from here
to here, we have the, so called mushy zone. We have,so called mushy zone, andhere, we
have liquid steel, we have liquid steel. Now, knowing that, the thermal conductivityof mushy
zone is smaller than thermal conductivity of steel, whereas, thermal conductivity of
air gap, as all of us know, thermal conductivity of air gap, is much smaller than thermal conductivityof
shell, solid shell,than thermal conductivity of shell. Now, as a result, if we want to
show the thermal gradient, or the temperature versus distance profile in mushy zone, solid
shell and mold and air gap, it can be shown, something this particular way.
So, first of all, we have the liquid steel. It comes over here. This is the profile. Now,
because the thermal conductivity of mushy zone is smaller than thermal conductivity
of steel,so, temperature gradient will be steeper in this mushy zone. Now, solid shell
has a higher thermal conductivity. So, the gradient will not be that steep, but it will
be somewhat flattener. This is the thermal gradient in steel shell. Now, the most important,
is the thermal gradient or the steep temperature drop in the air gap, because of the very small
thermal conductivity of air gap. So, the temperature drops suddenly in the air gap and then, again,
the temperature decreases. So, that is how, this particular…This is the temperature
versusdistance profile, shown normal to mold surface, during ingot solidification, and
it strongly depends upon the thermal conductivity, as shown in the figure.
Now, suppose, if we pour a killed steel into the mold. And, you would like to see, how
the macro structure is developed in the ingot. So, for that, let us see the macro structure
of the cross section of an ingot. So, this is, the blue one, blue color, that is, this
one is a chilled layer; mold is water cooled; as you pour the liquid, the liquid which is
in contact with the surface of the mold, it gets immediately chilled. So, as such, we
have the chilled layer; then, I have said that because of the segregation, a mushy zone
will form. In the mushy zone, which consists of dendrites and inter-dendritic liquid,so,
this,so, this will be so-called columnar zones, or which also consist of dendrites. In the
central portion, the central portion, we have a central zone, we have a central zone, which
consists of, which consist of dendrites, but they are randomly oriented and we call dendrites
which are randomly oriented and we call it Equi-axed zone.
So, now, with this background on principle of solidification, let us see now, the ingot
casting; let us see now, ingot casting, the first in the casting process. Now, ingot casting,
is done in cast iron mold, is done in cast ironmolds. The molds can have square, round,
or polygonal cross section, orpolygonal cross section. Typical ingot weight, typical ingot
weight, it varies, for rolling,5 to 20 tons; whereas, for forging, huge size, that is,
from few 100, few 100 tons to as high as 300 tons. Mold design,mold design, two design,one,
narrow end up, narrow end up, which automatically means, wide end down; that means, if I can
show it over here, this is the bottom plate; this is the narrow end up, or wide end down.
Second design is, narrow end down; narrow end down, automatically means, wide end up.
If I can show over here,so, this is the narrow end down; this is the narrow end up.
Now, the question comes, why cast iron molds; why cast iron mold and a conical shape, because,
thermal coefficient of cast iron, is different from that of steel. Steel contracts more than
cast iron;therefore, detachment becomes easier. Now, that is what, the importance in the design.
Conical shape, it facilitates ingot, while pulling ingotfrom the mold through a crane.
Now, here, narrow end up molds, they are most convenient for stripping the ingot, after
solidification. So, that is what the answer, for why cast iron molds and why there is a
conical shape, because of the easier stripping of the ingot, from the mold, after solidification.
Another important thing, that, inner walls of the mold, inner walls of the mold are coated.
They are coated by tar, oblique, fine carbon. Again, the question is why. If you do not
coat it, what will happen? The solidified shell, which is in contact with the mold,
it may get stick. So, during the stripping, it becomes very difficult. The coated material,
it decomposes, which prevents sticking of solidified ingot on the mold surface.
So, that is what, why inner walls of the mold are coated by some coating material. Next
thing, teeming; how the liquid steel is teemed in the ingot. One can have top pouring; bring
a ladle, put it on top of the ingot, slide gate is opened; the liquid steel is teemed
into the ingot and that is the toppouring. Another is, say, bottom pouring. Another is
the bottom pouring. In the bottom pouring, the liquid steel is poured into a channel,
and this channel, now, subdivides into two ingots, or two ingot mold, and the liquid
steel, which is coming from the ladle, is bifurcated into the two streams, and the liquid
steel begins to fill in the mold, from bottom to the top; that is what the bottom pouring
consist of. Now, another important thing, is the classification of steel ingots. We
can produce different types of steel, which are, which are classified, based on the oxygen
contentof the steel, or based on the oxygen killing. So, as such, we have three different
types of ingot that are being produced.
Major three types of…One, is the killed steel; second is the semi-killed steel and
third is the *** steel. For orientation, I will put, killed steel is a; semi killed
steel is b, and c is the *** steel. Between semi killed and *** steel, that is b,
between b and c, some different type of ingot can also be produced, which will depend upon
the *** action; that, I will tell you, what does it mean. So, first of all, let us
concentrate on the killed steel. The structure, or the macro structure, I have shown. This
is the ingot, which is the top of the mold; this is the bottom of the mold; and because
of the contraction of the steel, in case of killed steel, a pipe formation occurs. Now,
this pipe formation is, because of the contraction of steel. The steel, which will be last to
solidify, the amount of steel, is not sufficient to meet the contraction, which is occurred
during solidification. Hence, the top most part of the ingot has a pipe, or a shrinkage
cavity, which has to be scarved during processing. So, this is about the macro structure of killed
steel. Now, in semi-killed steel, the steel is not
killed completely; part of the oxygen is left over. So, on solidification, whenever the
solubility between the carbon and oxygen arrives, the carbon monoxide bubble forms. And hence,
the macrostructure of the semi-killed steel is characterized by, theseare the,so called
blow holes; and these blow hole forms when? Say,60, 70 percent of the steel is solidified;
because, oxygen content is still not sufficient,so that, it can form right from the beginning;
steel is also not fully killed. c is the *** steel. Now, in the ***
steel, no deoxidation is done. Because of thereaction between carbon and oxygen, that
occur during solidification, large amount of carbon monoxide bubbles are generated during
solidification. These bubbles may eject the droplets and the droplets fall back into the
liquid steel. They bring oxygen. So, on account of that, there is large amount of CO, that
can be entrapped. So, these are the blowholes. Now, because of the formation of CO, and this
is called *** action. Now I can use the term ***, because of the *** action,
the gradients will be minimized, and the steel which is solidifying, in contact with the
mold, it is a very clean steel, which is free from segregation. And, this sort of a layer,
which is just close to the wall of the mold; this is the clean metal, the clean metal.
Now, let me write down the characteristics of all these ingots. Now, first is the killed
steel. Now, here, oxygen is killed. How it is killed? By aluminium. So, killed steel,
as you recall, from deoxidation is done by aluminium. And, killed steel is used, or killed
steel ingots are used, when, first, homogeneous structure is required in finished steel; homogeneous
structure is required, in finished steel; that is the number 1. Number 2, typically,
alloy steels, alloy steels, forging grade steels, forging steelsand steels for carburized
type, they are produced as a killed steel ingots. Certain extra, certain extra deep-drawing,
deep-drawing steels, certain extra deep drawing steels, a very low carbon,for example, less
than 0.12 percent maximum; these steels are also killed.
But, the killed steel solidification, has two problems. One, the pipe formation, as
we are seeing, this is the pipe formation for killed steel; and second problem for killed
steel is, the alumina content of the steel, which is being obtained by use ofaluminium.
So, these are the important issues for killed steel ingot. Now, about the semi-killed, we
have, say, percentage carbon for semi-killed steel, the percentage carbon is in between
0.15 to 0.3 percent. Now, here, you must be wondering, why there is no solidification
shrinkage, or pipe formation is occurring, though steel is semi killed. It is not killed
with aluminium; maybe by silicon and manganese.
Because of the evolution of carbon monoxide during the solidification, it counter-balances
the solidification shrinkage. So, in killed steel, there is no CO evolution, because,
there is no oxygen over there. In semi-killed steel, partly there is oxygen; it is not killed
completely; but it is semi-killed. In between semi-killed and *** one, or in the ***
steel, this another grade, is produced, which is called caped steel. In the caped steel,
it is the variation of *** steel practice. The *** action is allowed to begin in
the beginning, and is then terminated, after a minute or two, by closing the top of the
mold, by a cast iron cap. In fact, this caped steel is also a modified form of *** steel
practice, which is done, as I have said earlier. A little amount of CO is allowed to occur.
But soon after, the top of the mold is closed by a cap, and more, is that, the close by
the cap and this steel, this caped steel ingot, they are produced for carbon to be greater
than 0.15 percent. Now, this practice of killed, caped steelingot production, is used for,
say, sheet, strip, wire and bars. About, say, *** steel, in the ***
steel, no deoxidation is done. No deoxidation is done. The percentage carbon, is in between
0.1 to 2.15 percent. Because of the large evolution of carbon monoxide, the steel, or
the solidified layer in contact with the mold is very clean; and it will also be low in
the solute, because of continuous stirring, that is provided by the CO bubbles. And, this
rimmed quality ingot, they are suited for steel sheets. So, that is what the different
type of ingots, that is being produced and they are useful and so on.
So, now with this, let us see, ingot defects and their remedies. Ingot defectsand remedies,
and remedies.Now, the first ingot defect is pipe; why it forms? It forms because of shrinkage;
because, liquid steel shrinks on solidification; pipe is a shrinkage cavity, which is formed
at the top of the killed steel ingot. Mind you, it forms only in the killed steel. What
is its effect? During reheating, oxidation, and formation of oxide, oxidation and formation
of oxide scale. What will it do? It will prevent welding, during, hot rolling; because, when
you hot roll it, oxide scale is very brittle, so, the welding will not occur. Hence, pipe
portion of the killed steel ingot has to be rejected. How, to eliminate? Now, pipe formation
is eliminated, by putting hot top at the, by putting hot top at mold top. So, what is
hot top?
The hot top keeps the ingot top, hot and molten, for a longer period. As a result, liquid steel
from top compensates the shrinkage, which has occurred during solidification. So, that
is what, in fact, hot top is; that means, top of the ingot is kept hot, so that, the
liquid steel remains molten for a longer period. Sometimes, use of insulating and exothermic
materials on the top of ingot, further ensures, availability of hot metal at the ingot top.
Pipe formation, in fact, pipe formation, which occurs due to shrinkage of liquid steel on
solidification, it can be minimizedalso, by casting in wide end up molds. That means,
there are two ways, in which it can be done; one way, by hot top; another way, is to casting
in wide end up molds. By casting in wide end up molds, what happens,the pipe, though it
will form, or the shrinkage cavity will form, but it will form at the top, it is shallow
and wider in nature.
So, I show you, by way of this sketch. In the figure a, anarrow end up mold is used
for casting. So, you see, this is the ingot top; and the, because of the solidification
shrinkage, the pipe formation extended deep into the ingot. And, this is, in fact, the
primary pipe, and to some extent, secondary pipe has also been formed. Now, if I take
wide end up mold, in the wide end up mold, you can see, this is, now, the pipe formation;
and, pipe formation is now occurring at the top of the ingot, and you see also, it is
shallow in nature and little wider in nature. So, you can use more amount of ingot, as compared
to, when casting is done in narrow end up. Now, if I provide a hot top;so, this is the
hot top; may have exothermic mixture, or whatever way; this is the hot top. Now, the level of
steel is ingot is same here, as well as here. Now, you see, if I provide hot top with the
wide end up mold, and if I cast it, what will happen? The pipe formation, it goes in the
hot top, and the entire ingot remains free from solidification shrinkage. That is how
the solidification shrinkage is eliminated. Next defects, we will discuss in the next
lecture.