Have you ever wondered why ceramics are hard and brittle while metals tend to be ductile? Why some materials conduct heat or electricity while others are insulators? Why adding just a small amount of carbon to iron results in an alloy that is so much stronger than the base metal? In this course, you will learn how a material’s properties are determined by the microstructure of the material, which is in turn determined by composition and the processing that the material has undergone. This is the second of three Coursera courses that mirror the Introduction to Materials Science class that is taken by most engineering undergrads at Georgia Tech. The aim of the course is to help students better understand the engineering materials that are used in the world around them. This first section covers the fundamentals of materials science including atomic structure and bonding, crystal structure, atomic and microscopic defects, and noncrystalline materials such as glasses, rubbers, and polymers.
2.28 Continuous Transformation (CCT) Diagrams
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Del curso dictado por Georgia Institute of Technology
Material Processing
142 calificaciones
De la lección
Kinetics of Structural Transformations
If thermodynamics, which we covered in the previous module, tells us how a material wants to change, then kinetics tells us how and how quickly that transformation occurs. This module starts by explaining the driving force for phase transformations. We will cover the nucleation and growth of precipitates, solidification, and sintering. Finally, there are a number of lessons which apply all that has been covered in the course to understanding carbon steels.
Conoce a los instructores
Thomas H. Sanders, Jr.Regents' Professor
School of Materials Science and Engineering
Welcome back.
We're going to start talking about the development of continuous cooling
transformation diagrams or they're sometimes referred to as CCT diagrams.
And as the name implies, the part is undergoing a continuous
cooling as opposed to an abrupt rapid quench that we have
in the isothermal types of diagrams that we've talked about earlier.
Now in order for us to be able to construct these continuous cooling
diagrams, what we need to have is a machine specimen.
It has specific dimensions, because it's going to have a specific thermal mass
that ultimately is going to be cooled by a waterjet which you see at the bottom.
So there's a fixture in which this specimen that appears to look
like a rivet, fits into and when it's in that position,
a water jet turns on and rapidly cools.
And what you see is the picture to the right which shows as you move
up away from the water jet, the cooling rate is decreasing.
So the process begins by taking that rivet material,
putting it into the single phase austenite range,
holding it there until it's completely homogenized then drop it or
position in two as rapidly as possible into the fixture.
And then the water comes on and as soon as that happens the material begins to cool
and you see the cooling rights that are plotted here.
And of course the cooling rights can be determined experimentally by putting in
temperature measure devices along the surface of the specimen.
Now it turns out that if you look at a variety of different materials and
what we're looking here is the effect of cooling rate on
a parameter that we refer to as the hardened ability of a steel.
And what we mean by the hardenability is the likelihood that that
microstructure under certain coolings will produce the highest strength or
the highest hardness, and therefore has the highest hardenability.
So let's take a look at the first plot that we have, and this is for
steel which is labeled 4340.
And this is a high strength steel that has many
applications in structural components, and
what you can see is plotted along the top,
is the cooling rate that tells you what that cooling rate is,
as a function of the distance from the water jet.
And of course, what you see here is that the flatness of this line,
which is a line that fits the hardness data.
And so what you're looking at is the hardness
as a function of distance away or as a function of cooling rate.
And what you're seeing is that the hardness that develops after that material
is quenched winds up producing a high hardenability material all the way up
to about a cooling rate of about 11C per second.
Now if we look at another alternative steel,
namely 4140, that particular steel has a shorter time or
a shorter distance or shorter cooling rate than does the 4340.
So for example in that particular case,
you have to make sure that you have a higher minimum cooling rate in
order to have the maximum hardness or the maximum harden ability for that steel.
And looking at all the different steels you one you see that is the most
sensitive of the cooling rate is the steel that's labelled 1040.
What is meant by 1040?
It's a plain carbon steel.
It contains four-tenths of a percent carbon.
And of course what you see is for all the cooling rates that we have,
there is a rapid drop in the Hardness of the material, it doesn't maintain
the strength of the martin sight that you would have, and therefore,
you would say that this is essentially a very low harden ability steel.
So these plots then give you how the material
performs as a function of the cooling rate.
We're going to look at how these CCT diagrams are actually constructed.
What we're going to do is to look at various cooling rates and
determine how it affects the start and the end of the transformation.
Then what we'll be able to do is to study this fictitious transformation that
I'm referring to as alpha to beta.
And I'm going to consider different linear cooling rates and
will determine at what time and what temperature the start and
the finish of the reactions occurred at the different cooling rates.
And then once I have all that,
then I'll take those points and I'll plot them to construct my CCT curves.
Now, we construct our CCT curves on a time temperature axis,
we're plotting the log of time and we're plotting the temperature.
The specimen is seeing as it cools from the transformation temperature.
So the transformation temperature above that I'm in the single alpha phase field.
I quench below that line and I begin to develop, depending upon how long I'm at
that particular temperature, and will form progressive
amounts of the beta that's occurring and replacing the alpha phase.
So what we'll do then, is we'll look at a variety of different cooling rates, and
what I'm doing here is to plot different cooling rates on my diagram.
And what I'm looking here is as I go from left to right,
I am seeing a decrease in cooling rate.
Now what I want to do is to take a look at specimens at various locations.
It turns out that first blue point that I see there at time t1 is
the first point at which for that particular cooling rate,
I begin to see the transformation occur.
When I go to my second cooling rate, I find that the combination of time and
temperature where the transformation just starts is at the second
point designated as t2.
I go on to position t3 and then eventually to position t4.
So now I have to start of all the different transformations
and I'm going to take those and
make a nice graph that fits all of those points together onto a single curve.
That single blue curve represents the start of the transformation
that's build up as a result of different cooling rates.
Now I'm going to do the same thing with the end of the transformation, and
you see the points t1 through t4.
And I'm going to connect all of those, and now I have the continuous
transformation diagram for this particular phase change,
where the alpha phase is being replaced by the beta phase.
Now, I can determine what the structure is when I cool all
the way down to room temperature by looking at my specimens.
And when I cool that specimen that cooled through that cooling rate
that's represented by t4, what it tells me is that the specimen that has gone all
the way down to room temperature hasn't changed once it's passed the t4 point.
Once it's passed that red point, it just continues to drop in temperature, and
the beta phase remains the same.
The same thing happens when we go from the blue t2 to the red t2.
What we're seeing is a continuous amount of the beta phase
forming as we go down in temperature.
But when we examine the material down there at room temperature again we
have 100% beta.
Now, if we look at the curve at which we're going to refer to as
cooling curve t1, where t1 blue and
t1 red represent the start and the finish of the transformation.
What we see is there is approximately 50% of the material
that is left over is still the alpha phase.
So when I cool the material down it's forming between the start and
finish of t1, but I have not completely transformed all of the alpha phase.
So this now gives me an indication of what's happening during that
transformation process and of course, the thing is as long as you cross
the start and the finish the material has completely transformed.
Now I've indicated a blue point, a green line on here and
that green line represents what we call the critical cooling rate.
And what that says is, any cooling rate that is faster than that cooling rate
will wind up avoiding the transformation and I will go directly
from above the transformation temperature to down to room temperature,
and I will have 100% of my alpha phase.
Any cooling rate that's faster than that critical cooling rate will do exactly
the same thing.
So this gives us a way of determining the critical cooling rate.
And depending upon where we are in our specimen and therefore, what the cooling
rate's going to be, we will see different amounts of material transformed.
And of course, those transform regions are occurring over a variety of temperatures
which means that they're not going to be all the same uniformity and size.
And so here is a particular CCT curve.
It looks very similar to what we talked about before.
This happens to be the continuous cooling transformation diagram for
a eutectoid steel and what we are going to do, we are going to examine what happened.
So if we look at the red portion, what's happening along the red portion?
The material is cooling all the way up to the start line.
What we have is 100% austenite as soon as we hit that start line
the austenite begins to transform into pearlite.
And so between those two points what we're seeing is
the increased amount of pearlite until you pass the complete line.
And when you pass that complete line, you have 100% pearlite and
it's stable all the way down to room temperature.
Now when we go to room temperature we see we are now in
the region where we have effectively 100% pearlite.
Now, if we look at the next cooling curve, what we're going to see is,
we're coming down and we're looking in the austenite phase field.
We continue to cool down from the start line to that line that appears
to be slightly horizontal and what's happening of course,
in there is having my austenite transform to my pearlite.
And when I come all the way through, and I drop below the martensite line,
any of that austinite that was left over when I crossed that
line will transform into martensite.
So I have now pearlite and martensite because all the austenite is now gone.
And if I look at this last line,
that last line represents again the critical cooling rate.
And so as a consequence for that particular cooling rate what I'm going to
have is a 100% martensitic structure.
Thank you.
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