2.26 Off-Eutectoid Isothermal Transformation (IT) Diagrams

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From the course by Georgia Institute of Technology

Material Processing

136 ratings

Georgia Institute of Technology

Material Processing

136 ratings

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.

From the lesson

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.

2.17 Developing High Strength Alloys6:58

2.18 The Iron-Carbon System7:37

2.19 Diffusional/DiffusionlessTransformations6:13

2.20 Heat Treating a Plain Carbon Eutectoid Steel2:49

2.21 Formation of Pearlite in Eutectoid Steel9:06

2.22 Formation of Bainite in a Eutectoid Steel5:07

2.23 Formation of Martensite9:49

2.24 Heat Treatments of Austenite Decomposition Products2:11

2.25 Isothermal Transformation (IT) Diagrams for a Eutectoid Steel6:31

2.26 Off-Eutectoid Isothermal Transformation (IT) Diagrams7:18

2.27 4340 Isothermal Transformation (IT) Diagram5:12

Meet the Instructors

Thomas H. Sanders, Jr.
Regents' Professor
School of Materials Science and Engineering

Up until this point, we've been focused on alloys

whose composition is at the eutectoid composition.

But now what we're going to do is to turn our attention to off eutectoid alloys.

Now here is the section of the iron carbon phase diagram, and

I'm calling your attention to the fact that we're going to look at a couple of

different compositions along the carbon editions from the on the order of

about 0.4 up to about 1.0 carbon with the eutectoid in the middle.

So what we have are the equilibrium phase boundaries and I have also indicated

the martensite start temperature as a function of carbon content.

We haven't talked about this, but

as you change the composition of the alloy in terms of carbon, what you find is

that the martensite start temperature depends upon what the carbon content is.

And so we see that increasing the carbon content decreases

the martensite's start temperature.

It will also decrease the 50% lines, and

the martensite finish lines as well.

So, when the material quenches from the austenite

phase field down through the two phase field of ferrite and

cementite, because we are in a section of the diagram where

we have the proeutectoid ferride form,

that alpha is forming as we cool down when we reach the eutectoid line.

Then structure that's left over of austenite is the material that

will ultimately transform depending upon the temperature to either perlite,

bainite, or martensite.

And what we will see is that the temperature at which the martensite

forms is higher than the temperature at which the martensite

would form if we were looking at a eutectoid steel.

And if we looked at a composition that was to the right, or

hypereutectoid, the structure that we would see would be

proeutectoid cementite coming down through that range.

And what we would find is that there is a continued decrease and

in the martensite temperature with the increased carbon content.

Now what we'll do is make a comparison between the equilibrium diagram and

the isothermal transformation diagram which is given to the right.

So up until this point, our IT diagrams are diagrams that were

associated with the eutectoid composition.

But now, because we are hypoeutectoid,

we have to consider the development of the proeutectoid phase.

And that's what happens in the upper portion of the IT diagram.

So as we cool down, we produce the proeutectoid phase ferrite,

and it's in this region where the ferrite forms.

Then I want to draw your attention as

we go down to lower temperatures and we look at lower temperatures,

we see that proeutectoid region narrowing down to a point.

And we'll discuss this in a bit more detail.

So, our proeutectoid ferrite forms,

and now we have the martensite start temperature.

So we know where the martensite lines are going to be.

And again, it's in comparison to the martensite temperature we have for

the eutectoid steel.

Now, one of the things that I want to point out is,

we look at how narrow that proeutectoid region gets and what it's telling us

is if we can quench our micro structure down below where that point is,

the micro structure that we will get will be 100% bainite.

There will be no proeutectoid phase.

Whereas if we go to a temperature above that critical point,

what we find is we will get austenite decomposing into proeutectoid ferrite,

followed by the austenite that transforms into perlite.

Okay.

So let's look at this in a bit more detail.

So what we're going to do is we're going to come along this line and

we're going to quench and hold..

And when we hold it, what we're going to see is austenite, 100%.

And now we're going to hold it for a slightly longer period of

time in that region where the proeutectoid ferrite forms.

And so now we get austenite and the proeutectoid ferrite.

Now we can't really tell exactly how much of this phase that we have,

because we're cooling down to different temperatures.

And so consequently, all we can really say from this diagram is that we will have,

at this particular ccomposition will have austenite and

we'll have some eutectoid alpha that has formed.

Generally speaking that eutectoid alpha is going to form at

the grain boundaries of the austenite.

If we hold it for a longer period of time, any of the austenite that have left over,

that was not consumed by the formation of the proeutectoid alpha,

winds up going into being able to transform

from austenite into perlite at this particular temperature.

So, what we get is proeutectoid alpha.

We're still in that region, so all of the sustenite has not decomposed and

we have perlite.

And we get over to this final point, and

now what we get is a microstructure that contains proeutectoid alpha and perlite.

Now let's go down to a lower temperature and consider what happens

if we happen to quench in the region that we refer to as bainite.

So now we hold it for the amount of time.

We get 100% austenite because I haven't started

the transformation into the bainite two phase region.

And so we will wind up with, at our next point,

50% austenite and 50% bainite.

And if we continue on holding it, we wind up getting 100% bainite,

and so at those three structures, we have austenite,

we have 50% austenite, 50% bainite, and

finally all the austenite is gone, and we have only the bainite phase present.

Thank you.

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