Georgia Institute of Technology

Material Processing

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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.

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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

Rencontrer les enseignants

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

In this lesson we're going to describe how you go about heat treating

a plain carbon steel whose composition is the Eutectoid.

What we see on the diagram are the regions where Pearlite forms,

and Pearlite is forming as a result of holding the Austenite

below the Eutectoid temperature for a certain period of time.

And what you do is to produce the Pearlite,

which is the two phase microstructure of ferrite and

cementite and that is occurring by a diffusional process.

When we quench to a lower temperature and hold.

For example, if we were to quench the 300 degrees and hold it for

various periods of time, we would see that up until we reach that first

line that separates the Austenite from the Austenite,

Bainite phase field, what we find is we have 100% Austenite.

When we reach the line at 300 degrees,

we begin to form structure that we refer to as Bainite.

Again, FE3C plus ferrite and it continues on and

the Austenite continues to decompose,

forming more and more Bainite until we reach the dotted line where we have

50% Bainite and then eventually we have 100% Bainite.

Now, if we were to quench from above the Eutechtoid temperature,

this time down to the Lower temperature.

We would wind up producing the structure that we refer to as martensite.

Now remember, martensite is a single phase material.

It occurs by a diffusionless transformation.

And it's referred to as an athermal transformation.

In other words let's quench the Austenite Down to the 50% line.

When we hold it at the 50% line, we can hold it indefinitely and

what we should have after we've held it for that period of time is still 50%

Austenite that has not decomposed plus the 50% martensite that did.

So, we have those horizontal lines.

Now when we look at the various temperatures to which we quench.

Starting up there with the Austenite transforming into these various

decomposition products, what we see is that there becomes

a progressively finer micro structure that develops and

ultimately when we quench down and product Martensite.

We're going to get a very fine microstructure that has a very high

strength.

Thank you.