2.22 Formation of Bainite in a Eutectoid Steel

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来自 Georgia Institute of Technology 的课程

Material Processing

152 个评分

Georgia Institute of Technology

Material Processing

152 个评分

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.

从本节课中

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

与讲师见面

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

In this lesson we're going to focus on the development of bainite that

comes out of a eutectoid steel.

And we're going to follow the same progress that we did

as we did when we described the decomposition of austenite into pearlite.

So now here is our eutectoid diagram again,

here is the temperature range over which we're interested.

We see that first red dot represents the austenite, and

the lower temperature is where the bainite is going to begin to form.

So, what happens in the case of forming the bainite,

which is different than the way the pearlite forms, is basically,

we start out with our austenite grain boundaries.

And I've indicated these structures that are looking

like laths that have grown from the austenite grain boundary

in toward the center of the austenite grains.

I've shadowed these with some colorant to indicate that around those

particles, which turn out to be ferrite, is where the carbon is going.

So the carbon is leaving the ferrite and

it has a cloud around the individual ferrite structures,

and eventually what's going to happen is those clouds will begin to

form the Fe3C inside of the bainite lamellar or

the bainite structures that we have, the laths of bainite.

So there's our carbon clouds.

And now what we're going to do is to allow some more time so

that these structures begin to form.

And we can produce some more of the structure where

those that formed initially turn out to begin to

have the structure of the ferrite in these little grains,

and the cementite that comes out.

And at this next time step, what's happened is we have nucleated and

grown some more of these alpha phases from the austenite grain boundary.

So the austenite supplies the location for

where the transformation is going to occur.

The alpha phase forms and then we get the distribution of carbon

around because of the low carbon content of the ferrite.

And then eventually what happens is the ferrite begins to

have some precipitation of the phase Fe3C or the iron carbide.

And so that's what those little hot dog shaped

particles are that are in the structure.

So depending upon the cooling rate and the holding temperature,

we're going to wind up getting different morphologies of bainite.

And as it turns out there are two types of bainites that form, high,

upper bainite and lower bainite and

the principle difference between the two of these is the fineness of the structure.

And so what you find is in the case of lower bainite,

those particles of Fe3c are very fine and

as a result are going to help contribute to the overall strength of the material

that will be the result when all the austenite has decomposed and

we're left with this two phase microstructure of alpha ferrite and Fe3c.

So when we look at the micrographs we have a micrograph where the bainite

form at an elevated temperature 495 C and we have these

types needle like structures which are growing into the austenite.

And what you begin to see is as you go to a lower temperature,

you get the development of very tiny particles inside, and

those tiny particles are the ones that are helping to contribute

to the strength of the material as a result of their fine distribution.

So the structure that we're looking at is given

to the left here, where we have the schematic of what the bainite looks like.

So these two structures that form,

the pearlite morphology that develops and the bainite morphology,

both of these develop from the single phase austenite and develop these

two different morphologies depending upon the temperature to which they're quenched.

Thank you.

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