2.27 4340 Isothermal Transformation (IT) Diagram

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来自 Georgia Institute of Technology 的课程

Material Processing

139 个评分

Georgia Institute of Technology

Material Processing

139 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

Our focus up until this point has been to look at essentially plain carbon steels,

but it turns out that there are an abundance of different compositions and

an abundance of different materials.

One of the really high strength materials that are fabricated

from steels is an alloy that's referred to as 4340.

And we're going to take a look at it, simply because it represents a difference,

because there's a change in the alloy composition.

There are other elements that have been added.

And what we see is as we begin to add other alloying elements,

the phase boundaries begin to change, or the boundaries of the start and

finish begin to change.

So, we'll look at 4340 and

examine the isothermal transformation diagram for that particular material.

We see that we have the regions that we normally do.

We have an austenite range.

We have an austenite ferrite range where were forming proeutectoid.

So we are obviously at a composition which is no longer eutectoid.

So we have that AF field and then we go beyond that and

what we're going to wind up seeing is the formation of perlite in that next region.

And so when all of the austenite is transformed,

we find ourselves in the region that is ferrite plus pearlite.

So, we'll go through these different regions of the diagram and

describe what's going on.

So generally, the way these diagrams are setup, they're setup so

that the A represents the austenite, the F is the ferrite phase

that forms as a proeutectoid and the pearlite is given here as P.

Sometimes what a number of diagrams contain rather than using P,

they will actually just use ferrite plus semminite and they will take those

two ferrites and put them together and you'll just have ferrite plus cementite.

But the reality of it is whenever you form proeutectoid ferrite,

you will have ferrite from that decomposition as well

as ferrite that forms the pearlite two-phase microstructure.

Now when we go down to low temperature, what we begin to see is the A+B range.

And again, the A+B range tells us that we have austenite plus bainite and

you can see that dotted blue line and

that dotted blue line represents the 50% line for the bainite structure.

When you get all the way over to the solid black line and

you're in the B range, that's where the bainite is.

When we go down to lower temperatures, what we see is a 10%, 50 and 90% lines and

those represent the transformations for the development of more insight.

Now, what's interesting is one of the things that's important about adding

a number of these alloying elements is that the high temperature phases

that are diffusion controlled tend to be pushed to slightly longer periods of time.

Ultimately, that's going to lead to materials that are not quite so

quiet sensitive and we'll see more about that when we get into

actually cooling curves, but we shift that curve slightly to

the right in comparison to eutectoid and off eutectoid alloys.

And so, those processes are delayed with respect to

the eutectoid compositions or plain carbon steels.

The other thing that happens is you see a clear delineation between the region

in which you form pearlite as opposed to the region where you form bainite.

So those regions become physically separated and you would expect they

would because they are actually formed by different kinds of mechanisms

with respect to the pearlite forming as a result of the motion of iron and

carbon along the austenite pearlite interface and the precipitation of

cementite out of the ferrite when we are forming the development of bainite.

So, they are different and we're not too surprised then that we

see these transformation curves that are shifted.

In addition, when you add alloying elements with the exception of

maybe cobalt, generally, what happens is as you add out other alloying

elements like carbon and chromium and nickel and other elements,

what you find is the martensite start and finish temperatures begin to depress.

So, these diagrams are different and each alloy will have its own isothermal

diagram that has been determined by investigators over the years.

Thank you.

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