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.