And what we mean by the hardenability is the likelihood that that
microstructure under certain coolings will produce the highest strength or
the highest hardness, and therefore has the highest hardenability.
So let's take a look at the first plot that we have, and this is for
steel which is labeled 4340.
And this is a high strength steel that has many
applications in structural components, and
what you can see is plotted along the top,
is the cooling rate that tells you what that cooling rate is,
as a function of the distance from the water jet.
And of course, what you see here is that the flatness of this line,
which is a line that fits the hardness data.
And so what you're looking at is the hardness
as a function of distance away or as a function of cooling rate.
And what you're seeing is that the hardness that develops after that material
is quenched winds up producing a high hardenability material all the way up
to about a cooling rate of about 11C per second.
Now if we look at another alternative steel,
namely 4140, that particular steel has a shorter time or
a shorter distance or shorter cooling rate than does the 4340.
So for example in that particular case,
you have to make sure that you have a higher minimum cooling rate in
order to have the maximum hardness or the maximum harden ability for that steel.
And looking at all the different steels you one you see that is the most
sensitive of the cooling rate is the steel that's labelled 1040.
What is meant by 1040?
It's a plain carbon steel.
It contains four-tenths of a percent carbon.
And of course what you see is for all the cooling rates that we have,
there is a rapid drop in the Hardness of the material, it doesn't maintain
the strength of the martin sight that you would have, and therefore,
you would say that this is essentially a very low harden ability steel.
So these plots then give you how the material
performs as a function of the cooling rate.
We're going to look at how these CCT diagrams are actually constructed.