In the last lecture, I mentioned about acicular ferrite several times, so what is acicular
ferrite?
In fact, acicular ferrite steels can also be regarded as HSLA steel since we can obtain
acicular ferrite structure when the hot-rolled microalloy steel is subject to fast cooling.
As the name suggests, acicular suggests, the microstructure consists of needle shape or
lenticular shaped grains.
If you look at the microstructure in detail such as by TEM, you can see they also contain
several subunits.
Actually, the acicular ferrite are randomly oriented ferrite grains formed within prior
austenite grains.
Actually, if you look at these two figures, one acicular ferrite, on the right hand side
the bainite the morphology looks quite similar but actually there are differences.
The difference is, as you can see in this left figure, acicular ferrite forms within
prior austenite grain boundary with random orientations.
On the other hand in the case of bainite, bainite consists of packet which contains
parallelly oriented laths.
So morphologically, bainite is more or less similar to martensite structures.
On the other hand, acicular, when you look at the low magnification such as optical micrograph,
it looks similar to bainite.
But if you look at detail, the grains are randomly oriented so it will give you much
final effective grain size.
Actually, you cannot make any acicular ferrite using any compositions.
The problem is the steel should have enough hardenability, which can be done by adding
alloying elements such as molybdenum and also by using the fast cooling.
So depending on the hardenability, that is, depending on the composition and the cooling
rate, steel can contain various phases other than the acicular ferrite such as bainitic
ferrite, quasi-polygonal ferrite and MA, martensite-austenite constituent phases.
In fact most of the acicular ferrite steels will consist of acicular ferrite and some
of the second phases such as bainitic ferrite, quasi-polygonal ferrite and MA constituent
phase.
Acicular ferrite has some important characteristics.
The one of the most important ones is acicular ferrite has a fine effective grain size than
martensite or bainite.
The effective grain size is actually meaning the crack propagation will change the direction
when you meet certain boundary.
So the boundary, mean-free path, is much smaller in the case of acicular ferrite than those
of martensite or bainite.
If you look at this figure, it is actually, this structure consist of three phases, acicular
ferrite, polygonal ferrite and martensite.
When crack propagate in the martensite, the crack doesn't change the direction.
That means martensite’s effective grain size is very big.
But if the crack propagate within the ferrite, within the acicular ferrite, crack path become
tortuous, that is, they will change the direction.
And that means acicular ferrite will have much finer effective grain size than the martensite
or even than the bainite.
So because of such characteristics, acicular ferrite steels show much better combination
of strength and toughness than the ferrite based steels.
Another important characteristic of acicular ferrite is that it shows higher strengths
than ferrite based steels after UOE pipe forming process.
The main reason is, in the case of ferrite based steel they show the discontinuous yielding
behaviour.
But on the other hand, acicular ferrite steel shows continuous yielding behavior.
So when you are doing the UOE pipe forming, first you are making a pipe into U-shape and
you form the upper part to make an O shape.
After that, you weld the upper part and you will expand.
So this is called U making U shape, O shape and expanding.
So this is one of the method which make certain kind, certain shape of the pipe.
So if you look at this figure one more time, although ferrite based steel has a higher
strength, than higher yield strength than the acicular ferrite steels, after you do
U and O forming, their strength becomes a little smaller than the original strength.
After you did expand the steel, strength increases a little bit, but in the case of acicular
ferrite steels now you are starting from the base material and after that you do U and
O shaping and you do the E expansion, the strength keeps on increasing.
So the final result is, acicular ferrite steel will have higher strength of the pipe than
that made by ferrite based steels.
Now this one is, this point is the property of steels after making flat product because
it's almost impossible to make a tensile sample out of the pipe.
So to match the properties you have to make a flat shape after you cut the pipe and making
flat.
So even in this case it shows that acicular ferrite steel has higher strength than that
of ferrite-pearlite steel.
