Perlite is a microstructure found in some steels within a certain range of compositions. But before I dive headlong into the guts of the definition, I will give a brief metallurgy primer to get things underway.

For those that don't know, a steel is basically iron with carbon in it. You might think that steels can only vary in one way, therefore: more "carbonny" or less "carbonny". You would be wrong in two ways. First of all, carbonny is not a word. Secondly, there can be a great deal of difference between two steels with identical carbon content. This is due to the fact that time is a factor in the formation of microstructures in metals: if you cool something quickly (throw it in cold water perhaps) then it has little time to arrange itself before it solidifies.

Metallurgists use a visual construction called a phase diagram to explain this, but it makes things more confusing to the beginner, so I'll resist the temptation and leave that link for the over-eager.

So on with the show... Perlite occurs when a steel with a composition close to the eutectoid composition (0.76% carbon by weight) is cooled slowly. This slow cooling gives the iron and carbon plenty of time to arrange themselves into the most energetically favourable microstructure. By a magical dice-roll of the rules of Gibbs free energy, it turns out that this structure is a lamellar arrangement of alpha-ferrite (iron with a little carbon dissolved in there) and cementite (iron and carbon in the form Fe3C). Ferrite is a soft, ductile phase; Cementite is a hard, brittle phase. This layered arrangement of the two phases therefore provides a best of both worlds scenario with moderate strength and toughness: a steel with a lot of perlite is something of an all-rounder.

Recognising a perlitic microstructure under a microscope is fairly easy. Simply cut up your sample and polish it to a smooth surface finish (a 1 micron finish should do nicely). Then etch it to show the carbon content and examine it under a microscope. Perlite will usually appear at grain boundaries and will look kind of like photocopies of fingerprints, but less regular in shape.

I have gone into some depth here, but possibly not enough if you are still reading this. My intention with this piece was partly to show you that no matter how nicely I try to present it, the world of metallurgy is exceedingly dull and will always be that way.

Pearlite is a microstructure occuring in Steel. It consists of alternating layers of Iron in Body Centered Cubic structure (Ferrite), and Fe3C (Cementite).

It is called Pearlite because of the fact that when viewed under low magnification, it looks a bit like mother of pearl.

Compared to other Steel microstructures, specifically Bainite and Martensite, it's not too strong or hard. However, it is quite ductile, which often comes in handy if you wish to avoid sudden catastrophic failure of whatever it is you're building.

How do you make Pearlite? Well, you take some Austenite, basically any steel above 727C, and let it cool. Pearlite will only be formed if it's allowed to cool between 540C and 727C. At the lower temperature, it forms quite quickly, within 10 seconds. At the higher temperatures, it can take quite a bit longer, taking longer the closer it is to 727C.

There are generally two classifications of Pearlite, Fine Pearlite, and Coarse Pearlite. The physical difference between the two is in the thickness of the layers the Pearlite is made of.

The finer the layers making up the Pearlite, the more they're going to resist slipping relative to each other, and the stronger the Steel is going to be. Conversely, the more coarse the Pearlite, the more the layers will be able to stretch out, and the more ductile it'll be.

Finer Pearlite is formed at lower temperatures than Coarser Pearlite, and thus it has to be cooled down quicker than the Coarse. Generally, if you wish to form Fine Pearlite, simply pulling it out of the furnace and letting it cool in the air will be sufficient. To form Coarse Pearlite you have to stick it in another furnace, one that is kept between 600-700C.

Also, Pearlite will generally not form on its own, depending on the Carbon content of the Steel. If the Carbon content of the Steel is less than 0.76%, some of the Austenite will have already transformed into Ferrite before it reaches 727C, and if it's above 0.76% Carbon, some of it will have turned into Cementite.

Usually these will form branches that will clump together a bit, and then the Pearlite will fill the gaps between them. This is called Pearlite in a Ferrite (or Cementite) matrix.

Of course, if you've got exactly 0.76% Carbon, you can get pure Pearlite.

The less Carbon you have, the more Ferrite you'll have compared to Pearlite, and the more Carbon you have, the more Cementite you'll have crowding out the Pearlite, up until 6.7% Carbon, at which point it'll be all Cementite.

And, if you really want to get funky, you can cool it down below 540C while the Pearlite is still only half done forming. If you do this, the Austenite that hasn't turned into Pearlite yet will turn into either Bainite or Martensite, depending on how fast you cool it.

This is how you get some funky properties, but it's beyond the scope of this node.

William D. Callister, Jr., Materials Science and Engineering An Introduction, 5th Edition. John Wiley & Sons, Inc.

My memory from ENME421, Mechanical Engineering, Materials I, at the University of Calgary.

Pearl"ite (?), Pearl"stone` (?), n. Min.

A glassy volcanic rock of a grayish color and pearly luster, often having a spherulitic concretionary structure due to the curved cracks produced by contraction in cooling. See Illust. under Perlitic.


© Webster 1913.

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