Pearlite

The pearlite is a lamellar arranged, eutectoid structure component of the steel.

It is a phase mixture of ferrite and cementite that occurs by coupled crystallization in iron -carbon alloys with carbon contents between 0.02 % and 2.06 %. The eutectoid point is at 723 ° C and 0.83 % C. Up to 2.06% C, the pearlite is present as a separate structural component, above 2.06% C, it is part of the ledeburite II ( eutectic structure ).

Often this is called a " pearlite ", as measured by the lamellar spacing in pearlite, thin-strip (obsolete: sorbitol ) and very feinststreifigen (obsolete: troostite ) perlite is divided. Since the disk packs in perlite are arranged randomly and are cut in the ground in different directions that correspond to visible in the micrograph fin spacing not the actual (usually less ) intervals.

Representation

The steel is ground with the usual method in metallography and polished and then etched with dilute nitric or picric acid. By etching the ferrite is more attacked than the cementite, which is why the cementite sublime come forward and throw in some oblique illumination shadow lines. The raised cementite also act as an optical grating, occurs in the iridescent color by interference of white light light. This is reminiscent of mother of pearl effect owes its name to the perlite.

Pearlite

In the formation of eutectoid pearlite, the structure is depleted locally in carbon, while the neighboring areas by diffusion continues to accumulate carbon. Due to the variety of low-carbon and - rich areas and causes the typical lamellar structure. Now Adapts to a carbon content of 0.02 % in the low-carbon blade, fold the fabric of the lamella into ferrite ( α - Fe). The carbon content in the carbon-rich lamella, however, rises to 6.67 %, so it forms cementite ( Fe3C ). Since it arises secondarily from the austenite ( γ - Fe) (as opposed to primarily from the melt), it is referred to as secondary cementite ( Fe3CII ). This front of cementite and ferrite simultaneously grows into the austenite.

Cools the structure further, then falls out of the α - Fe, due to the declining ability of carbon to bind ( 0.00001 % carbon at room temperature), further cementite from that one now that he fails for α - Fe, Tertiärzementit ( Fe3CIII ) calls.

When hypoeutectoid pearlite, ie with a carbon content between 0.02 Ma % < C < 0.80 % by mass, is formed in the temperature reach A3 ( GOS line in the iron -carbon diagram ), due to the decreasing solubility of the austenite ( γ - solid solution ) for carbon, already ferrite, so-called voreutektoider ferrite. Upon further cooling of the remaining austenite is carbon-rich, until it has a concentration of 0.80 % by mass of C, it is now at 723 ° C for eutectoid transformation, and the austenite transforms to pearlite to.

In a hypereutectoid pearlite, ie with a carbon content of 0.8 Ma. % < C < 6.67 Ma. %, Is created before the pearlite cementite. In contrast to the arising in the pearlite cementite cementite this is not in lamellar form, but is formed mainly at the grain boundaries and thus to distinguish microstructure default.

The starting temperature is small, so that there may be no diffusion of carbon, pearlite may occur. Instead, formed during the cooling, the bainite bainite.

Effect of cooling rate

Ideally obey the iron -carbon diagram showing the equilibrium lines. If you cool now but from a higher speed, these equilibrium lines no longer apply and the Perlitpunkt ( 0.8 % carbon, 723 ° C) expands to a Perlitgebiet at lower temperatures. This makes it possible to convert purely pearlitic under and hypereutectoid steel. The increased speed also leads to feinlamellarem perlite, so to sorbitol or troostite.

Increases the cooling rate to a value greater than the rate of diffusion of carbon, so it may be no pearlite and martensite is formed.

• Materials Science