It is a phase mixture of ferrite and cementite , which coupled by crystallization in iron - carbon - alloys occurs at a carbon content between 0.02% and 6.67%. The eutectoid point (100% conversion to perlite) is at 723 ° C and 0.83% C. Up to 2.06% C, the perlite is present as a separate structural component, above 2.06% C it is part of the Ledeburit II ( eutectic structure).
Often one speaks of a “pearlite step”, which is divided into pearlite, fine-striped (outdated: sorbitol ) and very fine-striped (outdated: troostite ) pearlite , measured by the lamellae spacing . Since the lamella packs are randomly arranged in the perlite and are cut in a wide variety of directions in the cut, the lamellar spacing visible in the cut does not correspond to the actual (mostly smaller) spacing.
The piece of steel is ground and polished using the usual metallographic methods and then etched with dilute nitric or picric acid . The etching attacks the ferrite more than the cementite, which is why the cementite lamellae stand out in a raised manner and cast shadow lines when the lighting is slightly oblique. The raised cementite lamellas also act as an optical grid in which interference from white light creates colored iridescent light. Perlite owes its name to this effect, reminiscent of mother-of-pearl .
When the eutectoid pearlite is formed, the structure is locally depleted of carbon, while the neighboring areas are increasingly enriched in carbon through diffusion . Due to the alternation of low-carbon and high-carbon areas, the typical lamellar structure is created. If the carbon content in the low-carbon lamella is now 0.02%, the structure of the lamella turns into ferrite ( α-Fe ). The carbon content in the carbon-rich lamella, on the other hand, rises to 6.67%, so cementite (Fe 3 C) is formed. Since it arises secondarily from austenite (γ-Fe) (in contrast to primarily from the melt), it is referred to as secondary cementite (Fe 3 C II ). This front made of cementite and ferrite grows into the austenite at the same time.
If the structure continues to cool, then, due to the decreasing ability to bind carbon (0.00001% carbon at room temperature), further cementite falls from the α-Fe, which is now, since it precipitates from α-Fe, tertiary cementite (Fe 3 C III ).
In hypoeutectoid pearlite formation, i.e. with carbon contents of 0.02% by mass <C <0.80% by mass, when the temperature A 3 is reached (line GOS in the iron-carbon diagram ), due to the decreasing solubility of the austenite (γ mixed crystal ) for carbon already ferrite, so-called pre-eutectoid ferrite. With further cooling, the remaining austenite becomes richer in carbon until it has a concentration of 0.80 Ma% C; now at 723 ° C the eutectoid transformation takes place and the austenite converts to pearlite.
In the case of hypereutectoid pearlite formation, i.e. with a carbon content of 0.8% by mass <C <6.67% by mass, cementite is formed before the pearlite transformation. In contrast to the cementite that forms during pearlite formation, this cementite is not in the form of lamellae, but rather forms primarily at the grain boundaries and is therefore structurally differentiated.
Influence of the cooling rate
Ideally, the iron-carbon diagram obeys the equilibrium lines. But if you cool down at a higher speed, these equilibrium lines no longer apply and the pearlite point (0.8% carbon, 723 ° C) expands to a pearlite area at lower temperatures. This makes it possible to convert hypereutectoid and hypereutectoid steel into a pure pearlitic form. The increased speed also leads to fine lamellar perlite (according to the old definition, sorbitol or troostite).
If the cooling rate increases to a value greater than the diffusion rate of carbon, no pearlite formation can occur and martensite forms .
The machinability, i.e. the machinability by drilling, milling, turning, etc., is significantly influenced by the mechanical properties. The hardness is around 210 HV, the tensile strength is 700 N / mm² and the elongation at break is 48%. The values are therefore in the middle range compared to other components of steel. The cementite is mostly in the form of finely divided lines, but through heat treatment it can also be in a globular (spherical) form. Because of its high hardness compared to ferrite, pearlite causes higher abrasive wear and greater cutting forces . However, it is less prone to sticking and the formation of built-up edges . The chip forms are cheaper and the surface qualities that can be achieved are better because it does not tend to form burrs.
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- Fritz Klocke, Wilfried König: Production Process Volume 1: Turning, Milling, Drilling , Springer, 8th edition, 2008, p. 274 f.
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