Perlite

Perlite is a structural component of steel. It is formed from the two phases ferrite and cementite. Pearlite can form during the cooling of hypoeutectoid or hypereutectoid steel.

Perlite

Pearlite is an eutectoid structural component of steel, which is arranged lamellarly. Perlite is a mixture of the two phases ferrite and cementite. It occurs at carbon contents of between 0.02 and 6.67 % during coupled crystallization in iron-carbon alloys. Thus, it occurs in steel and iron.

Formation

The formation of pearlite results in a local depletion of carbon in the structure. In return, the neighboring areas become increasingly enriched with carbon by diffusion. The lamellar structure typical of pearlite is created by the alternation of low and high carbon areas in the microstructure. If the carbon content in the low-carbon lamella reaches the value 0.02 %, the structure changes from pearlite to ferrite (α-Fe). Accordingly, cementite (Fe3C) is formed when the carbon content in the carbon-rich lamella increases to 6.67%. This is referred to as secondary cementite (Fe3CII) because it is formed secondarily from austenite (γ-Fe). The resulting front of cementite and ferrite grows into the austenite. 

As the microstructure cools further, more cementite falls out of the ferrite. This is caused by the ever decreasing ability to bind carbon. The resulting phase mixture is called tertiary cementite (Fe3CIII).

Cooling of hypoeutectoid steel

If steel has a carbon content of 0.02 Ma % < C < 0.80 Ma %, hypoeutectoid formation of pearlite occurs. If the temperature A3 (corresponds to the line GOS in the iron-carbon diagram) is reached, so-called pre-eutectoid ferrite is formed. This occurs due to the removable solubility of austenite (γ solid solution) for carbon. If the steel cools further, the austenite is enriched with further carbon. As soon as the austenite has a concentration of 0.80 Ma % C, eutectoid transformation occurs. At a temperature of 723 °C, the austenite transforms into pearlite. 

Hypereutectoid formation

A hypereutectoid formation of perlite is present at a carbon content of 0.80 Ma % < C < 6.67 Ma %. In this case cementite is formed during the perlite transformation. A decisive difference to cementite, which is formed during the perlite transformation, is the shape. The cementite is mainly formed at the grain boundaries and is therefore not present in the lamellar form that is otherwise common. 

A low starting temperature can prevent carbon diffusion. Thus, if the steel is cooled, bainite can be formed instead of perlite.

What is the effect of the cooling rate?

If steel is cooled at a higher rate than that specified in the iron-carbon diagram, the equilibrium lines no longer apply. As a result, the known pearlite point (0.8 % C, 723 °C) expands to a pearlite region at lower temperatures. This makes it possible to transform hypoeutectoid and hypereutectoid steel into pure pearlite. A further side effect of the higher cooling rate is that the lamellar shape of the perlite becomes finer.

Machinability 

The machinability of pearlite is influenced to a large extent by its mechanical properties. The phase mixture has a hardness of 210 HV, a tensile strength of 700 N/mm2 and an elongation at break of 48 %. It is therefore in the middle range when compared with other steel components. The abrasive wear of pearlite is higher compared to ferrite. This is due to its great hardness. This also results in higher cutting forces. However, pearlite has the advantage that it tends to have less built-up cutting edges and less stickiness. In addition, pearlite forms significantly fewer burrs. This leads to better surface qualities and more favorable chip shapes.