In this column, we will discuss the non-martensitic transformation products of pearlite and bainite.
Pearlite was named by Henry Howe for the pearly phase constituent discovered by Henry Clifton Sorby, who was an English microscopist. It consists of alternating plates of ferrite and cementite (Fe3C). It has a lamellar morphology and is formed from the eutectoid reaction. The lamellae spacing is roughly correlated to the cooling rate or temperature of formation. Faster cooling or low temperature formation yields finer spacing; with slow cooling, or high temperature of formation, the spacing is generally coarser.
The pearly appearance of pearlite is related to the lamellae of pearlite colonies. Metallographic etching attacks the ferrite more severely than the cementite. The raised spacing of the cementite acts as a sort of diffraction grating and the pearl-like appearance is produced by the diffraction of light.
Transformation of Austenite to Pearlite
When a plain carbon steel of approximately 0.80 percent carbon is slow cooled from the austenite temperature, it will transform completely to pearlite. At carbon contents below 0.80 percent carbon (the eutectoid point), there will be excess ferrite formed, which will form at the grain boundaries of the prior austenite grains. At carbon contents above 0.80 percent carbon, there will be excess cementite, and the cementite will precipitate on the prior austenite grain boundaries. The remaining microstructure will be predominantly pearlite.
This is a soft phase that is generally not desired in hardened steels. It has low toughness. However, it is the desired microstructure for annealed materials. The microstructure of pearlite is shown in Figure 1.
In any steel, pearlite microstructures are softer than either bainite or martensite. They tend to be less ductile than bainite, and far less ductile than tempered martensite at the same hardness. As the transformation temperature decreases within the temperature range of pearlite, the interlamellar spacing decreases, producing very fine and sometimes unresolved pearlite in an optical microscope. As the transformation temperature decreases, the hardness and toughness increase.
If we consider a eutectoid steel with 0.77%C it is fully austenitic down to the A1 temperature of 727°C. If the steel is cooled slowly through this temperature, the phase diagram shows that it will transform into a mixture of ferrite (α) and Fe3C in a classical eutectoid reaction:
This shows that the two phases have a fixed composition. It also shows that the reaction is reversible upon heating or cooling. Using the Lever-Rule and this reaction, it is possible to closely estimate the carbon content of a steel that has been slowly cooled, by measuring the relative amounts of ferrite and Fe3C.
Pearlite initially nucleates at ferrite or cementite grain boundaries. Eventually, the grain boundaries get saturated with nucleation sites for pearlite. At this point, nucleation ends, and growth proceeds by edgewise growth, the ends of the cementite lamellae growing into the grain.
The rate of pearlite formation is governed by several factors. The diffusion of carbon is necessary to achieve the carbon atom arrangement in pearlite. This also explains why the lamellar spacing decreases as the cooling rate is increased. At the lower temperatures during cooling, carbon can only diffuse so far to distribute the carbon between the ferrite and cementite phases.
The alloy content of the alloy also plays an important role in the rate of formation and structure of pearlite. Carbide formers, such as manganese, chromium, and molybdenum, tend to migrate to the cementite phase, while ferrite stabilizers partition to the ferrite phase.
This distribution of alloying elements to either ferrite or Fe3C slows down pearlite formation. This is shown in Figure 2. For chromium- and molybdenum-containing alloys, growth of pearlite is slowed because Cr and Mo must diffuse. Chromium and molybdenum atoms are much larger than carbon atoms, and the diffusion rate is much slower. Carbon atoms, being smaller, can diffuse through interstitial sites, while Cr and Mo can only diffuse by the primary atom sites. This effect is valuable, because it becomes part of the hardenability of the alloy, allowing slower quench rates to occur before pearlite can form.
In this column, I have described the two-phase structure of pearlite, and described some of the factors that affect transformation and the rate of growth.
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- G. Krauss, Steels — Processing, Structure, and Performance, 2nd ed., Metals Park, OH: ASM International, 2015.