Introduction
Pearlite is a common essential of an extensive variation of steels and it provides a considerable contribution to the strength. An importance of pearlite that should be emphasized is that pearlite is a bi-crystal. Normally, a colony of pearlite consists of two interpenetrating single crystals of ferrite and cementite (Fe3C) also known as iron carbide, which are primary ordered as alternating plates. Fines plates structure of pearlite is header and stronger than pearlite that consists of coarse plates because of pearlite spacing. Due to the evolution of the austenite/pearlite phase transformation during the production process, this morphology was able to be determined.
Dislocation mechanism of the pearlite formation is very important. Lamellar morphology of cementite and ferrite in pearlite is determine by specific features of the transformation mechanism such as thermoplastic deformation of overcooled austenite that caused by cooling process, and the formation of a polygonised structure in the form of flat dislocation walls and it is perpendicular to the planes of easy slip; regular distance (arrangement) of flat dislocation walls in austenite is determined by thermodynamics process. If the distance is smaller, the more overcooled the austenite. Flat cementite nuclei are form due to the elastic interaction of dislocations, flat walls formation with the atoms of carbon.
Dependence of interlamellar distance in pearlite on undercooling degree obeys the parabolic law, the relationship is the largest values of interlamellar distance in pearlite will correspond to small undercooling degree while the lowest values of interlamellar distance is correspond to greater undercooling. When the austenite cooling rate at temperature A1 (eutectoid temperature) increases, interlamellar distance in pearlite decreases and this is due to less intensive development of dislocation annihilation processes with growing thermoplastic deformation rate, determined by the cooling rate. When the interlamellar spacing is large, the diffusion distances for the transport of solute will be larger and it causes the growth of pearlite to slow down. Higher undercoolings can promote higher growth rate as the free energy change accompanying the transformation increases. However, the diffusion distances should be decrease to compensate for decrease in diffusivity since the reaction is diffusion-controlled.
Nucleation mechanism of pearlite involves the formation of two crystallographic phases. In the case of hypo-eutectoid steels, the pro-eutectoid ferrite nucleates first and it will continue to grow with the same crystallographic orientation during the pearlite formation as part of a pearlite colony. In this case, the cementite nucleation is the rate-limiting step in the formation of pearlite. On the other hand, the roles of ferrite and pearlite in hyper-eutectoid steels is completely reversed and in perfect eutectoid steel. The nucleation sites can be grain boundaries, edges or inclusions and once either the ferrite or cementite is nucleated, the conditions surrounding the new nucleus are ripe for nucleation of the other and pearlite grows in a co-operative manner.
In some of the special case with a set of condition, pearlite might exist as spheroids cementite in the matrix of ferrite, also known as “divorced eutectoid”. The naming of microstructure is because in recognition of the fact that there is no co-operation between ferrite and cementite as in the case of lamellar pearlite. Such structures are produced by spheroidising annealing treatment where the primary objective is to reduce the hardness in order to achieve good machinability as in the case of bearings steels. The presence of finely spaced pre-existing cementite particles in the austenite matrix is they key to promote the formation of divorced eutectoid.
The important steps to achieve a completely spheroidised structures is depends on heat treatment process. Avoidance of the formation of lamellar structure should be made when the steel is being cooled from the austenising temperature.
Formation of pearlite
Pearlite is formed when the slow cooling in an iron-carbon system is sufficiently at the eutectoid point in the Fe-C phase diagram (723?, eutectoid temperature). In the case of pure Fe-C alloy, it contains about 88 vol.% ferrite and 12 vol.% cementite. Pearlite is known for being tough and, when highly deformed, extremely strong.
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Figure 1: Pearlite transformation
As the transformation temperature increases, the pearlite spacing will increase. This will lead to an decrease in strength and hardness. In the case of fine austenite grain size, the nucleation will be high, the pearlite spacing is small. On the other hand, coarse austenite grain size will cause lower nucleation rate, and the pearlite spacing will be larger. Thus, strength and hardness of pearlite is very much depends on pearlite spacing, lower pearlite spacing gives better strength.
Bainite is a mostly metallic substance that exists in steel heat treatments. An accurate term to describe bainite is granular bainite. It is very common to be used to describe partly bainitic microstructure obtained from a continuous cooling process. The granular appearance is due to the gradual transformation that occurs during continuous cooling and this causes the formation of coarse sheaves of bainite. Inverse bainite forms in hyper-eutectoid steel, with cementite precipitating first and ferrite forming consequentially on the precipitated cementite plates. Ferrite in bainite plates possess different orientation relationship relative to the parent austenite than does the ferrite in pearlite. Columnar bainite is another structure associated with hyper-eutectoid steel compositions. The morphology is a non-lamellar arrangement of cementite and ferrite in the shape of “an irregular and slightly elongated colony”, however the mechanism of formation is reconstructive.
Upper bainite and lower bainite
Bainite can be classified into two distinctly different forms: upper bainite and lower bainite. The characteristics of a upper bainite are lath shape, comprised of ferrite subunits of matching crystallographic orientation arranged in units called ‘sheaves’. The subunits are separated by carbide precipitates and can be either plate or lath morphology. Each sheaf is in the form of a wedge-shaped plate on a macroscopic scale. The sheaves inevitably nucleate heterogeneously at austenite grain surfaces. The cementite precipitates from the carbon-enriched austenite between the ferrite plates; the ferrite itself is free from carbides. An action can be made to prevent the precipitation of cementite from austenite is increasing the silicon concentration to about 1.5 wt%; the reason behind this is because silicon is insoluble in cementite. Silicon-rich bainite steels can have very good toughness because the absence of brittle cementite. Bainite is quite brittle due to high percentage of cementite. Therefore, by adding the silicon concentration, it can reduce the brittleness of bainite.
These subunits in the lower bainite tends to be coarser than those in the upper bainite but the morphology of lower bainite and upper bainite are quite similar as far as microstructure and crystallography are concerned. However, there is still a little difference between lower bainite and upper bainite because the individual ferrite subunits in lower bainite contains a fine distribution of carbide particles in addition to the interplatelet carbides.
Upper bainite and lower bainite forms at two different temperature. Normally, upper bainite will form at higher temperature that is around 550? – 350? than lower bainite that is around 350?-250? in the same steel. However, the transition is determined by the carbon content to an extent in the steel. Isothermal transformation will results the mixtures of upper and lower bainite. Bainite contains nonlamellar eutectoid structure of ? ferrite and cementite.
Summary of description of nucleation and growth of pearlite
Firstly, Fe3C nucleus forms at the austenite (?) grain boundary. In second stages, ?-Fe now nucleated besides Fe3C platelets. In the stage three, new Fe3C plates nucleated next to ferrite grains producing lamellar structures of ferrite and cementite.
Summary of description of nucleation and growth of lower bainite and upper bainite
Lower bainite – In first stage, ?-Fe nucleus forms at the austenite (?) grain boundary (just below the nose of TTT curve). Next, Fe3C particles nucleated besides ?-Fe nucleus. During the third stage, ?-Fe grains continue to growth with Fe3C embedded. In stage four, feathery bainite is formed.
Upper bainite- Firstly, super-saturated solid solution (4S) of ferrite is formed on the austenite (?) grain boundary (far below the nose of TTT curve). After sometimes, the Fe3C particles begins to nucleated in side the 4S ferrite. Last stage, the shape formed is call acicular bainite (needle-like)
Application of pearlite
The common characteristic of pearlite is hard as well as strong because of the layered structure, and it is used in various types of applications. Pearlite could be wear resistant because of a strong lamellar network of ferrite and cementite. Although pearlite is quite hard and strong but is it not pretty tough. The steels with pearlitic microstructure can be drawn into thin wires. These wires are often bundled into rope type and they are actually commercially used for piano wires or ropes for bridges suspension.
High degrees of wire drawing (logarithmic strain above 3) leads to pearlitic wires with yield strengths of several gigapascals. This makes pearlite as one of the strongest structural bulk materials on earth. In the case of hypereutectoid pearlitic steel wires, when cold wire drawn to true (logarithmic strains above 5), can even show a maximum tensile strength that above 6 GPa.
Pearlite is also used to make cutting tools, knives, chisels and nails because pearlite has wear resistant.
In conclusion, the transformation difference between pearlite and bainite is due to cooling rate. Cooling rate is the critical factor that will affect the transformation. If the carbon steel is left for slow cooling (low cooling rate), pearlite microstructure will form while for moderate cooling (moderate cooling rate), bainite microstructure will form. The bainite has higher strength, hardness compares to the pearlite. In the case of pearlite, there are coarse pearlite and fine pearlite. Fine pearlite has higher strength, hardness compares to coarse pearlite but lower than the bainite. Thus, the ranking of strength and hardness ranking is bainite > fine pearlite > coarse pearlite. On the other hand, pearlite are generally has better ductility than bainite.
