Introduction of PearlitePearlite is a common essential of an extensive variation of steels and it provides a considerable contribution to the strength CITATION SEO03 l 17417 (S.E. Offerman, L.J.G.W. van Wilderen, N.H. van Dijk, J. Sietsma, M.Th. Rekveldt, S. van der Zwaag, 2003). A very important point 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. Fine 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 pearlite formationDislocation mechanism of the pearlite formation is very important. Specific features of the transformation mechanism will direct determine the lamellar morphology of cementite and ferrite in pearlite 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. The austenite will get more overcooled as the distance is smaller. Flat cementite nuclei are form due to the elastic interaction of dislocations, flat walls formation with the atoms of carbon.
Factors that affect interlamellar distance in pearliteDependence 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.
Figure 1: microstructure of pearlite
Nucleation mechanism in hypo-eutectoid steelsNucleation 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. Irony, 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 CITATION Mad04 l 17417 (Durand-Charre, 2004).
Occurrence of SpheroiditeIn 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.
Pearlite TransformationPearlite 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. Generally, in order to form pearlite, the carbon steel must be heated up to austenitizing temperature (above 723?), and then cool down to around 580? to 650? just above the nose of TTT curve, pearlite will form,
From figure 2, it shows that if the temperature is around 650?, and cool isothermally, coarse pearlite will form while if the temperature is around 580? just above the nose of curve, and cool isothermally, fine pearlite will form.
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Figure 2: Formation of coarse pearlite and fine pearlite at TTT Curves
Figure 3: Pearlite formed when cooling from austenite
Relationship between transformation temperature and pearlite spacingAs the transformation temperature increases, the pearlite spacing will increase. This will lead to a decrease in strength and hardness. In the case of fine austenite grain size, the nucleation will be high, the pearlite spacing is small. Conversely, 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 depending on pearlite spacing, lower pearlite spacing gives better strength. Generally, lower transformation temperature will have higher driving force and this will lead to a smaller interlamellar spacing in pearlite.
Pearlite is an equilibrium structure and it can be seen in Fe3C (iron carbide) phase diagram. From the figure 4, it is show that the pearlite is formed when it cools from austenite A3 to around 600?.
Figure 4: Iron Carbide (Fe3C) Phase Diagram
Figure 5: Bainite formation shows in TTT Curve
Introduction to bainiteBainite is a mostly metallic substance that exists in steel heat treatments. Granular bainite 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
Figure 6: Upper bainite and Lower bainite
Bainite can be classified into two distinctly different forms: upper bainite and lower bainite. The characteristics of an 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 CITATION HKD80 l 17417 (H.K.D.H. BHADESHIA and D.V EDMONDS, 1980). 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.
7924802482850Figure 7: Evolution of a bainite sheaf as a function of time
020000Figure 7: Evolution of a bainite sheaf as a function of time
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.
Non-equilibrium structure (Bainite)In the case of bainite, bainite is not shown in phase diagram because bainite is a non-equilibrium structure or also known as metastable structure. The formation of bainite only can be shown in CCT Curves. In order to form bainite, it must be quench down below the nose of the curve and cool isothermally, then bainite can be formed.
Transformation temperature of lower bainite and upper bainiteUpper 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.
For a plain carbon steel, it is not possible to form a bainite structure. Therefore, alloy element is required to fill inside the carbon steel in order to shift the curves in the TTT diagrams. In order to form bainite structure, the carbon steel with alloy element is heated up to austenitizing temperature, and then quench down rapidly so that there is no diffusion of carbon. The quenching is crucial because it needs to quench until it below the nose of TTT curve, so that the transformation of pearlite will not occur.
Figure 8: Growth and Nucleation of Pearlite and Bainite
Summary of description of nucleation and growth of pearliteFirstly, 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 bainiteLower 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 pearliteThe 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. Piano wires and wire for bridge suspension are comes from the thin wires that have pearlite microstructure. The thin wires are tie up and become a rope for these types of applications.
High degrees of wire drawing (logarithmic strain above 3) leads to pearlitic wires with yield strengths of several gigapascals. This unique property leads the pearlite as one of the strongest structural bulk material 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 6GPa.
Pearlite is also used to make cutting tools, knives, chisels and nails because pearlite has wear resistant.
Application of bainiteThe application of bainite is very wide as the steel with strengths that less than 1000MPa and has less than 2 wt.% of alloy concentrations been very popular in markets. Due to low content of alloy concentrations, bainite steels are easier to weld than other normal steel. This is why bainite steel has many civil engineering applications such as beams, wires, and re-bars.
Recently, a modern type of bainitic steels has been developed with a few of unique properties such as lower carbon content and lower alloying element concentrations. These bainitic steels are processed by using accelerated cooling. This will provide a necessary bainitic microstructure. These refined bainitic microstructure will provides larger strength and better weldability. Furthermore, bainitic steel are very useful in automobiles industry. This is due to their easy formability and strength, these properties make them as crash reinforcement bars to protect against sidewise impact, in cam shafts.
Another type of bainitic steel is creep resistant steels. These steels have been developed to withstand constant load applied and high temperature environment CITATION VCI16 l 17417 (V.C. Igwemezie, C.C. Ugwuegbu, and U.Mark, 2016). These properties make them to become useful in power plants. The components in steam power plant are boilers, turbines or steam lines. These components require a material that able to withstand constant load and high temperature. Thus, creep resistant steel is a wise selection.
Figure 9: TTT diagram for Hypereutectoid Steel
In the case of hypereutectoid steel, the TTT diagram is a little bit different for TTT diagram in eutectoid steel as transformation of austenite to cementite occurs. This is shown in figure 4.
Figure 10: TTT diagram for Hypo-eutectoid Steel
Similarity to TTT diagram for Hypo-eutectoid Steel, it is also slightly different compare to TTT diagram for eutectoid steel. There is another transformation of austenite to ferrite occurs. This can be proved in figure 4.
Figure 11: Comparison of hardness between bainite and pearlite
Figure 12: Comparison of tensile strength between bainite and pearlite
Note: The results that shown in Table 1 are obtain from the figure 11 to 12.
Table 1: Difference between Pearlite and Bainite
Pearlite Differences Bainite
Two-layered phase of alternating layers of ferrite and cementite Microstructure Plate-like structure
Below 727?Temperature require to form 125?- 550?Lower Cooling rate Higher
Lower Carbon content Higher
Lower Tensile strength Higher
Lower Ductility Higher
Lower Toughness Higher
Ferrite form first followed by Cementite Mechanism Cementite form first followed by Ferrite
When austenite cools below its eutectoid temperature 727?Formation Austenite quench down below the nose of CCT curve and cools isothermally
Occurs in steel and cast iron Occurrence Occurs in steel
Equilibrium Structure Non-equilibrium
ConclusionIn conclusion, the transformation difference between pearlite and bainite is due to cooling rate. Cooling rate and cooling temperature are the critical factors that will affect the transformation. If the carbon steel is left for slow cooling from austenite state, also known as air cool (low cooling rate), pearlite microstructure will form while for fast cooling from austenite state, quench (moderate cooling rate) isothermally, bainite microstructure will form. In terms of mechanical properties, bainite shows a better overall result compare to the pearlite. Other than that, fine pearlite also shows better mechanical strength compare to coarse pearlite. However, coarse pearlite has higher ductility compare to fine pearlite. Pearlite has a lamellar structure while bainite has dual structure. Example, upper bainite has feathery structure while lower bainite has acicular structure. Bainite is a metastable form while pearlite is an equilibrium structure. The applications for pearlite and bainite are very wide. However, pearlite is always chosen because the formation of bainite is very much complex and difficult to obtain compare to pearlite. Thus, pearlites are cheaper and easier to obtain.
