In this article we will discuss about:- 1. Introduction to TTT Diagrams 2. Effect of Alloying Elements on TTT Curve 3. Limitations.
Introduction to TTT Diagrams:
Solid state transformations, which are very important in steels, are known to be dependent on time at a particular temperature, as shown in figure.
Isothermal transformation diagram, also called as TTT diagram, measures the rate of transformation at a constant temperature i.e. it shows time relationships for the phases during isothermal transformation. Information regarding the time to start the transformation and the time required to complete the transformation can be obtained from set of TTT diagrams.
One such set of diagram for reaction of austenite to pearlite in steel is shown in below figure. The diagram is not complete in the sense that the transformations of austenite that occur at temperatures below about 550°C are not shown. This curve also termed as S-curve or Ben-curve.
As thickness of layers in pearlite depends on the temperature at which the transformation occurred. If the transformation took place at a temperature that is just below the eutectoid temperature, relatively thick layers of a-ferrite and cementite are produced and are termed as coarse pearlite.
This is because of high diffusion rates of carbon atoms. Thus with decreasing transformation temperature, very slow movement of carbon results in thinner layers a-ferrite and cementite i.e. fine pearlite is obtained.
At transformation temperatures below 550°C, austenite results in different product known as bainite. Bainite consists of α-ferrite and cementite phases.
Bainite forms needles or plates, depending on the temperature of the transformation; the microstructural details of bainite are so fine that they can be seen only through electron microscope.
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It differs from pearlite in the sense that different mechanism is involved in formation of bainite which does not have alternating layers of α-ferrite and cementite. In addition, because of equal growth rates in all directions pearlite try to form spherical colonies, whereas bainite grows as plates and has an acicular (needlelike) appearance.
Upper bainite, formed at the upper end of the temperature range (550°C-350°C), is perceived by relatively coarse, irregular shaped cementite particles in α-ferrite plates. If the transformation is taking place at lower temperatures (350°C-250°C), the α -ferrite plates turned out to be a more regular needlelike shape, and the obtained product is called lower bainite. At the same time carbide particles become smaller in size and appear as cross- striations making an angle of about 55 to the axis of the α -ferrite plate.
Upper bainite has large rod-like cementite regions, whereas lower bainite has much finer cementite particles as a due to very slow diffusion of carbon atoms at lower temperatures. Lower bainite is considerably harder than upper bainite.
Another characteristic of bainite is that as it has crystallographic orientation that is similar to that of ferrite nucleating from austenite, it is thought that bainite is nucleated by the formation of ferrite.
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This is opposite to pearlite which is thought to be nucleated by formation of cementite.
Bainite is a transformation product that is not as close to equilibrium as pearlite. The most confusing feature of the bainite reaction is its dual nature.
In terms of numbers, it reveals properties that are typical of a nucleation and growth type of transformation such as occurs in the formation pearlite and also a mixture of α-ferrite and cementite despite the quite different morphology (no alternate layers), but at the same time it differs from the Martensite as bainite formation is athermal and diffusion controlled though its microstructure is founded by acicular (needlelike) appearance.
The time-temperature dependence of the bainite transformation can also be presented using TTT diagram. It occurs at temperatures below those at which pearlite forms i.e. it does not form until the transformation temperature falls below a definite temperature, designated as BS.
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Above this temperature austenite does not form bainite except under action of external stresses. Below BS, austenite does not transform completely to bainite. The amount of bainite formed increases as the isothermal reaction temperature is lowered.
By reaching a lower limiting temperature, Bf, it is possible to transform austenite completely to bainite. The Bs and Bf temperatures are equivalent to the Ms and Mf temperatures for Martensite.
In simple eutectoid steels, pearlite and bainite transformations overlap, thus transition from the pearlite to bainite is smooth and continuous i.e. knees of individual pearlite and bainite curves are merged together. However each of the transformations has a characteristic C-curve, which can be distinguishable in presence of alloying elements.
As shown in complete TTT diagram for eutectoid steel in figure above approximately 550°C-600°C, austenite transforms completely to pearlite. Below this range up to 450°C, both pearlite and bainite are formed. Finally, between 450°C and 210°C, the reaction product is bainite only.
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Thus bainite transformation happened at a high degree of supercooling, and the pearlite transformation at a low degree of supercooling. In middle region, pearlitic and bainitic transformations are competitive with each other.
Martensitic transformation can dominate the proceedings if steel is cooled rapid enough so that diffusion of carbon can be seized. Transformation of austenite to Martensite is diffusion-less, time independent and the extent of transformation depends on the transformation temperature.
Martensite is a meta-stable phase and decomposes into ferrite and pearlite but this is extremely slow (and not noticeable) at room temperature.
Alloying retard the formation rate of pearlite and bainite. Start of the transformation is designated by MS, while the completion is designated by Mf in a transformation diagram. Martensite forms in steels possesses a body centred tetragonal crystal structure with carbon atoms occupying one of the three interstitial sites available. Tetragonal distortion caused by carbon atoms increases with increasing carbon content and so is the hardness of Martensite.
Mechanically, Martensite is extremely hard, thus its applicability is limited by brittleness associated with it. However, structure and the properties can be altered by tempering, heat treatment observed below eutectoid temperature to permit diffusion of carbon atoms for a reasonable period of time.
During tempering, carbide particles attain spherical shape and are bifurcated in ferrite phase – structure called spheroidite. Spheroidite is the softest vet toughest structure of the steel.
At lower tempering temperature (250° to 650°), a structure called tempered Martensite with similar microstructure as like spheroidite except that cementite particles are very, much smaller. The tempering heat treatment is also applicable to pearlitic and bainitic structures.
At higher temperatures of with increasing tempering times, precipitation of cementite begins and is finished by dissolution of the unstable carbide. Furthermore, the Martensite loses its tetragonality and becomes BCC ferrite, the cementite coalesces into sphered. A schematic of possible transformations involving austenite decomposition are shown in below figure.
Tempering of some steels may result in a reduction of toughness called as temper embrittlement. This may be avoided by (1) compositional control, and/or (2) tempering above 575 or below 375, followed by quenching to room temperature.
Impurities responsible for temper brittleness are- P, Sn. Sb and As. Si reduces the risk of embrittlement by carbide formation. Mo has a stabilizing effect on carbides and is also used to minimize the risk of temper brittleness in low alloy steels.
TTT diagrams have less of practical importance since an alloy has to be cooled rapidly and then kept at a temperature to allow for respective transformation to take place. However, most industrial heat treatments involve continuous cooling of a specimen to room temperature.
Hence, Continuous Cooling Transformation (CCT) diagrams are generally more appropriate for engineering applications. CCT diagrams measure the extent of transformation as a function of time for a continuously decreasing temperature. Both TTT and CCT diagrams are, in a sense, phase diagrams with added parameter in form of time. Each is experimentally determined for an alloy of specified composition. Thus, as shown in below figure, region representing austenite-pearlite transformation terminates just below the nose.
Effect of Alloying Elements on TTT Curve:
Alloying elements like Cr, Ni, Mo and W can have two effects:
i. To shift the nose of Austenite to pearlite transformation diagram to longer times.
ii. Formation of a separate Bainite Nose.
Limitations of TTT Diagram:
i. Only applicable for iron-carbon alloy of eutectoid composition thus for other compositions, curves will have different configurations.
ii. These plots are accurate only for transformations in which the temperature of allow is held constant throughout the duration.