Eutectoid transformation in Fe-C alloy (steel)
A eutectic system is known as a mixture of a few chemical compounds or elements that has a single chemical composition which will solidifies at a lower temperature. Therefore, this composition is known as the eutectic composition and the temperature for it is call the eutectic temperature. Eutectoid transformation is known when precipitation transformations are applied to phase transformation. This phenomenon is taken when there is a transformation from austenite to pearlite in iron-carbon (Fe-C) steel. The eutectoid reaction is usually defined as ?(0.76 wt% C) ? ? (0.022 wt% C) +Fe_3 C. From here, it can said that every transformation are able to observe through the iron-iron carbide phase diagram.
Figure 1.0.1: Iron-iron carbide phase diagram
Pearlite refers to an iron-based compound phase. In other words, it is known as a steels which consists of two-phase. With that, it is known as a layered structure which is consists of alternating layers of ferrite of 88wt% and also with cementite of 12wt% as shown in Figure 1.1.1 below. In the micrograph, dark regions are cementite and bright regions are ferrite. From here, ferrite are soft and cementite are brittle.
Figure 1.1.1: Lamellar structure of Pearlite
This phenomenon occurs in some steels and cast irons. Pearlite can be obtained during slow cooling of iron alloys where it forms by a eutectoid reaction. This is where the austenite is being cooled below the eutectoid temperature which is 723°C. The pearlite is formed by eutectoid decomposition of austenite upon cooling through the diffusion of carbon atoms.. However, when mild steel are used where carbon steel up to around 0.2wt% C. It consists mostly of ferrite and this is where the pearlite will tend to increase when the carbon content increased. Pearlite can also be achieved in two parts which is fine pearlite at low temperature and coarse pearlite at high temperature.
Bainite is known as a phase which exist in steel microstructure after a specific heat treatment are performed on it. From here, bainite is the decomposition of products that may be formed when the austenite is cooled down the eutectoid temperature which is around 250°C to 550°C. The appearance of bainite is that it is a fine non-lamellar structure. Furthermore, bainite is commonly consists of ferrite, carbide and, retained austenite. Other than that, bainite is formed at a cooling rate which is slower than that for the martensite formation but it is faster than that for ferrite and pearlite formation. From here, two kinds of bainite can be achieved which is known as the upper bainite and also the lower bainite as shown in Figure 1.1.1 below. It is known that lower bainite is obtained by transformation at a lower temperature (250°C to 350°C) and in contrast upper bainite is obtained at higher temperature (350°C to 550°C). Bainite is known to be generally stronger but less ductile than pearlite.
Figure 1.1.1: Microstructure of Upper Bainite and Lower Bainite
2.1 Austenite to pearlite transformation and undercooling
During the upper temperature, only austenite is present. This is where the 0.8% carbon being dissolved in solid solution within the FCC. As it is cooling down and through the temperature of 723°C, there are several changes occur simultaneously. As mentioned, pearlite is formed during sufficiently slow cooling in an iron-carbon system which locate at the eutectoid point as shown in Figure 1.0.1 above which is at the temperature of 723°C. During the cooling, the iron needs to change crystal structure from the FCC austenite to the BCC ferrite, but the ferrite can only contain 0.02% carbon in solid solution. Consequently, the excess carbon is then rejected and will then form the carbon-rich intermetallic which is known as the cementite. On the other hand, the net reaction at the eutectoid is the formation of pearlitic structure As shown in Figure 2.1.1 below, it can be seen that the carbon diffuse into cementite during the process to achieve BCC state and form pearlite. Through diffusion, a layered structure are formed which is due to the redistribution of C atoms between ferrite and cementite.
Moving on, after the temperature had pass the temperature of 723°C, an undercooling can be done to acquire different types of pearlite. For instance, a large undercooling will result in fine pearlite and in contrast a small undercooling will result in coarse pearlite. The differences can be explained using the Figure 2.1.2 below where the larger the difference in temperature means more undercooling which will result in fine pearlite and vice versa.
Figure 2.1.2: S-shaped curves at different temperature
As mentioned before, pearlite can be formed into two types which is the fine pearlite and coarse pearlite. This phenomenon can be further discussed as the s-shaped curves are used to construct the TTT diagram of the transformation for pearlite as shown in Figure 2.1.3 below. It can be observed that in the TTT diagram the austenite is being cooled down to a significant temperature as fine pearlite need to be cooled until lower temperature and coarse pearlite need to be cooled at a higher temperature. From here, the absolute layer thickness depends on the temperature during the transformation. This can be deduce where a higher temperature will form a thicker layer which results in coarse pearlite.
Figure 2.1.3: TTT diagram of formation of pearlite
2.2 Nucleation and Growth of Pearlite
Figure 2.2.1: Nucleation and growth of pearlite.
Based on Figure 2.2.1 above, it shows that nucleation of pearlite and how it continues to grow. From here, it can be seen that at the second phase the cementite nucleus starts to form at the grain boundary by heterogeneous nucleation. Following by that, ferrite will then form alongside of the cementite. As the formation continues where new cementite plate nucleates next to ferrite grains and producing a lamellar structure of ferrite and cementite as shown in the fourth and fifth phase above.
In addition, the nucleation and growth is said to result in reaction rate. For instance, taking Figure 2.2.2 below as an example. As the structure is cooled down just below T_E which is 723°C the nucleation rate is low and the growth rate is high. For the second one where it is cooled moderately below? T?_E, both the nucleation and growth rate is medium. However, for the third one where it is cooled way below the? T?_E, the nucleation rate is high and the growth rate is low. It can be observed that the third structure is the finest of all three.
Figure 2.2.2: Examples of structure cooled down to different temperature
2.3 Hypo-eutectoid steels
Figure 2.3.1: Microstructure of hypo-eutectoid steels
Hypo-eutectoid steels are known as the steels which are having less than 0.8% of carbon. The formation started where at high temperature the material is entirely austenite. After that, as it cools it enters a region where the stable phases are ferrite and austenite. Moving on, the low carbon ferrite will then nucleates and grows, where it leaves the remaining austenite richer in carbon. Continuing at 723°C, the remaining austenite is then assumed to be a eutectoid composition where it is 0.8% carbon. From there, it continues for further cooling and transform it to pearlite. Lastly, the resulting structure is a mixture of pro-eutectoid ferrite and regions of pearlite where pro-eutectoid is known as the ferrite that forms before the eutectoid reaction. The following reaction can be observed in Figure 2.3.2 below and the microstructure is as shown in Figure 2.3.1 above.
Figure 2.3.2: Formation of Hypo-eutectoid steel
2.4 Hyper-eutectoid steels
Figure 2.4.1: Microstructure of Hyper-eutectoid steels
Hyper- eutectoid steels are known to have more carbon than the amount of eutectoid as hyper means “greater than”. The process of formation ae similar to the hypo-eutectoid steel mentioned in part 2.3 above. The only differences I that the pro-eutectoid phase at hypo-eutectoid transformation is now cementite instead of ferrite. This phenomenon occurs as the carbon-rich phase nucleates and grows, the carbon content in the remaining austenite will then decrease and again reaching the eutectoid composition at 723°C. Moving on, the austenite will then transform to pearlite during slow cooling through eutectoid temperature. Therefore, the final structure formed will then consists of primary cementite and pearlite. However, the constant network of primary cementite will result in brittle material. The following reaction can be observed in Figure 2.4.2 below and the microstructure is as shown in Figure 2.4.1 above.
Figure 2.4.2: Formation of Hyper-eutectoid steels.
3.1 Formation of bainite
Figure 3.1.1: TTT diagram
Based on the TTT diagram shown in Figure 3.1.1 above, it can be observed that bainite forms through the decomposition of austenite which is same as pearlite. However, the temperature of decomposition is done above Ms but below the temperature where fine pearlite is formed. In this process, the material is kept isothermally in an appropriate medium which is the bainitic stage. It is then hold until the transformation is completed, then it will then be cooled down to room temperature. Likewise, as mentioned before that bainite is a mixture of ferrite and cementite. In bainite, the cementite is not neatly arranged in parallel plates as it may occur inside small grains of ferrite which consist of many irregular precipitates that are full of other defects such as dislocation. Bainite can be obtained as the pearlite structure gets finer and finer when the cooling rate increase. From here, there are two forms of bainite, known as upper and lower bainite as shown in Figure 3.1.2 below.
Figure 3.1.2: Types of bainite
NUCLEATION AND GROWTH OF BAINITE
3.2 Upper bainite
Figure 3.2.1: Upper bainite
Upper bainite as shown in Figure 3.2.1 refers to a structure which consists of needle ferrites that are separated by long cementite particles. In other words, it consists of clusters platelets of ferrite that are adjacent to each other and in almost identical crystallographic orientation. This is form so that a low-angle boundary arises whenever the adjacent platelets touch. Upper bainite commonly forms at temperature between 350 to 550°C. In general, low carbon steel exhibit fine bainitic laths where it is nucleated through shear mechanism at the austenite grain boundaries. Likewise, the carbon solubility of ferrite in bainite is much lower than austenite. This will then cause the carbon to be rejected into the surrounding. The cementite will then nucleates as discrete particles when the carbon concentration is high enough. From here, the cementite filament becomes more continuous. Therefore, the structure appeared are said to be feathery bainite. The mechanism are illustrated as shown in Figure 3.2.2 below.
Figure 3.2.2: Nucleation and growth of upper bainite
3.3 Lower Bainite
Figure 3.3.1: Lower Bainite
Lower bainite as shown in Figure 3.3.1 refers to a structure which consists of thin plates of ferrite that contains very fine rods of cementites. For lower bainite, the structure commonly forms at a temperature between 250 to 350°C. From here, the nucleation transformation is similar to upper bainite which is through partial shear. Moreover, the lower temperature does not allow diffusion of carbon easily. Thus, the iron carbides from are around 50-60° to the longitudinal axis of the main lath, contiguously with the bainitic ferrite. The carbide may precipitate as discrete particles due to low carbon. Therefore, the appearance of lower bainite look alike with martensite which is known as acicular bainite. The mechanism are illustrated as shown in Figure 3.3.2 below.
Figure 3.3.2: Nucleation and growth of lower bainite
DIFFERENCE BETWEEN PEARLITE AND BAINITE
DEFINITION A type of microstructure in steels that have two-layered phase of alternating layers of ferrite and cementite. A type of microstructure in steel having a plate-like structure
TEMPERATURE OF FORMATION When austenite cools below its eutectoid temperature (723°C) When austenite cools to a temperature where its structure is thermodynamically unstable
TYPES OF STRUCTURE Fine pearlite and coarse pearlite Upper bainite and lower bainite
OCCURRENCE Occur in steel and cast iron Occurs in steel