As a result, it can dissolve more carbon in its solid solution. Ferrite is an allotrope of iron known as alpha-phase-iron. It has a ceramic-like appearance, and it is paramagnetic. It has the body-centered cubic structure. Moreover, the dissolution of carbon in this allotrope is poor. Furthermore, this material is a ceramic-like material. It has many applications in electronic devices. Since it is hard and brittle, we can find this iron in cast iron and steel.
It has a metallic appearance, and it is soft relatively. When the austenite in such steels is cooled, ferrite first forms in the Mn-depleted regions. Ferrite has a very low solubility for carbon which partitions into the Mn-enriched regions which on further cooling, transform into bands of pearlite. The banded microstructure is illustrated in Fig. Martensite transformation begins when austenite is cooled to a temperature below M S on the time-temperature-transformation diagram.
It is a diffusionless transformation achieved by the deformation of the parent lattice into that of the product. The different colours indicate the displacements caused when martensite forms.
This physical deformation is described on a macroscopic scale as an invariant-plane strain Fig. The martensite therefore forms as a thin plate in order to minimise the strain energy. All of the displacive transformation products are therefore in the form of thin plates.
We have emphasised that the discipline motion of atoms cannot be sustained across austenite grain boundaries and hence plates of martensite, unlike allotriomorphs, are confined to the grains in which they nucleate Fig. The austenite grain boundaries are thus destroyed in the process of forming allotriomorphic ferrite or pearlite.
This is not the case with displacive transformation products where even if all the austenite is consumed, a vestige of the boundary is left as the prior austenite grain boundary. Austenite grain boundaries and indeed, prior austenite grain boundaries, absorb detrimental impurities.
One consequence is that strong steels based on microstructures obtained by displacive transformation become susceptible to impurity embrittlement. The grains simply separate at the grain surfaces with little absorption of energy during fracture. In contrast, lower alloy steels transform almost completely to martensite when cooled sufficiently rapidly.
Therefore, the microstructure appears different Fig. Transmission electron microscopy can reveal the small amount of inter-plate retained austenite in low-alloy steels Fig. Tempering at a low temperature relieves the excess carbon trapped in the martensite, by the precipitation of cementite.
The retained austenite is not affected by tempering at temperatures below M S , Fig. In some steels containing a strong carbide-forming elements such as Mo or V, tempering at temperatures where these solutes are mobile leads to the precipitation of alloy carbides Fig.
The Bain strain which converts austenite into martensite is a huge deformation; to mitigate its effects there are other deformations which accompany the transformation. These change the overall shape deformation into an invariant-plane strain.
One consequence is that there are lattice invariant deformations such as slip and twinning on a fine scale. Slip simply leads to steps in the interface, whereas twinning also introduces interfaces inside the martensite plate, as illustrated in Fig.
The atomic mechanism of bainite is similar to that of martensite Fig. Plates of bainite form without any diffusion, but shortly after transformation, the carbon partitions into the residual austenite and precipitates as cementite between the ferrite platelets - this is the structure of upper bainite Fig. Lower bainite is obtained by transformation at a lower temperature; the carbon partitioning is then slower, so some of the excess carbon has an opportunity to precipitate inside the ferrite plates and the rest of it precipitates from the carbon-enriched austenite as in upper bainite, Fig.
The difference between bainite and martensite is at primarily at the nucleation stage. Martensitic nucleation is diffusionless, but it is thermodynamically necessary for carbon to partition during the nucleation of bainite.
Bainite also forms at temperatures where the austenite is mechanically weak. The shape deformation due to the bainite transformation is therefore casues plastic deformation in the adjacent austenite. This deformation stops the bainite plates from growing and transformation then proceeds by the nucleation of further plates, which also grow to a limited size.
We have categorised transformations into displacive and reconstructive, with the former being strain dominated and the latter diffusion dominated. Displacive transformations are also known as military transformations by analogy to a queue of solidiers boarding a bus. The soliders board the bus in a disciplined manner such that there is a defined correspondence between their positions in the bus and those in the queue.
Near neighbours remain so on boarding. There is thus no diffusional mixing and no composition change. Because the soldiers are forced to sit in particular positions, there will be a lot of strain energy and this is not an equilibrium scenario. A civilian transformation is one in which the queue of civilians board the bus in an un-coordinated manner so that all correspondence between the positions in the bus and the queue is lost.
Civilians occupy the positions they prefer to occupy, a situation analogous to diffusion. There is a third kind of transformation, paraequilibrium in which the larger atoms in substitutional sites move in a discipline manner without diffusion whereas the faster moving interstitial atoms diffuse and partition between the phases. As a consequence, two back-to-back plates which accommodated each others shape deformation grow simultaneously.
This dramatically reduces the strain energy, but requires the simultaneous nucleation of appropriate crystallographic variants.
As a consequence, the probablity of nucleation is reduces and the microstructure is coarse. The following micrographs are courtesy of Arijit Saha Podder.
The samples were polished to a mirror finish prior to heat treatment. The thermal grooves reveal the austenite grain boundary structure. The photographs are taken using ordinary reflected light microscopy.
Here the sample is transformed to allotriomorphic ferrite to avoid surface relief effects. The photographs are taken using Nomarsk interference light microscopy. First Name required. Last Name required. Your Company required. Industry required. Input Your Industry required. Your Email required. Your Phone. City required. For more specific information on exact austenitic steel grades, please see our article on the many different grades of austenitic stainless steel.
Ferritic steels are made up of ferrite crystals, a form of iron which contains only a very small amount up to 0. Ferrite absorbs such a small amount of carbon because of its body centred cubic crystal structure - one iron atom at each corner, and one in the middle. This central iron atom is what gives ferritic stainless steels their magnetic properties.
Ferritic stainless steels are less widely-used due to their limited corrosion resistance and average strength and hardness. Austenitic stainless steels contain austenite, a form of iron which can absorb more carbon than ferrite. Austenite is created by heating ferrite to degrees C, at which point it transitions from a body centred cubic crystal structure to a face centred cubic crystal structure. When austenite cools, it generally reverts back to its ferrite form, which makes austenite difficult to utilise at anything below the extreme temperatures of a smelting furnace.
Austenite can be forced to retain its crystal structure at low temperatures with the inclusion of chemical additives, such as the nickel and manganese found in many austenitic stainless steels. Austenitic stainless steels cannot be significantly hardened by heat treatment, but can be hardened by cold working. Austenitic stainless steels are widely used, particularly in stainless steel screws , due to their excellent resistance to corrosion.
Cementite is a form of iron which contains even more carbon than ferrite and austenite.
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