#cementite

Fe, C 0.8 (wt%) steel, eutectoid transformation, normalised at 900ºC

Processing Normalised 900 ºC [...] Sample preparation Nital Technique Reflected light microscopy Length bar 400 μm / 100 µm Further information This steel is of the eutectoid composition. Once the temperature is lowered below the eutectoid temperature the steel becomes simultaneously supersaturated with both ferrite and cementite. A eutectoid transformation results (g to a + Fe3C). The resultant microstructure, known as pearlite, comprises lamellae of cementite (dark) embedded in ferrite (white). The platelets are parallel to each other and do not follow a specific crystallographic direction. Contributor Dr R F Cochrane Organisation Department of Materials, University of Leeds

Le de burite, Austenite cementite                   je t’aime                                  je t’aime                                                 je t’aime...

8 saattir şu grafiğe bakınca aşk şiiri bile yazdım ve bir mühendisin aşk şiiri böyle oluyor. grafikten aldığım kelimeleri, bildiğim tek fransızca kelimeyle birleştirip, biraz da şekilli yazınca çok güzel bir aşk şiiri ortaya çıktı. Sevdiceğinize, flörtünüze falan gönderebilirsiniz. Şiirin anlamını sorarsa “eriyorum, içinde çözülmek istiyorum, seni seviyorum...” dersiniz, bunu da uydurdum ama mantık olarak yanlış değil.

Fe, C 1.0 (wt%), hypereutectoid alloy

Processing: Normalised [...] Sample preparation: Etched in 3% nital Technique: Reflected light microscopy /  Scanning electron microscopy (SEM) Length bar: 400 μm / 40 µm / 50 µm / 10 µm Further information: This is a hypereutectoid alloy and illustrates the effect of a fast cooling rate on the resultant microstructure. The first phase to form from the austenite is proeutectoid cementite. With faster cooling there is less time for carbon to diffuse and the microstructure is more refined and  may form the initial cementite as Widmanstätten side plates. / This secondary electron SEM image shows the cementite delineating prior austenite grain boundaries with a thin layer. The amount of proeutectoid phase is very low, with the majority of the area being taken by the pearlite eutectoid. Again each pearlite cell has a different orientation with the ferrite phase being selectively etched. Contributor: Prof T W Clyne Organisation: Department of Materials Science and Metallurgy, University of Cambridge 

Thwarting oil-pipeline corrosion by identifying a nanoscale villain

Steel pipes rust and eventually fail. To preempt disasters, oil companies and others have created computer models to predict when replacement is needed. But if the models themselves go wrong, they can be modified only through experience, a costly problem if detection comes too late.

Now, researchers at Sandia National Laboratories, the Department of Energy's Center for Integrated Nanotechnologies and the Aramco Research Center in Boston, have found that a particular form of nanoscale corrosion is responsible for unpredictably decreasing the working life of steel pipes, according to a paper recently published in Nature's Materials Degradation journal.

Using transmission electron microscopes, which shoot electrons through targets to take pictures, the researchers were able to pin the root of the problem on a triple junction formed by a grain of cementite—a compound of carbon and iron—and two grains of ferrite, a type of iron. This junction forms frequently during most methods of fashioning steel pipe.

Proposed engineering method could help make buildings and bridges safer

Pearlitic steel, or pearlite, is one of the strongest materials in the world and can be made into thin and long wires. The strength of pearlite allows it to sustain very heavy weight, however what makes it special is its ability to stretch and contract without breaking (ductility). Ductility is important for building bridges, as even if a material is strong enough to support heavy weight, it can break when subjected to stretching if it is not ductile enough. This is why structures made of concrete can still collapse during violent earthquakes. Pearlite is used for suspension bridges to help them withstand strong shaking while supporting heavy weight.

Pearlite is made of alternating nanolayers of cementite and ferrite. The cementite helps make it strong, while the ferrite helps make it ductile. However, until now researchers did not know exactly how the two worked together to give pearlite its special quality, or better yet, how to control their working together to engineer an even better material.

Researchers at Kanazawa University have discovered that disruptions, or dislocations, in the arrangement of atoms along the interface between a cementite and a ferrite layer protect the cementite from fracturing under stretching or compression. Their study was published last month in the journal Acta Materialia.

Carbon Steel Microstructures: Cementite

Also known as iron carbide, cementite ( 1 , 2 , 3 ) is a compound of iron and carbon that can be found in most steels and cast irons. Its formula is Fe3C and it tends to form lamellar structures with ferrite, creating structures known as pearlite and bainite. Cementite is essentially a ceramic when in its pure form and is therefore hard and brittle. 

Hypoeutectoid steel, normalised at 1100°C

Processing: Normalised 1100°C

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Sample preparation: Nital

Technique: Reflected light microscopy

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Further information: A hypoeutectoid alloy (carbon composition less than eutectoid). The first phase formed upon cooling from the austenite phase field is proeutectoid ferrite. Due to the lower solubility of carbon in ferrite, carbon is partitioned into the remaining austenite. At the eutectoid point the remaining carbon enriched austenite transforms to pearlite (a mixture of ferrite and cementite) which is the darker region of the micrograph. The proportion of pearlite is dependent upon the overall composition. The ferrite (light areas) is a good example of an allotriomorphic ferrite. This means that its shape does not reflect its internal crystalline symmetry as it nucleates on the austenite grain boundaries and hence follows the shape of the boundaries, the remaining austenite within the ferrite then transforms to pearlite, and is surrounded by the ferrite. The large size of the areas of pearlite arises due to the high normalisation temperature which causes the austenite grains to grow large.

Contributor: Dr R F Cochrane

Organisation: Department of Materials, University of Leeds