What is the microstructure of cementite?
Cementite (or iron carbide) is a compound of iron and carbon, more precisely an intermediate transition metal carbide with the formula Fe3C. By weight, it is 6.67% carbon and 93.3% iron. It has an orthorhombic crystal structure. The carbide therefore cemented the iron.
How does the cooling rate affect the microstructure and why?
Higher cooling rates lead to a decrease of ferrite grain size and formation of high strength, hardness, dislocation density, and fine phases because it suppresses the atomic diffusion. Whereas slow cooling rates lead to transformation into soft, coarse and less dislocated phases like polygonal ferrite.
What microstructure is obtained if austenite with the composition of eutectoid is slowly cooled down?
If we cool a steel of eutectoid composition (0.80 wt% C) below 723 °C pearlite nodules nucleate at grain boundaries (Figure 12.3) and the microstructure transforms to pearlite.
What is Sorbite microstructure?
in physical metallurgy, a structural component of steels and cast irons. It is a dispersed variety of pearlite, which is a eutectic mixture of ferrite and cementite. The ferrite-carbide mixture formed as a result of hardening and high-temperature tempering is sometimes called tempered sorbite. …
Is cementite a phase or microstructure?
H. K. D. H. Bhadeshia
| Phase | Symbol |
|---|---|
| Lower bainite | αlb |
| Acicular ferrite | αa |
| Martensite | α’ |
| Cementite | θ |
How is cementite formed?
Cementite can be made directly from Hägg carbide through the reaction Fe+Fe2C\rightarrow Fe3C (Hofer:1950). Alternatively, powdered cementite can be made by heating Hägg carbide, which is richer in carbon, in a nitrogen stream at 800°C for some 20 min (Herbstein:1964).
How does cooling rate affect grain size?
The results show that the higher the cooling rate, the smaller the grain size of the alloy and the smaller the number of precipitated phases in the matrix. Uniform grain size of the alloy could be obtained at a stable cooling rate.
What is the effect of cooling on grain formation?
Thus, for a transformation where the ratio of nucleation to growth rate increases with a decrease in temperature, the grain size of the new phase decreases with an increase in cooling rate. This is why ferrite grains become refined by accelerated cooling.
What is the micro structure of steel above eutectoid temperature?
austenite
The phase just above the eutectoid temperature for plain carbon steels is known as austenite or gamma. We now consider what happens as this phase is cooled through the eutectoid temperature (723°C).
What is the thickness ratio of ferrite to Cementite?
Pearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons.
What is Troostite microstructure?
An obsolete term, formerly used to describe the structure obtained when martensite is lightly tempered. The structure is now known to be cementite precipitated in ferrite, however, the precipitate is so fine that it can not be seen clearly using an optical microscope. …
What is Troostite and Sorbite?
Structures of the lower pearlite stage with very fine flakes are referred to as sorbite and troostite. Their structure can no longer be seen under an optical microscope. Generated pearlite with a ball-like or concentrated cementite phase is the exception.