Week 2 Atomic structure and Steel Flashcards
Who is the biggest steel producer?
China is just over half the total produced and india is 2nd place. Germany is a lot in Europe.
What is the composition of steel?
Iron and carbon where the carbon content determines the maximum amount that can be dissolved in iron
Why is the carbon content in steel kept quite low?
For steel it contains between 0.01-2wt% C and it is kept low by the addition of alloying elements to resist the urge for the material to form carbides which can be detrimental and effects corrosion resistance.
What are the two distinct crystallographic forms of iron?
Body centred cubic )bcc) up to 910 deg C FERRITE and the fcc (face centre cubic) up to 1394 deg C AUSTENITE. then it will reverts to bcc again
under 10GPa excess can have hexagonal close packed form.
Number of atoms per unit cell?
fcc has halves at each face and 1/8 at each corner
bcc has a whole 1 in the middle and 1.8 at each corner
hcc has 12 1.6 at each corner, 2 1/2 at each top and bottom face and 3 whole ones in the middle
How does the volume change of pure iron with temperature?
As you heat iron it will naturally expand with the increase in temperature. When it changes from bcc to fcc the volume decreases massively. Then as T continues to increase the volume continues to increase and as it becomes bcc again there is a massive increase in volume as you get towards the melting temperature.
Why do you get a change in volume when pure iron changes from bcc to fcc with an increasing temperature?
In the bcc structure the aroms are less densely packed than the fcc as well as the bcc having less atoms per unit cell than fcc. The temperature causes them the atoms to vibrate and be more closely packed which decreses interatomic distance and increases packing efficiency.
What is equilibrium?
A state in which no further change will happen or is perceptible no matter how long time passes.
This is always when a material has its lowest free energy.
What are the 4 kinds of equilibrium?
Equilibrium - at its lowest free energy
Metastable equilibrium- not at lowest free energy but in a dip.
unstable equilibrium - where something is sitting at its lowest possible energy at a particular point but it is very unstable. Any very small factor could make it move.
Unstable - movement down the energy gradient
What is a phase?
A portion of the system(any isolated) whose properties, composition and structural arrangement are homogenous and distinct and mechanically separable (at least in principle) from the other parts of the system.
How is phase and equilibrium represented on an iron/ iron carbide diagram?
Where temperature is plotted against Wt%.
For pure iron with increasing temperature it can go from room temperature (alpha ferrite-bcc) to 910 deg C (austenite-fcc-gamma on graph) to 1380deg C (delta ferrite-bcc) to high (liquid-liquid).
Assumed over infinite time to meet equilibrium and it can be a mix of phases with varying wt%.
What happens to steel when more carbon is added past the solubility limit?
Where no more carbon can be added iron carbide is formed Fe3C(cementite) which is metastable.
What are the 3 main complex reactions in the iron carbide diagram?
Peritectic reaction, Eutectic and Eutectoid reactions.
What is the Eutectic reaction?
A reversible reaction where the liquid at the specific Wt% and temperature are going to change straight into gamma(austenite) and iron carbide. Liquid to 2 solids.
What is the Eutectoid Reaction?
A reversible reaction where the solid at the specific Wt% and temperature are going to change straight into alpha ferrite and iron carbide. solid to 2 solids.
What is the peritectic?
A reversible reaction where the liquid and solid at the specific Wt% and temperature are delta ferrite and Liquid go to gamma (austenite). Solid & Liquid to 1 solids.
What is interstitial atoms in steel?
Where there is a smaller atom sitting in between larger atoms in a gap. For example in the iron carbide diagram carbon is a smaller atom in the steel arrangement. But in this case it causes dislocations.
why is there a double yield point in steel? and what is happening in each?
The upper yield point is in the elastic stage. Then in the lower yield point where the dislocation (small atom) would like to move in a direction with the force applied but the larger atom wants to fill the gap and move in the opposite direction causing a build up in stress. It will get enough energy to move and will then swap places with the smaller atom - this is the jagged swiggle of interstitial locking.
What is interstitial locking?
What is pearlite?
It is not a phase so it doesn’t appear on the phase diagram however it can be formed
its a constituent of the microstructure
it looks like mother of pearl - it is etched and viewed through cross polarised light.
how it pearlite formed?
Eutectoid reaction 0.8% wt @ 723 deg C, it changes directly to where from austenite it changes to alpha ferrite + Fe3C.
There are 3 phases in equilibrium.
all phases are solid, all pearlite consists of interconnected lamellae in 3D
It is not a phase it is a transformation product/ constituent in the material.
IT forms from the alpha ferrite and the carbide.
Diffusion is thermally activated so if the cooling rate increases there shorter time for diffusion - the lamellae of pearlite become finer.
What is the existing parameters in place for the development of pearlite?
alpha ferrite is nucelated in the austenite grain boundary so the carbon is ejected forming a carbide. So there is an increase in carbon concentration forming cementite in between the layers of ferrite. The grain will then grow in the direction of pearlite in line(orientated) with the original austenite structure.
What happens with the pearlite in a hypoeutectoid composition?
where the Carbon content is <0.8wt% below the eutectoid reaction it is a double phase field. where there is gamma and gamma and alpha so pro eutectoid alpha ferrite is formed on the prior austenite grain boundaries, the remaining austenite (which now has eutectoid composition) transforms to pearlite. The microstructure consists of both proeutectoid ferrite and pearlite.
What happens with the pearlite in a hypereutectoid composition?
where the Carbon content is >0.8wt% this is also a double phase. on cooling the austenite transforms into austenite and iron carbide. pro-eutectoid iron carbide forms on the prior austenite grain boundaries. The remaining austenite transforms into pearlite so the remaining microstructure consists of both iron carbide and pearlite.
