Steels Flashcards
What is a steel?
Alloy that contains iron and carbon
Carbon content determined by max amount that can be dissolved in solid iron.
Usually 0.01-2wt%C
What are the two distinct crystallographic forms of iron?
Body centred cubic - up to 910C (eg. Ferrite or alpha symbol)
Face centred cubic - up to 1394C (eg. Austenite or gamma symbol) - goes back to bcc (delta form)
Describe body centred cubic structure
Atom at each corner and one in the middle
Describe a face centred cubic structure
Atoms at each corner and an atom at the centre of each face
What is a lattice parameter?
Distance between atoms in the cubic structure - a=b=c
What is an interstitial atom?
Different sized atom that fits in between a lattice of another set of atoms
What is a substitutional atom?
Atom of the same size as the lattice atoms that replace an atom
What is packing efficiency of cubic structures and why does it cause a volume change as structures change?
BCC fills 68% of space, FCC fills 74% of space - if you go from BCC to FCC, you will reduce volume cos its more close packed
What defines an equilibrium state?
No further change is perceptible - always when there is lowest free energy
What is a phase?
A portion of the system whose properties, composition, and structural arrangement are homogenous and distinct and mechanically separable from other portions of the system.
Why does Fe3C (cementite) form in steels?
When carbon is added beyond solubility limit of iron, Fe3C is formed which is metastable
What’s the difference between eutectic and eutectoid?
Eutectic - 1 liquid goes to 2 solids
Eutectoid - 1 solid goes to 2 solids (solid state reaction)
Where are eutectic and eutectoid reactions on the iron carbide diagram?
Eutectoid - bottom left V
Eutectic - upper right V
Describe the process of pearlite from a eutectoid composition
0.8wt%C steel (goes through eutectoid point) cools below 723C which is A1 temp.
Transforms to alpha (ferrite) which has very low Carbon solubility so rejects some as Fe3C.
Forms lamallae of ferrite and cementite - grows in horizontal grains
What’s the difference between hypo and hyper eutectoid compositions?
Hypo - less than eutectoid composition
Hyper - above eutectoid composition
Pro-eutectoid ferrite is formed on gamma grain boundaries
Remaining gamma turns into pearlite
Sketch a rough iron carbide diagram with general areas and percentages, temperatures etc.
Look at a diagram
What is the lever rule?
Opposite side to what you think, Co - 0.022/0.76 - 0.022 for pearlite
What’s an example of a non-equilibrium transformation?
If austenite cools slowly, carbon can diffuse out and change to ferrite.
If it cools quickly or quenches, carbon is trapped in interstitial sites.
As ferrite has a lower solubility, if the carbon can’t get out, it will hinder the transformation from FCC to BCC form.
What is the radius ratio of carbon atoms to iron atoms?
Carbon are 0.6 of iron so enter as interstitial rather than substitutional atoms
What atoms are substitutional for iron?
Manganese, Nickel, Chromium etc.
Why can the FCC dissolve a lot more carbon than BCC even though its closer packed?
FCC has less interstitial holes but they are bigger
Describe the differences in tetrahedral and octahedral interstitial sites?
Tetrahedral - atom is sat in a tetrahedral shape formed by corner atom and surrounding face atoms
Octahedral - atom in centre of cube edges
In a BCC lattice, which type of interstitial site has a greater strain energy?
Octahedral only strains by moving 2 neighbours - but distorts them apart by a large amount
Tetrahedral involves moving 4 neighbours
If you place an interstitial atom into a BCC lattice in an octahedral position (on the centre of a face) what are the distances to the nearest atoms?
Looking from top, a/sqrt2 to corner atom,
Looking from side, a/2 to body centre atom
a/2 is smaller distance than a/sqrt2 so the cube is distorted in the direction of the new atom moving away from the body centre atom
What are the limitations of the equilibrium phase diagram?
Only shows stability under equilibrium conditions - equilibrium cooling isn’t achieved practically
Doesn’t give info of meta stable phases or transformations from non-equilibrium cooling like martensite or bainite
No information about transformation kinetics, size of phases or property information
