Materials- Metals: Steels Flashcards
Generic positives and negatives of steel
Positives: strong, cheap, tough, versatile, easily welded, hard, stiff.
Negatives: CO2 produced when making, heavy, poor corrosion resistance.
What happens when carbon content in steel increases?
Metal becomes harder and stronger but less ductile and more difficult to weld and it has a lower melting point.
What happens to ferrite at 912°C?
If the temperature increases to this, the ferrite experiences a polymorphic transformation to FCC austenite.
What happens to austenite when the temperature is increased to 1394°C.
Austenite reverts back to a BCC phase known as δ-ferrite
Describe the composition of iron carbide and give its other name
6.70 wt% carbon. Cementite.
Where are the carbon atoms in a solid solution with iron?
In the interstitial positions of the iron crystal structure.
Where in FCC and BCC iron crystal structures are the holes that carbon can exist?
In BCC, they are the tetrahedral holes between the two edge and two central atoms in the structure.
In FCC, they are at the centres of the cube edges and are then surrounded by six atoms in the form of an octagon so are octahedral holes.
Which crystal structure has bigger holes for carbon?
FCC
Which is magnetic, α-ferrite or austenite?
α-ferrite
Approximate wt% C in steels and cast irons
Steels: <1.5%
Cast irons: 2-6%
When does pearlite form?
When the austenite is cooled gradually at the eutectoid composition (0.76 wt% C) and changes into α-ferrite and Fe3C which form in lamellae.
Describe the formation of pearlite
An initial cementite nucleus forms at an austenite grain boundary. This cementite plate grows and α-ferrite nucleates. α plate grows and cementite nucleates. This repeats. A new cementite plate can nucleate on an α plate with a different orientation to form a new colony. All the pearlite grows into both grains from the grain boundaries.
Describe the properties of pearlite relative to the materials it’s made from
Its properties are intermediate between the soft, ductile ferrite and the hard, brittle cementite.
What are hypo-eutectoid steels?
Steels with carbon weight percentages to the left of the eutectoid
Describe the formation of hypo-eutectoid steels
Austenite with a carbon weight percentage less than eutectoid is cooled. Most ferrite particles will form along original austenite grain boundaries. Cooling just above eutectoid produces an increase in the α phase. When cooling passes the eutectoid temperature, the remaining austenite will transform to pearlite and α already present won’t change. The pearlite exists in an α matrix.
