6. Three invariant reactions
A horizontal line always indicates an invariant
reaction in binary phase diagrams
Peritectic reaction at 1495˚C and 0.18%C,
d-ferrite + L↔ g-iron (austenite)
Eutectic reaction at 1147˚C and 4.3 %C,
L ↔ g-iron + Fe3C (cementite) [ledeburite]
Eutectoid reaction at 727˚C and 0.77%C,
g-iron ↔ a–ferrite+Fe3C (cementite) [pearlite]
9. Fe-C alloy classification
Metals
Ferrous metals
Steels
Non-ferrous metals
Cast Irons
Plain carbon steels
Grey Iron
Low carbon steels
White Iron
Medium carbon steels
Malleable & Ductile Irons
High carbon steels
Low alloy steels
High alloy steels
Stainless & Tool steels
10. Fe-C alloy classification
Fe-C alloys are classified according to wt.% C
present in the alloys
Commercial pure irons % C < 0.008
Low-carbon steels
0.008 - %C - 0.3
Medium carbon steels 0.3 - %C - 0.8
High-carbon steels
0.8- %C - 2.14
Cast irons
2.14 < %C
11. Cast irons
Cast irons that were slowly cooled to room
temperature consists of cementite, look whitish
– white cast iron.
If it contains graphite, look grayish – gray cast
iron.
It is heat treated to have graphite in form of
nodules – malleable cast iron.
If inoculants are used in liquid state to have
graphite nodules – spheroidal graphite (SG)
cast iron.
18. Phase vs. Microconstituents
A phase or a mixture of phases which has a
distinct identity in a microstructure is called a
microconstituent
Pearlite is not a phase.
It is a microconstituent and is a mixture of two
phases a- Ferrite and Fe3C.
19. a-Ferrite
Known as a -iron
Pure iron at room temperature
Body-centered cubic structure
Soft & ductile and imparts these
properties to the steel.
Less than 0.01% carbon will dissolve in
ferrite at room temperature
High temperature form is d ferrite, but
the two forms are identical.
Pure ferritic steels are rare
20. Austenite
Known as g -iron
Face-centered cubic
Much softer than ferrite
Not present at room temperatures.
More easily hot worked
21. Cementite
Iron Carbide - an intermetallic compound
Hard, brittle, white
melts at 1837 C , density of 7.4 g/cc
On the phase diagram, cementite corresponds
to a vertical line at 6.7% C
Engineers care only about compounds with less
carbon
Its presence in steels causes an increase in
hardness and a reduction in ductility and
toughness
22. Pearlite
A laminated structure formed of alternate
layers of ferrite and cementite with average
composition 0.83% carbon
Pearly lustre in the microscope
Interference of light in its regular layers
Most common constituent of steel
It combines the hardness and strength of
cementite with the ductility of ferrite and is the
key to the wide range of the properties of
steels.
The laminar structure also acts as a barrier to
crack movement as in composites. This gives
it toughness
23.
24. Phase Transformations
Involve some alteration of microstructure
1. No change in number or composition of the
phases present, diffusion- dependent.
Solidification of pure metals, allotropic
transformation.
2. Some alteration in composition and no of
phases, diffusion – dependent. Eutectoid
reaction
3. A metastable phase is produced,
diffusionless. Martensitic transformation.
25. Phase Transformations
At least one new phase is formed
Do not occur instantaneously
Begin by the formation of small particles
of new phase – nucleation
Homogenous – occurs uniformly throughout
the parent phase.
Hetrogenous – preferentially at grain
boundaries, impurities, dislocations
Size of these particles increase in size
until completion - growth
26. Phase Transformations
Dependent on
Temperature
Time
Composition
Require some finite time for completion
Equilibrium is rarely achieved in solids
Metastable – intermediate between
initial and equilibrium states.
34. CCT diagram
Usually materials are cooled continuously, thus
Continuous Cooling Transformation diagrams
are appropriate than TTT diagrams
For continuous cooling, the time required for a
reaction to begin and end is delayed, thus the
isothermal curves are shifted to longer times
and lower temperatures.
Main difference between TTT and CCT
diagrams: no space for bainite in CCT diagram
as continuous cooling always results in
formation of pearlite.