Topic related to material science and metallurgy, Includes basic information about steel.Also the Iron-Iron Carbon Diagrams and its structures with various features of fe-c diagram.
3. STEEL
Steel is also produced from pig iron by
removing the impurities and by decreasing
the carbon content.
Pig iron is again heated and the excess
carbon is removed as CO2 gas and the
oxides of other impurities form a slag on top
of the molten steel.
4. Steel Production
• Ferrous metals are those metals that
contain Iron.
• The steel production process might be
divided into three phases:
• Reduction of iron to pig iron
• Refining pig iron to steel
• Forming the steel into products
5. Composition of Steel
• The essential difference between cast iron and
steel is in the amount of carbon contained in the
constituency of the metal.
• Steel is fundamentally an alloy of iron and carbon
with carbon content less than 1.5% while cast
iron is an alloy of iron and carbon with carbon
content ranging between 1.5 to 4% .
6. FACTORS THAT AFFECT
PROPERTIES OF STEELS
Carbon content
Heat treatment and shaping method
Presence of harmful elements
Presence of alloying materials.
7. Allotropic forms of iron
• Elements or compounde exist in more
than one crystalline form under different
conditions of temperature and pressure.
• This phenomenon is called allotropy or
polymorphism.
• Allotropy is characterised by a change
in atomic structure which occure at a
difinite transformation temperature.
8. Fe-C True Equilibrium
Diagram
8
A phase diagaram
shows us the
microstructure
within a material
as function of the
material
composition and
material
temperature
1600
1400
1200
1000
800
600
400
0 1 2 3 4 90
L
g +L
+ Graphite
Liquid +
Graphite
(Fe) Co, wt% C
0.65
740°C
T(°C)
g + Graphite
100
1153°Cg
Austenite 4.2 wt% C
+ g
10. Definition of structures
Various phases that appear on the Iron-
Carbon equilibrium phase diagram are
as under:
•Austenite
•Ferrite
•Pearlite
•Cementite
•Martensite*
•Ledeburite
11. Definition of structures
• Ferrite is known as α solid solution.
• It is an interstitial solid solution of a small
amount of carbon dissolved in α (BCC) iron.
• stable form of iron below 912 deg.C
• The maximum solubility is 0.025 % C at
723C and it dissolves only 0.008 % C at
room temperature.
• It is the softest structure that appears on the
diagram.
12. Definition of structures
• Pearlite is the eutectoid mixture
containing 0.80 % C and is
formed at 723°C on very slow
cooling.
• It is a very fine platelike or
lamellar mixture of ferrite and
cementite.
• The white ferritic background or
matrix contains thin plates of
cementite (dark).
13. Definition of structures
• Austenite is an interstitial solid solution of
Carbon dissolved in g (F.C.C.) iron.
• Maximum solubility is 2.0 % C at 1130°C.
• High formability, most of heat treatments
begin with this single phase.
• It is normally not stable at room
temperature. But, under certain conditions it
is possible to obtain austenite at room
temperature.
14. Definition of structures
• Cementite or iron carbide, is very hard,
brittle intermetallic compound of iron &
carbon, as Fe3C, contains 6.67 % C.
• It is the hardest structure that appears on the
diagram, exact melting point unknown.
• Its crystal structure is orthorhombic.
• It is has
• low tensile strength (approx. 5,000 psi),
but
• high compressive strength.
15. Definition of structures
Martensite - a super-saturated solid solution of
carbon in ferrite.
It is formed when steel is cooled so rapidly that
the change from austenite to pearlite is
suppressed.
The interstitial carbon atoms distort the BCC
ferrite into a BC-tetragonal structure (BCT).;
responsible for the hardness of quenched steel
16. Definition of structures
• Ledeburite is the eutectic mixture
of austenite and cementite.
• It contains 4.3 percent C and is
formed at 1130°C.
17. Various Features of Fe-C diagram
Peritectic L + d = g
Eutectic L = g + Fe3C
Eutectoid g = + Fe3C
Phases present
L
Reactions
d
BCC structure
Paramagnetic
g austenite
FCC structure
Non-magnetic
ductile
ferrite
BCC structure
Ferromagnetic
Fairly ductile
Fe3C cementite
Orthorhombic
Hard
brittle
Max. solubility of C in ferrite=0.022%
Max. solubility of C in austenite=2.11%
18. The Iron-Iron Carbide
DiagramThe diagram shows three horizontal lines which
indicate isothermal reactions (on cooling /
heating):
• First horizontal line is at 1490°C, where peritectic
reaction takes place:
Liquid + d ↔ austenite
• Second horizontal line is at 1130°C, where
eutectic reaction takes place:
liquid ↔ austenite + cementite
• Third horizontal line is at 723°C, where eutectoid
reaction takes place:
austenite ↔ pearlite (mixture of ferrite &
cementite)
19. The Iron-Iron Carbide Diagram
• A map of the temperature at which different
phase changes occur on very slow heating
and cooling in relation to Carbon, is called
Iron- Carbon Diagram.
• Iron- Carbon diagram shows
• the type of alloys formed under very slow
cooling,
• proper heat-treatment temperature and
• how the properties of steels and cast irons
can be radically changed by heat-treatment.
20. Three Phase Reactions
• Peritectic, at 1490 deg.C, with low wt% C
alloys (almost no engineering importance).
• Eutectic, at 1130 deg.C, with 4.3wt% C,
alloys called cast irons.
• Eutectoid, at 723 deg.C with eutectoid
composition of 0.8wt% C, two-phase mixture
(ferrite & cementite). They are steels.
21. References
1. ) Article of Properties of steel from
Wikipedia
2) Book Of Material Science and
Metallurgy By K. I. Parashivamurthy
3) Photos from Google.