Phase diagram

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1. Iron carbonIron carbon
Phase diagramPhase diagram
DefinitionsDefinitions

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3. The best way to understand the metallurgy of The best way to understand the metallurgy of
carbon steel is to study the ‘Iron Carbon carbon steel is to study the ‘Iron Carbon
Diagram’. The diagram shown below is based Diagram’. The diagram shown below is based
on the transformation that occurs as a result of on the transformation that occurs as a result of
slow heating. Slow cooling will reduce the slow heating. Slow cooling will reduce the
transformation temperatures; for example: the transformation temperatures; for example: the
A1 point would be reduced from 723°C to 690 A1 point would be reduced from 723°C to 690
°C. However the fast heating and cooling rates °C. However the fast heating and cooling rates
encountered in welding will have a significant encountered in welding will have a significant
influence on these temperatures, making the influence on these temperatures, making the
accurate prediction of weld metallurgy using this accurate prediction of weld metallurgy using this
diagram difficult diagram difficult

7. AusteniteAustenite      This phase is only possible in This phase is only possible in
carbon steel at high temperature. It has a carbon steel at high temperature. It has a
Face Centre Cubic (F.C.C) atomic Face Centre Cubic (F.C.C) atomic
structure which can contain up to 2% structure which can contain up to 2%
carbon in solution. carbon in solution.
FerriteFerrite    This phase has a Body Centre This phase has a Body Centre
Cubic structure (B.C.C) which can hold Cubic structure (B.C.C) which can hold
very little carbon; typically 0.0001% at very little carbon; typically 0.0001% at
room temperature. It can exist as either: room temperature. It can exist as either:
alpha or delta ferrite. alpha or delta ferrite.

8. Carbon Carbon   A very small interstitial atom that A very small interstitial atom that
tends to fit into clusters of iron atoms. It tends to fit into clusters of iron atoms. It
strengthens steel and gives it the ability to strengthens steel and gives it the ability to
harden by heat treatment. It also causes harden by heat treatment. It also causes
major problems for welding , particularly if major problems for welding , particularly if
it exceeds 0.25% as it creates a hard it exceeds 0.25% as it creates a hard
microstructure that is susceptible to microstructure that is susceptible to
hydrogen cracking. Carbon forms hydrogen cracking. Carbon forms
compounds with other elements called compounds with other elements called
carbides. Iron Carbide, Chrome Carbide carbides. Iron Carbide, Chrome Carbide
etc etc

9. CementiteCementite    Unlike ferrite and austenite, Unlike ferrite and austenite,
cementite is a very hard intermetallic cementite is a very hard intermetallic
compound consisting of 6.7% carbon and compound consisting of 6.7% carbon and
the remainder iron, its chemical symbol is the remainder iron, its chemical symbol is
Fe3C. Cementite is very hard, but when Fe3C. Cementite is very hard, but when
mixed with soft ferrite layers its average mixed with soft ferrite layers its average
hardness is reduced considerably. Slow hardness is reduced considerably. Slow
cooling gives course perlite; soft easy to cooling gives course perlite; soft easy to
machine but poor toughness. Faster machine but poor toughness. Faster
cooling gives very fine layers of ferrite and cooling gives very fine layers of ferrite and
cementite; harder and tougher cementite; harder and tougher

10. PearlitePearlite    A mixture of alternate strips of ferrite A mixture of alternate strips of ferrite
and cementite in a single grain. The distance and cementite in a single grain. The distance
between the plates and their thickness is between the plates and their thickness is
dependant on the cooling rate of the material; dependant on the cooling rate of the material;
fast cooling creates thin plates that are close fast cooling creates thin plates that are close
together and slow cooling creates a much together and slow cooling creates a much
coarser structure possessing less toughness. coarser structure possessing less toughness.
The name for this structure is derived from its The name for this structure is derived from its
mother of pearl appearance under a mother of pearl appearance under a
microscope. A fully pearlitic structure occurs at microscope. A fully pearlitic structure occurs at
0.8% Carbon. Further increases in carbon will 0.8% Carbon. Further increases in carbon will
create cementite at the grain boundaries, which create cementite at the grain boundaries, which
will start to weaken the steel. will start to weaken the steel.

12. Cooling of a steel below 0.8% carbon Cooling of a steel below 0.8% carbon When When
a steel solidifies it forms austenite. When the a steel solidifies it forms austenite. When the
temperature falls below the A3 point, grains of temperature falls below the A3 point, grains of
ferrite start to form. As more grains of ferrite ferrite start to form. As more grains of ferrite
start to form the remaining austenite becomes start to form the remaining austenite becomes
richer in carbon. At about 723°C the remaining richer in carbon. At about 723°C the remaining
austenite, which now contains 0.8% carbon, austenite, which now contains 0.8% carbon,
changes to pearlite. The resulting structure is a changes to pearlite. The resulting structure is a
mixture consisting of white grains of ferrite mixed mixture consisting of white grains of ferrite mixed
with darker grains of pearlite. Heating is with darker grains of pearlite. Heating is
basically the same thing in reverse basically the same thing in reverse

13. Marten siteMarten site
If steel is cooled rapidly from If steel is cooled rapidly from
austenite, the F.C.C structure rapidly changes to austenite, the F.C.C structure rapidly changes to
B.C.C leaving insufficient time for the carbon to B.C.C leaving insufficient time for the carbon to
form pearlite. This results in a distorted form pearlite. This results in a distorted
structure that has the appearance of fine structure that has the appearance of fine
needles. There is no partial transformation needles. There is no partial transformation
associated with martensite, it either forms or it associated with martensite, it either forms or it
doesn’t. However, only the parts of a section doesn’t. However, only the parts of a section
that cool fast enough will form martensite; in a that cool fast enough will form martensite; in a
thick section it will only form to a certain depth, thick section it will only form to a certain depth,
and if the shape is complex it may only form in and if the shape is complex it may only form in
small pockets. The hardness of martensite is small pockets. The hardness of martensite is
solely dependant on carbon content, it is solely dependant on carbon content, it is
normally very high, unless the carbon content is normally very high, unless the carbon content is
exceptionally low. exceptionally low.

15. TemperingTempering
The carbon trapped in the martensite The carbon trapped in the martensite
transformation can be released by heating the transformation can be released by heating the
steel below the A1 transformation temperature. steel below the A1 transformation temperature.
This release of carbon from nucleated areas This release of carbon from nucleated areas
allows the structure to deform plastically and allows the structure to deform plastically and
relive some of its internal stresses. This reduces relive some of its internal stresses. This reduces
hardness and increases toughness, but it also hardness and increases toughness, but it also
tends to reduce tensile strength. The degree of tends to reduce tensile strength. The degree of
tempering is dependant on temperature and tempering is dependant on temperature and
time; temperature having the greatest influence. time; temperature having the greatest influence.

16. AnnealinAnnealin
This term is often used to define This term is often used to define
a heat treatment process that produces a heat treatment process that produces
some softening of the structure. True some softening of the structure. True
annealing involves heating the steel to annealing involves heating the steel to
austenite and holding for some time to austenite and holding for some time to
create a stable structure. The steel is then create a stable structure. The steel is then
cooled very slowly to room temperature. cooled very slowly to room temperature.
This produces a very soft structure, but This produces a very soft structure, but
also creates very large grains, which are also creates very large grains, which are
seldom desirable because of poor seldom desirable because of poor
toughness. toughness.

17. NormalisingNormalising Returns the structure back Returns the structure back
to normal. The steel is heated until it just to normal. The steel is heated until it just
starts to form austenite; it is then cooled in starts to form austenite; it is then cooled in
air. This moderately rapid transformation air. This moderately rapid transformation
creates relatively fine grains with uniform creates relatively fine grains with uniform
pearlite. pearlite.

WeldingWelding If the temperature profile for a If the temperature profile for a
typical weld is plotted against the carbon typical weld is plotted against the carbon
equilibrium diagram, a wide variety of equilibrium diagram, a wide variety of
transformation and heat treatments will be transformation and heat treatments will be
observed. observed.