microestructuras aco ao carbono

7/30/2019 Microestructuras Aco Ao Carbono

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Professora Ana Mara Prez Ceballos

Engenheira Metalrgica, M. Sc.

METALOGRAFIA

Estruturas em aos ao carbono


2/17

http://www.doitpoms.ac.uk/miclib/pds.swf?targetFrame=Fe-C-X


3/17

This is a hypoeutectoid alloy, which has been

air cooled from the austenite phase field at

950 C. The first solid to form is proeutectoid

ferrite, its morphology being determined by

the cooling rate. At slow cooling rates(furnace cooling) there is sufficient time for

the carbon rejected from the austenite to

diffuse and equilibrium solidification occurs.

With faster cooling the microstructure also

depends on the original austenite grain size.

Fast cooling and large grain size favours

ferrite forming as Widmansttten side plates

from the grain boundaries. Small grain sizes

imply a high number of nuclei and hence the

ferrite grows as grain boundary

allotriomorphs. In this case air cooling is

sufficiently slow to produce allotriomorphic

ferrite. The majority of the austenite has

changed to ferrite leaving only a small

amount to be transformed to pearlite,

therefore the microstructure shows large

ferrite grains with small islands of pearlite.

Hypoeutectoid alloy C 0.1 (wt%), normalised at

950C.

http://www.doitpoms.ac.uk/miclib/micrograph_record.php?id=17


4/17

Normalised carbon steel. Low carbon steel with a microstructure consisting mostly of ferrite with the darker

pearlite regions around the ferrite grains. Upon cooling the steel the ferrite forms initially, either on austenite

grain boundaries or inclusions. This causes carbon to be partitioned into the austenite. Eventually the remaining

austenite will be at the eutectoid condition and the transformation to pearlite will then take place. Micrograph

was taken transverse to direction of casting, therefore no directionality is seen. 0,08%C

http://ww

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5/17

Normalised carbon steel (fast cool). Low carbon steel with a microstructure consisting mostly of ferrite with the

darker pearlite regions around the ferrite grains. Upon cooling the steel the ferrite forms initially, either on

austenite grain boundaries or inclusions. This causes carbon to be partitioned into the austenite. Eventually the

remaining austenite will be at the eutectoid condition and the transformation to pearlite will then take place.

0,15%C

http://ww

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6/17

As cast carbon steel. Low carbon steel with a microstructure consisting mostly of ferrite with the

darker pearlite regions around the ferrite grains. Upon cooling the steel the ferrite forms initially,

either on austenite grain boundaries or inclusions. This causes carbon to be partitioned into the

austenite. Eventually the remaining austenite will be at the eutectoid condition and the transformationto pearlite will then take place. 0,2 %C

http://www.doitpoms.ac.u

k/miclib/full_record.php?id=20

9


7/17

Normalised carbon steel at 1000C. Low carbon steel with a microstructure consisting mostly of ferrite with the

darker pearlite regions around the ferrite grains. Upon cooling the steel the ferrite forms initially, either on

austenite grain boundaries or inclusions. This causes carbon to be partitioned into the austenite. Eventually the

remaining austenite will be at the eutectoid condition and the transformation to pearlite will then take place.This sample has been normalised, removing the directionality caused by casting. 0,2 %C

http://ww

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hp?id=211


8/17

Hypoeutectoid steel, normalised at 1100C. A hypoeutectoid alloy (carbon composition less than eutectoid). The first

phase formed upon cooling from the austenite phase field is proeutectoid ferrite. Due to the lower solubility of carbon in

ferrite, carbon is partitioned into the remaining austenite. At the eutectoid point the remaining carbon enriched

austenite transforms to pearlite (a mixture of ferrite and cementite) which is the darker region of the micrograph. The

proportion of pearlite is dependent upon the overall composition. The ferrite (light areas) is a good example of an

allotriomorphic ferrite. This means that its shape does not reflect its internal crystalline symmetry as it nucleates on the

austenite grain boundaries and hence follows the shape of the boundaries, the remaining austenite within the ferrite

then transforms to pearlite, and is surrounded by the ferrite. The large size of the areas of pearlite arises due to the high

normalisation temperature which causes the austenite grains to grow large. 0,4 %C


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normalisation temperature which causes the austenite grains to grow large. 0,4 %C


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10/17









normalisation temperature which causes the austenite grains to grow large. 0,4 %C, 0,8 %Mn.


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11/17

Fe, C 0.3 (wt%) steel, spheroidised carbide. A hypoeutectoid alloy (composition less than eutectoid). The first phase

formed upon cooling from the austenite phase field is proeutectoid ferrite. Due to the lower solubility of carbon in

ferrite, carbon is partitioned into the remaining austenite. At the eutectic point the remaining carbon-enriched austenite

transforms to pearlite (a mixture of ferrite and cementite), which is the darker region of the micrograph. The proportion

of pearlite is dependent upon the overall composition. Subsequent to casting the sample was annealed for a long period

just below the transformation range. This induces the cementite to take on a spherical appearance. This results in a softer

and more ductile alloy. This process is known as spheroidisation. The changes to the morphology of the cementite cannot

be seen clearly in this micrograph, but they are more apparent at higher magnification (see micrograph 242).


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12/17

Fe, C 0.3 (wt%) steel, spheroidised carbide. A hypoeutectoid alloy (composition less than eutectoid). The first phase

formed upon cooling from the austenite phase field is proeutectoid ferrite. Due to the lower solubility of carbon in

ferrite, carbon is partitioned into the remaining austenite. At the eutectic point the remaining carbon-enriched austenite

transforms to pearlite (a mixture of ferrite and cementite), which is the darker region of the micrograph. The proportion

of pearlite is dependent upon the overall composition. Subsequent to casting the sample was annealed for a long period

just below the transformation range. This induces the cementite to take on a spherical appearance. This results in a softer

and more ductile alloy. This process is known as spheroidisation. The changes to the morphology of the cementite cannot

be seen clearly in this micrograph, but they are more apparent at higher magnification (see micrograph 242).


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13/17http://www.doitpoms.ac.uk/miclib/pds.swf?targetFrame=Fe-C-X


14/17

This steel is of the eutectoid composition. Once

the temperature is lowered below the eutectoid

temperature the steel becomes simultaneously

supersaturated with both ferrite and cementite.

The resultant microstructure, known as pearlite,

comprises lamellae of cementite (dark)

embedded in ferrite (white). The platelets are

parallel to each other and do not follow a

specific crystallographic direction.

Changes in the apparent interlamellar spacing

from colony to colony in the photograph aredue to differences in the lamellae spacing with

respect to the polished surface. The coarseness

of the pearlite is determined by the

interlamellar spacing. This spacing is inversely

proportional to the undercooling. This is

primarily because of the increased rate of

carbide nucleation with increased undercooling.

The pearlite in this sample is coarse due to it

being slowly cooled. The undercooling is low so

the lamellae spacing is relatively large resulting

in a coarse microstructure.

Eutectoid alloy C 0.8 (wt%), Slow cooled.

http://www.doitpoms.ac.uk/miclib/micrograph_record.php?id=14


15/17

Fe, C 0.8 (wt%) steel, eutectoid transformation, normalised at 900C. This steel is of the eutectoid

composition. Once the temperature is lowered below the eutectoid temperature the steel becomes

simultaneously supersaturated with both ferrite and cementite. A eutectoid transformation results ( to

+ Fe3C). The resultant microstructure, known as pearlite, comprises lamellae of cementite (dark)

embedded in ferrite (white). The platelets are parallel to each other and do not follow a specific

crystallographic direction.


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16/17

Fe, C 0.8 (wt%) steel, eutectoid transformation, normalised at 900C. This steel is of the eutectoid composition. Once the

temperature is lowered below the eutectoid temperature the steel becomes simultaneously supersaturated with both

ferrite and cementite. A eutectoid transformation results ( to + Fe3C). The resultant microstructure, known as pearlite,

comprises lamellae of cementite (dark) embedded in ferrite (white). Subsequent to casting, the alloy was normalised

(annealed) just below the eutectoid temperature, in order to induce the carbides to take a spherical appearance, which

results in the steel becoming softer and more ductile. This is known as spheroidisation. The changes to the morphology

of the cementite cannot be seen clearly in this micrograph, but they are discernible at higher magnification (seemicrographs 266 and 267).


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17/17

Fe, C 0.8 (wt%) steel, eutectoid transformation, normalised at 900C. This steel is of the eutectoid composition. Once the

temperature is lowered below the eutectoid temperature the steel becomes simultaneously supersaturated with both

ferrite and cementite. A eutectoid transformation results ( to + Fe3C). The resultant microstructure, known as pearlite,

comprises lamellae of cementite (dark) embedded in ferrite (white). Subsequent to casting, the alloy was normalised

(annealed) just below the eutectoid temperature, in order to induce the carbides to take a spherical appearance, which

results in the steel becoming softer and more ductile. This is known as spheroidisation. The changes to the morphology

of the cementite cannot be seen clearly in this micrograph, but they are discernible at higher magnification (seemicrographs 266 and 267)


k/m

iclib/micrograph_

record.p

hp?id=267

microestructuras aco ao carbono

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