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Fig* 235 • The effect o f the cooling rate on the size
and shape of crystals:
(a)pure metals and intermetallic compounds,
(b)solid solution alloys,
(c)eutectics*
Foundry Department |
"Casting |
defects and |
Dr. V. Bastrakov |
measures of |
their prevention" |
On the other hand?the rate of cooling of light and heavy parts of the casting is different as well as that of remote from or neighbouring with ingates parts of the casting due to the difference of mould temperatures in these places* Such changes in the rate of cooling or temperature gradients may lead to different types of macro-and micro-structures of solid solutions and other constituents in different regions of the casting*,
v |
Taking into account the mentioned above it is possible |
to |
conclude that to regulate the cast structure is possible |
by |
the following ways* |
-by some treatment of a liquid alloy priory the pouring with the aim of its refining and increasing of the total
number of heterogeneous nuclei in the alloy,
-by the temperature o f superheating and pouring of the
alloy,
-by the rate of cooling during crystallisation,
-by physical methods of coercion on the solidifying
alloy,
-by herat treatment*
Obtaining fine equiaxed solid crjrstals in slowly cooled casting has been a major problem of cashing metall urgy* When the finest possible grains are needed i t ' i s possible to increase the rate of nucleation by adding more heterogeneous nuclei to the liquid alloy (grain refinement by inoculation treatment).
nucleation in central regions of the mould* The second effect is produced, by higxh frequency or ultrasonic vibrat
ions, provided these are of sufficiently high energy. Quan
titatively, neither |
of these physical, effects has been fully |
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explained, |
and furthermore |
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metho7g |
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to |
apply |
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physical are d ifficu lt |
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in |
general |
foundry |
practice |
for |
contro.lling |
the |
structure |
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of |
cast |
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But they make it possible |
to obtain |
shaped |
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alloys* |
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castings |
with fine |
grain structure, even of alloys laible |
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to |
transcrystallisation. |
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Sometimes |
a heat treatment |
is the only method to |
impove |
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or |
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to change the cast structure* |
Whether |
a casting |
should |
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be heat-treated or not depends on the characteristics of |
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the |
cast |
structure |
of the alloy, the castirig design and |
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the |
operations |
that |
follow |
the casting |
process. |
In general, |
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a heat treatment |
is |
used |
to |
introduce |
such changes |
in the |
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structure |
as w ill |
either |
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ur |
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operations |
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improve the manufactigg |
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of |
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the |
process |
or w ill |
lead |
to |
improvement |
in |
the |
properties |
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of |
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pastings relevant to |
their |
performance in service* |
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Apart from the cast structure and different kinds of wrought structure that can be obtained from cast structure by subsequent working and heat treatment processes* Solid objects in alloys can Ыво he formed from metal powders
or by deposition from the gaseous state or by electrolysis*
2A4* Djqcreo^ncy of |
mechanical |
properties |
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The extent of the |
general |
application |
of an alloy in |
industry depends on the |
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l i |
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availably of raw materials, the |
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technical feasibility and the eoonomy of the manufacturing
prooess, |
and the possibility |
of obtaining a wide range of |
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product |
properties for |
different conditions of their app |
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lication* |
Industrially |
more |
important amongst casting |
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properties are various mechanical jaroperties because they |
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are |
structure-sensitive |
ahd can be correspondingly ohanged |
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in |
the required direction* |
Applications of castings are |
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therefore often controlled by the sucess of achieving the |
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correct |
cast structure |
to meet the property requirements* |
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In practice |
this maans not only controlling the structure |
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of main crystals but more often than not controlling various types of imperfections and defects, particularly those associated with cavities and inclusion as the letters also
determine the mechanical properties of the alloy of castings*
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As it was shown ii^revious paragraphs Imperfections |
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of |
an alloy composition and |
structure are |
the |
main reason |
of |
imperfections of casting |
properties* |
These |
imperfect |
ions can also occur on a micro*—or on a macro-scale and not