3218
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This defect can he caused hy improper control of |
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melting and pouring; |
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skimming the slag from the metal surface |
in |
a ladle; |
-gating desigri;
-moulding sand practice.
At the pouring of metal through the ladle nose slag
formed either in |
the |
furnace or in the ladle ELS a result |
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of |
an Interaction |
of |
the metal with refractory materials |
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of |
their lining can enter |
the running system and then the |
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casting. |
To prevent slag |
inclusions entering the metal |
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from the |
ladle, it should he skimmed before pouring. |
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Por this purpose, it is better to overheat the metal in order to decrease its viscosity and to hold it in the la dle for a time sufficient to float slag particles to the metal surface, where they can he skimmed. The use of tea pot or bottom-pouring ladles also helps to avoid the ent rance of slag inclusions from the ladle.
Slag and dross can be additionally removed from the
metal by |
use of |
special |
designs of runner systems. Pour |
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ing basins should be employed so that any slag entering |
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from the |
ladle |
w ill |
rise |
and not pass into the runner sys- |
em ( Pig. |
2,29,a). |
The |
runner system its e lf should be des |
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igned to |
exert |
additional skimming action on the flowing |
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metal. |
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Sometimes, also* serrated ’ teeth” are molded in the
top of the runner to trap foreign particles or the swirl gate ( Fig. 2 ,29,d) employing centrifugal force" to aid
in the separation of slag is used. In the latter |
gate , |
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the lighter, foreign materials are forced |
to the |
center |
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of the swirl and upto a riser or vent. |
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In light, alloy casting, unpressurized gating systems |
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are; used, and described |
above measures |
of |
slag eliminating |
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in ranners are-useless. |
3n this |
case |
fo r |
metal cleaning, |
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it is necessary to add screens to |
the |
runner system} per |
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forated .tin-plated sheet is normally us6d for this purpose.
Screens may be inserted in |
gating |
systems at the sprue |
base, |
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in the runner or |
gates,..or |
at the |
juncture |
between the |
ru |
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nner and ga'tes ( |
FigV 2 .29 , |
e) • |
Wherever |
screen |
are loca |
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ted, it is usually desirable to enlarge thb cross |
section |
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of the flow channel at that point as otherwise the screen will act as a "choice” •
Strainer cores ( disk-shaped sand and ceramic cores with small holes) are sometimes used in place pf metal sc reens, ©specially for metals other than a^minium and mag nesium ( Fig. 2.29,b ) . These cores also act as a "choke” , and enlarging cross sections must be used at the point of their location too*
will minimize any tendency to entrap air or form oxides
and dross |
inclusions during the mould -fillin g operation* |
As regard |
to the gating system design, everything, what |
has been mentioned above conformably to prevention of slag inclusions, is true and may be used for eliminating the dross inclusions*
It should be noted that the .interruption of a stream at the pouring of easily oxidizing alloys can cause, under
the proper conditions, the exfoliation of metal into some
layers* The same defect may be found in centrifugal cas
tings when the alloy are poured slowly into a mould rota
ted with a high |
speed, i*e* |
when the speed of rotation is |
not agreed with |
the rate of |
pouring, |
Careful control of mould permeability and moisture and gas content reduce the danger of entrapping air ( or mould gases) and minimize any tendency to form dross from mould-metal reactions.
When magnesium base alloys are cast, moulding sand mixtures must contain special flu oric additions to prevent the und esirable reaction between the alloy and mould material.
Sand spots are caused by the metal washing particles from the runner system or mould walls. They appear as ent ail irregularly shaped depressions in the finished casting
Probably |
it should be clear |
how to |
prevent this def |
ect formation* |
The main measures |
to do |
it are the follow |
ing t - |
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-qualitative working-out and severe oarring-out of the mould and core production technology;
-accurate observance of rules of pouring a liquid metal;
-correct organization of metal streams in the mould cavity ;
-raising the total culture in the foundry.
Unfused ch ills and chaplets.
Unfused chaplets and internal ch ills , as well as
blows from chaplets and ch ills, are prevalent enough to
make foundrymen wary of their use. |
Poor fusion is |
due |
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to low pouring temperatures or the use of chaplets or ch |
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ills so large that they cool the metal before surface fu |
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sion can occur. Blows are due to moisture or other cont |
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amination such as |
o il, dust, |
or oxide |
accumulated |
on the |
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chill or chaplets |
before or |
after |
it |
is placed in |
the mould. |
External chills sometimes cause surface blows by similar contamination.
