3218
.pdfdepends on the thickness of casting «alls |
and may be taken |
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from table |
2.8, |
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Daring the pouring of cast |
iron into |
moulds |
particles |
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of the coal dust are burnt down, |
isolating |
vifalat^ |
substance |
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CO and OOg» |
The |
letters create |
a gas layer between molten |
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metal and mould |
and provide the |
reducing atmospheifc which |
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prevents the formation of oxides promoting the penetration of the metal into pores due to the increase of wetting sand grains by molten metal.
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Table 2.8, |
Goal dust content in a moulding sand |
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Thickness of |
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Goal dust |
Seave No. |
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casting walls» |
content , |
(fineness of |
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in mm |
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in % |
milling) |
3-5 |
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0 |
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'6b |
5-10 |
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l3 |
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оебз |
10-25 |
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3- 4 |
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0l6 |
25-50 |
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4-5 |
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02 |
50 |
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6-8 |
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0315" |
Blaole oif^eavy |
oils majr be |
used as antiburing-on |
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additions to mixture©when iron and cupper alloy castings |
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are produced |
in |
green |
sand moulds. |
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Their action |
is |
likely to coal dust but the amount of a black |
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oil addition |
is |
less |
(1*1»5$)* |
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-яе-
The metal penetration may be decreased or even eliminated with ihe increase of mould hardness* ®t is
explained by the fact that under the action of a high
pressure |
clay is displaced from the surface of sand grain |
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to |
mould |
pores and shuts them up. |
Therefore, |
it |
is |
posail |
to |
expect |
the more smooth surface |
of casting |
w ill |
be |
obtai |
when they have been made in high pressured moulds. |
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The |
static pressure in moulds for small |
castings is, |
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as |
a rule, insufficient to |
metal penetration, |
but |
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the defect can occur on such the oastings as a result of
dynamic pressure. This may occur even in case of moulds ma
by high pressure squeezing, but usually, with small qastin
made in hard-rammed moulds, metal |
penetrates to a depth of |
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one or two sand grains only* |
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Chemical burning-on is most likely to occur on castii |
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made of steel,iron, |
bronze and is |
formed as a result of |
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physiBal-chemical processes during |
solidification |
and cool |
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ing |
the casting at |
high |
temperature* In contrast |
to |
mecha |
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ica l |
burning-on,in |
this |
case sand |
grains are bonded |
by not |
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got |
cold |
metal |
but |
by complicate chemical combinations-pro |
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cts |
of |
interaction |
of metal oxides with materials |
of a ra |
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or |
core. |
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A liquid metal being poured |
into a mould is |
always о |
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dized |
in |
a more |
or |
less degree. |
The metal is being oxidij |
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oxigen |
of |
bthe |
air |
during its pouring into moulds and afte |
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the |
pouring-by |
oxigen having been |
in pores of the |
mould, t |
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water steam being formed in any mould, by oarbonic acid gas being the product of the burning down of organic components from moulding mixtures. The content of an oxide film depends firstly on the metal chemical composition. For example, carbon steel is covered by the film mainly consi sting of ferrous oxides, the film on steel containing 0.12% Cj0.23%Mn. 0.6% Si, 13.6% Cr, 0.19% Ni consists of 32,2% Сг20з> 24.4% Si02,9.56% FeO,l6.7% Mh0,l7%Al20j (16).
In |
such a way, |
molten metal always contains some amount |
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of oxigen in sfeape of either oxides or a solution in the |
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metal. |
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Besides |
the |
surface |
of |
a casting, having a-high temperature, |
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is oxidized |
^tensively |
especially from the beginning* |
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2 Fe |
+ |
02 |
* 2 FeO |
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2 Mn |
+ 02 |
* 2 Mn0 |
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Following the diagram of condition shown in fig*2tL3« FeO is melted at the temperature 1380°C,
They may |
be manganeseore, VgOg , and others. But it |
should |
be noted |
that additions containing oxigdn decrease |
the |
vitality bf moulding 6ahd& fend tsfeh be Ufesd em&j* in ^xtrfto^d* inary cases. The blowing of air through the mould during period of cooling a easting is the more rational way.
The appearance of chemical burfiing-on is influenced by the presence of oxides of alkaline and alkaline-earth
metals forming with ferrous oxide (FeO) silicates of a low melting point in sand mixtures. These silicates can penetrate into pores between sand grains and form a burn ing-on cake. Therefore, in order to decrease the chemical burning-on formation it is necessary to use moulding sands with minimum content of -hurmful admixtures like oxides of alkaline and alkaline-earth metals.
Sometimes, when used moulding materials are of an
unsatisfactory quality* s p e cia lly i f their refractoriness
is low, they can interact between themself and fuse Under
the action of a high temperature on the boundary of metal
with the mould. As a result of melting and slagging the mould materials, thermal burningaon is formed pn surfaces
of castings.
From this point of view in order to eliminate the thermal burning-on it is necessary to use moulding mater ials of the higher refractoriness* But in practice it
is not true for all cases. ©»F. Berg (14), estimating the influence of the refrac^Cness on burning-on,found that
the largest burning-on is discovered when moulding mixt ures of an average refractoriness are used* Th;.- burxin?-
The burning-on оan decrease at
the use of mixtures of both the highest and lowest refract
oriness* For example, the burning-on decreases when steel (the high pouring temperature) is poured intosand moulds
made of mixtures with liquid glass |
as a binder, |
while the |
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burning-on is to |
high when cast iron |
(the lower pouring |
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temperature) is poured into the same moulds* |
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Burning-on increased with the increase of the felds |
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pars content in |
mixtures |
up to 205$, |
The further |
increase |
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o f their content |
caused |
the |
decrease |
of burning-on (19)* |
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For the estimation |
of |
moulding material refractoriness |
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the special significance is given to the rate of the app earance of those properties which characterise ‘'refractorin(
ess", i*e* softening, melting, and caking.
According to this P*F. Berg (14) does a raw of consequences!
1* The chemical compoieiton of moulding mixtures cannot
characterise the refractoriness, as a different distri bution of elementsbetwoen grains nay be seen at the
invariable |
chemical |
composition* |
I-ean—while processes |
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of melting, softening, and caking |
begin |
on the grain |
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surface* |
Therefore, |
the |
processes |
w ill |
be developed |
the faster, the finer |
are |
grains and the more uniformly |
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they are mixed between themself* That is why the ref ractoriness of a silicate is always lower than this
one of a mechanical mixture of different grains-
with the same chemical composition.
2.The refractoriness of separate components of a mixture
can have the much less significance. For example, a mixture may be less refractory, when a big amount of
Q h i g h m o$<?ry clay, is added into it , than at the additio$^h lower refractory one but in a less amount.
3» The refractorina%and materials is more reasonable determined by the way of comparision with t he refract
oriness of standard piroscopes. They are thrihedral pyramids of 30 mm high and with sides of top and bottom bases correspondingly 2 and 8 mm.
They are put into a furnace together with such pyramids made of the testing material and are heated up to the temperature 1500°C with the rate 10-15 degrees per min, but after 1500°C the rate is 5 degrees per min.
Nevertheless, thermo-physical properties of moudfing materials are of a greate importance. A special attention
at this is given to their heat conductivity and heajr accumul ating ability. Anti-burning-on chromium-magnesite and ohromium stone mixtures are widely spreaded in practice? These mixtures possess of a higher heat activity and, besides, form a solid skin of steel on the surface of a casting creating a high hydraulic resistance of the face layer as they are used in a fine crushed condition.