3218
.pdfIt was established (20) that for large thick walls cafetinge mixtures of the higher heat activity are needed than for thin walls castings. It is explained by the considerable influence of a liquid metal convection on the temperature of a casting surface.
Mixtures must also possess of a high refractoriness and chemical inertiiese to steel oxides. In an Opposite case dimensions of mixture porqgican be increased as a result of the caking of mixture grains or an Erosion by liquid oxides and the liquid metal can penetrate itno large voides.
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In order to decrease a possibolity of the chemical |
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and |
thermal burning-on formation it is necessary first of |
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a ll |
to use |
moulding sands with minimum content of hurn^u1. |
admixtures |
as oxides |
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suafiMST* alkaline and alkaline-earth metals. |
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Aiso it is reasonable to use useful properties of chrom ium-magnesite and chromium iron-stone mixtures possessing
the |
high heat conductivity |
and heat |
accumulating ability |
to |
form quickly a solid skin of metal |
on the casting surfcc |
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The content of a gas medium being formed around the |
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easting during the pouring |
and solidification influences од |
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burning-on formation too. |
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Therefore the creating of |
the proper |
gas medium is one of |
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the methods of a chemical burning-on prevention. For example, the oreating of a reducing atmosphere promotes the elimination of burning-on at the casting of iron and
Anti-burning-on paints on the base of chromium-magne-
si-te and chromium iron-stone are lised for the coating of
moulds and cores for large steel castings* At this chromium
iron-stone and chromium magnesite are slagged and., prevent
the oxidation of costing surfaces. Before the use, these materials must be milled in powder and seaved through the
seave with cidls lxlmm.
Chromium iron stone must contain not loss than 36% chromium oxide (CrgO^)* Compositions of these paints are shown in
table |
2*10* |
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_Table 2.10 |
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Composition |
it) of paints for moulds and cores |
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for large |
steel'castings* in‘ Wt.% |
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NO, |
grain |
components |
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binders |
A) |
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Chromiumchromium |
liquidл\ |
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molasses'ЗУ .dextrin ; |
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magnesite |
iron-stone |
glaslr ' |
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powder |
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1 |
92 |
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- |
8 |
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m m |
Ш Л |
2 |
92 |
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- |
- |
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8 |
0.2 |
3 |
- |
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90 |
_ _• |
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- - |
0.2 |
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10 |
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Notes |
1. |
Water is added until |
the |
need viscosity |
is |
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obtained. |
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2» Modulus a 2.5 - |
2.9 |
, density = 1.48 -1.5 g/c^ |
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3# |
Density e 1.3 |
g/on? |
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4. |
Above 100%. |
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When moulds and cores are made of liquid glass mi: special paints are needed for coating. Composition of
paints, find pastes for ooating such moulds and cores are
shewn in Table 2.11.
Table 2.11
Conporition of points and pastes for moulds made
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of |
liquid glass mixtures |
— |
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grain components |
>M, «-■ |
шГМЫшТшт.... |
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No black |
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Binders |
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h,lvery |
aLogon |
mareha* bento sulphite |water |
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sirean |
lite |
nite |
liquor |
lution |
pa<graphite graphite pov.cler |
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St- |
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15."o1\ |
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molass |
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1 |
43.5 |
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3.5 |
10.0 |
.Л |
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2 |
43.5 |
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- |
- |
- |
3.5 ■ 10.0 |
<* |
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3 |
rr |
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90 |
- |
2.0 |
8.0 |
A*.09.0 |
4 |
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m* |
- |
60 |
1.0 |
- |
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5"J |
27.0 |
1 |
13?0 |
- |
2.5“ |
7 .5 |
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l__ |
—_ . |
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Notes |
1. |
Pastes No-.l and 2 are |
used for |
painth |
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moulds of ir >n castings but pastes N0.3,* |
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and 5-for steel castings. |
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2. Compositions of paints of p - N o . 1,21
3 are given in weight %9 but composition
paints N6,4ani5in volume%.
3.Pastes No.land? are diluted to the densii 1.23-1.3 g/cF^» but No.3,4" and 5-to the
density 1.58-1.6 g/cm^.
*t is very seldom that the proper kind of burnini exists"independently. More often mechanical and chemici
o f |
chemical |
and thermal burning-on arise on |
surfaces o‘ |
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the casting |
simultaneously. In |
this case |
it |
is necessai |
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to |
use measures preventing the |
formation |
of |
both cheroi( |
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and mechanical or both chemical and thermal burning-on The best way of the burning-on prevention is the appl*1 tion of mould ana core paints and dfiessings with fine grain structure and high rofractoringss.
W.A
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2,2 |
internal |
casting |
defects |
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The biost spreaded |
internal |
casting |
defects ore the fo ll |
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owing : |
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hot and cold |
cracks |
and tears |
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cavities |
of |
different origin |
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porosity |
and crumbly; |
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oxide, dross and sand |
inclusions |
inside |
castings; |
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exfoliation into some |
layers. |
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All of these defects can or not be |
found on surfaces |
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of castings, i.e . they can reach the surface of |
a casting |
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and be found visually. But more often they are subsurface defects occuring as a result of an imperfection of a tec hnological process of casting manufacture or some deviat
ions from it . The defect may occur in different stages
of the technological process of producing a casting but
usually they arise |
either when metal or alloy is partly |
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liquid |
or |
in |
stage |
of the metal |
solidification . |
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f |
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Some |
of |
these |
defects do not influence on the indi |
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ces of |
a casting work because of |
their small dimensions. |
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It is possible to guess that some of castings, which are Unloaded during their service, are used with unknown int ernal defects upto the end of their service as not ail castings are inspected before their use. But in any case the presence of defects in Casting deteriorates the str*-
ength and qualitative indices of the casting work.
Hot tears and cracks*
At |
solidification |
and |
cooling the metal a casting |
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con’jrcicts |
and unless it |
can |
contract Relatively |
freely |
in the mould as it cools |
from the temperature of |
solidi |
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fication to the room temperature, hot tears and cracks' may occur. Different reasons my cause this defect* Oa
of |
th@m is |
stresses |
developed in the casting by resist |
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nee |
of |
the |
mould ( |
Pig.2.14 ) . When the casting contra* |
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Cts it presses a core or mould by projecting parts. |
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Simultaneously the mould restricts the contraction |
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nding |
causing |
in |
the |
casting. |
The |
is ductili! |
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andvstres3Qs |
more |
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of |
the |
mould and core |
the |
less are |
stresses |
and the lesfl |
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is the probability of hot crack formation. If the value
of stresses |
exceeds the |
critica l |
tensil3,strength, c.raob |
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or tears |
are |
inevitable* |
It |
is |
possible |
to estimate tli( |
Value of |
stresses needed to |
distort the |
casting. |
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Hot cracks arise in castings at temperatures^not far from solidus one. Stresses in the mould as a resuj cf the casting contraction can be determined in accojJ nee to the formula t
m Shr,
Fig*2*ЗА* Defects due "to a mould*s preventing ~ь^д
casting from contracting freely [6j •
(a) and (b )- stresses developed in the casting by
resistance of the mould| |
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(c) and (ft)— thinner parts |
of |
the easting resist поз>- |
mal contraction, while heavier' sections cool more |
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slowly| |
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(e) and ( f ) - spokes that are |
curved rather straight |
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reduce warping or tearing |
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(g)- uniform section cools without tearing!
(h)~ nonuniform section or joined sections provide potential site of tearing*
Foundry Department |
" Casting defects and |
Dr. V. Bastrakov. |
measures of their prevention" |
Where
-the mixture modulus of elasticity}
£- the deformation of a moulding sand
bet us put the following significations
- a temperature range in which stresses occur 3?т - the area of pressure ( 5’ig,2,15 );
£- the area of a stressed cross section of the casting j
&- coefficient of thermal contraction of a ca sting metal*
The dependence of stresses in the casting from a relative deformation is as follows s
The conditions of equilibrium is expressed by the equartion ;
from where
Putting this into the previous formula we shall obtain
7
or
In such a way, it is cleaij that contraction stre
sses as a result of hindered shrinkage depend on physi cal-mechanical properties of the casting and mould in the given range of temperatures and eh relation -t- ,
For example, in fig , 2,16 the change of mechanical pro perties of cost iron at different temperatures is shown,
It may be seen that under high temperature, especially j the solidification interval, the mechanical properties о cast iron are very low.
It is obvious that the hot crack formation is more pro]), able at this period of cooling the metal. Cracks are foi med under conditions which decrease the strength of the metal in the corresponding interval of high temporaturee as well as under conditions which increase hindering.