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Proper procedure
in the use of ingot moulds exert a profound influence upon the
life of ingot moulds. Whatever care is taken in making a good
ingot mould casting including control of chemical composition
and metallographic structure, can be easily nullified if
adequate care and a proper procedure is not followed in using
the moulds.
Hence, preparation of the mould, bottom plate setting with
refractory brick flow channels, mould setting over the bottom
plate, mould assembly, teeming and ingot mould residence time
are important to ensure adequate ingot mould life and quality
of ingots. These are discussed below:
1.
AGEING OF NEW MOULDS
This refers to manufacture of mould to start of first pouring
of steel into the mould. This time period has a considerable
influence on ingot mould life and may be attributed to
internal residual stresses. Wherever ageing (natural stress
relieving) is not adequate before first pouring of steel, a
stress relieving heat treatment is required by heating for
approximately three hours at degree C.
2.
PREHEATING OF MOULDS
This is done at 100-120 °C to reduce internal stresses.
3.
COOLING OF TEEMED MOULDS
Immediately after
stripping, the mould temperature ranges between 700 °C and 950
°C or even higher depending upon the steel, and ingot mould
residence time. The moulds are to be air cooled even if rate
of cooling is slow. Water cooling should be prohibited. If
required, after stripping, the moulds are placed over special
grates to allow better air circulation.. Too rapid a cooling
increases stress build up, intensifying development of crazing
and even cracking. Plants practising wholly air cooling have
high mould life.
4.
PREPARATION OF MOULD
(a) CLEANING OF
MOULD
The moulds,
especially the internal faces, are required to be thoroughly
cleaned. Appropriate wire brushes may be used.
(b) REMOVAL OF ANY
OVERFLOW METAL / FINS etc.
This is required as otherwise it will cause greater strains on
the mould during next stripping and may cause transverse
cracks.
(c)
DRESSING OF MOULD
For better mould life and ingot quality, the cleaning and
coating of the moulds is essential. The inside surface
condition affects the ingot quality but the outside is most
important from heat extraction point. If the mould is being
used for the first time, the outer surface must be fully
cleaned of the adhered sand which, if not removed, will cause
higher temperature of the mould after teeming.
The object of mould coating is to prevent the metal from
adhering to mould wall and to separate the mould wall and
ingot to prevent crack and the break in ingot skin.
Coating is not to be done for atleast 30 minutes after the
moulds have been cooled and cleaned with water to allow the
water to evaporate out. The coating is normally done with a
mixture of tar and varnish. Tar coating helps in improving the
surface of the steel ingot and prolongs the life of the mould
by retarding the oxidation and decarborisation of the internal
surface of the mould and reduce sticking of the ingot in the
mould.
Proprietary refractory coatings are also available which
greatly improve the mould life and surface quality of the
steel ingot.
The temperature of the mould at the time of coating needs
control. A temperature lower than 90 °C will give excessive
thickness undecomposed volatile matter, while a temperature
above 150 °C will cause complete decomposition and cause
safety hazard.
5. MOULD
ASSEMBLY OVER BOTTOM PLATE
The bottom plate is placed flat on the ground and the hollow
refractory bricks of the channels and pouring basin (trumpet)
are carefully placed in position. The moulds are placed
correctly centered, otherwise the mould wall at the bottom,
nearer the steel entry gate will get eroded fast. While
stripping the ingot from such a mould will eventually cause a
piece of the mould bottom to tear off, making the mould
unusable for further use. Cracks at the mould bottom will take
place in off- centered placement of mould.
During mould setting, the bottom plate and the moulds should
be perfectly matched. Inclined placing, uneven thickness of
bottom plates or wrongly machined bottom face of moulds can
cause trouble. If moulds are not straight, the metal stream
will impinge ion the mould wall and damage the mould.
6. AVOID
USE OF HOT MOULDS
This will hasten crazing and cracking, drastically reducing
mould life. The mould re-use temperature should be 50 °C - 80
°C, which is most desirable.
Extra stock of moulds should be kept to avoid reuse of hot
moulds. Hot and cold moulds both adversely affect the life of
moulds and quality of the ingot as the maximum temperature
developed in a mould is a function of the initial mould
temperature. Higher the initial temperature, greater is the
structural change and increased is the tendency to crazing,
oxidation and growth.
7.
TEEMING PRACTICE
(a) TYPE OF
STEEL
The chemical
composition of steel determines the teeming temperature. With
killed steel, the average life of the mould is reduced,
because of longer holding time before stripping. Rimming
steels, being very low in carbon (0.05-0.08%) are tapped at
higher temperatures.
(b) TEEMING
TEMPERATURE
The steel pouring
temperature during teeming affects mould life. Temperatures
above 1570 °C, affects moulds especially those used for the
first time or during the first few heats. High incidence of
mould stickers are a result of high pouring temperatures.
Teeming temperatures should be as low as possible, in the
process of mould filling.
(c) AVOID
RUNNING STOPPER
For obvious
reasons
(d) AVOID COLD
HEATS
(e) AVOID METAL
OVERFLOWS
The ingot moulds
should be filled upto with atleast 3 inches less than the
mould heights.
8. INGOT
RESIDENCE TIME
Ingot residence time should be controlled to a minimum time,
as this is the most important factor in mould life. Just after
teeming, the inside mould wall acquires almost the temperature
of the molten metal while the outside is hardly about 100 °C.
The outer surface however gradually gets heated to higher
temperatures with increasing ingot residence time. Cast iron
loses its strength very fast with rise of temperature. Any
lateral pressure due to relative movement of ingot and ingot
mould at the time of stripping may lead to bottom breaking of
mould and / or scooping out of the mould metal from the inner
surface. Scooping out of the mould metal also takes place if
the ingot is sticking in any area of the mould, causing hard
release during stripping.
Structural transformation is considerable leading to greater
growth and a greater tendency to failure by crazing. Scooping
out of the mould metal also takes place if the ingot is
sticking in any area of the mould, causing hard release during
stripping.
9.
STRIPPING PRACTICE
Special care has
to be exercised while stripping the ingot from the hot mould.
The strength of the iron mould in a heated condition is very
low and hence cracks any develop in the mould if it is handled
roughly by hammering or hitting. Swinging of moulds against
solid objects for removal of steel ingot should be prohibited
or else moulds will crack. Mechanical strippers are useful
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