Transcript
UNIVERSITI TUN HUSSEIN ONN MALAYSIA
Beg Berkunci 10186400 Parit Raja, Batu Pahat
Johor Darul Takzim
THE FACULTY OF MECHANICAL & MANUFACTURING ENGINEERING
BDD 4063CASTING PROCESS
PROBLEM BASED LEARNING
NAME MATRIC NO.
NOR HASBULLAH BIN IBRAHIM CD090049
TUAN MOHD HAFEEZ BIN TUAN IBRAHIM CD090047
PUAN HUEY KIM AD080344
ELIZACORINA SIGURU AD080301
AZZA BIN AB RAHAMAN CD090082
MUHAMAD FUAD BIN ABDUL HALIM CD090085
MUHAMAD RIDZUAN BIN MD DIN CD090359
LECTURER : DR. ROSLI BIN AHMAD
SECTION : 6
Title Of Problem: The Cast-Oil Field Fitting
A common problem in casting process is concerning to defect failure. The initial
investigation found that the defect occurrence depends on type of material, casting process
used, geometry of the die, fluid flow and heat transfer of the molten metal. Apart from the
hazardous situation when dealing with hot temperature of molten metal as show in Figure
1, some defects such as gas bubbles, penetration and enlargement also occur in the cast
component.
A cast iron, T-type fitting is being produced for the oil drilling industry, using a no bakes
sand for the both mold and the core. Silica sand has been used in combination with a
binder. Figure 2 shows a cross section of the mold with the core in place (part a), and a
cross section of the finished casting (part b). The final casting contains several significant
defects. Gas bubbles are observed in the bottom section of the horizontal tee. A penetration
defect is observed near the bottom of the inside diameter, and there is an enlargement of the
casting at location C. Your company has been handed a special task to investigate the
typical defects failure. As a project leader of engineer, you required to write a technical
report on casting defects occurred and suggestions on how to obtain a casting product free
of defect.
Problem to be solved.
1. What is the most likely source of the gas bubbles and why are they present only at
the location noted?
2. What factors may have caused the penetration defect and why is the defect near the
bottom of the casting, but not near the top?
3. What factors led to the enlargement of the casting at Point C and what would you
recommend to correct this problem?
4. Could these molds and cores be reclaimed (recycled) after breakout?
Bubbles
Solution.
1.
Gas porosity is the formation of bubbles within the
casting after it has cooled. This occurs because most liquid materials can hold a
large amount of dissolved gas, but the solid form of the same material cannot, so the
gas forms bubbles within the material as it cools. Gas porosity may present itself on
the surface of the casting as porosity or the pore may be trapped inside the metal,
which reduces strength in that vicinity. Nitrogen, oxygen and hydrogen are the most
encountered gases in cases of gas porosity.
For this case, the binder for the no-bake sand is a polymerizable alkyd-oil/urethane
material. Gases can be evolved from the binder when it is heated and the polymer
material begins to depolymerize. In fact, there are two possibilities for gas problems
with this material. If the binder had been completely polymerized during the
manufacture of the core, the high temperature of the cast iron could break down the
binder into small fragments having low molecular weight and low boiling point,
thus producing the bubbles. In addition, this particular type of binder has a long
curing time --12 to 24 hours are required for the polymerization to complete at
room temperature. If the core or the mold were not completely cured, there would
already be low molecular weight, low boiling point, constituents present that could
form gases as soon as the liquid iron entered the mold cavity.
The gases are located near a surface, just beneath the core. It appears that the gas
bubbles formed, started to float, and were trapped by the core. Vents could be added
to the core and/or mold to give the gases an easier path to escape through the sand,
rather than becoming trapped in the liquid metal. In addition, we want to make sure
Penetration
that the binder is completely cured prior to pouring. Coarser grained sand with a
narrow distribution of sand grain sizes will provide higher permeability and permit
easier gas removal. Finally, a switch to a different type of binder could reduce the
amount of gas produced from that of the oil/urethane.
In addition, the gas bubble is most likely formed during pouring and filling due to
the interaction of molten metal and the surrounding air. Within Liquid metals have
a significant amount of dissolved gasses. When these metals solidify they
sometimes cannot accommodate the gases, and gas bubbles are formed. The gas
bubbles present in the figure is due to these processes and also due to the location;
the flow at the location is most likely a turbulent flow. The solution to this problem
is solved by using a melting process under vacuum conditions, and under protective
flux that excludes contact with the air. Controlling the flow of molten metal to
minimize turbulence, and gas flushing; the passing of reactive gases through the
metal.
2.
When molten metal enters the gaps between the sand grains, the result would be a
rough casting surface. This is due to either use of coarse sand grains in mould
material or no use of mould wash. The fluidity also has to do with penetration. High
fluidity causes erosion of the gating system and the metal to fill the mold cavity,
also any small voids between the particles of a (sand) mold. This causes the surface
to have small particles of embedded sand or foreign particles.
This can also be caused by higher pouring temperature. Because of high
temperatures metal-mold reactions are accelerated; this metal mold reaction causes
Penetration near bottom
defects, due to the changed chemical compositions of the metal-mold surfaces. The
defects are on the inside of the casting because that’s where the metal flow occurs
and also where the highest temperature is present. Choosing appropriate grain sizes,
together with proper mould wash should be able to eliminate this defect.
For this case, Penetration occurred by
liquid metal flowing between the sand grains of the core. It appears that the core
was not properly compacted, with relatively large voids between the sand grains.
The core may have also had very large sand grains with a very narrow distribution
of sizes (although this is contrary to the conclusion of question 1. The core also gets
hotter than the mold, since the core is completely surrounded by liquid metal. In
addition, the region showing the penetration is adjacent to the gate where it will
have received the molten metal first and would have been hotter longer than the
remainder of the mold. The long exposure to high heat may have led to the
breakdown of the binder and helped the liquid metal penetrate the sand. Finally, the
defect was only noted near the bottom of the casting because of the higher
metallostatic pressure head (the pressure of the column of molten metal) helping to
force the metal between the sand grains.
3. An enlargement or bulging of the casting surface resulting from liquid metal
pressure is also known as swell defect. It occurs due to poor ramming of the mould
or not properly reinforcing deep moulds.
Enlargement Casting Defect
The enlargement could have occurred because the mold was weak
and the high metallostatic pressure crushed the sand, thus
enlarging the mold cavity. Better compaction during mold making
would produce denser, and stronger, sand. Using a larger amount
of binder might also help, but gas problems would tend to become
more severe. Another possible cause would be erosion, because
the enlargement occurred next to the gate where all of the liquid
metal entered the mold cavity. The sand near the gate becomes
the hottest, and the binder may have decomposed prematurely.
Swells can be avoided by proper ramming of the sand and providing adequate
support to the mould and also by using of several gates, rather than just
one.
4. Both the molds and the cores could be reclaimed. The binders are
organic, and, with luck, most of the organic material will have
broken down during the casting and cooling process. If the organic
breakdown is not sufficient, some form of reclamation process can
be used. A mechanical reclamation system would perhaps fire the
sand grains at a hard metal plate, where the impact would break
the brittle polymer binder of f of the sand grain surface. A thermal
reclamation system, in which the sand is heated to a high
temperature (usually above 1000°F), will burn off any residual
binder. The processed sand is then carefully screened to assure
the proper size and distribution of sizes prior to rebounding and
reuse.
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