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.