Mercast process

In this process, Frozen Mercury is used for producing precision castings, So this process is called as Mercast process (or) Frozen mercury Moulding.

CUPOLA FURNACE

          For casting, metal has to be heated above its melting point. The heating is done in a furnace. Depending upon the fuel used, the furnace may be classified as Electric, Oil fired (or) Coal fired etc., Where metal free from any impurities is required, electric furnaces are used. In oil fired and coal fired furnaces, the flame plays upon the hot metal and the molten metal picks up impurities by coming in contact with flames. Electric furnaces are costly and equally costly to operate. Usually, for non-ferrous metals and alloys, oil fired crucible furnaces are used. The metal is placed in large graphite crucibles and heated on the outside surface of crucibles, so that flames do not come in actual contact with metal.

CONSTRUCTION:

          For melting cast iron, a cupola furnace is used. It is one of the most economical and convenient ways of providing a supply of molten cast iron. Cupola uses coke as fuel. Coke is produced by heating ordinary steam coal in an inert atmosphere. It gives more intense heat than coal.

          Cupola consists of a long cylindrical steel shell with its interior lined with refractory fire-bricks. It is erected vertically up and rests on short pillars about 0.85 meters above the ground level. The bottom of the cupola is provided with steel doors which are also lined with fire resistant material and covered with a layer of good quality sand.

          At a suitable height, near the top of this steel shell, an opening is cut, which is used for charging fuel and raw materials into the furnace. A wind box, connected to a motorized blower is provided at a height of one meter or so above the bottom closing doors. From this wind box, small air passages are provided into the cupola shell for supply of air to aid combustion of fuel. These air passages are called Tuyeres. At the bottom, above the door, a tapping spout is provided to tap molten metal and about 350 mm above the tapping hole another hole called the Slag Hole is provided at the back of the cupola, through which liquid slag can be forced out under air pressure.

          The cylindrical space provided between the tap hole and slag hole is called the molten metal well, in which the molten metal accumulates between two-taps. All these features are depicted in below Fig.,

OPERATION OF CUPOLA

          In the cupola the first operation is to repair the lining of the door and area around it and then closing the hinged doors. Doors are jammed shut, so that there is no possibility of their opening while the cupola is in operation. Then fire is lighted at the bottom with the help of some wood and kerosene oil rags. When the fire is burning strongly, coke is added from the top charging door until the height of coke bed reaches about half a meter above Tuyeres. After those tuyeres are opened partially, air blower is switched on and alternate layers of metal, flux (in the form of limestone pieces) and coke are charged from the top. These alternate layers settle down on coke bed. When the cupola is filled up to the level of charging door, the tuyeres are opened fully and the heating of charge begins. The coke near the tuyeres start burning and the coke bed becomes intensely hot. The metal in lower layers near the coke bed starts melting. Lime stone breaks down to CaO and CO2. The calcium oxide reacts with impurities like silica and other oxides forming slag (CaSiO3). Slag is lighter and floats upon the molten metal layer. Ultimately. when enough metal has melted, the slag is blown out by opening the slag hole. The metal is then tapped by puncturing the tap hole with a long steel rod with one end shaped like a cone. The molten metal will start flowing into the metal chute and is collected into ladles (refractory lined steel buckets to which long handles are welded) and taken away for pouring into mould's. The tap hole id then closed by plugging it with a lump of fire clay.

          The properties of cast iron improves with addition of small amounts of Ferro-Manganese and Ferro-Silicon. Since most of the manganese and silicon already present in scrap cast iron, pig iron and a little bit of thin steel scrap, which forms the metallic charge dropped into cupola, is oxidized and lost, addition of Ferro Manganese and Ferro Silicon has to be done to the molten metal in the ladles before pouring.

          After the day's work is over, extra coke is charged into the cupola along with last charge. After all the metal has melted, the air blower is switched off and the bottom door of cupola opened. What-ever unburnt coke etc., is left, is allowed to fall to the ground beneath the cupola door. This is necessary otherwise the left over coke, slag and metal etc., may join up in one mass, then its removal will become extremely difficult. The size of a cupola is denoted by its internal diameter.     

TYPES OF CASTING DEFECTS

          Casting defects are the unwanted irregularities that appear in the casting during metal casting process. Casting process involves a number of variables and a loss of control in any of these variables can cause defects under certain circumstances. Some of the common defects, their features and remedies to prevent such defects are described below:

1. Porosity (Blow-holes and pin holes):

          They appear as small holes in the casting. They may be open to surface (or) they may be below the surface of the casting. They are caused due to entrapped bubbles of gases. Molten metal absorb gases from various sources such as fluxes, moisture in sand , additives and normal atmosphere gasses and nitrogen. They may be causes by excessively hard ramming, improper venting, excessive moisture (or) lack of permeability in the sand.

Remedies:

1) Avoid excess ramming of mould.

2) Provide proper vent.

3) Avoid use of excess carbonaceous (or) other organic material in the sand/core binders, because these materials react with the molten metal producing large amount of gasses.  

2. Shrinkage cavity:

Some times due to faulty design of casting consisting of very thick and thin sections, a shrinkage cavity may be caused at the junction of such sections. Shrinkage cavity is totally internal. It is illustrated in Fig., It is caused due to shrinkage of molten metal. It is sometimes called as 'Pipe'.

Remedies: 

1) Use large sprue and riser to promote directional solidification.

2) Use either a chill (or) relocation of risers. (Chill is an object used to promote solidification in a specific portion of a metal casting).

3) Gates to be cut as as wide as possible.

3. Misrun:

This denotes incomplete filling of mould cavity. It may be caused by bleeding of molten metal at the parting of cope and drag, inadequate metal supply (or) improper design of gating.

Remedies:

1) Fluidity of the metal should be high.

2) Pouring rate and time should be controlled.

3) Thin sections should be suitable designed.

4) Pouring temperature should be high.

4. Cold shut:

A cold shut is formed within a casting, when molten metal from two different streams meets without complete fusion. Low pouring temperatures may be the primary cause of this defect.

Remedies:

1) Place gates and risers at proper locations.

2) Metal fluidity should be high.

3) Proper pouring temperature of metal should be maintained.

 5. Mismatch:

This defect takes place when the mould impression in the cope and drag do not sit exactly on one another but are shifted a little bit. This happens due to mismatch of the split pattern (dowel pin may have become loose) (or) due to defective clamping of cope and drag boxes.

This defect is of two types (1) Core shift, (2) Mould shift.

Remedies:

1) Proper alignment of cope and drag box.

2) Proper handling of assembled cope and drag boxes during operations.

3) Proper dowel pins are made in split pattern ( they not to be loose).

6. Drop:

This happens when a portion of the mould sand falls into the molten metal. Loose sand inadequately rammed (or) lack of binder may cause this defect.

7. Scab:

This defect occur when a portion of the face of a mould lifts (or) breaks down and the recess is filled up by molten metal.

8. Hot tear:

These cracks are caused in thin long sections of the casting, if the part of the casting cannot shrink freely on colling due to intervening sand being too tightly packed, offers resistance to such shrinking. The tear (or) crack usually takes place when the part is red hot and has not developed full strength, hence the defect is called "Hot Tear". Reason may be excessively tight ramming of sand.

Remedies:

1) Provide adequate fillets at sharp corners.

2) Proper metallurgical and pouring temperature.

3) Place gates and risers at proper locations.

4) Proper mold design to be maintained.

9.Penetration:

When fluidity of liquid metal is high, it may be penetrated into the sand mould/core (into the voids between the sand particles) causing a fused aggregate of metal and sand on the surface of the casting leading to defect.

Remedies:

1) Sand should be properly rammed.

2) Mould sand/core sand should not be too coarse to promote metal penetration.

3) Control proper metal temperature.

10.Sand wash:

Surface dip that results from erosion of the sand mold during pouring. This counter is formed in the surface of the final cast part.

11) Other defects include scars, blisters, sponginess (due to a mass of pin holes at one location) and slag inclusions etc.,