Lecture 11

Melting Practices

Melting is an equally important parameter for obtaining a quality castings. A number of furnaces can be used for melting the metal, to be used, to make a metal casting. The choice of furnace depends on the type of metal to be melted. Some of the furnaces used in metal casting are as following:.

  • Crucible furnaces
  • Cupola
  • Induction furnace
  • Reverberatory furnace

Crucible Furnace.

Crucible furnaces are small capacity typically used for small melting applications. Crucible furnace is suitable for the batch type foundries where the metal requirement is intermittent. The metal is placed in a crucible which is made of clay and graphite. The energy is applied indirectly to the metal by heating the crucible by coke, oil or gas.The heating of crucible is done by coke, oil or gas. .

Coke-Fired Furnace(Figure 13) .

  • Primarily used for non-ferrous metals
  • Furnace is of a cylindrical shape
  • Also known as pit furnace
  • Preparation involves: first to make a deep bed of coke in the furnace
  • Burn the coke till it attains the state of maximum combustion
  • Insert the crucible in the coke bed
  • Remove the crucible when the melt reaches to desired temperature

Figure 13: Coke Fired Crucible Furnace

Oil-Fired Furnace.

  • Primarily used for non-ferrous metals
  • Furnace is of a cylindrical shape
  • Advantages include: no wastage of fuel
  • Less contamination of the metal
  • Absorption of water vapor is least as the metal melts inside the closed metallic furnace

Cupola

Cupola furnaces are tall, cylindrical furnaces used to melt iron and ferrous alloys in foundry operations. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. A schematic diagram of a cupola is shown in Figure14 . This diagram of a cupola illustrates the furnace's cylindrical shaft lined with refractory and the alternating layers of coke and metal scrap. The molten metal flows out of a spout at the bottom of the cupola. .

Description of Cupola

  • The cupola consists of a vertical cylindrical steel sheet and lined inside with acid refractory bricks. The lining is generally     thicker in the lower     portion of the cupola as the temperature are higher than in upper portion
  • There is a charging door through which coke, pig iron, steel scrap and flux is charged
  • The blast is blown through the tuyeres
  • These tuyeres are arranged in one or more row around the periphery of cupola
  • Hot gases which ascends from the bottom (combustion zone) preheats the iron in the preheating zone
  • Cupolas are provided with a drop bottom door through which debris, consisting of coke, slag etc. can be discharged at      the end of the melt
  • A slag hole is provided to remove the slag from the melt
  • Through the tap hole molten metal is poured into the ladle
  • At the top conical cap called the spark arrest is provided to prevent the spark emerging to outside

Operation of Cupola

The cupola is charged with wood at the bottom. On the top of the wood a bed of coke is built. Alternating layers of metal and ferrous alloys, coke, and limestone are fed into the furnace from the top. The purpose of adding flux is to eliminate the impurities and to protect the metal from oxidation. Air blast is opened for the complete combustion of coke. When sufficient metal has been melted that slag hole is first opened to remove the slag. Tap hole is then opened to collect the metal in the ladle.

.Figure 14: Schematic of a Cupola

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