JNTU B.Tech III FABRICATION PROCESSES Previous Question Paper (January/February - 2021 - R16)

The selection of a furnace for melting metals is a critical decision in foundry operations, governed by several technical, economic, and operational factors:

1. Type of Metal and Alloy: The melting point, chemical reactivity, and susceptibility to gas absorption of the metal are primary considerations. For instance, high-melting-point alloys like steel require electric arc or induction furnaces, whereas low-melting-point non-ferrous metals like aluminum or zinc can be handled in crucible or reverberatory furnaces.
2. Production Volume and Rate: The required output determines the furnace capacity and melting rate. Continuous production lines often favor cupolas or large induction furnaces, while small-batch or job-shop production may utilize crucible or pit furnaces for flexibility.
3. Quality Requirements: Metallurgical purity is paramount. Electric induction furnaces provide excellent control over composition and temperature, minimizing oxidation and gas pickup, which is essential for high-quality castings. In contrast, fuel-fired furnaces may introduce impurities from combustion gases.
4. Fuel Availability and Cost: The local availability and cost of energy sources (coke, oil, gas, or electricity) significantly impact the choice. Regions with inexpensive electricity favor induction furnaces, while areas with abundant coal or coke may still utilize cupolas for cast iron production.
5. Flexibility and Versatility: If a foundry needs to produce a variety of alloys, a furnace that allows for easy cleaning, rapid alloy changes, and precise temperature control is preferred. Crucible furnaces offer high flexibility for small-scale alloy changes compared to large, fixed-hearth furnaces.
6. Capital and Operating Costs: The initial investment for high-tech equipment like vacuum induction furnaces is substantial compared to simple crucible or oil-fired furnaces. Operating costs, including maintenance, refractory lining life, and labor, must be balanced against the value of the final product.
7. Environmental Regulations: Modern foundries must comply with strict emission standards. Electric furnaces are generally cleaner and easier to equip with pollution control systems than coal or oil-fired furnaces, which may require complex scrubbing systems to manage smoke and particulate emissions.
8. Space and Infrastructure: The physical footprint of the furnace and the necessary auxiliary equipment (such as transformers, cooling systems, or charging mechanisms) must fit within the existing foundry layout and support infrastructure.

Turbulence in the gating system and mould cavity occurs when the metal flow velocity is too high or the design of the gating system causes sudden changes in direction or cross-section. This leads to several undesirable consequences that compromise the integrity and quality of the casting:

• Oxide Inclusion Formation: Turbulent flow increases the surface area of the molten metal exposed to air, promoting the formation of metal oxides. These oxides become entrapped within the solidifying metal, acting as stress concentrators and creating weak points that reduce the mechanical strength of the casting.
• Gas Entrapment: High-velocity turbulent flow causes the metal to splash and fold over itself, trapping air or mould gases within the molten stream. These trapped gases manifest as porosity or blowholes in the final casting, which are detrimental to both structural integrity and surface finish.
• Mould Erosion: Excessive turbulence and high velocity cause the molten metal to impinge forcefully against the sand mould walls. This leads to the erosion of sand particles, which are then carried into the casting as inclusions or cause dimensional inaccuracies by altering the mould cavity shape.
• Increased Dross Formation: The agitation of the melt increases the rate of chemical reactions between the molten metal and the atmosphere or mould material, leading to significant dross formation. This dross can be carried into the mould cavity, resulting in surface defects and internal contamination.
• Premature Solidification: Turbulence can disrupt the thermal gradients within the mould. In some cases, the increased exposure to the mould walls or the air can cause localized cooling, leading to premature solidification or 'cold shuts' where two streams of metal fail to fuse properly.

To mitigate these effects, gating systems must be designed to promote laminar flow by using tapered sprues, avoiding sharp corners, and ensuring the total cross-sectional area of the gating system is optimized to control the velocity of the molten metal.