Jawaharlal Nehru Technological University Hyderabad B.Tech III MANUFACTURING PROCESS AND MACHINE TOOLS Previous Question Paper (October - 2020 - R16)

Pattern allowances are necessary modifications made to the dimensions of a pattern to ensure the final cast product meets the required specifications after solidification and cooling. The primary allowances are:

• Shrinkage Allowance: Metals shrink upon cooling from pouring temperature to room temperature. Patterns are made larger than the required size to compensate for this liquid and solid contraction.
• Machining Allowance: Castings often require surface finishing. Extra material is added to the pattern surfaces that need machining so that the final dimensions are achieved after removing the excess metal.
• Draft Allowance: A slight taper is provided on all vertical surfaces of the pattern to allow for its easy withdrawal from the sand mold without damaging the mold walls.
• Shake or Rapping Allowance: Before withdrawal, the pattern is shaken to loosen it in the mold, which slightly increases the mold cavity size. To compensate, the pattern is made slightly smaller.
• Distortion Allowance: For castings with complex shapes or thin sections, metal may warp during cooling. The pattern is intentionally distorted in the opposite direction to ensure the final casting retains the desired shape.

The Heat Affected Zone (HAZ) is the portion of the base metal adjacent to the weld bead that has not been melted but has had its microstructure and mechanical properties altered by the heat of the welding process. The temperature reached in this region is high enough to cause metallurgical changes, such as grain growth, phase transformations, or precipitation of brittle phases.

The implications of the HAZ in arc welding include:

• Reduced Mechanical Properties: The HAZ is often the weakest point in a weldment. Changes in grain structure can lead to a decrease in tensile strength, ductility, or impact toughness compared to the parent metal.
• Hardness Variations: Rapid cooling rates can result in the formation of brittle martensite in the HAZ, increasing the risk of cracking.
• Residual Stresses: Non-uniform heating and cooling cycles create significant thermal stresses, which can lead to distortion or stress-corrosion cracking over time.
• Corrosion Susceptibility: Metallurgical changes, such as the sensitization of stainless steels (chromium carbide precipitation), can make the HAZ prone to intergranular corrosion.

Control of the HAZ is typically managed through preheating, controlled cooling rates, and post-weld heat treatment to restore desired material properties.