Question

Sticking friction is more prominent at the centre of the forging. Explain why and what problem can be arises because of that.

Answer 1

Ans.In forging metals or alloys are plastically deformed to shapes by compressive force with pair of dies.

This improves strength and toughness. Mechanical properties are also improved. In this unidirectional fibre distorts and closely control grain flow in a specific direction is achieved.

In closed die forging excess metal added to stock to ensure complete cavity filling. It acts as cushion against impact blows from finishing impression helps in filling of ribs and bosses of dies. This is called flash.

Amount of flash depends on forging size(10-15%).

Thin flash cools rapidly and because of its frictional resistance it creates the material in the cavity to high pressure, thereby increasing the filling of die cavity.

As sticking friction is more at the center of the forging this will result in accumulation of more filling of cavity.

To prevent this we provide Gutters which provides space for excess material and ensure complete closure of dies & defect free forged parts.

If this was not provided there will be chances of forging defects such as

1.scale pits,

2.forging laps

3. Hot tears and thermal cracking.

Sometimes it is necessary to provide lubrication to avoid friction resistance and to obtain defect free forged parts.

In hot forging graphite, molten glass is used as lubricant.

In cold forging mineral oils and soaps are used as lubricant.