Question

In manufacturing of self-lubricating bearings by powder metallurgy, an important secondary operation that is carried out after sintering is
1. Cold isostatic pressing
2. Hot isostatic pressing
3. Impregnation
4. Infiltration

Answer 1

Correct Answer - Option 3 : Impregnation

Explanation:

Steps involve in manufacturing self-lubricating bearings by powder metallurgy:

1) Production of copper powder, Tin powder, and Graphite powder

2) Mixing of elemental powder

3) Compaction

4) Sintering

5)Post sintering operations:

• Impregnation
• Sizing

6) Bearing preparation

Sintering: Sintering is a heat treatment applied to a powder compact in order to impart strength and integrity. The temperature used for sintering is below the melting point of the major constituent of the Powder Metallurgy material.

It is done in three stages:

• Burn off zone or preheat zone (400°C - 450°C) 
• Sintering zone (800°C - 850°C)
• Cooling zone

Post sintering operations:

1) Impregnation: It refers to the filling of pores in the part with oil.

The technique used is the combination of vacuum and pressure and it involves the below steps:

• The sintered part is placed in the chamber and subjected to a vacuum
• Oil is introduced and external pressure of 4-5 atm is applied
• Pressure will be brought to 1 atm and excess oil is drained 

2) Sizing: It is done to control the dimensions of the bearings within the tolerances and done by forcing the part into a die whose dimension is smaller than the part.