Hot & Cold Working

Q.1. What is hot working ? State its principles, advantages and disadvantages. Ans. Mechanical working of metals above its recrystallisation temperature is known as hot working. Advantages : 1. Larger deformation can be accomplished and more rapidly by hot working since the metal is in plastic state. 2. Porosity of the metal is considerably minimised. 3. Concentrated impurities, if any in the metal are disintegrated and distributed throughout the metal. 4. Grain structure of the metal is refined and physical properties improved. Disadvantages : 1. Due to high temperature a rapid oxidation or scale formation takes place on the metal surface, leading to poor surface finish and loss of metal. 2. On account of the lost of carbon from the surface of the steel piece being worked the surface layer loses its strength, which is a disadvantage when the part is put to service. 3. This weakening of the surface layer may give rise to a latigue crack which may ultimately result in fatigue failure of the part. 4. Close tolerances cannot be maintained. 5. It involves excessive expenditure on account of high cost of tooling. This, however, is compensated by the high production rate and better quality of products.