Heat treatments

Question 1

What are some uses of heat treatments?

Answer 1

• increase strength, hardness & wear resistance
• increase ductility and softness
• increase toughness
• obtain fine grain size
• to remove internal stresses induced by differential deformation e.g. high temps

Question 2

What is annealing?

Answer 2

A heat treatment process in which material is taken to an elevated temp, kept there for some time and then left to cool, usually in an off furnace.

Question 3

What are the purposes of annealing?

Answer 3

• relieves internal or residual stresses
• increase softness, ductility, toughness and machinability

Question 4

What are the stages of annealing?

Answer 4

1. material is heated to required temp
2. soak the treated material at the required temp by leaving it soaking for half-hour for every 25mm thickness
3. material is placed inside at an off furnace to cool at around 50-100 degrees per hour

Question 5

What is normalising?

Answer 5

A material is taken to an elevated temp, kept there for some time and then left to cool AT A FASTER RATE than annealing by placing it in the air to cool

Question 6

What are the purposes of normalising?

Answer 6

• controls much more precisely a defined grain size to produce finer mixtures and phases

Question 7

What are the stages of normalising?

Answer 7

1. material is heated to required temp
2. it is then soaked for 10-20 mins for every 10mm of thickness
3. the treatment is then completely cooled by cooling in air

Question 8

What are the stages of hardening and tempering?

Answer 8

1. a steel is heated to required temp and held there
2. the steel is then quenched in water or oil
3. this is followed by tempering (a soak at a lower temperature) which develops the final mechanical properties and relieves stresses.

Question 9

What are the purposes of hardening and tempering?

Answer 9

• inducing high hardness and wear resistance
• it is done to all heavy- duty carbon steel machine parts

Question 10

What is case hardening?

Answer 10

A process used with mild steel to give it a hardened skin.

Question 11

What are the stages of case hardening?

Answer 11

1. the metal is heated to the required temp until it is a cherry red colour
2. it is then dipped in carbon powder
3. this is repeated 2-3 more times before quenching the metal in water to harden the skin

Question 12

What are the purposes of case hardening?

Answer 12

• hardens the surface
• improves resistance to surface indentation, fatigue and wear
• can be used to make the gear teeth, cams, bearings etc