Heat treatments Flashcards
What are some uses of heat treatments?
- 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
What is annealing?
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.
What are the purposes of annealing?
- relieves internal or residual stresses
- increase softness, ductility, toughness and machinability
What are the stages of annealing?
- material is heated to required temp
- soak the treated material at the required temp by leaving it soaking for half-hour for every 25mm thickness
- material is placed inside at an off furnace to cool at around 50-100 degrees per hour
What is normalising?
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
What are the purposes of normalising?
- controls much more precisely a defined grain size to produce finer mixtures and phases
What are the stages of normalising?
- material is heated to required temp
- it is then soaked for 10-20 mins for every 10mm of thickness
- the treatment is then completely cooled by cooling in air
What are the stages of hardening and tempering?
- a steel is heated to required temp and held there
- the steel is then quenched in water or oil
- this is followed by tempering (a soak at a lower temperature) which develops the final mechanical properties and relieves stresses.
What are the purposes of hardening and tempering?
- inducing high hardness and wear resistance
- it is done to all heavy- duty carbon steel machine parts
What is case hardening?
A process used with mild steel to give it a hardened skin.
What are the stages of case hardening?
- the metal is heated to the required temp until it is a cherry red colour
- it is then dipped in carbon powder
- this is repeated 2-3 more times before quenching the metal in water to harden the skin
What are the purposes of case hardening?
- hardens the surface
- improves resistance to surface indentation, fatigue and wear
- can be used to make the gear teeth, cams, bearings etc