Heat Treatments

Question 1

What is Process Annealing

Answer 1

Heating of steelwith less than 0.3% carbon to above red hot to relieve stress from distorted grains caused by cold working

• Ferrite grains reform as unstressed while pearlite remains in deformed (elongated) state

Question 2

What is Full Annealing

Answer 2

Heating of 0.8% or less carbon into the austenite region (FCC), Steel is then cooled very slowly in a furnace

Results in large equiaxed grains that are unstressed which means metal is softer and more ductile

Allows machining to occur easier

Question 3

What is Normalising

Answer 3

Heating steel up into austenite region (FCC) but then its cooled in still air

Takes less time than annealing and produces finer grained structure (pearlite layers closer together) and thus a stronger steel

Question 4

What is Quenching

Answer 4

BCC Steel is heated into FCC structure (austenite) and quenched (cooled very rapidly) thus forming a BCT martensite structure

This occurs because quenching doesn’t allow the carbon to escape from the FCC structure thus forming BCT martensite

Results in very hard but brittle metal

Question 5

What is Tempering

Answer 5

Process of decreasing hardness and increasing toughness to reduce brittleness of hardened metal

Steel heated between 200 - 600 degrees and cooled slowly in still air. This allows some of the BCT martensite to turn back into BCC ferrite thus increasing toughness and decreasing hardness

Higher temp = more tough and less hard

Question 6

What is Case Hardening

Answer 6

1) Soaking metal at high temp in carbon rich (used when metal contains too little carbon to heat treat) or nitrogen rich environment
2) Flame hardening (Application of flame to a local area of the steel) or Induction hardening (Application of an electric current to a local area)

Results in a hardened, wear resistant surface while maintaining soft and tough interior

Often used for gears

Question 7

What is Precipitation hardening

Answer 7

1) Heat alloy until structure is homogenous (completely dissolving second phase)
2) Quench alloy to retrain structure
3) Natural Ageing, over time second phase (alloying metal) will precipitate out introducing foreign atoms into the crystal lattice thus hindering the movement of dislocations and making the metal harder
4) Heating to slightly elevated temperature will accelerate hardening process - Artificial ageing
- Over aging can cause material to become too brittle as bonds in the lattice break.

Question 8

What is Vulcanization

Answer 8

Process of heating rubber with Sulphur to enhance properties of polymer. Forms cross-links between long rubber molecules thus achieves improved elasticity, resilience, tensile strength, hardness and wear resistance