CARBON STEEL Microstructure

1
CARBON STEELMicrostructure Mechanical
properties

• EXPERIMENT 10
• Instructor M.Yaqub

2
TYPE OF IRON

• a-Iron (Ferrite) Stable up to 912 oC.
• BCC in Structure. Soft in pure state.
• Can disolve up to .02 Carbon.
• g-Iron (Austenite) Stable between 912 oC and 1394
  oC.
• FCC in Structure
• Cementite (Iron Carbide) Fe3C Hard and Brittle
• Complex Structure.

3
TYPE OF STEEL

• Hypo-Eutectoid Steel of Carbon lt 0.7
• Eutectoid Steel of Carbon 0.7
• Hyper-Eutectoid Steel of Carbon gt 0.7

4
LOWER CRITICAL TEMPERATURE

• When steel heated, microstructure changes (new
  grains forms). The temperature where this growth
  starts is called Lower Critical Temperature, fix
  for all of C, 727oC

5
UPPER CRITICAL TEMPERATURE

• The temperature where new grains formation
  completes, (All old grains replaced by new
  grains) is called Upper critical temperature.
• This temperature depends upon of Carbon in
  steel.
• Can be taken from Iron-Carbon Diagram .

6
IRON-CARBON DIAGRAM
7
HEAT TREATMENT PROCESSES
8
ANNLEALING

• Heating the steel above upper critical
  temperature and then cool in furnace (very slow
  cooling, cooling rate 10 oC / hour).
• Annealing reduces the hardness and improve
  ductility.
• Structure after annealing is coarse pearlite.

9
NORMALIZING

• Heating the steel above upper critical
  temperature and then cool in air (cooling rate
  100 oC / hour).
• Structure after normalizing is fine pearlite.
• Hardness more than the Anealed steel.

10
QUENCHING

• Heating the steel above upper critical
  temperature and then cool in water or in oil
  (very fast cooling).
• After quenching, steel is very hard and brittle
  and practically no use.
• Structure after quenching is fine martensite
  which is complex, hard and brittle structure. .

11
TEMERING

• Reheat the quenched steel up to intermediate
  temperature (below lower critical temperature)
  and then cool.
• The structure is called tempered martensite.
• After tempering, steel become tough and looses
  some hardness. It become use able now.

12
SUMMARY
13
PEARLITE

• A homogenous mixture of Cementite Fe3C and a-Iron
  in solid state.
• For Hypo Eutectoid steel, final structure is
  Ferrite (a-Iron) and pearlite.

14
PEARLITE (Contd.)

• For Hyper Eutectoid steel, final structure is
  cementite and pearlite.

15
PEARLITE (Contd.)

• For Eutectoid steel, final structure is only
  pearlite.