CARBON STEELCARBON STEEL

Microstructure Microstructure &&

Mechanical propertiesMechanical properties

EXPERIMENT # 10EXPERIMENT # 10

Instructor: M.YaqubInstructor: M.Yaqub

TYPE OF IRONTYPE OF IRON Iron (Ferrite)Iron (Ferrite) Stable up to 912 Stable up to 912 ooC.C. BCC in Structure. Soft in pure state.BCC in Structure. Soft in pure state. Can disolve up to .02% Carbon. Can disolve up to .02% Carbon.

Iron (Austenite)Iron (Austenite) Stable between 912 Stable between 912 ooCC and and 13941394 ooC.C.

FCC in StructureFCC in Structure

Cementite (Iron Carbide) FeCementite (Iron Carbide) Fe33CC Hard and Brittle Hard and Brittle Complex Structure. Complex Structure.

TYPE OF STEELTYPE OF STEEL

Hypo-Eutectoid Steel:Hypo-Eutectoid Steel: % of Carbon < 0.7% of Carbon < 0.7

Eutectoid Steel:Eutectoid Steel: % of Carbon = 0.7% of Carbon = 0.7

Hyper-Eutectoid Steel:Hyper-Eutectoid Steel: % of Carbon > 0.7% of Carbon > 0.7

LOWER CRITICAL TEMPERATURELOWER CRITICAL TEMPERATURE

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

UPPER CRITICAL TEMPERATUREUPPER CRITICAL TEMPERATURE

The temperature where new grains formation The temperature where new grains formation completes, (All old grains replaced by new completes, (All old grains replaced by new grains) is called Upper critical temperature. grains) is called Upper critical temperature.

This temperature depends upon % of Carbon This temperature depends upon % of Carbon in steel. in steel.

Can be taken from Iron-Carbon Diagram .Can be taken from Iron-Carbon Diagram .

IRON-CARBON DIAGRAMIRON-CARBON DIAGRAM

HEAT TREATMENT HEAT TREATMENT PROCESSESPROCESSES

ANNLEALINGANNLEALING

Heating the steel above upper critical Heating the steel above upper critical temperature and then cool in furnace (very slow temperature and then cool in furnace (very slow cooling, cooling rate 10 cooling, cooling rate 10 ooC / hour). C / hour).

Annealing reduces the hardness and improve Annealing reduces the hardness and improve ductility. ductility.

Structure after annealing is coarse pearlite.Structure after annealing is coarse pearlite.

NORMALIZINGNORMALIZING

Heating the steel above upper critical Heating the steel above upper critical temperature and then cool in air (cooling temperature and then cool in air (cooling rate 100 rate 100 ooC / hour). C / hour).

Structure after normalizing is fine pearlite.Structure after normalizing is fine pearlite. Hardness more than the Anealed steel. Hardness more than the Anealed steel.

QUENCHINGQUENCHING

Heating the steel above upper critical Heating the steel above upper critical temperature and then cool in water or in oil temperature and then cool in water or in oil (very fast cooling). (very fast cooling).

After quenching, steel is very hard and After quenching, steel is very hard and brittle and practically no use. brittle and practically no use.

Structure after quenching is fine Structure after quenching is fine martensite which is complex, hard and martensite which is complex, hard and brittle structure. .brittle structure. .

TEMERINGTEMERING

Reheat the quenched steel up to Reheat the quenched steel up to intermediate temperature (below lower intermediate temperature (below lower critical temperature) and then cool. critical temperature) and then cool.

The structure is called tempered The structure is called tempered martensite. martensite.

After tempering, steel become tough and After tempering, steel become tough and looses some hardness. It become use looses some hardness. It become use able now. able now.

SUMMARYSUMMARY

PEARLITEPEARLITE

A homogenous mixture of Cementite FeA homogenous mixture of Cementite Fe33C and C and

Iron in solid state. Iron in solid state. For Hypo Eutectoid steel, final structure is For Hypo Eutectoid steel, final structure is

Ferrite (Ferrite (Iron) and pearlite.Iron) and pearlite.

PEARLITE (Cont’d.)PEARLITE (Cont’d.)

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

PEARLITE (Cont’d.)PEARLITE (Cont’d.)

For Eutectoid steel, final structure is only For Eutectoid steel, final structure is only pearlite.pearlite.