CARBON STEEL Microstructure & Mechanical properties EXPERIMENT # 10 Instructor: M.Yaqub

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.

TYPE OF STEEL Hypo-Eutectoid Steel: % of Carbon < 0.7 Hyper-Eutectoid Steel: % of Carbon > 0.7

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

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 .

IRON-CARBON DIAGRAM

HEAT TREATMENT PROCESSES

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.

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.

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. .

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.

SUMMARY

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.

PEARLITE (Cont’d.) For Hyper Eutectoid steel, final structure is cementite and pearlite.

PEARLITE (Cont’d.) For Eutectoid steel, final structure is only pearlite.