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Heat treatment

it  is defined as an operation or combination of operation ,involving heating and cooling of metal and alloys in its solid state with the object of changing the characteristic of material

OBJECTIVES OF HEAT TREATMENT

• TO IMPROVE MACHINEABILITY 
• TO CHANGE OR REFINE GRAIN SIZE 
• TO RELEIF THE STRESSES OF THE METAL INDUCED DURING COLD OR HOT WORKING
• TO IMPROVE MECHANICAL PROPERTIES 
• TO IMPROVE ELECTRICAL PROPERTIES
• TO INCREASE RESISTANCE TO WEAR HEAT  AND CORROSION
• TO  PRODUCE A HARD SURFACE ON A DUCTILE INTERIOR

CONSTITUENTS OF IRON  AND STEEL

• FERRITE
• CEMENTATION
• PEARLITE
• MARTENSITE
• AUSTANITE
• TROOSITE
• SORBITE

1. FERRITE 

• IRON WHICH CONTAINS LITTLE OR NO CARBON IS CALLED FERRITE
• IT IS VERY SOFT AND DUCTILE AND IS KNOWN AS ALPHA IRON BY THE METALLURGIEST
• IT DOES NOT HARDENED WHEN COOL RAPIDLY

2. CEMENTITE

• IT IS A DEFINATE CARBIDE  OF IRON WHICH IS EXTREMELY HARD
• IT CONTAIN 6.6% CARBON
• IT IS MAGNETIC BELOW 250'C
• ITS PRESENCE IN IRON OR STEEL DECREASES THE TENSILE STRENGTH BUT  INCREASES THE HARDNESS AND CUTTIN G QUALITIES

3. PEARLITE

• IT IS THE NAME GIVEN TO THE MIXTURE OF ABOUT 87.5% FERRITE AND 12.5% CEMENTITE
• IT COMPRISES OF ALTERNATE LAYERS OF FERRITE AND CEMENTITE IN STEEL . THE THICKNESS OF ALTERNATE PPLATES AND A DISTANCE BETWEEN THEM IS  THE GOVERNED BY THE RATE OF COOLING , SLOW COOLING PRODUCE A CORES  STRUCTURE THEN RAPID COOLING

4. MARTENSITE

• IT IS A HARD BRITTLE MASS OF FIBROUS OR NIDDLE LIKE STRUCTURE AND IS THE CHEAF CONSTITUENT OF HARDEND STEEL
• IT IS NOT TOUGH AS AUSTANITE
• IT DIFFER IN AUSTANITE BEING MAGNETIC

5.  AUSTANITE

• IT IS A SOLID SOLUTION OF IRON CARBON WHICH IS STABLE ONLY WITH IN A PARTICULAR RANGE OF COMPOSITION AND TEMPERATURE
• IT IS NON MAGNETIC
• IT IS FORMED WHEN CARBON STEEL WITH MORE THAN 1.1% CARBON IS QUENCHED RAPIDLY FROM ABOUT 1000'C