13-08-2012, 04:36 PM
ALLOYS
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Pupose of Heat treatment
Improvement in magnetic and electrical properties
Refinement of grain structures
Removal of the imprisoned trapped gases
Removal of internal stresses
Improves fatique and corrosion resistance
Types of Heat treatment
Annealing
Hardening
Tempering
Normalizing
Carburizing
Nitriding
Annealing
Annealing means softening which is done by heating the metal to high T , followed by slow cooling in a furnace.
Full Annealing
This involves heating steel to a T about 30 to 50 degree
celsius above the higher critical temperature and follows
cooling process.
Annealing T of various grades of carbon steel are
Mild steel =840-870°C
Medium-carbon steel=780-840°C
High-carbon steel=760-780°c
Purpose
To increases the ductility and machinability.
To makes the steel softer , together with an appreciable increase in its toughness.
Tempering
The processes of heating the already hardened steel to a T lower than its own hardening temperature and then cooling it slowly.
Re-heating T not exceed to 400°C.
Purpose :
To removes stress and strains
To reduces the brittleness,hardness,ductility
Cutting tools likes blades,cutters always requires tempering
Carburizing
The mild steel article is taken in a cast iron box containing small pieces of charcoal.
It heated to 900 to 950°C and allow to keep it for sometimes,so the carbon is absorbed to required depth.
Then allow to cool slowly within the iron-box itself.
The outer skin of the article is converted into high-carbon steel(contains 0.8% t0 1.2% carbon).
Purpose:
To produces hard-wearing surfaces on steel article.