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HEAT TREATMENT CLASS NOTES FOR ENGINEERING

  HEAT TREATMENT

  CLASS NOTES FOR

  ENGINEERING

 

  

Heat treatment of steels gives different

forms of modified micro-structures and

modified desired physical, chemical and

strength properties. Process of heat

treatment does phase transformations

during heating and cooling after

changing a micro-structure in a solid

state. These processes are mostly thermal

only (Heating and cooling) and changes

only micro-structure. Heat the object,

held at that temperature and then cooled.

Heating rate and more important is the

cooling rate which controls the

micro-structure. It controls the physical,

chemical and strength properties.

 Processes of Heat Treatment

According to cooling rate, there are two main heat treatment processes:
1. Annealing – slow cooling rate (in air or within a furnace)
2. Quenching – fast cooling rate (in oil or in water)

Annealing

Produces equilibrium structures according to the Fe-Carbon diagram. It relieves the internal stresses, increases the ductility and improves the machinability.

Quenching

Gives non-equilibrium structures (martensite) and increases hardness, brittleness as well as strength.
TYPES OF ANNEALING
(i) Normalizing or Full Annealing
(ii) Spheroid zing
(iii) Stress relieving

Normalizing

This process increases ductility, toughness and strength. It is The cooled in STILL AIR. Heat the object to temperature 30-50°C above A3 (8700C) in Austenite field.  Hold it at this temperature (called soaking temperature) for few hours. The soaking temperature depends on carbon content. After soaking the object, cool in STILL AIR. This cooling produces small grain size structure. The small grain size structure improves both toughness and strength (especially yield strength). During normalizing, there is Allotropic transformation upon heating γ→α
Important: There is no change of grain size during cooling of Austenite in normalizing.

SPHEROIDIZING

Spheroid zing increases ductility. The process is limited to cold worked objects of steels containing carbon > 0.5%. Heat the object up to lower critical temperature A1 (727°C). At this temperature, cold worked Ferrite will recrystallize. Iron carbide present in the form of Pearlite appears as spheres or “balls ”. This change of carbides shape reduces the strength and hardness. Thus material becomes soft and ductile.

QUENCHING

Quenching increases hardness, water quenching gives more hardness than air quenching. Soaking temperature is 30-50°C above A3 (8700C) or A1(7270C). The critical cooling rate obtains non-equilibrium structure of martensite. During fast cooling Austenite cannot transform to Ferrite and Pearlite by atomic diffusion. The critical cooling rate is the slowest speed of quenching that will ensure maximum hardness (full Martensitic structure). Martensite is a body-centered tetragonal form of iron and contains some dissolved carbon. It is very hard and brittle. Marten site has a “needle-like” structure. It is a distorted lattice structure. The speed, type of quenching medium  & size of component affect the amount of Marten site formed. Quenching provides increased hardness.
TEMPERING

There increases in toughness. Do Tempering after quenching. Tempering of hardened steels removes internal stresses. It  reduces brittleness created by quenching. In tempering, heat steel to temperature between 200 and 600°C depending upon the final properties desired. This heating removes carbon atoms  of the distorted lattice structure of marten site.  Thus relieves some of internal stresses. Thus it reduce the hardness.  These increase  ductility and toughness. The higher is the tempering, the greater is the toughness to absorb bigger shocks.

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