SOLID STATE PHASE TRANSFORMATION

SOLID STATE PHASE TRANSFORMATION



Diffusion :                It is a flow process which governed the moments of atom and molecules in        solid.

Diffusion Example : Carburising, An-cooling and Normalizing, Oxidation of Metal,
                                  Phase Transfermation, Vibrational Energy, Corrosion Resistance.

Influence :                 Thermal Energy
                                  Stress Gradient
                                  Electrical and Magnetic Field Gradient
                                  Concentration Gradient



Diffusion Bending :  Brazing and Clading

diusion o material
Diffusion
Types of Diffusion : The moment of individual atoms and molecules usally follow a random path so  this kind of diffusion is called Microscopic DiffusionIn the state of without random moment or pre defind path, so this kind of diffusion is called Macroscopic Diffusion

Verious Mechanisim of diffusion : Diffusion of atoms is known as unit step or one jump of atom. There are four types of mechanisim.
1. Vaccany diffusion
2. Interstitial diffusion
3. Substitutional diffusion
4.Direct interstitial change mechanisim.

Potential energy barrier : The barrier in the two atoms is known as enthapy on potential energy barrier (Hm).

Thermal energy :  This energy is used to push the interstitial atom to ohter atoms gap and the thermal energy is converted in to vibrational energy by which the small atoms is jumps to another atoms place.

LAWS OF DIFFUSION / FICK'S LAW :


It is also known as classical linear law.

Diffusion can occurs under the influence of concentration gradient and thermal energy in all the directly with in a all material.

1st LAW -  

When two material system in which the atoms moves unidirectionaly in opposition direction and their influence of concentration gradient is generally express weight per unit wt or concentration gradient is express change in the mass per unit volume in one dimentional case. The first law can be written as

dn/dt =  - Dx . A dm/dx
diffusion path
diffusion path
dn/dt is express no. of atoms diffusion per uint time (x)
          cross sectional area (A)
dm/dx concentration gradient

where as dx = diffusion co-efficient along x-asix.
sometime dx is known as diffusion.

Diffusivity :  

It depend on temp concentration and nature of diffusing atom concentration change with respect is time
1. Dx indicate increase in diffusion down to the concentration gradient. this is known as steady state flow, where 2nd law is non steady state flows.

2nd LAW :  

It is applicable for non steady state flow and it deals with the practical situation of diffusion in material.
Where any metallic slap of thickness (DELx) along the diffusion direction 'x' this thickness enclosed between two plane like Xo DEL( x + DELx ) which are far to direction 'x'. As diffusion proceed the concentration change with time and diffusing atom accumulate in to the slap or moving outs of the slap.
The diffusion accumulation atom going in to the materials or depletion atom coming out of the material is difference in the in going and out going flux in a solid.

Jx = Diffusion flux per unit per cross section area per unit time.

Jx = atom/ cm^2/ sec.


MICRO CONSTITUENTS OF STEEL

ferrite structure
micro-structure of ferrite 

1. Ferrite :   It is a solution of carbon and alpha iron. It's carbon contain is 0.02% at room temperature. It is most ductile and softy. It is present in the forms of flakes.

pearlite structure
pearlite
2. Pearlite :  It is generally found in medium carbon steel. It is the mixer of ferrite and cementite. It have two type 1. Fine pearlite 2. Coarse pearlite states. This two state of pearlite is exists at about 723℃ to 625℃.

cementite micro structure
cementite
3. Cementite :  It is the carbide from of iron means Fe3C or the composition of the iron and carbon. It is so hard. The presence of carbide is increase the hardness but decrease the ductility. Here carbon is in the from of knots.

fig of ledeburite
ledeburite
4. Ledeburite :  It is the eutectic mixture of austenite and Cementite. It is available in the form of layer. It is found in the cast iron. The presence of ledeburite is become brittle and mechaniability.

austenite
austenite
5. Austenite :  It is a solid solution of iron carbide in gama iron is called austenite. Austenite is obtain where it is heated above higher critical temperature generally austenite is non magnetic in nature and observed excess ferrite and Cementite at higher temperature.

fig of martensite
martensite
6. Martensite :  It is the hardest constituent of steel and it has a needle like structure. Generally it a solid solution of Cementite in alpha iron. It is obtained in steel where it is heated above 723℃ and is rapidly cooled by sudden quenching.

bainite
bainite
7. Bainite :  It is the mixture of ferrite and Cementite and it is More finer crystal than the pearlite. It is formation starts below 625℃. This is obtained below 625℃.

troosite
troosite
8. Troosite :  It is also obtained by sudden quenching of heated steel and its structure is slightly granular. These are two types of troosite 1. Primary troosite - It is formed from  austenite and second is 2. Secondary troosite - it is formed by martensite.

sorbite
sorbite
9. Sorbite :  When reheating temperature is increase the decomposition of steel begins and causes change of troosite in to sorbite. The formation of sorbite doesn't takes place spontaneously rather then occurs gradually sorbide begins to fall at about  400℃ and ends at about 680℃. It's structure is in the forms of spheroids or globules.







References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharks,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
Lectures of IITs & BPUT (ODISHA).

Author :
Subir Kumar Sahu.
Metallurgist.

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