Annealing Process and types of annealing process.

        ANNEALING PROCESS

Many of us have heard about heat treatments. What else is the heat treatment process and what is heat treatments? And today I will learn about a chapter of that heat treatments "Annealing process". 

See how the Annealing process is,

It is the heating of steel to austenite temperature and then cooling slowly in the furnace.

Main objects of annealing : 

(i)   To reduce hardness

(ii)  To improve machinability

(iii) To increase or to restore              ductility

(iv)  To relieve internal stresses

(v)   To reduce or eliminate structural in homogeneity

(vi)  To refine grain size

(vii) To prepare steel for subsequent heat treatment

Slow cooling results in the formation of spheroidal carbied and coarse lamilar pearlite.These products are very soft.The cooling rate during annealing varies from  depending upon alloying element in the steel and lower rate of cooling is used for alloy steels as compared to plain C-steels. Annealing results in the formation of ferrite, spheroidal cementite and coarse pearlite. All theses phases and micro-constituents are relatively soft and therefore called softening treatment and produces relatively lower hardness values ​​while ductility increases.

Types of Annealing process : 

There are four types of anneling process: 

(i) Full Annealing: Primary objective of this process is to  reduce hardness and incrcase ductility. The process involves 

(a) Heating the steel ahout 50 to 

75 ° C above the upper critical temperature for hypoeutectoid steels and above the lower critical temperature for hyper eutectoid steel. 

(b) Holding it at this temperature for a sufficient time depending upon the thickness of object the holding time is 3-4 min / mm of thickness of the largest sections. 

(c) Slowly cooling in the furnace. The rate of cooling varies from 

30 ° C-200 ° C per hour depending upon the composition and stability of austenite, when slow cooling in done austenite is decompose into coarse pearlite and forrite structures in hypoeutectoid steels and in eutectoid steels is converted into pearlite. Hyper-eutectoid steels may undergo full annealing after hot working like cooling. Austenite deamposes into pearlite and cementite. 

(ii) Process Annealing : It is usually carried out to remove the effects of it suitable for further plastic deformation as in the case of sheet and wire industries. It is the re-crystallization of cold warked steel by heating below critical temperature. The exact temperature depends upon the extent.of cold working grain size composition and holding time During Re-crystallization, the deformed grains are reoriented to increase plasticity and remove internal stresses.It should be noted that this process produces no  change in microstructure and the process is primarily used for low carbon steels. 

(iii) Spheroidise Annealing: This process is applied to high carbon steels which are difficult to machine. It causes formation of all carbides in the steel in the form of very small globules or spheroids like spheres. The process consists of heating the sleel near the lower critical temperature (730-770° C), holding at this temperature and their cooling slowly to 600 ° C. The rate of cooling in the furnace is 25-30 C / hr. Varieties of heat. treatment can be used to produce a spheroidized structure but all of them are relatively lengthy and costly. 

(iv) Diffusion Annealing: Diffusion annealing or homogenizing is applied to alloy steel ingots and heavy complex castings for eliminating the chemical in homogeneity within the separate crystals by difusion. Homogonizing is carried out at temperature 1000-1200 °C. The optimum temperature being 1150 °C at which diffusion proceed quite easily. The steel is heated to 1150 °C as the highest rate available for the furnace. Holding time is kept at minimum, followed by cooling with the furnace for 6-8 hours to 800-850 C and then further cooling in air. After Homogenizing castings undergo full annealing to refine their structures. Higher temperatures are sclected to enable diffusion more and more.

#FIZAENGINEER........SK NAJMUL

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SK NAJMUL (munna)

Hi. I’m a Mechanical Engineer, are I’m Designer of Blog Magic. I’m Creative Website, Web Designer, Auto Cad 3D and Interaction Designer, Industrial Machine Designer, Web Developer, Business Enthusiast, CNC, Writer and Photo editing and some time research an Mechanical subjects, Inspired to make things looks better.

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