User:Rita Physics/sandbox

What is Annealing ? Annealing, in metallurgy and materials science, is a heat treatment that alters a material to increase its ductility and to make it more workable. It involves heating a material to above its critical temperature, maintaining a suitable temperature, and then cooling. Annealing can induce ductility, soften material, relieve internal stresses, refine the structure by making it homogeneous, and improve cold working properties.

Why Anneal ? There are two main reasons for annealing. The first is to soften it and remove stress. The second is to homogenize the structure.

Every time a piece of metal is worked it accumulates stress and gets harder. The harder it gets, the more difficult it is to work again. Take something as simple as a coin as an example. The cast slab of coinage alloy is rolled down to a plate. It becomes so hard that it must be annealed before it can be rolled further. It may undergo several such cycles before reaching the correct thickness. The coin sized blanks are then punched out of the strip. The cut faces are hard so the blanks are annealed again before they can be minted. No final anneal is needed as the hardness from minting process helps with wear in service. When a metal is cast, the solidification processes result in both macro and micro segregation of the alloying elements present. Macro segregation needs to be broken down by mechanical work, but micro segregation can often be homogenized by annealing.

How annealing done ? Annealing is basically a very simple process. The metal is heated up, held at temperature for a time, then it is slow cooled. If the condition of the surface does not matter or cleaning takes place later (e.g. castings) then it can be done in air. If the surface finish does matter then a protective atmosphere is used. Typically this would be nitrogen with a small hydrogen addition. Steel is a bit different from the rest of the metals so it will be addressed separately.

Why Annealing is done ??? (Annealing is performed to )
 * reduce hardness,
 * remove residual stresses,
 * improve toughness,
 * restore ductility, and
 * to alter various mechanical, electrical or magnetic properties of material through refinement of grains.

Advantages of Annealing : The following are some of the advantages of annealing • It softens the steel. • It enhances and improves the mach inability of steel. • It increases the ductility of steel • It enhances the toughness of steel • It improves the homogeneity in steel • The grain size of the steel is refined a lot by annealing • It prepares the steel for further heat treatment.

Types of Annealing : 1.	Full Annealing : The process involves heating the steel to 30 to 50 degrees Centigrade above the critical temperature of steel and maintaining the temperature for a specified period of time, then allowing the material to slowly cool down inside the furnace itself without any forced means of cooling. Hot Worked sheets, forgings, and castings made from medium and high carbon steels need full annealing.

2.	 Process Annealing : This process is mainly suited for low carbon steel. The material is heated up to a temperature just below the lower critical temperature of steel. Cold worked steel normally tends to posses increased hardness and decrease ductility making it difficult to work. Process annealing tends to improve these characteristics. This is mainly carried out on cold rolled steel like wire drawn steel, etc.

3.	Stress Relief Annealing : Large castings or welded structures tend to possess internal stresses caused mainly during their manufacture and uneven cooling. This internal stress cause brittleness at isolated locations in the castings or structures, which can lead to sudden breakage or failure of the material. This process involves heating the casting or structure to about 650 Degree centigrade. The temperature is maintained constantly for a few hours and allowed to cool down slowly.

4.	 Spheroids Annealing : This is a process for high carbon and alloy steel in order to improve their mach inability. The process tends to improve the internal structure of the steel. This can be done by two methods: a. The material is heated just below the lower critical temperature about 700 Degree centigrade and the temperature is maintained for about 8 hours and allowed to cool down slowly. b. Heating and cooling the material alternatively between temperatures just above and below the lower critical temperature

5 . Isothermal Annealing : This is a process where is steel is heated above the upper critical temperature. This causes the structure of the steel to be converted rapidly into austenite structure. The steel is then cooled to a temperature below the lower critical temperature about 600 to 700 Degree Centigrade. This cooling is done using a forced cooling means. The temperature is then maintained constant for a specified amount of time in order to produce a homogenous structure in the material. This is mainly applicable for low carbon and alloy steels to improve their mach inability.