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11-07-07, 03:37 PM #1
What Is Annealing?
Annealing is the process of heating the steel to a particular temperature in the austenite region and cooling down the steel very slowly. There are many derivatives of the annealing process, but generally the process is a slow cool process.
Another derivative of the annealing process is known as sub-critical anneal. This process involves soaking at a temperature below the lower transformation line, in the region of 1,200xF to 1,300xF, until the steel has equalized across its cross-section in temperature, followed by a slow cool. Slow cooling can mean a cooling rate between 5xF per hour up to 50xF per hour.
As can be imagined, the cooling period can be a considerable amount of time. It should be noted that the nickel alloyed steels and the A series tool steels should be cooled very slowly, as nickel will cause an air-hardening effect.
Other Types of Annealing:
* Bright Anneal. This method is a method of annealing which uses a protective atmosphere to prevent the steel surface from oxidation.
* Process Anneal. This procedure is done at a temperature close to the lower critical line on the iron carbon diagram. Sometimes confused with sub-critical annealing, it is used when considerable cold working is to follow.
* Recrystallization Anneal. Once again, this is a process often mistaken for subcritical annealing. It is used after cold working to produce a specific grain structure.
* Sub-Critical Anneal. This method is used on cold-worked steel and is carried out below the lower critical line on the iron carbon equilibrium diagram. It is sometimes applied to tool steels that have been over tempered and require annealing before hardening and tempering.
* Spheroidize Anneal. This process is a controlled heating and cooling procedure to produce spheroidal or globular cementite particles. It is usually applied on high carbon steels for good machining characteristics such as high alloy steels and tool steels.
* Isothermal Annealing. The process temperature of this procedure is determined by knowledge of the steel's carbon content. The steel is then taken to that temperature and cooled down to a holding temperature that allows the steel to transform isothermally.
* Full Anneal. This is a process that involves raising the steel's temperature up to the sustenite region followed by a slow cool.
What Is Normalizing?
Normalizing is a process that makes the grain size normal. This process is usually carried out after forging, extrusion, drawing or heavy bending operations.
When steel is heated to elevated temperatures to complete the above operations, the grain of the steel will grow. In other words, the steel experiences a phenomenon called "grain growth."
This leaves the steel with a very coarse and erratic grain structure. Furthermore, when the steel is mechanically deformed by the aforementioned operations, the grain becomes elongated.
There are mechanical property changes that take place as a result of normalizing - inasmuch as the normalized steel is soft, but not as soft as a fully annealed steel. Its grain structure is not as coarse as an annealed steel, simply because the cooling rate is faster than that of annealing. Usually the steel is cooled in still air and free from air drafts. The process temperature is virtually the same as for annealing, but the results are different due to the cooling rate.
The process is designed to:
* Give improved machining characteristics.
* Ensure a homogenous structure.
* Reduce residual stresses from rolling and forging.
* Reduce the risk of "banding."
* Help to give a more even response to the steel when hardening.
What Is Stress Relieving?
Stress relieving is an intermediate heat treatment procedure to reduce induced residual stresses as a result of machining, fabrication and welding. The application of heat to the steel during its machining or fabrication will assist in removing residual stresses that will, unless addressed during the manufacturing by stress relieving, manifest themselves at the final heat treatment procedure.
It is a relatively low temperature operation that is done in the ferrite region, which means that there is no phase change in the steel, only the reduction of residual stresses. The temperature region is usually between 800xF to 1,300xF. However, the higher that one goes in temperature, the greater the risk of surface oxidation there is. It is generally better to keep to the lower temperatures, particularly if the steel is a "pre-hard" steel. The hardness will be reduced if the stress relieve temperature exceeds the tempering temperature of the steel.
There is a general rule of thumb for time at temperature. It must be stated that the time is taken when the part is at temperature, not when the furnace is at temperature. The time at temperature for the processes of full anneal (not spheroidize anneal), normalize and stress relieve is 60 minutes at part temperature per one-inch of the maximum cross-sectional area.
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