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What is Annealing Thermal Cycle – Annealing Temperature – Definition

Time and annealing temperature are important parameters in these procedures. Especially the target temperature defines the annealing thermal cycle. Annealing Thermal Cycles

Hot working

Any annealing cycle consists of three stages:

  • heating to the desired temperature,
  • holding or “soaking” at that temperature,
  • cooling, usually to room temperature.

Time and annealing temperature are important parameters in these procedures. Especially the target temperature defines the annealing thermal cycle.

Annealing Thermal Cycles

In practice, specific thermal cycles of an almost infinite variety are used to achieve the various goals of annealing. These cycles fall into several broad categories that can be classified according to the temperature to which the steel is heated and the method of cooling used.

  • thermal annealingProcess Annealing. Process annealing is a specific heat treatment that restores some of the ductility to a product being cold-worked so it can be cold-worked further without breaking. It is commonly used during fabrication procedures that require extensive plastic deformation, to allow a continuation of deformation without fracture or excessive energy consumption. The temperature range for process annealing ranges from 260 °C to 760 °C, depending on the alloy in question. This process is mainly suited for low-carbon steel. This is mainly carried out on cold-rolled steel like wire-drawn steel, centrifugally cast ductile iron pipe etc.
  • Stress-relief Annealing. Stress-relief annealing is used to relieve stresses from cold working. In contrast to process annealing, this heat treatment is usually performed after the product has been made. Care must be taken to ensure uniform cooling, particularly when a component is composed of variable section sizes. If the rate of cooling is not constant and uniform, new residual stresses can result that are equal to or greater than those that the heat-treating process was intended to relieve. The annealing temperature is typically a relatively low one such that effects resulting from cold working and other heat treatments are not affected. Stress-relief heat treating can reduce distortion and high stresses from welding that can affect service performance.
  • Recrystallization Annealing. Recrystallization annealing of cold-worked steels (carbon content up to 0.5%) can produce a new grain structure without inducing a phase change.  Metal is heated to a temperature at which the hardening caused by the previous cold-working is removed. During recrystallization, the internal bonds between the atoms change, the crystal lattice does not change. Annealing temperatures are between 550-700 ° C and the endurance is about 1 hour or more, cooling in air. It is used as an inter-operational annealing in cold forming, especially for low-carbon parts.
  • Full Annealing. Full annealing produces a microstructure that is softer and more amenable to other processing such as forming or machining. The temperatures for full annealing are typically 50 °C above the upper critical temperature (A3) for hypoeutectic steels and the lower critical temperature (A1) for hypereutectoid steels. It is referred to as full annealing because it achieves full austenitization of hypoeutectoid steels. The alloy is then furnace cooled. That means, the heat-treating furnace is turned off, and both furnace and steel cool to room temperature at the same rate, which takes several hours. The cooling rate of the steel has to be sufficiently slow so as to not let the austenite transform into bainite or martensite, but rather have it completely transform to pearlite and ferrite or cementite. A full anneal typically results in the second most ductile state a metal can assume for metal alloy. The metal attain relatively low levels of hardness, yield strength and ultimate strength with high plasticity and toughness. Full annealing is often used in low- and medium-carbon steels that will be machined or will experience extensive plastic deformation during a forming operation. Stainless and high-alloy steels may be austenitized (fully annealed) and quenched to minimize the presence of grain boundary carbides or to improve the ferrite distribution.
  • Normalizing. Normalization is an annealing process applied to ferrous alloys to refine grain size, make its structure more uniform, make it more responsive to hardening, and to improve machinability. Normalizing is performed on steels that have been plastically deformed by, for example, a rolling operation. This cold worked steels consist of grains of pearlite, which are irregularly shaped and relatively large and vary substantially in size. Normalizing is an austenitizing heating cycle followed by cooling in still or slightly agitated air. Typically, the temperatures for normalizing are approximately 55 °C above the upper critical line. Normalization temperature is higher than temperature for full annealing, on the other hand the cooling more intense. Normalizing improves machinability of a component and provides dimensional stability if subjected to further heat treatment processes. The main difference between annealing and normalizing is that annealing allows the material to cool at a controlled rate in a furnace. Normalizing allows the material to cool by placing it in a room temperature environment and exposing it to the air in that environment.
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J. R. Lamarsh, A. J. Baratta, Introduction to Nuclear Engineering, 3d ed., Prentice-Hall, 2001, ISBN: 0-201-82498-1.

See above:
Thermal Annealing

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