What is Full Annealing - Definition

Full annealing produces a microstructure that is softer and more amenable to other processing such as forming or machining. 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.

The term thermal annealing refers to a heat treatment in which a material is exposed to an elevated temperature for an extended time period and then slowly cooled. This process alters the physical and sometimes chemical properties of a material to increase its ductility and reduce its hardness, making it more workable. In this process, atoms migrate in the crystal lattice and the number of dislocations decreases, leading to a change in ductility and hardness. Metal gets rid of stresses and makes the grain structure large and soft-edged so that when the metal is hit or stressed it dents or perhaps bends, rather than breaking. Typically, annealing is carried out to relieve stresses, increase softness, ductility, and toughness; and/or produce a specific microstructure.

Generally, in plain carbon steels, annealing produces a ferrite-pearlite micro-structure. Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability. The most common structural steels produced have a mixed ferrite-pearlite microstructure. Their applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways. These steels are relatively inexpensive and are produced in large tonnages.

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.

thermal annealing

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.

References:

Materials Science:

U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 1 and 2. January 1993.
U.S. Department of Energy, Material Science. DOE Fundamentals Handbook, Volume 2 and 2. January 1993.
William D. Callister, David G. Rethwisch. Materials Science and Engineering: An Introduction 9th Edition, Wiley; 9 edition (December 4, 2013), ISBN-13: 978-1118324578.
Eberhart, Mark (2003). Why Things Break: Understanding the World by the Way It Comes Apart. Harmony. ISBN 978-1-4000-4760-4.
Gaskell, David R. (1995). Introduction to the thermodynamics of Materials (4th ed.). Taylor and Francis Publishing. ISBN 978-1-56032-992-3.
González-Viñas, W. & Mancini, H.L. (2004). An Introduction to Materials Science. Princeton University Press. ISBN 978-0-691-07097-1.
Ashby, Michael; Hugh Shercliff; David Cebon (2007). Materials: engineering, science, processing and design (1st ed.). Butterworth-Heinemann. ISBN 978-0-7506-8391-3.
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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