What is Normalizing - Thermal Annealing - Definition

Normalizing 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.

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

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.

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.
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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.

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Thermal Annealing

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