Metal Annealing - Article to learn more

Metal Annealing

<a href='https://www.imetllc.com/metal-annealing/'>Metal Annealing</a>

Annealing is a common term used for metal heat treating. However, the term is sometimes used incorrectly to refer to different annealing processes and even for heat treatments that are not annealing processes, creating confusion. For the heat treatment used with cold-worked metal to reduce its strength and increase its ductility, the most accurate and least confusing term to use is recrystallization anneal.

This article discusses the recrystallization anneal process used with cold-worked metal to reduce its strength and increase its ductility.

Cold working

Cold working is used during metal production to reduce sheet and plate thickness, reduce bar, rod, and wire diameter, and modify tube diameter and wall thickness. During cold working the number of dislocations in a metal increases, resulting in increased yield and tensile strength as the amount of cold working increases. At the same time, the metal’s ductility decreases. There comes a point when additional cold working will cause the metal to crack. At this point, the metal must be annealed if additional cold working is required. The additional cold working might be at the mill or at a component fabricator.

Recrystallization annealing

During a recrystallization anneal, metallurgical changes occur that result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working. The metal must be heated above its recrystallization temperature for these changes to occur. The recrystallization temperature for a particular metal depends on its composition and amount of cold working. 

Metallurgical effects of recrystallization anneal

During a recrystallization anneal of a cold-worked metal, new grains form from the cold-worked grains. These new grains have a greatly reduced number of dislocations compared to the cold-worked metal. This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed.  Figure (a) shows the microstructure of the cold rolled sample. The cold-worked grains are elongated in the rolling direction. Figure (b) shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour. Small equiaxed grains are present.  Figure (c) shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour. Large equiaxed grains are present. For samples (b) and (c), new grains formed from the cold-worked grains during the annealing.

cold-worked and annealed metal
Figures a, b, and c.

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The figure (b) sample had yield strength of 11 ksi (75 MPa). Many small grains are present in this sample. The figure (c) sample had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the figure (b) sample.

The difference in grain size after annealing demonstrates the effect of temperature on grain size. Also, with continued annealing time, regardless of temperature, the newly formed grains undergo grain growth, when larger grains grow at the expense of smaller grains. There is a decrease in strength as the average grain size increases during the grain growth phase.

Annealed grain size

Several factors influence the annealed grain size – initial grain size, amount of cold working, and annealing temperature and time. In turn, the grain size affects metal strength, hardness, and ease of forming. This is illustrated in the figure below.

Annealing

To learn more

These courses, videos, and articles provide more information about dislocations, cold working, and recrystallization annealing.

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