When metal fabricators are tasked with forming complicated parts, they have quite a few tools, strategies, and processes in place to help them get the job done. One important component in their arsenal of tools and knowledge is a thorough understanding of the annealing process.
Annealing is a term that refers to the heat treatment processes carried out on soft metals. During the annealing process, metals–namely steels–are heated evenly to return them to as close to their pre-cold worked state as possible. This heat treatment process increases a metal’s ductility and ensures that metal forming and shaping are more efficient processes. Ideally, the annealing process should not cause damage to the tooling or the work piece.
As a process, annealing is necessary because materials tend to lose ductility while gaining yield strength after a certain amount of cold working. If cold working is needed continuously throughout the metal forming process, annealing becomes a necessary component of that process because it helps to restore the metal’s original properties.
During the standard annealing process, there are three stages: recovery, recrystallization, and grain growth. Here’s a rundown of these three stages:
Stage 1 – Recovery. In annealing, recovery is a process that acts to recover the physical properties of the metals such as thermal expansion, electrical conductivity, and internal energy. Essentially, it is this initial step that softens the metal. During recovery, dislocations or irregularities in the metal’s structure find their way into stress-free environments, usually the walls of pre-existing cells. As we’ll see in the recrystallization stage, it is these stress-free cells that help push the annealing process forward.
Stage 2 – Recrystallization. Recrystallization is a restorative process. In order for annealing to be effective, workpieces must be heated to a temperature that’s above its recrystallization temperature. During recrystallization, deformed grains of the metal’s crystal structure are replaced by the new stress-free grains that were developed during recovery. These undeformed, stress-free cells nucleate and grow until all of the original grains have been replaced. So, for example, low carbon steel should be annealed at around 900 degrees Celsius (or 1650 degrees F) because that is the temperature at which it’ll recrystallize.
Keep in mind, though, that it’s not always necessary to heat the metal into a critical temperature range. Mild steel products that need to be repeatedly cold worked can be softened by annealing at 500-650 degrees Celsius.
Stage 3 – Grain Growth. Grain growth is the third stage and this happens when annealing is allowed to continue after recrystallization is completed. In this stage, the microstructure of the metal becomes coarse and makes the workpiece lose some of its strength. Strength can typically be regained through a process called hardening. For steel, quenching and tempering is the hardening process you would use.