Often during fabrication and manufacturing different processes are used to alter the natural state of a metal. One of the most commonly used methods or processes of changing a metal is through annealing. Annealing stainless steel, like other metals, is important for many different reasons based on the specific application of the metal after the process.
In essence, during the annealing of stainless steel, the metal is heated to a specific temperature and held or kept at this temperature for a specific length of time. As there are different alloys of stainless steel, the temperatures and duration of time at that temperature will vary. When annealing stainless steel, having the ability to completely control the process and the temperature is essential to create the desired end change in the structure of the metal.
The Stages with all Steels
As with all alloys and metals, there are several different steps involved in annealing stainless steel. Attention has to be given to each step, as something as simple as cooling too quickly or heating for too long or at too high of a temperature will result in a less than desirable end result.
The first stage of the annealing process for any metal is the heating stage. This is also known as recovery, as it actually allows all defects to be removed from the alloy at the grain boundaries, or between the small particles that make up the alloy.
Heating at this stage for process annealing of low-carbon steels will occur at about 1328°F for the austenitic range, or below that temperature for the ferritic range. With sub-critical annealing, which is used for high-carbon steels, the heat is below the 1328°F, but it is held for a longer period of time. Stainless steel is typically annealed at temperatures higher than 1900°F, but there are some alloys that can be annealed at a slightly lower temperature.
The second stage is known as recrystallization when annealing stainless steel. This allows the grains to form again within the alloy without any strain, defects or deviations from the norm. This process allows the final product to be softer and more workable metal that is not prone to breaking or being brittle.
This second stage is a long air cooling period for low-carbon steels and a similarly long and slower air-cooling phase for the high-carbon steels. It is also not uncommon to have multiple heating and slow cooling cycles were the temperature has very limited range of variance.
The result of annealing stainless steel, particularly in a controlled furnace, is a bright, reflective surface and a stainless that is workable and can be more easily machined.