What is the difference between Solution Annealed, Bright Annealed, Mill Annealed and Annealed/Cold Finished? The actual differences are subtle and can explain minor variations in strength, overall corrosion resistance but also surface finish quality. We talk primarily about Nickel based and Austenitic stainless grades here.
When you want the softest quality material to work with, you should always ask for Solution Annealed. Solution annealing will create a homogeneous structure through the material, putting all carbides into the grain - rather than sitting at the grain boundaries. A proper solution anneal will create a slightly larger grain size when compared to a Mill Anneal, primarily due to the higher annealing temperature attained and the grains are picking up elements that have precipitated out during processing. More on the Mill Anneal in a minute.
A Bright Anneal is very similar to a Solution Anneal, but can yield a finer grain size and a great surface finish.To make a Bright Anneal, you would grind or polish the material to remove surface imperfections and oxidation prior to the Solution Anneal. The Solution Anneal is performed in an inert atmosphere to prevent it from reoxidizing the finish. The final step is a very light cold pass on the material, reducing the thickness slightly and breaking the grain structure up a little - thereby improving the grain structure. Bright Annealed materials are usually desired for heavy forming, to reduce the grains pulling. Large grains, when heavily formed, can create an orange peel type surface. Smaller grains will result in a smaller pattern, or not as prominent in appearance. Bright Annealed products have a slightly higher hardness and an elevated tensile strength compared to Solution Annealed products.
Mill Annealed products are typically annealed at a lower temperature than their Solution Annealing temperature. The time at temperature can be longer than the Solution Annealed product, but the grain size is usually smaller and properties are slightly elevated. With a Mill Annealed product, you may worry about the homogeneity of the product, not necessarily from tip to tail, but in areas within a plate or sheet. There can still be some precipitates hanging out in the grain boundaries of a Mill Annealed product These precipitates, while normally essential to the alloy for strength, can present a problem with a corrosive media. Stress Corrosion Cracking (SCC) is typically an issue with Mill Annealed products that are borderline for the application. Solution Annealed products will always outperform a Mill Annealed product in corrosion resistance.
Annealed/Cold Finished product is very similar to a Bright Anneal, typically the difference is the product form. Bright Annealed is usually found in foil, strip, sheet or light plate. Annealed/Cold Finished is typically for bars and rods. In order to attain a better surface quality or a tighter size tolerance, the material is annealed and then put through a cold sizing pass. This sizing pass will help to round or straighten the product in order to prepare for tighter tolerances. After the sizing pass, further finishing may be performed to the surface to enhance the finish appearance or tolerance. The grain size is usually smaller (finer has a larger number in the ASTM grain numbering). Also, the mechanical properties (hardness, tensile and yield) are slightly elevated when compared to a Solution Annealed product.