Annealing is a heat treatment that causes physical changes to the material. This process involves heating a material to a certain temperature, maintaining that temperature for a specified period of time, then cooling in a specified period of time. Annealing is typically used to reduce hardness, increase ductility and eliminate internal stresses. There are two primary types of annealing for tubing: Bright Annealing and Open Air Annealing.
Open Air Annealing
Open Air Annealing is an annealing process performed with atmospheric oxygen. This exposure to atmospheric oxygen causes scaling on the surface. All relevant standards require that tubes be provided free of scale, so tubes are pickled to remove the layer of scale. Pickling uses a combination of nitric and hydrofluoric acid which chemically attacks the scale on the surface. This acid combination must be carefully removed from the tubing to prevent unintentional corrosion. This can be particularly challenging for small diameter tubes.
Bright annealing is an annealing process performed in a vacuum or a controlled atmosphere containing inert gases (such as hydrogen). This controlled atmosphere reduces the surface oxidation to a minimum which results in a brighter surface and a much thinner oxide layer. Pickling is not needed after bright annealing since the oxidation is minimal. Since there is no pickling, the surface is much smoother which results in better resistance to pitting corrosion.
In the USA and much of Europe, Bright Annealing is now the standard method for annealing tubing. After bright annealing, the OD is often mechanically polished with a 180-240 grit to give the ideal finish. For large OD tubing (>2″) and for pipe, annealing and pickling is still the standard practice for most manufacturers in the US and Europe.
Bright Annealing is becoming more common in other parts of the world as well, but much of the tubing that comes from China is still annealed and pickled. The OD is polished to 240 grit or higher to give a bright outer finish, but the inside of the tube is where the key differences will be noticeable.
Regardless of which annealing method is used, it is key to make sure that only high quality manufacturers are used and that the tubing is thoroughly inspected. Tubing Central stocks both bright annealed and annealed and pickled tubing. To insure all products meet our quality standards, Tubing Central audits all suppliers on a yearly basis, requires additional testing on annealed and pickled tubing, and performs visual and dimensional inspection on 100% of received shipments.