Bright annealed stainless steel tube

Most stainless steel tubing is heat treated by the bright annealing process, in which the tubing is annealed in a protective medium to prevent discoloration of the bright surface. Since austenitic stainless steel is the most often used tubing material, the heat treatment process is bright annealing of austenitic stainless steel in essence.

ASTM A269 Gr. TP 316 tubing: o.d = 38.1 mm, w.t = 1.65 mm.

The stainless steel tubing is generally produced by a cold-drawing process. A subsequent annealing process is required to eliminate residue stress and obtain improved softness and ductility. Meanwhile, since chromium carbides formed at the grain boundaries during high-temperature exposure may result in susceptibility of the austenitic stainless steel to intergranular corrosion, they must be dissolved during annealing before cooling begins. If the annealing is conducted in a protective atmosphere such as pure hydrogen or dissociated ammonia, the formation of an oxide layer on the tubing exterior and interior surfaces can be completely avoided. The stainless steel tubing maintains its original metal luster as well as excellent surface finish. There is no need for a subsequent pickling process.

The continuous bright annealing furnace for stainless steel tubes.

In engineering practice, the stainless steel tubing is usually bright annealed in dissociated ammonia, which is a medium cost furnace atmosphere providing a dry, carbon-free source of reducing gas. The typical composition of dissociated ammonia is 75% hydrogen, 25% nitrogen, less than 300 ppm residual ammonia and less than -50°C (-60°F) dew point. The gas as well as the furnace shall be dry and scrupulously clean. After bright annealing, the hardness of the tubing shall be less than 90 HRB.