Black annealed strictly speaking not an ordinary annealing, but the process and annealing almost. Steel black pipe skin is black oxide!

Annealing: annealing the metal alloy is heated to an appropriate temperature and to maintain a certain time, and then slow cooling heat treatment process. Annealed steel is the organization hypoeutectoid ferrite sheet pearlite; eutectoid steel or eutectoid steel is granular pearlite. In short annealing organization is close to an equilibrium state organization.

The purpose of annealing

① reduce the hardness of steel, improved ductility, in order to facilitate cutting and cold deformation processing.

② grain refinement, eliminate casting, forging, welding defects caused by the organization, the organization and composition of homogeneous steel, steel to improve performance or for the subsequent heat treatment for tissue preparation.

③ eliminate the internal stress in steel to prevent deformation and cracking.