Proper Selection of Annealing Atmospheres for Electrical Steels

摘要

Semi-processed cold-rolled electrical steel laminations require an anneal before their use in motors, transformers and generators. This anneal is almost always performed by the end user, and is frequently done in an exothermic atmosphere. The anneal's primary function is to refine the magnetic properties (permeability, coercivity, core loss) through stress relief, grain growth and decarburization. However, the aggressive nature of the exothermic atmosphere results in subsurface oxidation that prevents magnetic optimization. A more ideal atmosphere is hydrogen-nitrogen (8-15% H_2) in which the dew point is more controllable, and subsurface oxidation is reduced. Superior magnetic properties are achievable in hydrogen-nitrogen compared with exothermic, particularly in ultra-low carbon electrical steels which themselves are increasing in market share. Higher efficiency electrical steels particularly benefit from annealing in hydrogen-nitrogen.