Use of a box or annealing cover is preferred for either stress-relief or final process annealing. This makes it much easier to control the atmosphere properly and requires much less gas. When an inner cover is not used, a sufficient quantity of protective gas must be used to flush the furnace adequately and diminish the effect of leaks. Oxidation of electrical steels must be avoided because it impairs magnetic properties, particularly at high inductions. Annealing box covers should fit closely and original air should be flushed completely before heating.
To avoid distortion from cooling large batches of wide laminations too rapidly, cool the material slowly under protective atmosphere to a temperature of 600 to 800F (316 to 427C). For charges weighing only a few thou- sand pounds, the cooling rate should not be more than
50F (28C) per hour. Even slower cooling is advisable for heavier furnace charges.