2.3.3
The actual annealing can be done in a batch or a continuous annealing furnace. Both are used commercially, but most large production shops prefer the continuous furnace because it fits the scheduling of the other operations in the shop. A typical annealing furnace is a multichambered roller hearth furnace. The first chamber is a burn-off oven. It heats the laminations to less than 427°C (800°F) and evaporates the punching lubricant. Usually the transfer between the burn-off and the high-heat chamber is in a hooded open space that removes smoke that may escape from the burn-off oven. The steel enters the high-heat chamber through a flame curtain or a purgeable vestibule, or both. The laminations are heated to the soak temperature in the high-heat chamber, held at that temperature if necessary, and then slowly cooled to bluing temperature [less than 566°C (1050°F)]. Bluing can take place in the same furnace chamber that houses the high-heat zone or in a separate bluing chamber. The need for adequate moisture in the bluing zone is the controlling factor that will determine whether a separate chamber is necessary. If a steam blue is chosen, a purgeable transfer vestibule between the high-heat chamber and the bluing chamber is recommended.
The soak temperature and time in the high-heat chamber are functions of the steel composition and the amount of temper mill extension in the punched sheet. For low-alloy sheet with 0.02 percent carbon (200 ppm) and about 5 percent temper mill extension, a typical high-temperature annealing cycle is 788°C (1450°F) for 2 h in an atmosphere containing a hydrogen-to-water ratio of two to three. If the atmosphere is an EXO gas type with at least 5 percent carbon dioxide, the soak time can be reduced to about 1 h.
Steels with 1.3 to 2 percent silicon plus aluminum should be annealed at a higher temperature because of the sluggish grain growth at 788°C (1450°F). A typical soak temperature for these steels is 816 to 843°C (1500 to 1550°F). These higher-alloy steels will have the best magnetic quality if they are purchased as ultra-low-carbon (<50 ppm carbon) sheet and are annealed in an atmosphere with a hydrogen-to-water ratio greater than 20.
Very high alloy electrical sheet with alloy contents in excess of 2 percent should be annealed at temperatures around 899°C (1650°F). When the carbon content of
the steel is less than 50 ppm, there is no need to hold the laminations at high temperature in the high-heat chamber. The soak time previously mentioned is for decar-burization. The other metallurgical process that takes place at high temperature is grain growth. Grain growth is more sensitive to temperature than to time at that temperature, so the annealing cycle for ultra-low-carbon steels can be shortened by minimizing the time at a particular temperature.