Directional Silicon Steel Magnetic Yoke
Directional, or grain-oriented, silicon steel is annealed so the easy path for flux follows the rolling direction. On a curved induction coil that path has to follow the arc. Flat stacking would put the joint across the flux. Arc stacking, done on a locating fixture, keeps the sheets tight to the coil.
Current yokes use Nippon Steel oriented sheet, Nippon Steel 0.23-80 on the 5 ton vacuum yoke, grade 230 on the 70 ton yoke, and 23RJ100 on the 60 ton yoke. WISCO cold-rolled sheet is used where the drawing calls for it. Every sheet is looked at before it is stacked. A phlogopite mica board separates stainless covers from the laminations.
The 20 ton and 35 ton oriented yokes use copper tubes, two in and two out, and cover about 60 to 65 percent of the coil surface.