Oriented silicon steel is more restricted in composition than non-oriented varieties. The texture is developed by a series of careful working and annealing operations, and the material must remain essentially single-phase throughout processing, particularly during the final anneal because phase transformation destroys the texture. To avoid the y loop of the Fe-Si phase system, today`s commercial steel has about 3.25% Si. Higher silicon varieties, which might be favored on the basis of increased resistivity and lower magnetostriction, are precluded by difficulties in cold rolling.

Temperature, atmosphere composition, and dew point are closely controlled to decarburize the strip without oxidizing the surface. During this treatment, primary recrystallization occurs, forming small, uniform, equiaxed grains. The coating of magnesium silicate glass which forms will provide electrical insulation between successive laminations when assembled in a transformer core. At this stage, the steel is graded by cutting Epstein samples from the coil; the samples are stress relief annealed and flattened at 790°C, and tested for core loss.

Applications for oriented silicon steel include transformers (power, distribution, ballast, instrument, audio, and specialty), and generators for steam turbine and water wheels.

Lay-up cores, in general, utilize the whole spectrum of grain oriented quality and gages. The gage and grade of material for a given application are determined by economics, transformer rating, noise level requirement, loss requirements, density of operation, and even core size. Because the strip must be flat to produce a good core, coils are flattened after the high temperature anneal. Then, the strip is coated with an inorganic phosphate for insulation. Samples from each coil end are graded after a laboratory stress relief anneal, as previously described. From such strip, the transformer manufacturer cuts his required length improves the insulation of the strip. Consequently, it decreases the eddy current losses and heat buildup, which is of particular importance in transformers which must withstand an impulse test.

As noted earlier, an important requirement in the manufacture of lay-up cores is minimizing transformer noise. Noise is a function of manufacturing and core design factors, the core material characteristic being one of the most important. The dependence of magnetostriction on silicon content has already been noted. In addition, magnetostriction is reduced by improving the texture and by introducing tensile stresses through application of glass-type insulation coatings. Because compressive stresses affect magnetostriction adversely, it is important that the lamination remains flat for assembly. Operating induction is also a factor that affects noise, and indeed affects the transformer`s general operating characteristics. Operating inductions of lay-up transformers are usually in the 10,000 to 17,000 G range; power ratings extend over the 500 to 1,000,000 kVA range.

Wound cores are wound toroidally with the [100] crystallographic direction around the strip. Processing steps are somewhat different from those used for lay-up transformers though the starting material is the same-large toroidally annealed coil coated with magnesium silicate, which usually provides sufficient insulation.

For wound core application, unreacted MgO powder is removed from the strip surface, and a sample from each coil end is cut into Epstein strips to be tested as before. After being graded, the coil is shipped to the transformer manufacturer either as slit multiples or as a full-width coil for subsequent slitting. The slit multiple, wound to the given core dimension, must be stress relief annealed at 790°C in a dry nonoxidizing atmosphere. Annealing trays and plates must be of low carbon steel to eliminate any carbon contamination, which can be very detrimental to quality.

After being stress relief annealed, the cores are cut, and the transformer core is assembled by lacing the steel around the copper (or aluminum) current-carrying coils. In the stress relief annealed condition, grain-oriented steel is sensitive to mechanical strain; therefore, cores must be assembled carefully. Regardless of how carefully assembly is accomplished, the final core quality is always poorer than it was in the stress-relief annealed, uncut condition.

The difference in quality, commonly referred to as the "destruction factor", is due to the relative strain sensitivity of the grain-oriented steel, the handling procedure in fabrication, and the uniformity and amount of air gap in the core. Being a function of the transformer design and fabrication, the latter two factors are controlled best by the manufacturer. Most wound cores are utilized in distribution transformer applications of 25 to 500 kVA.