When low carbon steel is alloyed with small quantities of silicon, the added volume resistivity helps to reduce eddy current losses in the core. Silicon steels are probably of the most use to designers of motion control products where the additional cost is justified by the increased performance. These
steels are available in an array of grades and thicknesses so that the material may be tailored for various applications. The added silicon has a marked impact on the life of stamping tooling, and the surface insulation selected also affects die life. Silicon steels are generally specified and selected on the basis of allowable core loss in watts/lb.
The grades are called out, in increasing order of core loss by M numbers,
such as M19, M27, M36 or M43, with each grade specifying a maximum core loss. (Note that this means that material can be substituted up , as M19 for M36, but not vice versa.) The higher M numbers (and thus higher core losses) are progressively lower cost, although only a few percent is saved with each step down in performance. M19 is probably the most common grade for motion control products, as it offers nearly the lowest core loss in this class of material, with only a small cost impact, particularly in low to medium production quantities. In addition to grade, there are a number of other decisions to make regarding silicon steels. These are:
1. Semi vs. Fully processed material,
2. Annealing after stamping,
3. Material Thickness,
4. Surface insulation.
Fully processed material is simply material which has been annealed to optimum properties at the steel mill. Semi processed material always requires annealing after stamping in order to remove excess carbon as well as to stress relieve. The better grades of silicon steel are always supplied fully processed while semi processed is available only in grades M43 and worse. The designer considering semi processed M43 should evaluate Low Carbon Steel which may provide equivalent performance at lower cost.
Even though annealed at the mill, fully processed material may require further stress relief anneal after stamping. The stresses introduced during punching degrade the material properties around the edges of the lamination, and must be removed to obtain maximum performance. This is particularly true for parts with narrow sections, or where very high flux density is required. In one instance, a tachometer manufacturer was able to reduce the stack height in his product by 10% by annealing after stamping. The annealing cycle requires a temperature of 1350-1450 F in a non oxidizing, non carburizing atmosphere. Endothermic, nitrogen, and vacuum atmospheres all work well. The selection of lamination thickness is a fairly straightforward trade off of core loss versus cost. Thinner laminations exhibit lower losses (particularly as frequency increases), but thinner material is more expensive initially, and more laminations are required for a given stack height. The most common thicknesses are .014 in., .0185 in., and .025 in. (29 Gauge, 26 Gauge, and 24 Gauge, respectively.) These thicknesses are supplemented by thin electrical steels, available in .002, .004, and .007 in. thick. Thin electrical steels are available in one grade (Equivalent to M19) and are made by re-rolling standard silicon steel. Due to substantially higher material cost, thin electrical steel is used primarily for high performance and high frequency applications. In order to gain full advantage from a laminated core, the laminations must be insulated from one another. The simplest way to do this is to specify a surface insulation on the raw material. Silicon steels are available with several types of insulation:
Also called bare, or oxide coated. This is a thin, tightly adherent oxide coating put on the material at the steel mill, or during the annealing process after stamping. This is the lowest cost insulation, but offers little
Enamel or varnish coating which offers excellent insulation, but parts so coated cannot be annealed after stamping.
An inorganic coating providing higher resistance than C-0, but which will withstand annealing temperatures.
An improved inorganic coating similar to C-4 but with significantly higher resistance. It withstands annealing well in most cases. This is probably the best choice for most performance sensitive applications. The main drawback to C-5 is an increase in tool wear due to abrasiveness.