Advantages and disadvantages of adding silicon

Silicon (about 0.5–4.5 % by weight) is added to iron to make the silicon steel used in transformer and machine cores. It improves the magnetic properties but spoils the mechanical properties.

Advantages

1. Higher resistivity. Resistivity rises from about 0.1 µΩ·m (pure iron) to about 0.6 µΩ·m (4 % Si). Since $P_e \propto \dfrac{B_m^2 f^2 t^2}{\rho}$, eddy current loss falls sharply.
2. Lower hysteresis loss. The coercive force falls and the hysteresis loop becomes narrower.
3. No ageing. Silicon combines with or removes carbon and oxygen, so carbide precipitation, and with it the rise in loss over time, is avoided.
4. Higher permeability at low and medium flux densities, so magnetising current is less.
5. Less magnetostriction, so transformer hum is lower.

Disadvantages

1. Brittleness. Above about 4–4.5 % Si the steel becomes hard and brittle, so it cannot be rolled thin or punched easily.
2. Tool wear. Dies and punches wear quickly, which raises manufacturing cost.
3. Lower saturation flux density. Silicon is non-magnetic, so it reduces the iron content. The core must work at a lower $B_m$, or be larger.
4. Lower thermal conductivity, so heat leaves the core less easily.
5. Higher cost of the material.

 loss,          brittleness
 rho  ^ resistivity     /
      |      .-------  /
      |   .-'         /
      | .'  core loss/
      |\_           /
      |   ~~--.__  /
      +-----------+----> % Si
      0   1   2   3   4   5
     usual: 0.5-3.5 % (motors)
            4-4.5 % (transformers)

Practical choice