Magnetic Properties of Grey Cast Iron

The magnetic properties of gray cast iron vary widely, from low permeability and high coercive force to high permeability and low coercive force. These changes mainly depend on the mictrostructure of gray cast iron. Adding alloying elements to obtain the required magnetic properties is achieved by changing the structure of grey cast iron castings.

Ferrite has high magnetic permeability and low hysteresis loss; pearlite is just the opposite, it has low magnetic permeability and large hysteresis loss. Pearlite is formed into ferrite by annealing heat treatment, which can increase the magnetic permeability by four times. Enlarging ferrite grains can reduce hysteresis loss. The presence of cementite will reduce the magnetic flux density, permeability and remanence, while increasing the permeability and hysteresis loss. The presence of coarse graphite will reduce the remanence. The change from A-type graphite (a flake-shaped graphite that is uniformly distributed without a direction) to a D-type graphite (a finely curled graphite with a non-directional distribution between dendrites) can significantly increase the magnetic induction and coercive force.

Before reaching the non-magnetic critical temperature, the temperature rise significantly increases the magnetic permeability of gray cast iron. The Curie point of pure iron is the α-γ transition temperature of 770°C. When the mass percentage of silicon is 5%, the Curie point will reach 730°C. The Curie point temperature of cementite without silicon is 205-220°C.

The matrix structure of commonly used grades of gray cast iron is mainly pearlite, and their maximum permeability is between 309-400 μH/m.