According to different magnetism, all metals and alloys can be divided into two categories:

1) The permeability of ferromagnetic metals and alloys is much greater than that of vacuum;

2) The permeability of diamagnetic and paramagnetic metals and alloys is close to that of vacuum.

Carbon steel and some other steels and nickel are ferromagnetic metals and alloys. Heat resistant copper, brass, aluminum and white copper belong to diamagnetic and paramagnetic metals and alloys.

In addition to the change of resistance coefficient, the permeability of ferromagnetic metals and alloys also changes during heating.

In the heating process of diamagnetic and paramagnetic metals and alloys, only the resistance coefficient changes and the permeability remains basically unchanged. Therefore, when the diamagnetic and paramagnetic materials are heated, the current input depth increases slightly only due to the increase of electromagnetic coefficient.

When the metal is heated from the original temperature to the magnetic transition temperature, the permeability changes little. As soon as it exceeds the magnetic transition point (Curie point), the permeability decreases to a value close to the vacuum permeability, which remains until the end of heating.

The heating specification of ferromagnetic metals and alloys from original temperature to magnetic transition temperature is usually called cold specification, and the heating specification of ercong magnetic transition temperature to forging temperature is called thermal specification.