When metal is heat treated using a high-frequency induction annealing machine, what changes will occur to its physical properties?

When metal is heat treated using a high-frequency induction annealing machine, its physical properties and organizational properties will undergo some changes. Today, let’s take a look at the changes in its physical properties.

The changes in the physical properties of metal during the heating process mainly refer to the changes in resistivity and relative magnetic permeability. From the calculation formula of current penetration depth, it can be seen that their changes have a great influence on the distribution of current density.

Generally speaking, as the temperature of a metal increases, its resistivity increases. When the temperature rises from 0℃ to 850-900℃, the resistivity of steel increases 4-5 times. And it can be roughly believed that in the temperature range of 800-900°C, the resistivities of various types of steel are almost equal.

When the magnetic field intensity is constant, the relative magnetic permeability of steel decreases as the temperature increases. The decrease is very slow at the beginning and can generally be ignored. Then when it reaches a certain critical temperature, that is, near the Curie point, it decreases sharply, and the corresponding magnetic permeability value drops rapidly to 1. If the temperature continues to increase, The relative magnetic permeability also no longer changes.

When using a high-frequency induction annealing machine for induction heating, when the temperature is constant, the relative magnetic permeability decreases as the magnetic field intensity increases. The magnetic field strength inside a conductor is non-uniform and attenuates from the surface to the center. Therefore, the relative magnetic permeability increases from the surface to the center. During the heating process, since the surface temperature of the heated metal is high and the center is low, the resistivity and relative permeability of each point on the cross section are also different and constantly changing. For example, after the Curie point of steel, compared with before the Curie point, the current penetration depth increases by 7-9 times.

For non-magnetic materials, such as copper, aluminum and stainless steel, the relative magnetic permeability remains unchanged during the heating process, and only the resistivity increases as the temperature increases. For magnetic materials, since the resistivity and relative permeability change greatly during the heating process, both the power factor and the absorbed power change significantly.