Metal heating by high-frequency heating equipment, what are the characteristics of the heating process?

The process of metal heat treatment using high-frequency induction heating equipment is also a process in which the temperature of the workpiece continues to change. Industry insiders usually divide the temperature change process into three stages: the heating stage, the equalizing stage and the overheating stage. Each stage has its own characteristics and laws, and understanding its content plays an important role in formulating the induction heating heat treatment process. Today, let’s take a look at the characteristics of metal heating stages.

The OA section of the heating curve, that is, the room temperature starts to heat to point A, represents the heating stage of the induction heating metal. The heating temperature at this stage is mainly below the Curie point of the steel. Its heating temperature rise has the following characteristics.

(1) The metal surface temperature is higher than the core temperature. For ferromagnetic metals, the magnetic permeability of the metal is large, the resistivity is small, the skin effect of the current is significant, and the surface heats up quickly. The metal center relies on inward conduction heat from the surface to maintain heating. The core temperature is always lower than the surface temperature.

(2) The radial temperature difference between the metal surface and the center. During the heating stage, the radial temperature difference inside the metal gradually decreases as the heating temperature increases. The radial temperature difference generated by metal heating will have a greater impact on the quality of steel heat treatment. Therefore, the radial temperature difference should be minimized.

(3) The heating rate of metal and its changes. For ferromagnetic metals, the heating rate in the heating stage is bounded by the Curie point. Metals heat up quickly below the Curie point, and slow down significantly above the Curie point. Because when the heating temperature is higher than the Curie point, the penetration depth of the induced current increases sharply to 5-10 times below the Curie point. At this moment, the heating current density drops rapidly. Heating rate slows down. In addition, when the temperature exceeds 800°C, the increased radiation heat loss will also affect the heating rate of the metal.

After understanding the characteristics of the metal heating process during induction heating, we can formulate a heat treatment process suitable for the workpiece based on its heating characteristics when formulating the heat treatment process, thereby ensuring the heat treatment quality of the workpiece.