Analyze the characteristics of the heating process of metal heated by high-frequency induction heating machine

Characteristics of metal heating process

1) The metal surface temperature is higher than the core temperature. For ferromagnetic metals, the metal's magnetic permeability is large, its 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) 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. Below the Curie point, the metal heats up quickly; above the Curie point, the heating rate slows down significantly. 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. In summary, in order to improve the heating effect during the heating stage, increase the heating speed throughout the process, and reduce the radial temperature difference, dual-frequency power supply heating should be used. In the temperature zone below the Curie point, a lower frequency power supply with a larger current penetration depth is used for heating; in the temperature zone above the Curie point, a higher frequency power supply with a shallower current penetration depth is used for heating. This can not only achieve rapid heating and reduce the radial temperature difference, but also achieve good thermal efficiency.

Characteristics of the diathermy stage

1) The influence of penetration hot spot temperature on heat treatment process. The temperature of the penetration hot spot of ferromagnetic steel is related to the formulation of the heat treatment process. The steel is heated and heat treated using a high-frequency induction heating machine, and its transmission point temperature increases with the increase in heating speed. Under normal heating conditions, the penetration point of steel is near or slightly higher than the Curie point. When formulating the heating I for quenching and tempering treatment of steel, attention must be paid to the influence of the temperature difference between the front and rear surface and the center of the diathermy temperature on the heat treatment effect. If necessary, set up a temperature equalizing device after the quenching and tempering heating inductor to reduce or eliminate the temperature difference.

2) The influence of heating rate in the diathermy stage. It is necessary to control the heating rate of metal during the thermal penetration stage. When the heating temperature exceeds the penetration point and the temperature is higher than 800°C, due to the increase in the penetration depth of the induced current, surface radiation and convection heat losses increase sharply, and the steel surface heating rate increases. decreases, the central heating rate accelerates, and finally a reverse temperature difference phenomenon occurs where the central temperature is higher than the surface temperature. As the frequency of the heating power supply increases, the probability of occurrence of the reverse temperature difference phenomenon increases. When using a high-frequency power supply above 100kHz for heating, reverse temperature difference is more likely to occur. For this reason, when using a dual-frequency power supply for heating, especially when using an ultrasonic power supply to heat steel above the Curie point, in order to maintain a reasonable temperature difference between the surface and the center, the frequency of the power supply used should not be too high, and the power density of the heated surface should not be too high. large to maintain a suitable heating rate and avoid the temperature difference from expanding.

3) Important process parameters in the heat diathermy stage. The temperature corresponding to the metal penetration point is called the penetration temperature, and the corresponding heating time is the penetration time; the corresponding heating power density is called the penetration surface power density. Thermal penetration temperature, thermal penetration time, and thermal penetration surface power density are important parameters for formulating the rapid heat treatment quenching and heating process.