Internal temperature changes of workpiece heated by induction billet heating furnace
When the induction billet heating furnace is used for rapid heat treatment, the diathermy of the steel and the uniform temperature along the cross section play an important role in the uniformity of the mechanical properties after heat treatment. Therefore, it is very important to know the temperature distribution inside the heating process and master the time of uniform temperature.
When the induction billet heating furnace heat the billet, the metal in the current permeable layer is heated directly by the induction current, and the metal outside the current permeable layer is heated by heat conduction.With the continuous heating for the billet, the temperature difference between the metal center and the surface is gradually reduce. With the increased temperature, the penetration depth of the current increases, the heat transfer rate slows down, the heating rate decreases, and finally, the surface temperature is consistent with the central temperature so as to achieve homogenization of the whole steel.
Characteristics of the temperature increase process of induction heating
During the temperature increase process, the ferromagnetic steel temperature increase speed will change with the increase of temperature. The turning point of heating rate change is the Curie point, also known as the temperature of magnetic loss point. When the temperature is higher than the Curie point, the ferromagnetic steel loses magnetism and becomes a paramagnetic material. For carbon steel and low alloy steel, Curie points near will be 760~780 C.
Ferromagnetic steel has a sudden change in permeability at Curie point. The relative permeability of magnetic steel and low alloy steel dropped from about 200 to 1. Because of the relative permeability change, the heating rate of steel will slow down rapidly.
When the billet induction heating furnace heat the billet below the Curie point of ferromagnetic steel, the skin effect is remarkable as the relative permeability µr is big but the heating current frequency is fixed. At this time, the temperature difference between the surface and the center is large, and it is easy to heat conduction. When the temperature exceeds the Curie point, the relative permeability of the steel is µr=1, and the magnetism of the steel disappears into a paramagnetic material. At the same time, when heated under the same condition, the skin effect is weakened and the conduction velocity of the surface to central heat is slowed down because of the increase of the depth of the current, and the heating speed of the whole steel is slowed down.
In conclusion, when the ferromagnetic steel is heated at the temperature below Curie point, it has a faster heating rate. When the temperature exceeds the Curie point, the heating speed of the steel is slowed down when the temperature is above the same condition. The frequency of heating power should be adjusted in order to make Curie heating up and down at the same time. Low frequency power supply is used for heating below Curie point; high frequency power supply is used for heating above Curie point to achieve higher heating efficiency.
Contact person: Hailee Ma
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Specialist of steel pipe induction heating furnace in China; Glad to be your business partner in induction heating field.
Post time: 07-26-2018