The induction quenching is a quenching process that utilizes the thermal effect generated by the induction current passing through the forging to heat the surface and local part of the forging to the quenching temperature, followed by rapid cooling. During quenching, the forging is placed in a copper position sensor and connected to an alternating current of a fixed frequency to generate electromagnetic induction, which results in an induced current on the surface of the forging that is opposite to the current in the induction coil. The closed loop formed by this induced current along the surface of the forging is called eddy current. Under the action of the eddy current and the resistance of the forging itself, the electrical energy is converted into thermal energy on the surface of the forging, causing the surface to quickly heat up to the quenching overflow, after which the forging is immediately and rapidly cooled to achieve the purpose of surface quenching.
The reason eddy currents can achieve surface heating is determined by the distribution characteristics of alternating current in a conductor. These characteristics include:
- Skin Effect:
When direct current (DC) passes through a conductor, the current density is uniform across the cross-section of the conductor. However, when alternating current (AC) passes through, the current distribution across the conductor’s cross-section is uneven. The current density is higher on the surface of the conductor and lower at the center, with the current density decreasing exponentially from the surface to the center. This phenomenon is known as the skin effect of AC. The higher the frequency of the AC, the more pronounced the skin effect. Induction heating quenching utilizes this characteristic to achieve the desired effect.
- Proximity effect:
When two adjacent conductors pass through the current, if the current direction is the same, the induced back potential on the adjacent side of the two conductors is the largest due to the interaction of alternating magnetic fields generated by them, and the current is driven to the outer side of the conductor. On the contrary, when the current direction is opposite, the current is driven to the adjacent side of the two conductors, that is, the inner flow, this phenomenon is called the proximity effect.
During induction heating, the induced current on the forging is always in the opposite direction of the current in the induction ring, so the current on the induction ring is concentrated on the inside flow, and the current on the heated forging located in the induction ring is concentrated on the surface, which is the result of the proximity effect and the skin effect superimposed.
Under the action of the proximity effect, the distribution of the induced current on the surface of the forging is uniform only when the gap between the induction coil and the forging is equal. Therefore, the forging must be rotated continuously during the induction heating process to eliminate or reduce the heating unevenness caused by the unequal gap, so as to obtain a uniform heating layer.
In addition, due to the proximity effect, the shape of the heated area on the forging is always similar to the shape of the induction coil. Therefore, when making the induction coil, it is necessary to make its shape similar to the shape of the heating area of the forging, so as to achieve a better heating effect.
- Circulation Effect:
When alternating current passes through a ring-shaped or helical conductor, due to the action of the alternating magnetic field, the current density on the outer surface of the conductor decreases because of the increased self-inductive back electromotive force, while the inner surface of the ring attains the highest current density. This phenomenon is known as the circulation effect.
The circulation effect can improve the heating efficiency and speed when heating the outer surface of a forged piece. However, it is disadvantageous for heating the inner holes, as the circulation effect causes the current in the inductor to move away from the surface of the forged piece, leading to significantly reduced heating efficiency and slower heating speed. Therefore, it is necessary to install magnetic materials with high permeability on the inductor to improve heating efficiency.
The larger the ratio of the axial height of the inductor to the diameter of the ring, the more pronounced the circulation effect. Therefore, the cross-section of the inductor is best made rectangular; a rectangular shape is better than a square, and a circular shape is the worst and should be avoided as much as possible
- The sharp Angle effect:
When the protruding parts with sharp corners, edge edges and small curvature radius are heated in the sensor, even if the gap between the sensor and the forging is equal, the magnetic field line density through the sharp corners and protruding parts of the forging is larger, the induced current density is larger, the heating speed is fast, and the heat is concentrated, which will cause these parts to overheat and even burn. This phenomenon is called the sharp Angle effect.
In order to avoid the sharp Angle effect, when designing the sensor, the gap between the sensor and the sharp Angle or convex part of the forging should be appropriately increased to reduce the concentration of the magnetic force line there, so that the heating speed and temperature of the forging everywhere are as uniform as possible. The sharp corners and protruding parts of the forging can also be changed to foot corners or chamfers, so that the same effect can be obtained.
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Post time: Jul-24-2024
