The principle of induction hardening
Induction heating surface quenching, is the use of electromagnetic induction, skin effect, eddy current, and resistance heat, and other electromagnetic principles so that the surface of the workpiece quickly heating, and rapid cooling heat-treatment process.
Induction heating surface quenching, the workpiece is placed within the copper pipe made of sensors, when a certain frequency of alternating current through the inductor, induced current is generated in the alternating magnetic field, due to the effect of skin effect and eddy current, the resistance of the heat generated in the workpiece surface layer of high density alternating current, rapid heating workpiece surface, quickly reach the quenching temperature, then cooling, workpiece surface hardening.
During induction heating, the distribution of induced current on the workpiece section is related to the current frequency.The higher the current frequency, the stronger the skin effect, the thinner the surface layer where the induced current concentrates, and thus the thinner the heating and hardening layers.
Therefore, the different depths of the hardening layer can be obtained by adjusting the current frequency.
The difference and advantage between induction quenching and flame quenching
Surface heat treatment is to change the surface structure of parts to obtain martensite with high hardness, and retain the toughness and plasticity of the heart (namely, surface hardening), or change the chemical composition of the surface at the same time to obtain corrosion resistance, acid resistance, alkali resistance, and surface hardness higher than the former (namely, chemical heat treatment) method.
Induction quenching: Induction heating speed is very fast, only in a few seconds or more than ten seconds. The martensite of the quenching layer has a fine structure and good mechanical properties. The surface of the workpiece is not easy to oxidize and decarbonize, the deformation is small, and the depth of the hardening layer is easy to control, the quality is stable, the operation is simple, especially suitable for mass production.
Commonly used for medium carbon steel or medium carbon low alloy steel workpiece, such as 45, 40Cr, 40MnB, etc.It can also be used for high carbon tool steel or cast iron parts. When the depth of hardening layer is about 1/10 of the radius, the good match of strength, fatigue resistance and toughness can be obtained. Induction heating surface quenching is not suitable for the workpiece with complex shape, because the inductor is difficult to make.
The advantages of induction quenching：
1. Surface hardness is 2-3HRC higher than ordinary quenching, and has lower brittleness;
2, fatigue strength, impact toughness are improved, the general workpiece can be increased by 20-30%;
3. Small deformation;
4. Quenching layer depth is easy to control;
5, quenching is not easy to oxidation and decarburization;
6. Cheaper steel with low hardenability can be used;
7, easy to achieve mechanization and automation operation, high productivity;
8. The higher the current frequency, the thinner the penetration layer.
The principle of flame quenching
Flame surface quenching: it is a flame burning with acetylene-oxygen or gas-oxygen mixture, which is sprayed onto the surface of the parts for rapid heating. When the quenching temperature is reached, water or emulsion is used for cooling immediately.
The depth of the quenching and penetration layer is generally 2-6mm, too deep will often cause the surface of the parts serious overheating, easy to produce quenching cracks.
Surface hardness: steel up to HRC65, gray cast iron HRC40-48, alloy cast iron HRC43-52.
This method is simple and does not require special equipment, but it is easy to overheat and the quenching effect is unstable, which limits its application.
Suitable for single or small batch production of large parts and tools or parts that need local quenching, such as large shaft type, large modulus gear, etc.
Common steel is medium carbon steel, such as 35, 45 and medium carbon alloy structural steel (alloy-element & LT;3%), such as 40Cr, 65Mn, etc., can also be used in gray cast iron, alloy cast iron.
If the carbon content is too low, the hardness will be low after quenching, while if the carbon and alloying elements are too high, they will easily break. Therefore, carbon steel with a carbon content between 0.35-0.5% is the most suitable.
Medium carbon steels (0.4-0.5%C) and medium carbon alloy structural steels are commonly used, but high carbon tool steels and low alloy structural steels are also available, as well as cast iron.
For small 10-20mm diameter parts, it is recommended to use a deeper penetration layer depth, up to 1/5 of the radius; Generally, when the depth of the quenching and penetration layer is about 1/10 of the radius, the best match of strength, fatigue resistance, and toughness can be obtained. The larger section of the parts can be a shallow depth of the penetration layer, that is less than 1/10 radius.
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