Introduction to Heat Treatment Skills of Mechanical Knowledge Points_Latest News

Introduction to heat treatment skills of mechanical knowledge points

The heat treatment of steel refers to the“solid state”It is a technical method to change the internal structure of steel by different heating, heat preservation and cooling of steel, and then obtain the required structure and function.

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In mechanical manufacturing, all important tools and parts must undergo heat treatment, and some parts must be treated more than twice in the entire technical process. In addition to alloying, heat treatment methods are the main way to change the properties of metal materials.

Heat treatment is different from other processing procedures. Its purpose is not to change the shape and size of parts, but only to change its internal structure and function. It is an important process to ensure the internal quality of parts.

According to the purpose and requirements of heat treatment and technical methods, the heat treatment of steel is classified as follows:

(1) General heat treatment

Annealing The heat treatment technology of heating steel to a certain temperature, holding it for a certain period of time, and then cooling it slowly (furnace cooling, pit cooling, sand cooling, gray cooling) is called annealing. the

Normalizing Heat the steel to 30-50 degrees above the phase transition temperature, keep it warm for a certain period of time, and then air-cool the heat treatment technology. the

Quenching Heat the steel to a certain temperature above the phase transition temperature, and then rapidly cool it to room temperature (oil cooling or water cooling), and then improve the hardness and wear resistance of the steel. the

Tempering Heating the quenched workpiece to a temperature lower than the phase transition point, maintaining it for a period of time, and then cooling it to make the structure into a more stable state is called tempering. the

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Comparison of annealing and normalizing:

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Common point: It is mainly used for preparatory heat treatment of casting, forging and welding blanks to improve the machining performance of blanks, remove internal stress, prepare for final heat treatment, and can also be used for final heat treatment of mechanical parts with low performance requirements.

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The difference: the cooling rate is different: the normalizing cooling rate is higher, and the strength and hardness are also higher. Normalizing is shorter than annealing production cycle, equipment utilization rate is high, more economical.

[Note] If the parts are large in size or complex in shape, large internal stress and deformation may occur during normalizing, even cracking.

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The relationship between tempering and quenching:

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After quenching, although the hardness of the steel is high, the brittleness is too large and the structure is unstable, so it is not suitable for direct use. The hardness decreases through tempering, but the resistance is greatly improved. The purpose of tempering is to reduce the brittleness of quenched steel, reduce or eliminate internal stress, and avoid deformation and cracking of workpieces. Stabilize the unstable structure to maintain the shape and scale accuracy of the workpiece and obtain the required structure and function.

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[Note] Quenching and tempering must be used together, and it is meaningless to perform any one operation alone.

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(2) Surface heat treatment

Surface quenching Surface quenching is a heat treatment method that uses rapid heating to austenitize the surface layer without changing the chemical composition and core structure of the steel to strengthen the surface of the part. Induction heating surface hardening and flame heating surface hardening are widely used in current production.

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Chemical heat treatment is a heat treatment technology that puts steel parts in a specific medium at a certain temperature to keep the active atoms in the medium into the surface of the steel, and then changes the chemical composition and organization of the surface to change its performance. the

Chemical heat treatment methods include: carburizing, nitriding, carbonitriding, chromizing, aluminizing, boronizing, etc.

The most commonly used in current production are carburizing, nitriding and carbonitriding.