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[Surface treatment] heat treatment
06/09/2024
Since today's requirements for metal materials are more comprehensive, if you want to improve the strength and hardness of metal, you need to use "heat treatment" to achieve it.
The so-called "heat treatment" is to heat the metal material to a specified temperature, maintain the specified temperature for a certain period of time, and then lower the temperature to normal temperature or low temperature at a certain rate according to the required metal properties, changing the metal structure through changes in temperature. This leads to better materials.
Generally, the heat treatment methods we often hear include normalizing, annealing, quenching, tempering and surface hardening.
About heat treatment methods
Normalising
The steel is heated to austenite (30-50°C above Ac3) and maintained at processing temperature, and then cooled in air to obtain a structural balance of the metal. The function of normalizing is to improve the mechanical properties of the material, improve the cutting performance, refine the as-cast structure, eliminate network cementite, and prepare the structure for subsequent processing, spheroidizing annealing and quenching. The cooling speed of normalizing is faster than that of annealing, the metal structure is finer, and the hardness and strength are also higher. For hypoeutectoid steel, normalizing treatment has better mechanical properties. If the shape of the part is complex, normalizing cooling is fast but carries the risk of crack formation. Normalizing is low cost and easy to operate, so normalizing should be used first when conditions permit.
Annealing
The metal is heated above the recrystallization temperature, maintained at this temperature for a period of time and then cooled slowly. The function of annealing is to restore the properties of metal reduced by cold working, improve softness, ductility and toughness, release internal residual stress, and produce a specific structure. During the annealing process, the movement of atoms or lattice vacancies is often used to release internal residual stress. Through the process of the arrangement and reorganization of these atoms, the dislocations in the metal material are eliminated. This change also makes the dislocations in the metal easier to move, increasing the Its malleability.
Quenching
Heating alloy products or glass to a certain temperature and then rapidly cooling it in water, oil or air containing minerals is mainly used to improve the hardness and strength of the alloy. Quenching increases the strength and hardness of steel and cast iron. The rate of rapid cooling also affects the surface hardness and core hardness of the material. The material will become brittle after quenching, so it is usually tempered to reduce the brittleness of the material.
Tempering
Tempering is to heat the quenched steel to an appropriate temperature below the austenite transformation temperature, maintain high temperature heating for 1 to 2 hours and then cool. The structure of tempered steel tends to be stable, its brittleness is reduced, and its toughness and plasticity are improved. Tempering and quenching are closely connected processes and are the last process of heat treatment. They can eliminate or reduce quenching stress, stabilize the shape and size of steel, and prevent deformation and cracking of quenched parts. High temperature tempering can also improve cutting performance.
Tempering is further subdivided into low temperature tempering, medium temperature tempering and high temperature tempering.