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Principle of heat treatment

Heat treatment process generally includes heating, insulation, cooling three processes, and sometimes only heating and cooling two processes. These processes are interconnected and uninterrupted. Heating is one of the important processes of heat treatment. There are many heating methods for metal heat treatment. The first is to use charcoal and coal as heat sources, and then apply liquid and gas fuel. The application of electricity makes the heating easy to control without environmental pollution. The use of these heat sources can be directly heated, or by molten salt or metal, or the floating particle indirect heating. When the metal is heated, the workpiece is exposed in the air, often oxidation, decarburization (i.e. the carbon content of steel parts surface decreased), which have very adverse effects for the surface properties of the parts after heat treatment. As a result, metals should be heated in a controlled atmosphere or atmosphere, molten salt and a vacuum, and may be protected by coating or packaging. Heating temperature is one of the important process parameters of heat treatment process. The selection and control of heating temperature is the main problem to ensure the quality of heat treatment. The heating temperature varies with the purpose of the treated metal material and the heat treatment, but is generally heated above the phase transition temperature to obtain high temperature tissue. Another change will take some time, so when the metal surface to meet the requirements of the heating temperature, the temperature must be maintained for some time, inside and outside temperature, the microstructure change completely, this time called the holding time. When using high energy density heating and surface heat treatment, the heating speed is very fast, and generally there is no holding time, but the heat treatment time is longer.

Cooling is also an indispensable step in the process of heat treatment. The cooling method is mainly controlled by the cooling rate due to the different process. The cooling rate of the general annealing is the slowest, the normalizing cooling rate is faster, and the quenching cooling speed is faster. But also because different types of steel have different requirements, such as empty hard steel can use the same normalizing the cooling speed of quench.

The metal heat treatment process can be divided into three major categories: integral heat treatment, surface heat treatment and chemical heat treatment. Depending on the heating medium, the heating temperature, and the cooling method, each class can be divided into several different heat treatment processes. Different metals can be obtained with different heat treatment processes of the same kind of metal with different properties. Iron and steel is the most widely used metal in industry, and the microstructure of steel is also the most complex. Therefore, there are many kinds of heat treatment processes for steel. The overall heat treatment is a metal heat treatment process that heats the workpiece integrally and then cools at an appropriate rate to change its overall mechanical properties.

There are four basic processes, annealing, normalizing, quenching and tempering, for the whole heat treatment of steel. Annealing is the workpiece heated to the appropriate temperature, according to the material and the workpiece dimensions using different soaking time and then slowly cooled, the purpose is to make the internal organization of metal close to equilibrium, obtain good process performance and use of performance, or prepare for quench further. Normalizing the workpiece is heated to a suitable temperature after cooling in the air, the effect of normalizing and annealing are similar, but the organization received more fine materials commonly used to improve the cutting performance, is also sometimes used for some less demanding parts as the final heat treatment.

Quenching is the heating of the workpiece after thermal insulation, in water, oil or other inorganic salts, organic water solution, such as quenching medium quickly cooling. After quenching, the steel becomes hard, but becomes brittle at the same time. In order to reduce the brittleness of a steel member, the quenched steel member is subjected to prolonged heat insulation at a temperature higher than room temperature below 650 DEG C, and then cooled. This process is called tempering. Annealing, normalizing, quenching and tempering are four "fire" in the whole heat treatment, in which the quenching and tempering are closely related and often used together. "Four fires", with different heating temperature and cooling methods, have evolved different heat treatment processes. In order to obtain a certain strength and toughness, the process of combining quenching with high temperature tempering is called quenching and tempering. When some alloys are quenched to form a solid solution, they are kept at room temperature or a little higher at an appropriate temperature for a long time to increase the hardness, strength, electrical or magnetic properties of the alloy. Such a heat treatment process is called aging treatment.

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