Metal (die) heat treatment process includes heating, insulation, cooling three processes, sometimes only heating and cooling process of two. These processes are connected to each other, can not be interrupted.
Heating is one of the important processes of the heat treatment. Metal (die) many heating heat treatment method, the first is the use of charcoal and coal as a heat source, then application of liquid and gas fuel. Application of electric heating is easy to control, and no environmental pollution. The use of these heat sources can be directly heated, or by molten salt or metal, as well as floating particles 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. So the metal should normally be in a controlled atmosphere or the protection of the atmosphere, the molten salt and vacuum heating, can also be used to protect heating coating or packing method.
The heating temperature is one of the important parameters of the heat treatment process, selection and control of heating temperature, the main problem is to ensure the quality of heat treatment. The purpose of heating temperature of metallic materials and heat treatment processing vary, but are generally heated to above the phase transition temperature to obtain high temperature. 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. Using high energy density heating and surface heat treatment, heating fast, generally there is no holding time, and the chemical treatment of the holding time is often longer.
Cooling and heat treatment is an indispensable step in the process, the cooling method is different because of different process, mainly control the cooling rate. General speed of the slowest cooling annealing, normalizing the cooling speed, quenching cooling 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.
(die) metal heat treatment process can be roughly divided into the overall heat treatment, surface heat treatment and chemical heat treatment of three categories.According to the heating medium, heating temperature and cooling methods, each class can be divided into several different heat treatment process. With a metal with different heat treatments, available in different organizations, which have different performance. The iron and steel industry is the most widely used metal, and the microstructure of iron and steel is the most complicated, so many kinds of steel heat treatment process.
The overall heat treatment is heating the whole piece, and then at the appropriate rate of cooling, the metal heat treatment process to change the overall mechanical properties of the. The whole heat treatment of steel there are four basic process of annealing, quenching and tempering, normalizing.
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 workpiece after heat insulation, rapid cooling in water, oil or other organic and inorganic salt aqueous quenching medium. Quenched steel hardened, but brittle. In order to reduce the brittleness of steel, the steel after quenching for a long time at a proper temperature higher than room temperature below 650 DEG C and insulation, and then cooling, this process is called tempering. Annealing, normalizing, quenching and tempering is the whole metal (die) heat treatment in "four fire", which close to quenching and tempering, is often used in conjunction, are indispensable.
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