Heat treatment process generally includes three heating, heat preservation, cooling process, sometimes only two heating and cooling process. The process to connect to each other, completely. Heating is one of the important process of heat treatment. Heating of metal heat treatment method are many, the first is to use charcoal and coal as a heat source, and application of liquid and gas fuel. The application of electric heating is easy to control, and no environmental pollution. The heat source can be used to direct heating, but can be by molten salt or metal, and floating particles with indirect heating.
Metal heating workpiece when it is exposed to the air, often occur oxidation and decarburization (that is, the surface carbon content lower) steel parts, the performance on the surface of the parts after heat treatment have a bad effect. Thus metals usually should be in a controlled atmosphere or protective atmosphere, molten salt and a vacuum heating, coating or packing method to protect the heating is also available.
The heating temperature is one of the important process parameters of heat treatment process, selection and control of heating temperature, is a major problem to ensure the quality of heat treatment. Heating temperature is processed and varies the purpose of the metal material and heat treatment, but is usually heated to above phase transition temperature, in order to obtain high temperature. Another change will take time, so when the heating temperature of metal surface is achieved, still must in the temperature keeping time, make internal and external temperature, the microstructure change completely, this paragraph of time is called the heat preservation time. Heating with high energy density and surface heat treatment and heating speed, generally there is no heat preservation time, and the chemical heat treatment of heat preservation time tend to be long.
Cooling is indispensable in the process of heat treatment process steps, cooling method for craft different and different, main is to control the cooling rate. General annealing cooling speed of the slowest, normalizing the cooling speed faster, quenching cooling speed is faster. But also with different steel grade have different requirements, such as empty hard steel can be hardened by normalizing the same cooling rate.
Metal heat treatment process can be divided into the overall heat treatment, surface heat treatment and chemical heat treatment three categories. According to the heating medium, heating temperature and cooling method is different, each categories and can be divided into a number of different heat treatment process. The same kind of metal in different heat treatment process, can obtain different organizations, which have different performance. Steel is one of the most widely used industrial metals, and the microstructure of iron and steel is the most complex, therefore there are many different kinds of steel heat treatment process.
The overall heat treatment is heated for artifacts as a whole, and then cooling at the appropriate speed, to change its overall mechanical properties of metal heat treatment process. Steel overall roughly annealing heat treatment, the four basic normalizing, quenching and tempering process.
Annealing is the workpiece of heating to proper temperature, according to the material and workpiece size in different heat preservation time, and then slowly cooling, the purpose is to make the metal internal organization at or near equilibrium state and obtain good process performance and use of performance, or for further quenching organization. Normalizing is after the workpiece heated to proper temperature cooling in the air, the effect of normalizing and annealing are similar, just get the organization of a finer, commonly used to improve the cutting performance of material, also sometimes used for low requirements for some parts as the final heat treatment.
After quenching is the workpiece heating temperature, water, oil or other inorganic salts, organic aqueous solution fast cooling quenching cooling medium. Hardened steel after quenching, but at the same time become fragile. In order to reduce the brittleness of steel after quenching of steel is higher than the room temperature and under 650 ℃ at some stage in the proper temperature for a long time of heat preservation, again for cooling, this process is called tempering. Annealing, normalizing, quenching and tempering is the overall heat treatment of the "four" fire, closely related to the quenching and tempering, often, be short of one cannot.
"Four fire" as the heating temperature and cooling methods, and developed the different heat treatment process. In order to obtain a certain amount of strength and toughness, combine quenching and high temperature tempering process, known as conditioning. Some alloy after quenching formation of supersaturated solid solution, put it at room temperature or slightly higher under proper temperature to keep a long time, in order to improve the hardness and strength of the alloy or electrical magnetic, etc. The heat treatment process called aging treatment.
The pressure deformation and heat treatment processing effective and combined closely, obtain good strength and toughness of components with method called thermomechanical treatment; In negative pressure or vacuum atmosphere heat treatment is called vacuum heat treatment, it can not only make the workpiece no oxidation, no decarburization, keeping the surface is bright and clean after processing, improve the performance of the workpiece can also lead to infiltration agent for chemical heat treatment.
Surface heat treatment is the only heating workpiece surface to change the surface mechanical properties of metal heat treatment process. For heating the workpiece surface and not only make excessive internal heat into the workpiece, using the heat source must have a high energy density, the larger on unit area of the workpiece and the heat, or local can short the surface layers of components or instantaneous at high temperature. The main method of surface heat treatment has the flame hardening and induction heating heat treatment, the common source of heat oxygen acetylene and oxygen propane flame, induced current, laser and electron beam, etc.
Chemical heat treatment is by changing the workpiece surface chemical composition, microstructure and mechanical properties of metal heat treatment process. The chemical heat treatment and surface heat treatment difference is that the latter has changed the chemical composition of workpiece surface. Chemical heat treatment is artifacts in a medium containing carbon, nitrogen and other alloying elements (gas, liquid and solid) in heating, heat preservation for a long time, so that the workpiece surface infiltration of carbon, nitrogen, boron and chromium element. After the infiltration of elements, sometimes even on other heat treatment such as quenching and tempering. Chemical heat treatment of the main methods of carburizing, nitriding, metallic cementation.
Heat treatment is the mechanical parts and one of the important working procedure in process of mould manufacturing. In general, it can ensure and improve the properties of the workpiece, such as wear resistance, corrosion resistance, etc. Can also improve the organization and the stress state of blank, for all kinds of cold and hot working.
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