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Heat Treatment Process and Classification
Metal heat treatment is one of the crucial processes in mechanical manufacturing. Compared with other machining processes, heat treatment typically does not alter the shape or overall chemical composition of a workpiece; rather, it enhances or improves the workpiece’s service performance by modifying its internal microstructure or changing the chemical composition of its surface. A key characteristic of heat treatment is that it improves the intrinsic quality of the workpiece—quality that generally cannot be detected by the naked eye. As some people put it, machining is like surgery, while heat treatment is like internal medicine—a reflection of a nation’s core manufacturing competitiveness.
Process procedure
The heat treatment process generally consists of three stages: heating, holding, and cooling; sometimes it involves only two stages—heating and cooling. These stages are interconnected and must be carried out continuously without interruption.
(Heating)
When metals are heated, the workpieces are exposed to air, often leading to oxidation and decarburization (i.e., a reduction in carbon content on the surface of steel parts). This has a highly detrimental effect on the surface properties of the parts after heat treatment. Therefore, metals are typically heated either in a controlled atmosphere or a protective atmosphere, in molten salts, or under vacuum. Alternatively, they can be protected during heating by using coatings or packaging methods.
The heating temperature is one of the crucial process parameters in heat treatment. Selecting and precisely controlling the heating temperature is a key factor in ensuring the quality of heat-treated parts. The appropriate heating temperature varies depending on the metal material being treated and the specific purpose of the heat treatment; however, in general, the material is heated above its phase-transition temperature to obtain a high-temperature microstructure. Moreover, the phase transformation requires a certain amount of time. Therefore, once the surface of the metal workpiece reaches the desired heating temperature, it must be held at that temperature for a specified period to ensure uniform temperature throughout the part and complete transformation of the microstructure. This holding period is referred to as the soaking time.
(Thermal insulation)
When using high-energy-density heating and surface heat treatment, the heating rate is extremely fast, and typically there is no holding time. In contrast, chemical heat treatments often involve relatively long holding times.
(Cooling)
Cooling is also an indispensable step in the heat treatment process. The cooling method varies depending on the specific process, but the primary goal is to control the cooling rate.
Process Classification
Metal heat treatment processes can generally be categorized into three major types: bulk heat treatment, surface heat treatment, and chemical heat treatment. Each of these major categories can further be subdivided into several distinct heat treatment processes, depending on the heating medium, heating temperature, and cooling method used. By employing different heat treatment processes on the same metal, one can obtain varying microstructures, thereby achieving different material properties. Steel is the most widely used metal in industry, and its microstructure is also the most complex; consequently, there are numerous types of heat treatment processes for steel.
Overall heat treatment is a metal heat treatment process in which the entire workpiece is heated and then cooled at an appropriate rate to obtain the desired microstructure and thereby alter its overall mechanical properties. For steel, the four basic processes of overall heat treatment—annealing, normalizing, quenching, and tempering—are collectively referred to as the “four fires” of heat treatment.
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