A Comprehensive Guide to Heat Treatment Processes
Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing processes, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but by changing the microstructure inside the workpiece, or changing the chemical component of the workpiece surface. Give or improve the performance of the workpiece. It is characterized by improving the inherent quality of the workpiece, which is usually not the naked eye.
1.Technological Process
The heat treatment process generally includes three processes of heating, holding, cooling, sometimes only heating and cooling two processes. These processes are connected to each other, and they are uninterruptible.
①Heating
When metal is heated, the workpiece is exposed to air, often oxidation, decarburization (i.e., the surface of the steel parts is lowered), which is very disadvantageous for the surface properties of the parts after heat treatment. Therefore, the metal should usually be heated in a controlled atmosphere or a protective atmosphere, and the molten salt is heated in vacuo, and can also be protected by a coating or packaging method.
The heating temperature is one of the important process parameters of the heat treatment process, selects and controls the heating temperature, which is the main problem that guarantees heat treatment quality. The heating temperature varies from the purpose of the treated metal material and heat treatment, but it is generally heated to a phase change temperature to obtain a high temperature tissue. In addition, the transition requires a certain amount of time, so when the surface of the metal workpiece reaches the required heating temperature, it is necessary to maintain a certain period of time, so that the internal and outer temperature is consistent, which makes the microstructure transition, which is referred to as a holding time.
②Insulation
When using high energy density heating and surface heat treatment, the heating speed is extremely fast, and there is generally no incubation time, and chemical heat treatment time is often longer.
③Cool down
Cooling is also an indispensable step in the heat treatment process, and the cooling method is different from the process, mainly controlling cooling speed.
2.Process classification
The metal heat treatment process can be generally 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 major class can also distinguish between several different heat treatment processes. The same metal uses different heat treatment processes to obtain different tissues, thereby having different properties. Steel is the most widely used metal in industrial applications, and steel microstructure is also the most complicated, so the steel heat treatment process has a wide variety.
The overall heat treatment is to heat the workpiece, then cooled at an appropriate speed to obtain a metal heat treatment process of the desired mechanical properties of the overall mechanical properties. The overall heat treatment of steel is roughly annealing, normal fire, quenching and tempering four basic processes, i.e., heat treated “four fires”.