Do you know "four fires" in heat treatment of metal materials?
Date of release:2018-07-05 Author: Click:
Heat treatment is a common processing technology for metal materials. The purpose is to improve the properties of the material by changing the chemical composition or internal microstructure of the material without destroying the shape of the metal material. As the name suggests, heat treatment is based on temperature control. The process generally includes three links: heating, insulation and cooling. As early as the sixth Century B.C., iron and steel smelting technology was invented and popularized by human beings. At that time, people began to use simple heat treatment processes to increase the hardness of steel materials. With the continuous improvement of relevant scientific theories, in recent years, the processing technology of this metal material has been further developed. The more commonly used means are known as the "four fire".
The "four fires" in the heat treatment process are the general names of four different treatments, namely, annealing, normalizing, quenching and tempering. Because they all take a word "fire", so they are called "four fires". The "four fires" are related to the cooling process after heating, but the cooling method differs from that of the media.
When the metal is heated to the suitable temperature, the heat preservation reaches enough time. The length of the heat preservation time is determined according to the properties of the material and the shape and size of the workpiece, and then it is cooled slowly. This method is called annealing. After annealing, the hardness of the metal is reduced, the machinability is improved, the residual stress is eliminated, and it is not easy to deform or crack during the process.
Normalizing, also known as normalizing, is similar to annealing process, only when cooling needs to place the workpiece in the air, or to spray water, spray, blow and other means to assist the cooling. Normalizing the metal materials can also remove the internal stress and improve the cutting process. Only the hardness and microstructure of the material are more fine. Annealing and normalizing are usually used as a preparatory heat treatment for improving material properties, but for some less demanding materials, normalizing can also be used as an end heat treatment.
After heating and holding the workpiece to make all or part of the austenite 1, the process of rapid cooling of metal is called quenching by using water, oil or other inorganic salts and organic water solution as medium. Quenching can greatly improve the hardness of the workpiece, especially the surface quenching technology, because the oxidation and decarburization of the workpiece are less, often used in the machining and manufacturing of the spindle and gear of the machine tool. But after quenching, the brittleness of metal materials will also increase, and the next step of tempering needs to be carried out in time.
The tempering is to reheat the quenched workpiece to a suitable temperature below the critical temperature range, and then put it in an environment above room temperature and below 650 degrees centigrade for a long time heat preservation and cooling at the end. This process can effectively reduce the brittleness of steel parts and improve ductility and toughness. Tempering and quenching are often used. If the quenched workpiece is tempered by high temperature, the whole process is called quenched and tempered. If tempering is slightly higher than room temperature, the whole process is referred to as aging treatment.
Complex heat treatment
In addition to the simple "four fires", heat treatment processes are often combined with other processes to form more complex heat treatment processes to achieve better processing results. For example, in the process of heat treatment of materials, the pressure processing and deformation process is added. This combination can enhance the strength and toughness of the workpiece. This method is called deformation heat treatment. If the heat treatment process is carried out in negative pressure atmosphere or vacuum environment, the oxidation and decarburization of the workpiece will be effectively avoided. The surface of the workpiece treated is clean and the performance is improved. This treatment is called vacuum heat treatment. For example, in the heating process, different media are used to mix the surface of the workpiece with carbon, nitrogen, boron and other elements to change the chemical composition of the surface of the workpiece. This method is called chemical heat treatment.