Basic knowledge of metal heat treatment (1)

Metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium and maintained at a constant temperature for a certain period of time. 1. Metallic metal: A material that has opaque, metallic luster, good thermal conductivity and electrical conductivity, and whose electrical conductivity decreases with increasing temperature, and is characterized by ductility and malleability. The internal atoms of the metal have a regular arrangement of solids (ie, crystals). Alloy: A substance that has two or more metals or metals and non-metals and has metallic properties. Phase: The component of the alloy with the same composition, structure and properties. Solid solution: a solid metal crystal in which one (or several) elements of atoms (compounds) are dissolved in the lattice of another component while still maintaining the lattice type of another component, solid solution partitioned solid solution and replacement solid solution Two. Solid solution strengthening: Since the solute atoms enter the gap or junction of the solvent lattice, the lattice is distorted, and the hardness and strength of the solid solution are increased. This phenomenon is called solid solution strengthening. Compound: The chemical composition of the alloy components is combined to form a new crystalline solid structure with metallic properties. Mechanical mixture: An alloy composition composed of two crystal structures, although it is a two-sided crystal, is a constituent and has independent mechanical properties. Ferrite: A gap solid solution of carbon in a-Fe (body-centered cubic structure of iron). Austenite: interstitial solid solution of carbon in g-Fe (iron of face-centered cubic structure). Cementite: a stable compound (Fe3c) formed by carbon and iron. Pearlite: a mechanical mixture of ferrite and cementite (F+Fe3c contains 0.8% carbon). Leysite: a mechanical mixture of cementite and austenite (4.3% carbon). Metal heat treatment is in mechanical manufacturing. One of the important processes, compared to 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 composition of the surface of the workpiece, to give or improve the workpiece Use performance. It is characterized by improved internal quality of the workpiece, which is generally not visible to the naked eye.

In order to make the metal workpiece have the required mechanical properties, physical properties and chemical properties, in addition to the rational selection of materials and various forming processes, heat treatment processes are often essential. Steel is the most widely used material in the machinery industry. The microstructure of steel is complex and can be controlled by heat treatment. Therefore, the heat treatment of steel is the main content of metal heat treatment. In addition, aluminum, copper, magnesium, titanium, and the like can also be modified by heat treatment to obtain different mechanical properties, physical properties, and chemical properties. During the progress from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment was gradually recognized. As early as 770 BC to 222 BC, Chinese people have discovered in production practice that the performance of copper and iron will change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing agricultural tools. In the sixth century BC, steel weapons were gradually adopted. In order to improve the hardness of steel, the quenching process was rapidly developed. Two swords and a scorpion unearthed in Yanxia, ​​Yixian County, Hebei Province, China, have martensite in their microstructure, indicating that they are quenched. With the development of quenching technology, people gradually discovered the effect of quenching agent on quenching quality.

The three-nation monk Pu Yuan once made 3,000 knives for Zhuge Liang in the Shaanxi Diagonal Valley. According to legend, he sent people to Chengdu to take water and quench them. This shows that in ancient China, we noticed the cooling ability of different water quality, and also paid attention to the cooling ability of oil and urine. The Western Han Dynasty (206 BC ~ AD 24) unearthed in the tomb of Jingshan King, the carbon content of the heart is 0.15~0.4%, while the surface carbon content is more than 0.6%, indicating that the carburizing process has been applied. However, at that time, as a secret of personal "craftsmanship", it was not allowed to be rumored, so the development was very slow. In 1863, British metallographers and geologists showed six different metallographic structures of steel under the microscope, which proved that the steel would undergo internal changes during heating and cooling, and the phase of the steel at high temperatures would change during rapid cooling. It is a harder phase. The theory of homogeneity of iron established by the Frenchman Osmond and the iron-carbon phase diagram first established by the British Austin laid the theoretical foundation for the modern heat treatment process. At the same time, people have also studied the protection of metals during the heating process of metal heat treatment to avoid oxidation and decarburization of metals during heating. From 1850 to 1880, there were a series of patents for the protection and heating of various gases (such as hydrogen, gas, carbon monoxide, etc.). From 1889 to 1890, British Lake was awarded a patent for various metal bright heat treatments. Since the twentieth century, the development of metal physics and the application of other new technologies have made the metal heat treatment process more developed. A remarkable development was from 1901 to 1925, in which the converter was used for gas carburizing in industrial production; in the 1930s, a dew point potentiometer appeared, which made the carbon potential of the furnace atmosphere controllable, and later developed a carbon dioxide infrared detector. , oxygen probes and other methods to further control the carbon potential in the furnace; in the 1960s, the heat treatment technology used the role of the plasma field to develop ion nitriding and carburizing processes; the application of laser and electron beam technology, and the new metal Surface heat treatment and chemical heat treatment methods.

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