Metal heat treatment basics

Metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium and maintained at this temperature for a certain period of time, followed by cooling at different speeds.

1. Metal organization

Metal: Substances that have opaque, good heat conductivity and electrical conductivity of metallic luster, and whose electrical conductivity decreases with increasing temperature and which are rich in ductility and malleability. Metal internal atoms have a regularly arranged solid (ie crystal).

Alloy: A substance that consists of two or more metals or metals and non-metals and that has metallic properties.

Phase: The same composition, composition, and properties of the alloy.

Solid solution: a solid metal crystal in which one (or several) constituent atoms (compounds) dissolve into the crystal lattice of another constituent while still maintaining the crystal lattice type of another constituent, solid solution solid solution and substitution solid solution Two kinds.

Solid solution strengthening: As the solute atoms enter the gaps or nodes of the solvent crystal lattice, the crystal lattice is distorted and the solid solution hardness and strength are increased. This phenomenon is called solid solution strengthening.

Compounds: Alloying occurs between alloying components, creating a new crystalline solid state structure with metallic properties.

Mechanical Mixture: An alloy composition consisting of two crystal structures, although a two-sided crystal, is a component with independent mechanical properties.

Ferrite: The interstitial solid solution of carbon in a-Fe (body-centered cubic iron).

Austenite: Interstitial solid solution of carbon in g-Fe (face centered cubic structure of iron).

Cementite: A stable compound formed of carbon and iron (Fe3c).

Pearlite: mechanical mixture of ferrite and cementite (F+Fe3c 0.8% carbon)

Laimanite: mechanical mixture of cementite and austenite (carbon content 4.3%)

Metal heat treatment is one of the important processes in mechanical manufacturing. Compared with other processing technologies, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but by changing the microstructure within the workpiece, or changing the chemical composition of the workpiece surface , Give or improve the performance of the workpiece. Its characteristic is to improve the inherent quality of the workpiece, which is generally not visible to the naked eye.

In order to make the metal workpieces have the required mechanical properties, physical properties and chemical properties, in addition to the rational selection of materials and various forming processes, the heat treatment process is often indispensable. 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 their alloys can also be changed by heat treatment of their mechanical, physical and chemical properties to obtain different performance.

In the process 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, the Chinese had discovered in their production practice that the properties of copper and iron could change due to temperature and pressure deformation. The softening of white cast iron is an important process for making farm tools.

In the sixth century BC, steel weapons were gradually adopted. In order to increase the hardness of the steel, the quenching process was rapidly developed. The two swords and a beak that were unearthed in Yanxia, ​​Yi County, Hebei Province, China, have martensite in their microstructure, indicating that they have been quenched.

With the development of quenching technology, people gradually find the effect of quenching agent on quenching quality. The Three Kingdoms monk Pu Yuan once made 3000 knives for Zhuge Liang in Xiegu, Shaanxi Province. According to legend, he sent people to quench water in Chengdu. This shows that China had noticed the cooling capacity of different water quality in ancient times, and also paid attention to the cooling capacity of oil and urine. In the Western Han Dynasty (206 BC ~ 24 AD) unearthed in China, the sword in the Jingwang Tomb of Zhongshan had a carbon content of 0.15 to 0.4% at the heart, and a surface carbon content of 0.6% or more, indicating that the carburizing process has been applied. However, as the secret of the individual “craftsmanship” at that time, he refused to be circulated. Therefore, the development was slow.

In 1863, British metallographers and geologists showed six different microstructures of steel under a microscope, demonstrating that when the steel is heated and cooled, internal changes occur in the structure, and the phase in the steel at high temperatures changes during quenching. It is a harder phase. The iron allotropism theory established by the Frenchman Osmond and the iron-carbon phase diagram first developed by the British Austen laid the theoretical foundation for the modern heat treatment process. At the same time, people also studied the method of metal protection in the heat treatment of metal heat treatment to avoid metal oxidation and decarburization during heating.

From 1850 to 1880, there was a series of patents for protection heating of various gases (such as hydrogen, gas, carbon monoxide, etc.). From 1889 to 1890 the British Laker obtained a number of patents for bright heat treatment of metals.

Since the 20th century, the development of metal physics and the application of other new technologies have made the metal heat treatment process more developed. A remarkable progress was made in 1901 to 1925 when rotary drum furnaces were used for gas carburizing in industrial production; dew point potentiometers appeared in the 1930s and the carbon potential in the furnace atmosphere was controlled, and carbon dioxide infrared instruments were later developed. , Oxygen probes and other methods to further control the atmosphere carbon potential in the furnace; in the 60's, the heat treatment technology used the role of the plasma field to develop the process of ion nitriding and carburizing; the application of laser and electron beam technology also made the metal gain new Surface heat treatment and chemical heat treatment methods.

Source: China Metal Products Network