Basic knowledge of metal heat treatment

Metal heat treatment is a process in which a metal workpiece is heated to a suitable temperature in a certain medium, and after it is kept at this temperature for a certain period of time, it is cooled at different speeds.
Metal: A substance with opaque, metallic luster, good thermal and electrical conductivity, and its electrical conductivity decreases with the increase of temperature, and is rich in ductility and ductility. A solid (ie, crystal) in which atoms in a metal are arranged regularly.

Alloy: A substance with metallic characteristics composed of two or more metals or metals and non-metals.

Phase: The component of the alloy with the same composition, structure and performance.

Solid solution: It is a solid metal crystal in which one (or several) atoms (compounds) of one component is dissolved into the lattice of another component while still maintaining the lattice type of the other component. The solid solution is divided into interstitial solid solution and replacement solid solution Two kinds.

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

Compound: The chemical combination between alloy components produces a new crystal solid structure with metallic properties.

Mechanical mixture: An alloy composition composed of two crystal structures. Although it is a two-sided crystal, it is a component and has independent mechanical properties.

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

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

Cementite: a stable compound (Fe3c) formed by carbon and iron.

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

Leeburite: a mechanical mixture composed of cementite and austenite (4.3% carbon)

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 and overall chemical composition of the workpiece, but by changing the microstructure inside the workpiece or changing the chemical composition of the surface of the workpiece , To give or improve the performance of the workpiece. Its characteristic is to improve the 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 reasonable selection of materials and various forming processes, heat treatment processes are 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, etc. and their alloys can also change their mechanical, physical and chemical properties through heat treatment to obtain different performance properties.

In the process from the Stone Age to the Bronze Age and the Iron Age, the role of heat treatment has gradually become known to people. As early as from 770 to 222 BC, the Chinese found in production practice that the performance of copper and iron would change due to the influence of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing farm tools.

In the sixth century BC, steel weapons were gradually adopted. In order to increase the hardness of steel, the quenching process developed rapidly. Two swords and a halberd unearthed in Yanxiadu, Yixian County, Hebei Province, China, have martensite in their microstructures, indicating that they were quenched.

With the development of quenching technology, people gradually discovered the influence of quenching agent on quenching quality. Pu Yuan, a Shu native of the Three Kingdoms, once made 3,000 knives for Zhuge Liang in Xiegu, Shaanxi. This shows that China has paid attention to the cooling capacity of different water quality in ancient times, as well as the cooling capacity of oil and urine. The sword in the tomb of King Jing of Zhongshan in the Western Han Dynasty (206 BC to 24 AD) unearthed in China has a carbon content of 0.15 to 0.4% in the heart, but a carbon content of over 0.6% on the surface, indicating that the carburizing process has been applied. But at that time, as a secret of personal “craftsmanship”, he refused to spread it out, so its development was slow.

In 1863, British metallurgists and geologists showed six different metallographic structures of steel under the microscope, which proved that when steel is heated and cooled, the internal structure changes, and the phase of steel at high temperature changes when it is quenched. It is a harder phase. The allotrope theory of iron established by the Frenchman Osmond, and the iron-carbon phase diagram first established by the Englishman Austin, laid a preliminary theoretical foundation for modern heat treatment technology. At the same time, people have also studied the method of protecting the metal during the heating process of the metal heat treatment to avoid the oxidation and decarburization of the metal during the heating process.

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