Knowledge of heat treatment in metal industry

1. Concept: metal heat treatment: it is a process of heating the metal workpiece in a certain medium to a suitable temperature, and keeping it in this temperature for a certain period of time, and then cooling it at different speeds.

Metal: a substance having opaque, metallic luster, good thermal and electrical conductivity, and whose conductivity decreases with the increase of temperature, and which is characterized by ductility and malleability. A solid (that is, a crystal) in which the atoms are arranged regularly within a metal.

Alloy: substance having metallic properties consisting of two or more metals or of metals and nonmetals.

Phase: the parts of an alloy that have the same composition, structure, and function.

Solid solution: a solid metal crystal in which the atoms (compounds) of one (or more) constituent dissolve into the lattice of another constituent while retaining the lattice type of the other constituent.

Solid solution strengthening: because solute atoms enter the gap or junction of the solvent lattice, the lattice distortion, the hardness and strength of the solid solution increased, this phenomenon is called solid solution strengthening phenomenon.

Compound: a new crystalline solid structure with metallic properties resulting from the combination of alloy components.

Mechanical mixture: an alloying composition consisting of two crystalline structures, of course, two kinds of crystals, but of the same composition, having independent mechanical functions.

Ferrite: interstitial solid solution of carbon in a-Fe.

Austenite: interstitial solid solution of carbon in g-fe.

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

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

A mechanical mixture of cementite and austenite (containing 4.3% carbon).

Iii. Metal heat treatment process:

1, metal heat treatment technology is one of the important processes in mechanical manufacturing, compared with other processing technology, heat treatment generally does not change the shape of the workpiece and the overall chemical composition, but by changing the internal microstructure of the workpiece, or change the surface of the chemical composition of the workpiece, give or improve the function of the workpiece. Its feature is to improve the intrinsic quality of the workpiece, which is generally not visible to the naked eye.

2, in order to make the metal workpiece with the required mechanical function, physical function and chemical function, in addition to the reasonable selection of materials and a variety of forming process, heat treatment process is often essential. Steel is the most widely used material in the mechanical industry, steel microstructure is complex, can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. In addition, the mechanical, physical and chemical functions of aluminum, copper, magnesium, titanium and their alloys can also be changed through heat treatment to obtain different functions.

Iv. History of heat treatment:

The role of heat treatment became familiar in the evolution from the Stone Age to the Bronze Age and the iron age. As early as 770 b. c. to 222 b. c., the Chinese discovered in their production practices that the functions of copper and iron varied with the effects of temperature and pressure deformation. The softening treatment of white cast iron is an important process for manufacturing agricultural tools.

In the 6th century BC, steel blade was gradually adopted. In order to improve the hardness of steel, quenching technology was developed rapidly. Two swords and a halberd unearthed in yanxiadu, yi county, hebei province, China, have martensite in their microstructure, indicating that they were quenched. With the development of quenching technology, the influence of quenching agent on quenching quality is gradually discovered. Pu yuan, a native of shu of The Three Kingdoms, used to make 3000 dao for zhuge liang in the valley of shaanxi. This suggests that the Chinese in ancient times were aware of the cooling capacity of different water quality, as well as the cooling capacity of oil and urine. The sword unearthed in the tomb of king jing of zhongshan during the western han dynasty (206bc ~ AD 24) had carbon content of 0.15 ~ 0.4% in the heart, but more than 0.6% in the surface, indicating that the carburizing process had been applied. But at that time as a personal "craft" secret, refused to pass on, so the development was slow.

In 1863, British metallographers and geologists showed six different metallographic structures of steel under the microscope, and confirmed that the internal microstructure of steel changes when heated and cooled, and that the phase in steel at high temperature changes to a harder phase during sharp cooling. The isomeric theory of iron established by osmond, a Frenchman, and the iron-carbon phase diagram first developed by Austin, an Englishman, laid the theoretical foundation for the modern heat treatment process. At the same time, the protection of metal during heat treatment is also studied to avoid oxidation and decarbonization of metal during heat treatment.

Between 1850 and 1880, a series of patents were issued for the use of various gases (such as hydrogen, gas, carbon monoxide, etc.) for protective heating. In 1889 ~ 1890, the Englishman lacker obtained the patent of various metal bright heat treatment.

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. An obvious development is 1901 ~ 1925, the use of rotary furnace in the production of gas carburizing; In the 1930s, the potential difference meter of the flaring point made the carbon potential of the atmosphere in the furnace controllable. Later, the method of further controlling the carbon potential of the atmosphere in the furnace with carbon dioxide infrared meter and oxygen probe was studied. In the 1960s, the heat treatment technology used the role of plasma field, developed the ion nitriding, carburizing process; With the application of laser and electron beam technology, new surface heat treatment and chemical heat treatment methods have been obtained.

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