chemical heat treatment

Each chemical heat treatment process has its own characteristics. If it is necessary to improve the wear resistance, friction reduction, bite resistance, corrosion resistance, high temperature oxidation resistance and fatigue resistance respectively or simultaneously, the corresponding chemical heat treatment process should be selected according to the material and working conditions of the workpiece.

Chemical heat treatment is an ancient process, which can be traced back to the Western Han Dynasty in China. The sword unearthed by Liu Sheng, the king of Zhongshan in the Western Han Dynasty, has a carbon content of 0.6 ~ 0.7% on the surface and 0. 15 ~ 0.4% in the center, which has obvious carburizing characteristics. The book Tiangong Kaiwu written by Song Dynasty in Ming Dynasty records the carburizing process of low carbon steel with fermented soybean and animal bone charcoal as carburizing agents.

In the section of "Quenching Knife" in Fang Yizhi's "Essentials of Physics" in Ming Dynasty, it was also recorded that "coating the mouth with salt paste, calcining and quenching". Nitrate is a nitrogen-containing substance, which needs to be nitrided to a certain extent. This shows that chemical heat treatment processes such as carburizing, nitriding or carbonitriding have been mastered by working people since ancient times and are widely used as processes for making weapons and farm tools.

With the gradual improvement of the theory and technology of chemical heat treatment, chemical heat treatment has been widely used in industry since the beginning of the twentieth century. With the rapid development of machinery manufacturing and military industry, higher and higher requirements are put forward for various performance indexes of products. Besides carburizing, nitriding, carbonitriding and nitrocarburizing, the multi-element nitrocarburizing processes such as aluminizing, chromizing, boronizing, sulphurizing, sulphurizing and nitrocarburizing have been studied and improved.

With the appearance of electronic computer, the control of chemical heat treatment process is becoming more and more perfect, not only the automation degree of production process is getting higher and higher, but also the process parameters and treatment quality are more reliably controlled.

According to the properties of infiltrated elements, chemical heat treatment can be divided into two categories: nonmetallic infiltration and metallic infiltration. The former includes carburizing, nitriding, boronizing and various non-metallic elements, such as carbonitriding, carbonitriding and carbonitriding (thiocyanate). The latter mainly includes aluminizing, chromizing and zinc dipping. Titanium, niobium, tantalum, vanadium and tungsten are also commonly used surface alloying elements. Binary and multicomponent metallization processes, such as aluminizing, chromizing and chromizing, have been used in production. In addition, binary or multi-element * * * infiltration processes of metal and nonmetal elements are constantly emerging, such as Al-Si * * * infiltration and B-Cr * * * infiltration.

Chemical heat treatment of steel can be divided into ferritic chemical heat treatment and austenitic chemical heat treatment according to the basic structure of diffusion. The diffusion temperature of the former is lower than the precipitation temperature of iron and nitrogen, such as nitriding, sulphurizing, sulphurizing and oxynitriding. These processes can also be called low temperature chemical heat treatment. The latter diffuses above the critical temperature, such as carburizing, boronizing, aluminizing and carbonitriding. These processes all belong to the category of high temperature chemical heat treatment.

Carburizing is a chemical heat treatment process that makes carbon atoms penetrate into the surface of steel workpiece. After carburizing, the carbon content on the surface of the workpiece is generally higher than 0.8%. After quenching and tempering at low temperature, the hardness and wear resistance are improved, and at the same time, the core can maintain high toughness, bear impact load and have high fatigue strength. But the disadvantages are high processing temperature and large deformation of the workpiece.

Carburizing technology is widely used in important parts of aircraft, automobiles, machine tools and other equipment, such as gears, shafts and camshafts. Carburizing is the most widely used and fastest developing chemical heat treatment process. Microprocessor can realize the automation of the whole carburizing process and control the carbon content on the surface and the distribution of carbon in carburized layer.

Nitriding is a chemical heat treatment process that makes nitrogen atoms diffuse to the surface of metal workpiece. After nitriding steel, nitride-based surface layer can be formed. When steel contains nitrides such as chromium, aluminum and molybdenum, it can obtain higher hardness, wear resistance, corrosion resistance and fatigue resistance than carburized layer. Nitriding is mainly used for workpieces that require high precision, deformation, fatigue strength and wear resistance, such as boring machine spindle, boring bar, grinder spindle, cylinder liner and so on.

Carbonitriding and nitrocarburizing are chemical heat treatment processes in which carbon and nitrogen penetrate into the surface of metal workpiece simultaneously. The former is mainly carburizing. Compared with carburizing, carburized parts have less deformation during quenching, higher wear resistance and corrosion resistance, and better fatigue resistance than carburizing. Since 1970s, carbonitriding technology has developed rapidly, which can be used not only for several automobile and tractor parts, but also for surface strengthening of various gears and shafts. The latter is mainly nitriding, which is characterized by fast infiltration rate, short production cycle, small surface brittleness and lax requirements for workpiece materials. The disadvantage is that the infiltration layer of the workpiece is thin, which is not suitable for high load work.

Infiltration is a chemical heat treatment process that allows boron atoms to penetrate into the surface of the workpiece. Boron has little solubility in steel and mainly forms borides with some alloying elements in steel. The wear resistance of boronized parts is higher than that of nitrided and carburized layers, and it has high thermal stability and corrosion resistance. The boronizing layer is fragile and difficult to deform and process, so the workpiece should be finished before boronizing. This process is mainly used for structural steel parts of medium carbon steel and medium carbon alloy, and also for surface strengthening of non-ferrous metals and alloys such as titanium.

Boronizing technology has been applied to abrasive tools, drill bits of oil rigs worn by abrasive particles, parts of coal water pumps, crawler plates of tractors, valve stems and valve seats working in corrosive media or high temperature conditions. However, there are still some shortcomings in boronizing process, such as high treatment temperature, large deformation, difficult cleaning of molten salt boronizing parts and brittleness of boronizing layer.

Sulphurizing is a chemical heat treatment process in which sulfur reacts with the surface of metal workpiece to form a thin film. After sulphurizing, the hardness of the workpiece is low, but the antifriction effect is good, which can prevent the wear and seizure caused by friction heat and plastic deformation when the surface of the friction pair contacts.

Sulfur-nitrogen cementation and sulfur-nitrogen-carbon cementation are chemical heat treatment processes that simultaneously infiltrate sulfur, nitrogen or sulfur, nitrogen and carbon into the surface of metal workpieces. When the sulphurizing process is adopted, the friction reduction of carburized layer is good, but it will be destroyed quickly under high load. When nitriding or nitrocarburizing process is adopted, the carburized layer has good wear resistance and fatigue resistance, but poor friction reduction. Sulfur-nitrogen or sulfur-nitrogen-carbon cementation process can make the surface layer of workpiece have wear-resistant and friction-reducing properties.

Metal infiltration is a chemical heat treatment process in which one or several metal elements infiltrate into the surface of metal workpiece. Metal elements can be infiltrated simultaneously or successively in different ways. In the infiltrated layer, most of them exist in the form of intermetallic compounds, which can improve the wear resistance, corrosion resistance and high temperature oxidation resistance of the surface layer of the workpiece respectively. Commonly used aluminizing processes include aluminizing, chromizing, zincating, etc.

The development of chemical heat treatment will focus on expanding the application of low temperature chemical heat treatment; Improve the quality of infiltrated layer and speed up the process of chemical heat treatment; Develop special steel suitable for general chemical heat treatment process; Develop pollution-free chemical heat treatment process and composite infiltration process; A variety of chemical heat treatment processes are controlled by computers, and corresponding mathematical models are established, and sensors, external instruments and equipment suitable for various media are developed.

Chemical heat treatment: refers to the heat treatment process in which a metal or alloy workpiece is kept in an active medium at a certain temperature, so that one or more elements penetrate into its surface layer and change its chemical composition, microstructure and properties. Common chemical heat treatment processes include carburizing, nitriding, carbonitriding, aluminizing and boronizing. The main purpose of chemical heat treatment is to improve the hardness, wear resistance, corrosion resistance, fatigue strength and oxidation resistance of steel parts.