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Carburizing refers to a chemical heat treatment process in which a steel workpiece is heated to a high temperature in a carbonaceous medium to increase the carbon content of the surface layer of the workpiece. The material of the carburized workpiece is generally low carbon steel or low carbon alloy steel (carbon content less than 0.25%). After carburizing, the chemical composition of the steel surface can be close to high carbon steel. After the workpiece is carburized, it is quenched to obtain high surface hardness, high wear resistance and fatigue strength, and maintain the toughness of the low carbon steel after quenching in the core, so that the workpiece can withstand the impact load. Carburizing processes are widely used in mechanical parts such as gears, axles, camshafts, etc. for aircraft, automobiles and tractors.
The carburizing process can be traced back to 2000 before China. The earliest was carburizing with a solid carburizing medium. Liquid and gas carburizing appeared in the 20th century and is widely used. In the United States in the 1920s, gas drum carburizing was started. In the 1930s, continuous gas carburizing furnaces began to be used industrially. In the 1960s, high temperature (960-1100 °C) gas carburization was developed to the 1970s, and vacuum carburization and ion carburization occurred.
Principle: Carburizing, like other chemical heat treatments, also involves three basic processes.
Decomposition: Decomposition of a carburizing medium produces activated carbon atoms.
Adsorption: The activated carbon atoms are absorbed into the surface austenite after being absorbed by the surface of the steel, and the carbon content in the austenite is increased.
Diffusion: The surface carbon content increases and the concentration of carbon in the heart is poor, and the carbon enthalpy on the surface diffuses into the interior. The diffusion rate of carbon in steel depends mainly on the temperature, and is related to the difference between the internal and external concentrations of the infiltrated elements in the workpiece and the content of alloying elements in the steel. The material of carburized parts is generally low carbon steel or low carbon alloy steel (carbon content less than 0.25%). After carburizing, quenching must be carried out in order to fully exert the beneficial effect of carburizing. The surface microstructure of the workpiece after carburizing and quenching is mainly high hardness martensite plus retained austenite and a small amount of carbide, and the core structure is low-carbon martensite with good toughness or non-martensitic structure. However, ferrite should be avoided. Generally, the depth of the carburized layer ranges from 0.8 to 1.2 mm, and the depth of the carburized layer can reach 2 mm or more. The surface hardness can reach HRC 58-63, and the hardness of the core is HRC 30-42. After carburizing and quenching, compressive internal stress is generated on the surface of the workpiece, which is beneficial to improve the fatigue strength of the workpiece. Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts, thereby extending the service life of parts.
Classification According to different carbon-containing media, carburizing can be divided into solid carburizing, liquid carburizing, gas carburizing and carbonitriding.
Solid carburizing, the part is buried in a container filled with solid carburizing agent (the main component is charcoal, and sodium carbonate, barium carbonate, etc. are used as the infiltration agent), and is decomposed by the chemical reaction of carbon and the infiltration agent at high temperature. Activated carbon atoms penetrate into the surface of the part. Solid carburizing can be carried out in various heating furnaces, which is simple and easy, but the quality is not easy to control, the cycle is long, and the working conditions are poor.
Liquid carburizing The parts are immersed in a molten salt bath. The main components of the salt bath are sodium chloride, sodium carbonate and barium chloride. The composition of the various salts can be adjusted by different temperatures. Generally, thin layer carburizing is mostly carried out at a low temperature (850 to 900 ° C), a low concentration cyanide salt bath (also called liquid cyanide or liquid carbonitriding). Deep carburizing is mostly carried out at a higher temperature (900 to 950 ° C) and a cyanide content of 6 to 16%. The other is cyanide-free liquid carburizing. The main salt bath components are sodium chloride, potassium chloride and sodium carbonate, plus processed carburizing agents: carbon powder, silicon carbide and urea.
Gas carburizing Put the parts into a sealed carburizing furnace, using an endothermic gas as the carrier gas in the carburizing medium, and using natural gas or propane as the rich gas. A drip-type liquid carburizing agent can also be used, and kerosene, benzene, acetone or ethyl acetate is used as a strong osmotic agent, and methanol and ethanol are used as a diluent. These liquids are vaporized and decomposed at a high temperature after being dropped into the furnace to produce a stable carburizing gas. There are two main types of gas carburizing equipment: one is a continuous pusher without tank furnace, and the other is a periodic sealed box furnace and a pit furnace. Drip-type carburizing is mostly used in pit furnaces, and can also be used in periodic sealed box furnaces. The biggest advantage of gas carburizing is that the carbon content of the furnace gas can be controlled by the control system to control the amount of rich gas feed or the amount of infiltration agent to change the carbon potential of the furnace gas (see controlled atmosphere). Gas carburizing is suitable for mass production, easy to control quality and automation, and good working conditions.
Carbonitriding is a chemical heat treatment process in which carburizing is mainly carried out while nitrogen is infiltrated. At the time of co-infiltration, ammonia added to the gas carburizing atmosphere is decomposed into hydrogen and monoatomic nitrogen, and the nitrogen is adsorbed on the surface of the workpiece together with carbon from the carburizing gas. Due to the simultaneous infiltration of nitrogen, the eutectoid transformation temperature of iron carbon can be lowered, so that the eutectoid transformation can be carried out at a temperature lower than that of carburization, and thus the treatment temperature is low. At the same time, due to the action of nitrogen, the critical cooling rate of martensite (see quenching) is also reduced, which can be quenched in a milder quenching medium to reduce the tendency of quenching distortion and cracking. Carbonitrides in the carbonitriding layer increase hardness and improve wear resistance. The carbon and nitrogen content of the surface of the metal workpiece and the total depth of the layer are determined by the carbon potential, temperature and time in the atmosphere. The depth of the carbonitriding layer is shallower than that of carburizing, and is generally 0.05 to 0.75 mm. After quenching, the carbonitrided layer has martensite, retained austenite, carbide and carbonitride, and the core is low carbon martensite or contains non-martensitic structure. A small amount of ferrite may be allowed for parts with a small load. Carbonitrided workpieces are typically made of low or medium carbon steel and alloy steel for the treatment of wear parts and gearboxes and drive shafts for cars, light trucks. Carbonitriding, like carburizing, can also control the carbon potential and be quenched and tempered after treatment.
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