Nitriding Nitriding is a chemical heat treatment process in which nitrogen atoms are infiltrated into the surface of a workpiece in a certain medium at a certain temperature. Common liquid nitriding, gas nitriding, ion nitriding. The traditional gas nitriding is to put the workpiece into a sealed container, and to pass the flowing ammonia gas and heat it. After a long time of heat preservation, the ammonia gas is thermally decomposed to generate active nitrogen atoms, which are continuously adsorbed to the surface of the workpiece and diffused into the surface layer of the workpiece. , thereby changing the chemical composition and structure of the surface layer to obtain excellent surface properties. If carbon is simultaneously infiltrated during the nitriding process to promote the diffusion of nitrogen, it is called nitrocarburizing. Commonly used are gas nitriding and ion nitriding.
Principle The application of nitrogen infiltrated into steel on the one hand forms iron nitride with different nitrogen content from the surface and the iron. On the one hand, it combines with alloying elements in steel to form various alloy nitrides, especially aluminum nitride and chromium nitride. . These nitrides have high hardness, thermal stability and high dispersion, which results in high surface hardness, wear resistance, fatigue strength, seizure resistance, resistance to atmospheric and superheated steam in nitriding steel. Corrosion resistance, temper softening resistance, and reduced notch sensitivity. Compared with the carburizing process, the nitriding temperature is relatively low, so the distortion is small, but because the core hardness is low, the infiltration layer is also shallow, generally only meet the requirements of wearing light and medium load wear resistance and fatigue resistance, or Machine parts that are resistant to heat and corrosion, as well as a variety of cutting tools, cold work and hot work dies. There are many methods for nitriding, and gas nitriding and ion nitriding are commonly used.
The study of steel nitriding began in the early 20th century and was industrially applied after the 1920s. The initial gas nitriding was limited to steel containing chromium and aluminum and later expanded to other steel grades. Since the 1970s, nitriding has developed rapidly from theory to process and has become more and more perfect. The applicable materials and workpieces have also expanded and become one of the important chemical heat treatment processes.
Gas nitriding is generally aimed at improving the wear resistance of metals, and therefore it is required to obtain a high surface hardness. It is suitable for 38CrMnAc isotonic steel. The surface hardness of the workpiece after nitriding can reach HV850~1200. The nitriding temperature is low, the workpiece distortion is small, and it can be used for parts with high precision requirements and wear resistance requirements, such as trampoline masts and spindles, grinding machine spindles, cylinder sleeves, and the like. However, because the nitriding layer is thin, it is not suitable for bearing heavy-duty wear parts.
The gas nitrogen can be nitrided by general nitriding (ie isothermal nitriding) or multi-stage (two-stage, three-stage) nitriding. The former is that the nitriding temperature and the ammonia decomposition rate remain unchanged throughout the nitriding process. The temperature is generally between 480 and 520 ° C, the ammonia gas decomposition rate is 15 to 30%, and the heat preservation time is nearly 80 hours. This process is suitable for parts with shallow infiltration, strict distortion requirements, and high hardness requirements, but the processing time is too long. Multi-stage nitriding is carried out by using different temperatures, different ammonia decomposition rates and different times for nitriding and diffusion in different stages of nitriding. The whole nitriding time can be shortened to nearly 50 hours, and a deeper layer can be obtained, but the nitriding temperature is higher and the distortion is larger.
There is also a gas nitriding for the purpose of corrosion resistance, the nitriding temperature is between 550 and 700 ° C, the temperature is maintained for 0.5 to 3 hours, the ammonia decomposition rate is 35 to 70%, and the chemically stable compound layer can be obtained on the surface layer of the workpiece. Prevent the workpiece from being corroded by humid air, superheated steam, gaseous combustion products, and the like.
The normal gas nitriding workpiece has a silver-gray surface. Sometimes, it may be blue or yellow due to oxidation, but generally does not affect the use.
Ion nitriding, also known as glow nitriding, is carried out using the principle of glow discharge. The metal workpiece is placed as a cathode in a negative pressure vessel having a nitrogen-containing medium. After being energized, nitrogen and hydrogen atoms in the medium are ionized to form a plasma region between the anode and the cathode. Under the strong electric field in the plasma region, positive ions of nitrogen and hydrogen bombard the surface of the workpiece at high speed. The high kinetic energy of the ions is converted into heat, which heats the surface of the workpiece to the desired temperature. Due to the bombardment of ions, atomic sputtering occurs on the surface of the workpiece, which is purified, and at the same time, nitrogen enthalpy penetrates into the surface of the workpiece due to adsorption and diffusion.
Compared with general gas nitriding, the characteristics of ion nitriding are: 1 can shorten the nitriding cycle properly; 2 the nitriding layer is less brittle; 3 can save energy and ammonia consumption; 4 can not need nitriding part Shielded to achieve local nitriding; 5 ion bombardment has a purifying surface effect, can remove the passivation film on the surface of the workpiece, can directly nitriding the stainless steel "target=_blank> stainless steel, heat-resistant steel workpiece. 6 thickness and structure of the layer can be controlled Ion nitriding has developed rapidly and has been used in workpieces such as machine screws, gears, and molds.
Nitrocarburizing is also called soft nitriding or low temperature nitrocarburizing, that is, below the iron-nitrogen eutectoid transformation temperature, so that the surface of the workpiece penetrates into the carbon while mainly infiltrating nitrogen. The fine carbide formed after carbon infiltration promotes the diffusion of nitrogen and accelerates the formation of high nitrogen compounds. These high nitrogen compounds in turn increase the solubility of carbon. The mutual promotion of carbon and nitrogen atoms accelerates the rate of infiltration. In addition, carbon can also reduce brittleness in nitrides. The compound layer obtained after nitrocarburizing has good toughness, high hardness, wear resistance, corrosion resistance and seizure resistance.
Commonly used nitrocarburizing methods are liquid method and gas method. The treatment temperature is 530 to 570 ° C, and the holding time is 1 to 3 hours. Early liquid salt baths used cyanide salts, and later appeared in a variety of salt bath formulations. There are two commonly used: neutral salt through ammonia and urea plus carbonate-based salts, but these reaction products are still toxic. The gas medium mainly includes: an endothermic or exothermic gas (see controlled atmosphere) plus ammonia gas; urea thermal decomposition gas; and an organic solvent containing carbon and nitrogen, such as formamide or triethanolamine.
Nitrocarburizing can not only improve the fatigue life, wear resistance, corrosion resistance and seizure resistance of the workpiece, but also has simple equipment, low investment, easy operation, short time and small distortion of the workpiece, and sometimes gives the workpiece a beautiful appearance. .
Glow ion ion nitriding one, advantages: short nitriding time, easy to control quality, nitride layer fatigue resistance, high strength, due to nitriding temperature of 520-540, so the workpiece deformation is small, surface diamagnetic high.
Second, the shortcomings: the equipment control is complex, the furnace temperature uniformity is not good.
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