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Heat treatment of metals: Nitriding

Vacuum nitriding furnace.jpg

A chemical heat treatment process that causes nitrogen atoms to infiltrate into the surface of steel work pieces. Traditional gas nitriding is to put the workpiece into a sealed container and heat it with flowing ammonia. After a long time of heat preservation, ammonia decomposes to produce active nitrogen atoms, which are absorbed to the surface of the workpiece and diffused into the surface of the workpiece, thereby changing the chemical composition and structure of the surface layer, and obtaining excellent surface properties. If nitrogen is diffused into the nitriding process at the same time to promote nitrogen diffusion, it is called nitrocarburizing. The study of nitriding in iron and steel industry started in the early twentieth Century and was applied in industry after 20s. The initial gas nitriding was limited to steel containing chromium and aluminum and later expanded to other steel grades. Since the 1970s, nitriding has been developed rapidly from theory to technology, and the suitable materials and workpieces have been enlarged day by day. It has become one of the important chemical heat treatment processes.

 Nitrogen penetrating into steel forms iron nitride with different nitrogen content from surface to inner and various alloy nitrides, especially aluminum nitride and chromium nitride, combined with alloy elements in steel. These nitrides have high hardness, thermal stability and high dispersion, so that the nitrided steel can obtain high surface hardness, wear resistance, fatigue strength, anti-occlusion, resistance to atmospheric and superheated steam corrosion, resistance to tempering and softening, and reduce the notch sensitivity. Compared with the carburizing process, the nitriding temperature is relatively low and the distortion is small, but because of the lower hardness of the center and the shallower carburizing layer, it can only meet the wear and fatigue resistance requirements of light and medium loads, or the machine parts with certain heat and corrosion resistance, as well as various cutting tools, cold and hot working dies, etc. There are many ways of nitriding. Gas nitriding and plasma nitriding are often used.
 Gas nitriding is generally used to improve the wear resistance of metals, so it is necessary to obtain high surface hardness. It is suitable for nitriding steels such as 38CrMnAc. After nitriding, the surface hardness of the workpiece can reach HV850 ~ 1200. Nitriding temperature is low, workpiece distortion is small, can be used in parts with high precision and wear resistance requirements, such as boring bar and spindle, grinder spindle, cylinder sleeve, etc. However, due to the thinner nitriding layer, it is not suitable for heavy duty wear-resistant parts.
 Nitrogen gas can be nitrided by means of general nitriding or isothermal nitriding or multistage (two stage and three stage) nitriding. The nitriding temperature and the decomposition rate of ammonia remain unchanged throughout the nitriding process. The temperature is generally between 480~520 degrees, the decomposition rate of ammonia is 15 ~ 30%, and the holding time is nearly 80 hours. This process is suitable for parts with shallow penetration layer, strict distortion and high hardness requirements, but the treatment time is too long. Multi-stage nitriding is nitriding and diffusing at different temperatures, ammonia decomposition rates and time in different stages during the whole nitriding process. The whole nitriding time can be shortened to nearly 50 hours and a deeper nitriding layer can be obtained, but the nitriding temperature is higher and the distortion is larger.
 There is also gas nitriding for the purpose of corrosion resistance. The nitriding temperature is between 550 700, the holding time is 0.5 3 hours, the ammonia decomposition rate is 35 70%. The surface layer of the workpiece can be chemically stable compound layer to prevent the workpiece from being corroded by wet air, superheated steam, gas combustion products
The normal gas nitriding workpiece has a silver gray surface. Sometimes oxidation may also be blue or yellow, but generally does not affect the use.

Plasma nitriding, also known as glow nitriding, is based on the principle of glow discharge. The metal workpiece is placed as a cathode in a negative pressure vessel with a nitrogen-containing medium. After being electrified, the nitrogen and hydrogen atoms in the medium are ionized and a plasma region is formed between the cathode and the anode. The positive ions of nitrogen and hydrogen are bombarded at high speed to the surface of the workpiece under the action of strong electric field in the plasma region. The high kinetic energy of ions is changed into heat energy, and the surface of the workpiece is heated to the required temperature. Due to ion bombardment, the surface of the workpiece is purified by atom sputtering. At the same time, nitrogen penetrates into the surface of the workpiece due to adsorption and diffusion.
 Compared with general gas nitriding, the characteristics of ion nitriding are as follows: (1) the nitriding period can be shortened appropriately; (2) the brittleness of nitriding layer is small; (3) energy and ammonia consumption can be saved; (4) shielding the parts that do not need nitriding to realize local nitriding; (6) ion bombardment can purify the surface and remove the passive film on the workpiece surface. It can directly nitriding stainless steel and heat-resistant steel workpieces. The thickness and microstructure of the infiltrated layer can be controlled. Ion nitriding has been developed rapidly and has been used in machine tools such as screw rods, gears, moulds and so on.
 Nitrocarburizing is also known as soft nitriding or low temperature carbonitriding, that is, below the iron-nitrogen eutectoid transformation temperature, so that the surface of the workpiece in the main nitrogen infiltration at the same time also carburizing. The formation of fine carbides after carbon infiltration promotes the diffusion of nitrogen and speeds up the formation of high nitrogen compounds. These high nitrogen compounds can increase the solubility of carbon in turn. The mutual promotion of C and N atoms accelerates the penetration rate. In addition, carbon can also reduce brittleness in nitride. After nitrocarburizing, the compound layer has good toughness, high hardness, wear resistance, corrosion resistance and anti bite.
 The common nitrocarburizing methods include liquid method and gas method. The treatment temperature is 530~570 degrees and the holding time is 1~3 hours. In the early stage, cyanide salts in liquid salt bath were used to produce various salt bath formulations afterwards. There are two commonly used salts: neutral salt with ammonia and urea with carbonate as the main salt, but these products are still toxic. Gas media mainly include: endothermic or exothermic gas (see controllable atmosphere) with ammonia; urea thermal decomposition gas; dripping organic solvents containing carbon and nitrogen, such as methylamine, triethanolamine and so on.
Nitrocarburizing can not only improve the fatigue life, wear resistance, corrosion resistance and bite resistance of the workpiece, but also use simple equipment, less investment, easy operation, short time and small distortion of the workpiece, and sometimes give the workpiece a beautiful appearance.




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