Nitriding is a chemical heat treatment process in which nitrogen atoms penetrate into the surface of the workpiece at a certain temperature and medium. Common methods are liquid nitriding, gas nitriding, ion nitriding, the latter two types are the most commonly used. The traditional gas nitriding is to put the workpiece in a sealed container full of flowing ammonia gas, after heating and heat preservation for a long time, the ammonia decomposed to active nitrogen atoms adsorbed to the surface of the workpiece and diffused into it, changing the chemical composition and organization of the surface to obtain excellent surface performance.
The nitrogen infiltrating into the steel forms iron nitride with different nitrogen content on the metal surface and core and forms various alloy nitrides with alloying elements in the steel, especially aluminum nitride and chromium nitride. These nitride compounds have high hardness, thermal stability and high dispersion, which can make the nitriding steel get high surface hardness, wear resistance, fatigue strength, anti-bite, anti-atmospheric and superheated steam corrosion ability, anti-tempering softening ability, and reduce notch sensitivity.
If carbon is infiltrated in the process of nitriding to promote the diffusion of nitrogen, it is called nitrocarburizing. Compared with the carburizing process, nitriding temperature is relatively low, small distortion, but because of the low hardness of the heart, the permeability layer is shallow, generally can only meet the requirements of light and medium load wear-resistant, fatigue resistance, or certain heat resistance, corrosion resistance requirements of machine parts, as well as a variety of cutting tools, cold work and hot work die.
Gas Nitriding
The main purpose of gas nitriding is to improve the wear resistance and high surface hardness of the metal, suitable for 41CrAlMo74 nitriding steel. After nitriding, the surface hardness of the workpiece can reach HV850~1200. Nitriding temperature is low, small distortion can be used for high precision requirements, and wear-resistant requirements of the parts, such as boring machine boring rod and spindle, grinding machine spindle, cylinder sleeve, etc. But it is not suitable for wear-resisting parts under heavy load due to the thin nitriding layer.
Gas nitriding can be a general nitriding method (isothermal nitriding) or multi-stage (two, three) nitriding method. The former is nitriding temperature and ammonia decomposition rate remain unchanged in the whole nitriding process, generally between 480~520℃, ammonia decomposition rate of 15~30%, holding time of nearly 80 hours. This process is suitable for the parts with shallow seepage layer, strict distortion and high hardness, but the processing time is too long.
Multi-stage nitriding is to carry out nitriding and diffusion at different temperatures, different ammonia decomposition rates and different times in the whole nitriding process. The whole nitriding time can be shortened to nearly 50 hours, and a deeper infiltration layer can be obtained, but the nitriding temperature is higher and the distortion is larger.
There are anti-corrosion gas nitriding, nitriding temperature between 550~700℃, insulation 0.5~3 hours, ammonia decomposition rate of 35~70%, the surface of the workpiece can obtain chemical stability of high compound layer, prevent the workpiece by wet air, superheated steam, gas combustion products, etc., corrosion.
For a normal gas nitriding workpiece, the surface is silver gray. Sometimes may also be blue or yellow due to oxidation, but generally does not affect the usage.
Ion Nitriding
Also known as glow nitriding because of the glow discharge principle. The metal workpiece is placed in the negative pressure container of the nitrogen medium as the cathode, and the nitrogen and hydrogen atoms in the medium are ionized after electrification, and the strong electric field in the plasma region is formed between the anode and the cathode, and the positive ions of nitrogen and hydrogen “hit” the surface of the workpiece at high speed.
The high kinetic energy of the ions is converted to heat energy, heating the metal surface to the desired temperature. The ion bombard produces atomic sputtering on the surface of the workpiece, which is similar to the purification effect. At the same time, the adsorption and diffusion effect make nitrogen penetrate into the surface of the workpiece, which is used for the parts such as the screw, gear and mold of the machine tool.
Compared with the general gas nitriding, the advantages of ion nitriding are: shorten the nitriding cycle; Nitride temperature in 520-540, the workpiece deformation is small, nitride layer brittleness is small; Local nitriding can be realized, and the thickness and structure of the infiltration layer can be controlled. Ion bombardment can purify the surface and remove the passivation film on the surface of metal parts. Stainless steel and heat-resistant steel can be directly nitriding.
Nitrocarburizing
Low-temperature nitriding, also known as soft nitriding, that is, the metal surface is permeated with nitrogen and carbon below the iron – nitrogen eutectoid transition temperature. The micro carbides formed after the infiltration of a small amount of carbon can promote the diffusion of nitrogen and accelerate the formation of high nitrogen compounds, which increase the solubility of carbon in turn. In addition, carbon in nitrides can reduce brittleness. Nitrocarburizing can not only improve the fatigue life of the workpiece, as well as wear resistance, corrosion resistance and anti-bite ability but also have the advantages of low cost, simple operation, shorter time, small workpiece distortion and good appearance.But the general carbonitriding layer is shallow than the carburizing layer, so it is generally used to bear the light load and require high wear resistance parts.
The common nitrocarburizing methods are the liquid method and gas method. The treatment temperature is 530~570℃ and the preservation g time is 1~3 hours. The liquid method is a cyanide salt or a variety of salt bath formulations, commonly used are neutral salt + ammonia and urea + carbonate, but these reaction products are toxic. Gas medium is mainly endothermic or exothermic gas (controlled atmosphere)+ ammonia, urea thermal decomposition gas, dripping organic solvents containing carbon and nitrogen, such as formamide, triethanolamine and so on.
Cyanide cyaniding refers to high-temperature carbonitriding, which mainly uses carburizing to form nitrogen-containing high-carbon Austenite and nitrogen-containing high-carbon martensite after quenching due to its relatively high temperature and strong diffusion ability of carbon atoms. The infiltration of nitrogen promotes the infiltration of carbon so that the co-infiltration speed is faster. The infiltration layer of 0.5~0.8mm can be obtained by holding for 4~6h, which improves the stability of supercooled austenite.
