carburization

Carburizing is a kind of metal surface treatment. Most of the carburized materials are low carbon steel or low alloy steel. The specific method is to place the workpiece in an active carburizing medium and heat it to the single-phase austenite area of 900-950 ℃. After holding for enough time, the active carbon atoms decomposed from the carburizing medium will penetrate into the surface of the steel piece, so as to obtain high carbon on the surface and keep the original composition in the core A similar process is low temperature nitriding. This is a common heat treatment process for metal materials, which can make the surface of carburized workpiece obtain high hardness and improve its wear resistance.

Carburizing refers to the process of penetrating carbon atoms into the surface layer of steel. It also makes the low carbon steel workpiece have the surface layer of high carbon steel. After quenching and low temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still maintains the toughness and plasticity of low carbon steel.

The material of carburized workpiece is generally low carbon steel or low carbon alloy steel (carbon content is less than 0.25%). After carburizing, the chemical composition of steel surface can be close to that of high carbon steel. The workpiece shall be quenched after carburizing to obtain high surface hardness, high wear resistance and fatigue strength, and maintain the strength and toughness of low-carbon steel quenched in the center, so that the workpiece can withstand impact load. Carburizing process is widely used in mechanical parts of aircraft, automobiles and tractors, such as gears, shafts, camshafts, etc.

The material of carburized parts is generally low carbon steel or low carbon alloy steel (carbon content is less than 0.25%). After carburizing, quenching must be carried out to fully play the beneficial role of carburizing. The surface microstructure of the workpiece after carburizing and quenching is mainly high hardness martensite plus residual austenite and a small amount of carbide. The central structure is low carbon martensite with good toughness or structure containing non martensite, but ferrite should be avoided. Generally, the depth of carburized layer ranges from 0.8 mm to 1.2 mm, and the depth of carburized layer can reach 2 mm or more. The surface hardness can reach HRC58-63, and the core hardness is HRC30-42. After carburizing and quenching, compressive internal stress is generated on the surface of the workpiece, which is beneficial to improving the fatigue strength of the workpiece. Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts, so as to extend the service life of parts.

Carburizing can be divided into solid carburizing, liquid carburizing, gas carburizing and carbonitriding according to different carbon containing media;

nitriding

Nitriding is a chemical heat treatment process that allows nitrogen atoms to penetrate into the surface of the workpiece in a certain medium at a certain temperature. Liquid nitriding, gas nitriding and ion nitriding are common. The 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, the ammonia is decomposed to produce active nitrogen atoms, which are continuously adsorbed on the surface of the workpiece and diffused into the surface of the workpiece, so as to change the chemical composition and fabric of the surface layer and obtain excellent surface performance. If carbon is infiltrated simultaneously in the nitriding process to promote the diffusion of nitrogen, it is called nitrocarburizing. Gas nitriding and ion nitriding are commonly used.

On the one hand, nitrogen infiltrated into steel forms iron nitride with different nitrogen content with iron from the outside to the inside, on the other hand, it combines with alloy elements in steel to form various alloy nitrides, especially aluminum nitride and chromium nitride. 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 seizure, anti atmospheric and superheated steam corrosion, anti tempering softening, and reduce notch sensitivity. Compared with the carburizing process, the nitriding temperature is relatively low, so the distortion is small. However, due to the lower hardness of the core and the shallow carburizing layer, it can generally only meet the requirements of wear resistance and fatigue resistance to bear light and medium loads, or machine parts with certain requirements of heat resistance and corrosion resistance, as well as various cutting tools, cold and hot working molds, etc. There are many methods of nitriding, gas nitriding and ion nitriding are commonly used.

Gas nitriding

Generally, the main purpose is to improve the wear resistance of metals, so high surface hardness is required. It is applicable to 38CrMoAl and other nitrided steels. After nitriding, the surface hardness of the workpiece can reach HV850~1200. The nitriding temperature is low, and the distortion of the workpiece is small. It can be used for parts with high precision and wear resistance requirements, such as boring bar and spindle of boring machine, spindle of grinder, cylinder sleeve, etc. However, due to the thin nitriding layer, it is not suitable for wear-resistant parts bearing heavy loads.

General nitriding (i.e. isothermal nitriding) or multistage (two-stage, three-stage) nitriding can be used for gas nitriding. The former is that the nitriding temperature and ammonia decomposition rate remain unchanged throughout the nitriding process. The temperature is generally 480~520 ℃, the ammonia decomposition rate is 15~30%, and the holding time is nearly 80 hours. This process is applicable to parts with shallow infiltration layer, strict distortion requirements and high hardness requirements, but the treatment time is too long. Multi stage nitriding refers to the process of nitriding and diffusion at different temperatures, different ammonia decomposition rates and different times according to different stages in 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 greater.

There is also gas nitriding for the purpose of corrosion resistance. The nitriding temperature is 550~700 ℃, the heat preservation is 0.5~3 hours, and the ammonia decomposition rate is 35~70%. The surface of the workpiece can obtain a compound layer with high chemical stability to prevent the workpiece from being corroded by wet air, superheated steam, gas combustion products, etc.

The surface of normal gas nitriding workpiece is silver gray. Sometimes, it may be blue or yellow due to oxidation, but it generally does not affect the use.

Ion nitriding

Also known as glow nitriding, it is carried out using the principle of glow discharge. The metal workpiece is put into a negative pressure container with nitrogen containing medium as cathode, and the nitrogen and hydrogen atoms in the medium are ionized after being energized, forming a plasma area between the cathode and anode. Under the action of strong electric field in the plasma area, positive ions of nitrogen and hydrogen bombard the workpiece surface at high speed. The high kinetic energy of ions is converted into heat energy to heat the workpiece surface to the required temperature. As a result of the bombardment of ions, the surface of the workpiece generates atomic sputtering, which is purified. At the same time, due to adsorption and diffusion, nitrogen permeates into the surface of the workpiece.

Compared with general gas nitriding, the characteristics of ion nitriding are: ① the nitriding cycle can be shortened appropriately; ② The brittleness of nitrided layer is small; ③ Energy and ammonia consumption can be saved; ④ The parts that do not need nitriding can be shielded to achieve local nitriding; ⑤ Ion bombardment can purify the surface, remove the passive film on the surface of the workpiece, and make stainless steel and heat-resistant steel workpieces nitrided directly. ⑥ The thickness and structure of the infiltration layer can be controlled. Ion nitriding has developed rapidly and has been used for machine tool screw, gear, mold and other workpieces.