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Comparative study on vacuum nitriding and carburizing processes

Nitriding is a chemical heat treatment process that penetrates nitrogen atoms into the surface of the workpiece in a certain medium at a certain temperature. Commonly used are liquid nitriding, gas nitriding, and ion nitriding. Traditional gas nitriding is to put the workpiece into a sealed container, pass it through flowing ammonia gas and heat it. After being kept warm for a long time, the ammonia gas thermally decomposes to produce active nitrogen atoms, which are continuously adsorbed to the surface of the workpiece and diffuse into the surface layer of the workpiece. , thereby changing the chemical composition and structure of the surface layer and obtaining 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.

1.Principle of vacuum nitriding

On the one hand, the nitrogen that penetrates into the steel forms iron nitride with different nitrogen contents from the surface to the inside. On the other hand, it combines with the alloying elements in the 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 parts can obtain high surface hardness, wear resistance, fatigue strength, seizure resistance, resistance to atmosphere and superheated steam Corrosion resistance, resistance to temper softening, and reduced notch sensitivity. Compared with the vacuum carburizing process, the nitriding temperature is relatively low, so the distortion is small. However, due to the lower hardness of the core and the shallower carburizing layer, it generally can only meet the wear resistance and fatigue resistance requirements of light and medium loads, or Machine parts with certain requirements for heat resistance and corrosion resistance, as well as various cutting tools, cold work and hot work molds, etc. There are many methods of nitriding, the most commonly used ones are vacuum nitriding and ion nitriding.

2.What is vacuum carburizing?

Vacuum 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, and then undergoes vacuum quenching and low temperature vacuum tempering to make the surface layer of the workpiece have high hardness and wear resistance, while the central part of the workpiece still maintains the texture of low carbon steel. Toughness and plasticity.

Principle of vacuum carburizing: Carburizing, like other chemical heat treatments, also includes three basic processes: decomposition, adsorption and diffusion.

Decomposition: The decomposition of the carburizing medium produces active carbon atoms.

Adsorption: After the activated carbon atoms are absorbed by the surface of the steel part, the activated carbon atoms dissolve into the surface austenite, increasing the carbon content in the austenite.

Diffusion: As the carbon content on the surface increases, there will be a concentration difference with the carbon content in the center, and the carbon on the surface will diffuse to the inside. The diffusion rate of carbon in steel mainly depends on temperature, and is also related to the concentration difference between the inside and outside of the infiltrated element in the workpiece and the content of alloying elements in the steel.

The material of vacuum carburized parts is generally low carbon steel or low carbon alloy steel (carbon content is less than 0.25%). Vacuum quenching must be performed after vacuum carburizing to give full play to the beneficial effects of carburizing. The surface microstructure of the workpiece after carburization and quenching is mainly high-hardness martensite plus residual austenite and a small amount of carbides. The core structure is low-carbon martensite with good toughness or a structure containing non-martensite. However, ferrite should be avoided. Generally, the depth of the carburizing layer ranges from 0.8 to 1.2 mm, and can reach 2 mm or deeper during deep carburizing. The surface hardness can reach HRC58~63, and the core hardness is HRC30~42. After vacuum 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, thereby extending the service life of parts. The vacuum carburizing process is widely used in mechanical parts such as aircraft, automobiles and tractors. Such as gears, shafts, camshafts, etc.

3.The difference between vacuum nitriding and carburizing

(1) The biggest difference between vacuum carburizing and vacuum nitriding is that the media are different and the applicable steels are also different. Carburizing is suitable for low carbon steel, and nitriding is suitable for medium carbon steel.

(2)Vacuum carburizing: It is a kind of metal surface treatment. Most of the carburizing methods 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 900~950 degrees Celsius. In the single-phase austenite zone, after being kept warm for a sufficient time, the active carbon atoms decomposed in the carburizing medium can penetrate into the surface of the steel piece, thereby obtaining high carbon on the surface, while the core still maintains its original composition.

(3)Vacuum nitriding: It 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. Commonly used are liquid nitriding, gas nitriding, and ion nitriding. Traditional gas nitriding is to put the workpiece into a sealed container, pass it through flowing ammonia gas and heat it. After being kept warm for a long time, the ammonia gas thermally decomposes to produce active nitrogen atoms, which are continuously adsorbed to the surface of the workpiece and diffuse into the surface layer of the workpiece. , thereby changing the chemical composition and structure of the surface layer and obtaining excellent surface properties.

(4)Common points between vacuum nitriding and vacuum carburizing

Vacuum nitriding and vacuum carburizing are both methods of treating metal surfaces, both of which can improve metal surface properties.