Nitriding and Carburizing

Nitriding, carburizing and what are the similarities and differences

1, carburizing: a metal surface treatment.

2, nitriding: is at a certain temperature in a certain medium so that nitrogen atoms penetrate the surface of the workpiece chemical heat treatment process.

1, carburization: carburized steel carbon content are generally in the range of 0.15 - 0.25%, for heavy-duty carburized body, can be increased to 0.25 - 0.30% carbon carburized steel, the most used is 15 and 20 steel, they are carburized and heat-treated surface hardness of up to 56 - 62 HRC. commonly used quenching and low-temperature tempering. Get the surface high hardness heart high toughness wear-resistant impact parts.
The workpiece is placed into the active carburizing medium, heated to 900 - 950 degrees Celsius single-phase austenite area, insulation after sufficient time, so that the carburizing medium decomposition of active carbon atoms penetrate into the surface layer of steel, so as to obtain the surface layer of high carbon, the core is still to maintain the original composition.

2, nitriding: nitriding before the preparatory heat treatment tempering - nitriding workpiece should be tempered before nitriding to obtain tempered Sohnite organization, tempering treatment tempering temperature is generally higher than the nitriding temperature. This treatment workpiece deformation is small, can prevent water vapor, alkaline solution corrosion. However, the production cycle is long, the cost is high, the nitriding layer is thin and brittle/fragile, and it is not suitable for bearing concentrated heavy loads.

What is nitriding (nitriding)
Nitriding is at a certain temperature in a certain medium so that nitrogen atoms penetrate into the surface layer of the workpiece chemical heat treatment process. Common liquid nitriding, gas nitriding, plasma nitriding. The traditional gas nitriding is to put the workpiece into a sealed container, through the flow of ammonia and heating, insulation for a long time, the thermal decomposition of ammonia to produce active nitrogen atoms, constantly adsorbed to the surface of the workpiece, and diffusion of infiltration into the surface layer of the workpiece, so as to change the surface layer of the chemical composition and organization, to obtain excellent surface properties. If in the nitriding process at the same time infiltration of carbon to promote the diffusion of nitrogen, it is called nitrogen-carbon co-infiltration. Commonly used are gas nitriding and plasma nitriding.

Principle

On the one hand, the nitrogen infiltrated into the steel forms iron nitride with different nitrogen content from the surface to the inside, and 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, which results in high surface hardness, abrasion resistance, fatigue strength, resistance to galling, resistance to corrosion by atmospheric and superheated steam, resistance to temper softening and reduced notch sensitivity of the nitrided steel parts. Compared with the carburizing process, nitriding temperature is relatively lower, and thus the distortion is small, but due to the lower hardness of the core, the penetration layer is also shallow, generally can only meet the light, medium load wear resistance, fatigue resistance requirements, or a certain degree of heat, corrosion-resistant machine parts, as well as a variety of cutting tools, cold and hot molds and so on. Nitriding has a variety of methods, commonly used is gas nitriding and plasma nitriding.

Carburizing/carburization is the process of infiltrating carbon atoms into the surface layer of steel. It is also the process of making a low carbon steel workpiece have the surface layer of high carbon steel, and then after quenching and low temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the core of the workpiece still maintains the toughness and plasticity of low carbon steel.

Principle

Carburizing, like other chemical heat treatments, consists of three basic processes.

Decomposition

Decomposition of the carburizing medium produces active carbon atoms.

Adsorption

The active carbon atoms are absorbed by the steel surface and dissolve into the surface austenite, increasing the carbon content of the austenite.

Diffusion

The increase of carbon content on the surface will result in a concentration difference with the carbon content in the core, and the carbon on the surface will diffuse into the interior. The speed of carbon diffusion in steel depends mainly on the temperature and is related to the concentration difference between the internal and external elements in the workpiece and the content of alloying elements in the steel.

Carburized parts are generally made of low carbon steel or low carbon alloy steel (carbon content less than 0.25%). Carburizing must be quenched to give full play to the beneficial effects 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 heart of the organization for the good toughness of low carbon martensite or non-martensite organization, but should avoid the appearance of ferrite. The general depth of carburized layer ranges from 0.8 to 1.2 millimeters, and the depth of carburization can be up to 2 millimeters or deeper. The surface hardness can reach HRC58～63, and the hardness of the heart is HRC30～42. After carburizing and quenching, the surface of the workpiece generates compressive internal stress, which is favorable to improve the fatigue strength of the workpiece. Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts, in order to extend the service life of parts.

Carburizing process is widely used in aircraft, automobiles and tractors and other mechanical parts. Such as gears, shafts, camshafts and so on.

Nitriding and carburizing difference

The biggest difference between carburizing and nitriding is that the medium is different, the applicable steel is different, carburizing is applicable to low carbon steel or low alloy steel, nitriding is applicable to medium carbon steel.

1, carburizing is a kind of metal surface treatment, the use of carburizing for low carbon steel or low alloy steel, the specific method is to put the workpiece into the active carburizing medium, heated to 900 - 950 degrees Celsius single-phase austenite area, insulation after enough time, so that the carburizing medium decomposition of active carbon atoms penetrate into the surface of the steel parts, so as to obtain the surface layer of high carbon, the heart part of the original composition.

2, nitriding: is at a certain temperature in a certain medium so that nitrogen atoms penetrate into the surface layer of the workpiece chemical heat treatment process. Common liquid nitriding, gas nitriding, plasma nitriding. The traditional gas nitriding is to put the workpiece into a sealed container, through the flow of ammonia and heating, insulation for a long time, the ammonia thermal decomposition of reactive nitrogen atoms, constantly adsorbed to the surface of the workpiece, and diffusion of infiltration into the surface layer of the workpiece, so as to change the chemical composition and organization of the surface layer, to obtain excellent surface properties.

Common points of nitriding and carburizing

Nitriding and carburizing are both methods of metal surface treatment, which can improve the surface properties of metal.

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