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Introduction and principles of gas carburizing process

Introduction and principles of gas carburizing process
Gas 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 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. Similar to low temperature nitriding treatment. This is a common heat treatment process for metal materials. It can make the surface of the carbon-infiltrated workpiece obtain a high hardness and improve its wear resistance.
It also makes the low carbon steel workpiece have a surface layer of high carbon steel, and then undergoes vacuum quenching and low temperature 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 toughness of low carbon steel. and plasticity.
The material of gas carburizing workpieces is generally low carbon steel or low carbon alloy steel (carbon content is less than 0.25%). After carburization, the chemical composition of the steel surface can be close to that of high carbon steel. After carburizing, the workpiece must be quenched to obtain high surface hardness, high wear resistance and fatigue strength, and to maintain the strength and toughness of the quenched low carbon steel in the core, so that the workpiece can withstand impact loads.
The gas carburizing process is widely used in mechanical parts such as aircraft, automobiles and tractors, such as gear shafts, camshafts, etc.

The principle of gas carburizing
Contains 3 basic processes.
① Decomposition: The decomposition of carburizing medium produces active carbon atoms.
②Adsorption: Activated carbon atoms are absorbed by the surface of the steel and immediately dissolve into the surface austenite, increasing the carbon content in the austenite.
③Diffusion: 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 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 gas 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 retained 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 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 level of carbon concentration and how to prevent it

1. Excessive carbon concentration and prevention methods
Causes and hazards: If the carburizing process is heated rapidly, the temperature is too high, or a new carburizing agent is used during solid carburizing, or too much strong carburizing agent is used, it will cause the carburizing concentration to be too high. As the carbon concentration is too high, massive and coarse carbides or network carbides appear on the surface of the workpiece. Due to the generation of this hard and brittle structure, the toughness of the carburized layer drops sharply. Moreover, high carbon martensite is formed during quenching, and grinding cracks are prone to occur during grinding.

Ways to prevent:
① Do not heat rapidly. It is necessary to use an appropriate heating temperature to prevent the grains of the steel from growing. If the grains are coarse during carburizing, normalizing or two quenching treatments should be performed after carburizing to refine the grains.
② Strictly control the uniformity of the furnace temperature and avoid excessive fluctuations. Special attention must be paid to solid carburizing in the reverberatory furnace.
The material of carburized parts is generally low carbon steel or low carbon alloy steel (carbon content is less than 0.25%). Quenching must be carried out after 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 retained 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 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.

2. Carbon concentration is too low and prevention methods
Causes and hazards: Large temperature fluctuations or too little penetration agent will cause insufficient carbon concentration on the surface. The ideal carbon concentration is between 0.9-1.0%. If it is lower than 0.8%C, parts will wear easily. Since the carbon concentration is almost never too low in actual production, it is difficult to process even if it occurs in the later stage. Freezing treatment can improve the hardness.