Carburizing Process Introduction

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Carburizing refers to the process of infiltrating carbon atoms into the surface layer of steel. It is also to make the work-piece of low carbon steel have the surface layer of high carbon steel, and then through quenching and low temperature tempering, so that the surface layer of the work-piece has high hardness and wear resistance, while the central part of the work-piece still maintains the toughness and toughness of low carbon steel. plasticity.

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

The main mechanism is to let the steel surface receive the most various loads (wear, fatigue, mechanical load and chemical corrosion), and achieve high surface hardness, high wear resistance, fatigue strength and corrosion resistance by infiltrating elements such as carbon. , without having to process the entire material through expensive alloying or other complex processes. This can not only replace some of the more expensive high-alloy steels with low-cost carbon steel or alloy steel, but also maintain the strength and toughness of the low-carbon steel in the core after quenching, so that the work-piece can withstand impact loads. Therefore, it is fully in line with the direction of energy saving, consumption reduction and sustainable development.

The carburizing process in China can be traced back to before the 20th century. The earliest is carburizing with solid carburizing medium. Liquid and gas carburizing was introduced and widely used in the 20th century. The United States began to use rotary drum furnaces for gas carburizing in the 1920s. In the 1930s, continuous gas carburizing furnaces began to be used in industry. In the 1960s, high temperature (960 ~ 1100 ℃) gas carburizing was developed. In the 1970s, vacuum carburizing and ion carburizing appeared.

According to the different carbon-containing media, carburizing can be divided into gas carburizing, solid carburizing, liquid carburizing, and carbonitriding (cyanation).

Gas carburizing is to put the work-piece into a closed carburizing furnace, pass gas infiltrating agent (methane, ethane, etc.) or liquid infiltrating agent (kerosene or benzene, alcohol, acetone, etc.), and decompose activated carbon atoms at high temperature , a carburizing operation process that penetrates into the surface of the work-piece to obtain a high-carbon surface layer.

Solid carburizing is to put the work-piece and solid carburizing agent (composed of charcoal and accelerator) together in a closed carburizing box, put the box into a heating furnace and heat it to the carburizing temperature, and keep it for a certain period of time to make the activated carbon atoms One of the earliest carburizing methods for infiltrating the surface of work-pieces.

Liquid carburizing is the use of liquid medium for carburizing. Commonly used liquid carburizing medium are: silicon carbide, “603” carburizing agent, etc.

Carbonitriding (cyanidation) is further divided into gas carbonitriding, liquid carbonitriding, and solid carbonitriding.
Carburizing, like other chemical heat treatments, also includes three basic processes.
Decomposition→Adsorption→Diffusion
Decomposition: The decomposition of the carburizing medium produces activated carbon atoms.

Adsorption: After the activated carbon atoms are absorbed by the steel surface, they dissolve into the surface austenite, which increases the carbon content in the austenite.
Diffusion: When the carbon content on the surface increases, there is a difference in concentration between the carbon content in the core, and the carbon on the surface diffuses into the interior. The diffusion rate of carbon in steel mainly depends on the temperature, and it is also related to the difference between the inner and outer concentrations of the infiltrated elements in the work-piece and the content of alloying elements in the steel.

Low carbon steel carburizing: The material of carburizing 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 work-piece after carburizing and quenching is mainly high-hardness martensite plus retained austenite and a small amount of carbides, and the core structure is low-carbon martensite with good toughness or a non-martensite-containing structure. However, ferrite should be avoided. Generally, the depth of carburized layer ranges from 0.8 to 1.2 mm, and it can reach 2 mm or more during deep carburization. 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 work-piece, which is beneficial to improve the fatigue strength of the work-piece.

Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts, thereby extending the service life of parts
1. One-time heating, quenching, low temperature tempering, carburizing temperature 820～850℃ or 780～810℃
Features: For those with high requirements on the strength of the core, quenching at 820-850ºC is used, and the core structure is low-carbon martensite; for those with high surface hardness, the grains can be refined by heating and quenching at 780-810ºC
Scope of application: It is suitable for carbon steel and low alloy steel work-pieces after solid carburizing. Coarse-grained steel with thick gas and liquid carburizing, some work-pieces that are not suitable for direct quenching after carburizing, and parts that need to be machined after carburizing.

2. Carburizing, high temperature tempering, one-time heating quenching, low temperature tempering, carburizing temperature 840～860℃
Features: High temperature tempering decomposes martensite and retained austenite, and carbon and alloying elements in the infiltrating layer are precipitated in the form of carbides, which is convenient for machining and quenching. After quenching, the retained austenite is reduced
Scope of application: mainly used for CR-NI alloy steel carburizing work-pieces

3. Secondary quenching and low temperature tempering
Features: The first quenching (or normalizing) can eliminate the infiltration layer network carbide and refine the core structure. The second quenching mainly improves the structure of the infiltrated layer, but when the performance requirements of the core are higher, the core should be quenched above AC3
Scope of application: It is mainly used for important carburized work-pieces with high requirements on mechanical properties, especially for coarse-grained steel. However, two high-temperature heatings are required after carburizing, which increases the deformation and oxidative decarburization of the work-piece, and the heat treatment process is more complicated.

4. Secondary quenching, cold treatment, low temperature tempering
Features: Quenching at a temperature higher than AC1 or AC3 (core), the surface layer of high-alloy steel retains austenite, and cold treatment (-70～80ºC) promotes austenite transformation, thereby improving surface hardness and wear resistance
Scope of application: Mainly used for high-alloy steel work-pieces that do not require machining after carburizing

5. Direct quenching and low temperature tempering
Features: The grain of steel cannot be refined. The quenching distortion of the work-piece is large, the surface of the alloy steel carburized part has a large amount of retained austenite, and the surface hardness is low
Scope of application: simple operation and low cost. Pit furnaces are used to process parts that are not subject to deformation and shock loads, and are suitable for gas carburizing and liquid carburizing processes

6. Precooling, direct quenching, low temperature tempering, quenching temperature 800～850℃

Features: It can reduce the quenching distortion of the work-piece, the amount of retained austenite in the carburized layer can also be slightly reduced, the surface hardness is slightly increased, but the austenite grains have not changed

Scope of application: simple operation, small oxidation, decarburization and quenching deformation of work-piece. Widely used in various work-pieces made of fine-grained steel.
Common defects
1. Too high carbon concentration
1. Causes and hazards: If the carburizing is heated rapidly, the temperature is too high, or a new carburizing agent is used in solid carburizing, or too much strong catalyzing agent is used, the carburizing concentration will be too high. As the carbon concentration is too high, massive and coarse carbides or network carbides appear on the surface of the work-piece. Due to this hard and brittle structure, the toughness of the carburized layer drops sharply. In addition, high-carbon martensite is formed during quenching, and grinding cracks are prone to occur during grinding.

2. Methods of prevention
① It cannot be heated rapidly, and it is necessary to use an appropriate heating temperature so as not to make the grains of the steel grow. If the grains are coarse during carburizing, the grains should be refined by normalizing or twice quenching after carburizing.
②Strictly control the uniformity of furnace temperature and do not fluctuate too much. Special attention should be paid to solid carburizing in the reverberatory furnace.
③ When solid carburizing, the carburizing agent should be used in the ratio of new and old. It is best to use 4-7% BaCO3 as the infiltrating agent, and do not use Na2CO3 as the infiltrating agent.

2. The carbon concentration is too low
⒈ Causes and hazards: Large temperature fluctuations or too little infiltration agent will cause insufficient carbon concentration on the surface. The ideal carbon concentration is between 0.9-1.0%, below 0.8% C, the parts are easy to wear.

⒉ Methods of prevention:
①Carburizing temperature is generally 920-940℃. If the carburizing temperature is too low, the carbon concentration will be too low, and the carburizing time will be prolonged; if the carburizing temperature is too high, the grains will be coarse.
②The dosage of penetrating agent (BaCO3) should not be less than 4%.

3. Local carbon depletion on the surface after carburizing
⒈ Causes and hazards: During solid carburization, the charcoal particles are too large or mixed with impurities such as stones, or the infiltration agent and the charcoal are not evenly mixed, or the work-pieces come into contact with each other. Dirt on the work-piece surface can also cause carbon depletion.
⒉ Methods of prevention
①The solid carburizing agent must be prepared in proportion and stirred evenly.
②Be careful not to touch the work-pieces installed in the furnace. When solid carburizing, the carburizing agent should be rammed, and the work-piece should not be contacted by too much carburizing.
③Remove the dirt on the surface.

4. Carburizing concentration intensifies the transition
⒈ Causes and hazards: The sudden transition of carburizing concentration means that the change of carbon concentration on the surface and the center is intensified. It is not a uniform transition from high to low, but a sudden transition. The reason for this defect is that the carburizing agent has a strong effect (such as newly prepared charcoal, the old carburizing agent is added very little), and at the same time, alloying elements such as Cr, Mn, and Mo in the steel promote the formation of carbides, causing High concentration on the surface, low concentration in the center, and no transition layer. This defect causes considerable internal stress on the inside and outside, and cracks or spalling occurs during the quenching process or the grinding process.
⒉ Prevention method: The old and new carburizing agent is formulated according to the prescribed ratio to ease carburizing. It is better to use BaCO3 as the infiltrating agent, because Na2CO3 is relatively sharp.

5. Tempering and cracking during grinding
⒈ Cause: The phenomenon of softening of the surface of the carburized layer after grinding is called tempering caused by grinding. This is due to the fact that the machining feed rate is too fast during grinding, improper selection of the hardness and particle size of the grinding wheel or rotation speed, or insufficient cooling during the grinding process are all prone to such defects. This is because the heat during grinding softens the surface. When tempering defects occur during grinding, the wear resistance of the parts is reduced.

Hexagonal cracks appear on the surface. This is because the surface of the hard grinding wheel is excessively ground and heats up. It is also related to insufficient heat treatment and tempering and excessive residual internal stress. After etching with acid, any defective parts are black, which can be distinguished from those without defects. This is the heat tempering produced during grinding. The reason for the transformation of the horse body into the troostite structure. In fact, cracks can be seen with the naked eye after grinding.
⒉ Methods of prevention:
①After quenching, it must be fully tempered or tempered many times to eliminate internal stress.
②Use a soft or medium alumina grinding wheel with a grain size of 40~60, and the grinding feed rate is not too large.
③When grinding, open the coolant first, and pay attention to sufficient cooling during the grinding process.

Editor: Frank Lee
Copyright: SIMUWU Vacuum Furnace