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Basic overview of carburization


Sep 18,2018

Basic overview of carburization

Carburizing is the process of making carbon atoms infiltrate into the surface layer of steel. It also makes the workpiece of low carbon steel have the surface layer of high carbon steel, and then quenched and low temperature tempered 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 and plasticity of low carbon steel.

The material of carburized workpiece is generally low carbon steel or low carbon alloy steel (carbon content less than 0.25%). After carburizing, the chemical composition of the surface of the steel part 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 toughness of the quenched mild steel in the heart so that the workpiece can withstand impact loads. The carburizing process is widely used in aircraft, automobiles and tractors for mechanical parts such as gears, shafts and camshafts.

The main mechanism is to infiltrate carbon and other elements to achieve high surface hardness, high wear and fatigue strength, and corrosion resistance where the steel surface is subjected to the most various types of loads (wear, fatigue, mechanical loads, and chemical corrosion), without having to treat the entire material through expensive alloying or other complex processes. This not only replaces some of the more expensive high-alloy steels with inexpensive carbon or alloy steels, but also maintains the toughness of the quenched low carbon steel in the heart, allowing the workpiece to withstand impact loads. Therefore, it is fully in line with the direction of energy saving, consumption reduction and sustainable development.

Carburizing process in China can be traced back to before the 20th century. The earliest carburizing was done with solid carburizing medium. Liquid and gas carburizing emerged and was widely used in the 20th century. In the United States, gas carburizing was started in the 1920's in rotary furnaces, and in the 1930's, continuous gas carburizing furnaces were used in industry, and in the 1960's, high temperature (960-1100°C) gas carburizing was developed. In the 1970s, vacuum carburizing and ion carburizing were introduced.

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

Gas carburizing is a carburizing process in which the workpiece is loaded into a closed carburizing furnace, and gas carburizing agent (methane, ethane, etc.) or liquid carburizing agent (kerosene or benzene, alcohol, acetone, etc.) is introduced to decompose active carbon atoms at high temperature and penetrate into the surface of the workpiece to obtain a high carbon surface layer.

Solid carburizing is one of the earliest carburizing methods in which the workpiece and solid carburizing agent (charcoal plus accelerator) are packed together in a closed carburizing box, which is heated to carburizing temperature in a heating furnace and held for a certain period of time to make active carbon atoms penetrate into the surface of the workpiece.

Liquid carburizing is the use of liquid medium for carburizing, commonly used liquid carburizing medium are: silicon carbide, "603" carburizing agent, etc.

Carburizing (cyanidation) is also divided into gas carburizing, liquid carburizing and solid carburizing.

Like other chemical heat treatment, carburization also contains three basic processes.

Decomposition → Adsorption → Diffusion

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

Adsorption: The active carbon atoms are absorbed by the steel surface and then dissolved into the surface austenite to increase the carbon content in the austenite.

Diffusion: The carbon content of the surface increases and the carbon content of the heart of the concentration difference, the surface of the carbon is then diffused to the internal. The rate of carbon diffusion in steel depends mainly on the temperature and is related to the difference in concentration of the carburized elements inside and outside the workpiece and the content of alloying elements in the steel.

Low-carbon steel carburizing: Carburized parts are generally made of low-carbon steel or low-carbon alloy steel (carbon content less than 0.25%). After 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, and the core organization is ductile low-carbon martensite or non-martensite containing organization, but should avoid ferrite. Generally, the depth of carburizing layer ranges from 0.8 to 1.2 mm, and the depth of carburizing can be 2 mm or deeper. After carburizing and quenching, compressive internal stresses are generated on the surface of the workpiece, which is beneficial to improve the fatigue strength of the workpiece. Therefore, carburizing is widely used to improve the strength, impact toughness and wear resistance of parts to extend the service life of parts

 1, a heating quenching low-temperature tempering, carburizing temperature 820 ~ 850 ℃ or 780 ~ 810 ℃

Features: high strength requirements for the heart, using 820 ~ 850ºC quenching, the heart of the organization for low-carbon martensite; surface requirements for high hardness, using 780 ~ 810ºC heating and quenching can refine the grain

Scope of application: for solid carburizing after the carbon steel and low alloy steel workpiece. Gas, liquid carburizing thick coarse grain steel, some carburizing after direct quenching of the workpiece and carburizing parts to be machined.

2, carburizing, high-temperature tempering, once heated quenching, low-temperature tempering, carburizing temperature 840 ~ 860 ℃

Features: High-temperature tempering makes martensite and residual austenite decomposition, carbon and alloy elements in the carburized layer in the form of carbide precipitation, easy to work machining and quenching after the carburized layer residual austenite reduction

Scope of application: mainly used for CR-NI alloy steel carburizing workpiece

 3、Second quenching and low-temperature tempering

Features: the first quenching (or normalizing), can eliminate the carburized layer mesh carbide and refine the core organization. The second quenching mainly to improve the carburized layer organization, but the core performance requirements should be higher than the core AC3 quenching

Scope of application: mainly used for important carburized workpieces with high requirements for mechanical properties, especially for coarse-grained steels. However, after carburizing requires two high-temperature heating, so that the workpiece deformation and oxidation decarburization increases, the heat treatment process is more complex

 4、Second quenching and cold treatment low-temperature tempering

Features: higher than AC1 or AC3 (heart) temperature quenching, high alloy steel surface residual austenite more, cold treatment (-70 ~ 80ºC) to promote austenite transformation, thereby improving the surface hardness and wear resistance

Scope of application: mainly used for carburizing high alloy steel workpieces that do not require mechanical processing

5、Direct quenching and low-temperature tempering

Features: can not refine the grain of steel. Workpiece quenching distortion is larger, alloy steel carburized parts with more surface residual austenite, the surface hardness is lower

Scope of application: simple operation, low cost. Pit-type furnace used to deal with the deformation and impact load is not large parts, suitable for gas carburizing and liquid carburizing process

6、Pre-cooling direct quenching and low-temperature tempering, quenching temperature 800 ~ 850 ℃

Features: can reduce the quenching distortion of the workpiece, the amount of residual austenite in the carburized layer can also be slightly reduced, the surface hardness is slightly increased, but the austenite grains do not change

Scope of application: simple operation, workpiece oxidation, decarburization and quenching deformation are small. Widely used in various workpieces made of fine-grained steel.

Common defects

I. Carbon concentration is too high

⒈ causes and hazards: If the carburizing is heated sharply, the temperature is too high or solid carburizing with a new carburizing agent, or with a strong carburizing agent too much will cause the phenomenon of carburizing concentration is too high. With the high carbon concentration, the surface of the workpiece appears blocky and coarse carbide or reticulated carbide. Due to this hard and brittle organization, the toughness of the carburized layer drops sharply. And high carbon martensite is formed during quenching, which is prone to grinding cracks during grinding.

2. Methods of prevention

①Can not be sharply heated, need to use the appropriate heating temperature, not to make the steel grain growth is good. If the grain is coarse when carburizing, it should be normalized or quenched twice after carburizing to refine the grain.

② Strict control of furnace temperature uniformity, not fluctuate too much, in the reflection furnace in the solid carburizing need to pay special attention.

③When solid carburizing, the carburizing agent should be used in the ratio of new and old. It is better to use 4-7% BaCO3 as carburizing agent, and not to use Na2CO3 as carburizing agent.

II. Carbon concentration is too low

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

The methods of peal prevention.

① carburizing temperature is generally used 920-940 ℃, carburizing temperature is too low will cause the carbon concentration is too low, and prolong the carburizing time; carburizing temperature is too high will cause grain coarseness.

② The amount of carburizing agent (BaCO3) should not be less than 4%.

Three. After carburizing surface local poor carbon

⒈ causes and hazards: solid carburizing, charcoal particles are too large or inclusions of stones and other impurities, or carburizing agent and charcoal mixed unevenly, or the workpiece contact can cause local no carbon or poor carbon. The dirt on the surface of the workpiece can also cause carbon poverty.

The method to prevent peal

①The solid carburizing agent must be prepared according to the proportion and mixed evenly.

②The workpiece loaded in the furnace should be careful not to have contact. When solid carburizing, pound the carburizing agent solid, do not make the carburizing over collapse and make the workpiece contact.

③Remove the dirt from the surface.

IV. Carburizing concentration intensified transition

⒈ generated by the causes and hazards: carburizing concentration of sudden transition is the surface and the center of the carbon concentration changes intensify, not from high to low uniform transition, but a sudden transition. The reason for this defect is that the carburizing agent is very strong (such as newly prepared charcoal, the old carburizing agent added very little), while the steel has Cr, Mn, Mo and other alloying elements is to promote the formation of carbide is strong, and cause the surface high concentration, low concentration in the center, and no transition layer. After the production of this defect caused considerable internal stress on the surface, in the quenching process or grinding process to produce cracks or spalling phenomenon.

The method to prevent pealing: carburizing agent new and old according to the prescribed ratio system, so that carburizing moderate. It is better to use BaCO3 as a carburizing agent, because Na2CO3 is more acute.

V. Tempering and cracking during the grinding process

⒈ generated by: carburizing layer after grinding process caused by the softening of the surface phenomenon, called grinding process generated by tempering. This is due to too fast processing feed during grinding, improper selection of grinding wheel hardness and grain size or speed, or insufficient cooling during grinding, which can easily produce such defects. This is due to the softening of the surface caused by the heat during grinding. Tempering defects during grinding will reduce the wear resistance of the part.

Hexagonal cracks are produced on the surface. This is due to excessive grinding of the surface with a hard grinding wheel, which causes heating. It is also related to insufficient heat treatment and tempering, and excessive residual internal stress. After etching with acid, any defective parts are black and can be distinguished from those without defects. This is the heat tempering generated during grinding. Make the horse make the body transformed into the reason of flexural organization. In fact, cracks can be seen with the naked eye after grinding.

The methods of peal prevention.

①After quenching, it must be fully tempered or tempered several times to eliminate internal stress.

②Use a soft or medium quality alumina grinding wheel with 40~60 grit size and the grinding feed is not too large.

③Open the coolant first when grinding and pay attention to adequate cooling during grinding.

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