carburization
2023-09-20 14:12:36
Carburizing refers to the process of infiltrating 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 after quenching and low temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still maintains the toughness of low carbon steel. Plasticity.
The definition of carburizing is a kind of metal surface treatment. The carburizing is mostly low carbon steel or low alloy steel. The specific method is to place the workpiece into a single phase with an activated carburizing medium and heated to 900-950 degrees Celsius. In the austenite region, after sufficient time of heat preservation, the activated carbon atoms decomposed in the carburizing medium penetrate into the surface layer of the steel, thereby obtaining high carbon in the surface layer, and the core remains the original composition. Similarly, low temperature nitriding treatment is performed. This is a common heat treatment process for metal materials, which can achieve high hardness and improve the wear resistance of the surface of the carbonized workpiece.
Introduction Carburizing is the process of infiltrating carbon atoms into the steel surface layer. It also makes the low carbon steel workpiece have the surface layer of high carbon steel, and after quenching and low temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still maintains the toughness of low carbon steel. Plasticity.
The material of the 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 steel surface can be close to high carbon steel. After the workpiece is carburized, it is quenched to obtain high surface hardness, high wear resistance and fatigue strength, and maintain the toughness of the low carbon steel after quenching in the core, so that the workpiece can withstand the impact load. Carburizing processes are widely used in mechanical parts such as gears, axles, camshafts, etc. for aircraft, automobiles and tractors.
The carburizing process can be traced back to 2000 before China. The earliest was carburizing with a solid carburizing medium. Liquid and gas carburizing appeared in the 20th century and is widely used. In the United States in the 1920s, gas drum carburizing was started. In the 1930s, continuous gas carburizing furnaces began to be used industrially. High temperature (960 ~ 1100 ° C) gas carburization in the 1960s was developed. By the 1970s, vacuum carburization and ion carburization occurred.
Principle Carburizing, like other chemical heat treatments, also involves three basic processes.
1 decomposition: decomposition of the carburizing medium produces activated carbon atoms.
2 Adsorption: The activated carbon atoms are absorbed into the surface austenite after being absorbed by the surface of the steel, so that the carbon content in the austenite is increased.
3 Diffusion: When the surface carbon content increases, the concentration of carbon in the heart is different, and the carbon enthalpy on the surface diffuses into the interior. The diffusion rate of carbon in steel depends mainly on the temperature, and is related to the difference between the internal and external concentrations of the infiltrated elements 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 less than 0.25%). After carburizing, quenching must be carried out in order to fully exert the beneficial effect of carburizing. The surface microstructure of the workpiece after carburizing and quenching is mainly high hardness martensite plus retained austenite and a small amount of carbide, and the core structure is low-carbon martensite with good toughness or non-martensitic structure. However, ferrite should be avoided. Generally, the carburized layer has a depth ranging from 0.8 to 1.2 mm and a depth of carburizing up to 2 mm or more. 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 improve 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.
Classification According to different carbon-containing media, carburizing can be divided into solid carburizing, liquid carburizing, gas carburizing and carbonitriding.
Carburizing process 1. Direct quenching low temperature tempering structure and performance characteristics: The grain of steel cannot be refined. The quenching deformation of the workpiece is large, and the amount of retained austenite on the surface of the alloy steel carburized part is large, and the surface hardness is low.
Scope of application: Simple operation and low cost for handling parts with deformation and impact load, suitable for gas carburizing and liquid carburizing processes.
2, pre-cooling direct quenching, low temperature tempering, quenching temperature 800-850 °C structure and performance characteristics: can reduce the workpiece quenching deformation, the amount of retained austenite in the layer can also be slightly reduced, the surface hardness is slightly improved, but the Austrian There is no change in the grain of the body.
Scope of application: Simple operation, small oxidation, decarburization and quenching deformation of workpieces, widely used in various tools made of fine grain steel.
3, a heat quenching, low temperature tempering, quenching temperature 820-850 ° C or 780-810 ° C. Structure and performance characteristics: for the higher core strength requirements, using 820-850 ° C quenching, the heart is low carbon M, surface For those with high hardness, quenching at 780-810 °C can refine the grains. Scope of application: Applicable to carbon steel and low alloy steel workpieces after solid carburizing, gas and liquid carburized coarse grain steel, some workpieces that are not suitable for direct quenching after carburizing and parts that need to be machined after carburizing.
4, carburizing high temperature tempering, one heating and quenching, low temperature tempering, quenching temperature 840-860 °C. Structure and performance characteristics: high temperature tempering causes M and residual A to decompose, carbon and alloying elements in the layer are precipitated as carbides. It is easy to reduce the residual A after cutting and quenching. Scope of application: Mainly used for carburizing workpieces of Cr-Ni alloy
5, secondary quenching low temperature tempering structure and performance characteristics: the first quenching (or normalizing), can eliminate the carburized layer of network carbides and refine the heart tissue (850-870 ° C), the second quenching mainly Improve the layer structure, quenching between Ac1-Ac3 of the material when the performance of the core is not high, and quenching above Ac3 when the core performance is high.
Scope of application: Mainly used for important carburizing parts with high mechanical properties, especially for coarse grain steel. However, after carburizing, two high-temperature heating is required to increase the deformation and oxidative decarburization of the workpiece, and the heat treatment process is complicated.
6. Low temperature tempering structure and performance characteristics of secondary quenching and cold treatment: higher than Ac1 or Ac3 (heart) temperature quenching, more residual A in high alloy surface layer, and cold treatment (-70 °C / -80 °C) to promote A transformation Improve surface hardness and wear resistance.
Scope of application: Mainly used for high-alloy steel workpieces that are not machined after carburizing.
7. Induction heating quenching after carburizing Low temperature tempering structure and performance characteristics: It can refine the infiltration layer and the structure near the infiltration layer. The quenching deformation is small, and the hardened portion is not allowed to be pre-impregnated.
Scope of application: New developments in various gear and shaft carburizing processes
The carburizing process is a very old process, and in China, it can be traced back to 2000. At first it was carburized with a solid carburizing medium. Liquid and gas carburization occurred in the 20th century and is widely used. Later, vacuum carburization and ion carburization occurred. Up to now, the carburizing process still has very important practical value. The reason is that its reasonable design idea is to let the steel surface layer receive the most load (wear, fatigue, mechanical load and chemical corrosion), through the infiltration of carbon. The elements achieve high surface hardness, high wear resistance and fatigue strength and corrosion resistance without the need to treat the entire material by expensive alloying or other complicated 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 toughness of the low-carbon steel after quenching in the core, so that the workpiece can withstand the impact load. Therefore, it fully meets the direction of energy conservation, consumption reduction and sustainable development.
In recent years, a high-concentration carburizing process has occurred, which is different from the conventional process in carburizing in a completely austenite region (temperature at 900-950 ° C, surface carbon content after carburization is 0.85% to 1.05%). The non-uniform austenite state between Ac1 and Accm is carried out, and the carbon concentration on the surface of the layer can be as high as 2% to 4%. As a result, a uniform, dispersed distribution of fine particle carbides can be obtained. The carburizing temperature is reduced to a temperature range of 800 ° C to 860 ° C, which can achieve direct quenching after carburizing of general steel; since the high concentration carburizing layer contains a high amount (20% to 50%) of dispersed carbides, it is displayed It has better wear resistance, corrosion resistance, higher contact and bending fatigue strength, higher impact toughness, lower brittleness and better tempering stability than ordinary carburizing. The process also has the advantages of wide applicability, no special requirements for equipment, high economic efficiency and practical value, and has obtained competitive research and development at home and abroad in recent years.
In order to prevent coarsening of austenite grains during carburization, an appropriate amount of titanium is generally added to the steel to prevent grain growth by forming a grain boundary of the titanium carbonitride particles. The national standard stipulates that the amount of titanium added in the carburized steel is 0.04 to 0.08 wt%. However, recent research has shown that when the titanium content exceeds 0.032%, titanium nitride is precipitated when the carburized steel smelting ingot solidifies. The size of the titanium nitride is on the order of micrometers, which does not prevent the growth of austenite grains. Instead, it is the source of microcracks due to the sharp angle effect of the cubic particles and the discontinuity of the matrix structure. And the crack propagation of the relay station, seriously impairing the toughness and plasticity of the steel. The work also shows that reducing the titanium content to 0.02-0.032% can still effectively control the austenite grain growth, and avoid the formation of harmful titanium nitride particles, so it is recommended to be reasonable. Range of choice
(1) The carbon concentration is too high
1. Causes and hazards: If the carburization is heated rapidly, the temperature is too high, or a new carburizing agent is used when the carburizing is solid, or a strong infiltration agent is too much, the carburization concentration is too high. As the carbon concentration is too high, massive carbides or reticulated carbides appear on the surface of the workpiece. Due to the formation of such a hard and brittle structure, the toughness of the carburized layer is drastically lowered. And high carbon martensite is formed during quenching, and grinding cracks are likely to occur during grinding.
2. Method of prevention
1 can not be heated sharply, it is necessary to use appropriate heating temperature, so that the grain growth of steel is not good. If the grains are coarse during carburizing, the grains should be refined by normalizing or twice quenching after carburizing.
2 Strictly control the uniformity of the furnace temperature, and it should not fluctuate too much. Special attention should be paid to the solid carburization in the reverberatory furnace.
3 When carburizing solids, the carburizing agent should be used in new and old ratios. Preferably, the infiltration agent is 4-7% of BaCO3, and Na2CO3 is not used as the infiltration agent.
(2) The carbon concentration is too low
1. Causes and hazards: If the temperature fluctuates greatly or the infiltration agent is too small, the carbon concentration on the surface will be insufficient. The most desirable carbon concentration is between 0.9 and 1.0%, below 0.8% C, and the parts are prone to wear.
2. Methods of prevention:
1 Carburizing temperature generally adopts 920-940 °C. 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 grain will be coarse.
2 The amount of infiltration agent (BaCO3) should not be less than 4%.
(3) Local carbon deficiency on the surface after carburizing:
1. Causes and hazards: When solid carburizing, charcoal particles are too large or contain impurities such as stones, or the infiltration agent is not uniformly mixed with charcoal, or the workpiece is contacted to cause local carbon-free or carbon-poor. Contaminants on the surface of the workpiece can also cause carbon depletion.
2. Method of prevention
1 Solid carburizing agent must be prepared in proportion and stirred evenly.
2 Do not touch the workpiece of the furnace. When the solid is carburized, the carburizing agent should be compacted, so that the carburization is not caused to collapse and the workpiece is in contact.
3 except the dirt on the surface.
(4) The transition of carburizing concentration is intensified
1. Causes and hazards:
The sudden transition of carburizing concentration is a change in the carbon concentration between the surface and the center, not a uniform transition from high to low, 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 is added little), and the alloying elements such as Cr, Mn, Mo in the steel promote the formation of carbides, resulting in The surface is highly concentrated, the center is low in concentration, and there is no transition layer. The occurrence of this defect causes considerable internal stress in the surface, causing cracks or flaking during the quenching process or during the grinding process.
2. Methods of prevention:
The carburizing agent is old and new according to the prescribed ratio to make the carburization moderate. It is better to use BaCO3 as the infiltration agent because Na2CO3 is relatively sharp.
(5) tempering and cracking during grinding
1. Reasons for the occurrence:
The surface of the carburized layer is softened by grinding, which is called tempering caused by grinding. This is due to the fact that the machining feed rate during grinding is too fast, the hardness of the grinding wheel and the particle size or the rotational speed are not properly selected, or the cooling during the grinding process is insufficient, and such defects are easily generated. This is because the heat during grinding softens the surface. When the tempering defect occurs during grinding, the wear resistance of the part is lowered.
The surface produces a hexagonal crack. This is because the surface of the hard grinding wheel is excessively ground and heated. It is also related to insufficient heat treatment and tempering, and excessive residual internal stress. After etching with acid, any defective part is black, which can be distinguished from the defect. This is the heat tempering that occurs during grinding. The reason why the horse body is transformed into a troostite structure. In fact, the crack can be seen by the naked eye after grinding.
2. Methods of prevention:
1 After quenching, it must be fully tempered or tempered several times to eliminate internal stress.
2 The soft or medium-sized alumina grinding wheel with 40~60 grain size is used, but the grinding feed is not too large.
3 Open the coolant first during grinding and pay attention to the sufficient cooling during the grinding process [2]
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
The definition of carburizing is a kind of metal surface treatment. The carburizing is mostly low carbon steel or low alloy steel. The specific method is to place the workpiece into a single phase with an activated carburizing medium and heated to 900-950 degrees Celsius. In the austenite region, after sufficient time of heat preservation, the activated carbon atoms decomposed in the carburizing medium penetrate into the surface layer of the steel, thereby obtaining high carbon in the surface layer, and the core remains the original composition. Similarly, low temperature nitriding treatment is performed. This is a common heat treatment process for metal materials, which can achieve high hardness and improve the wear resistance of the surface of the carbonized workpiece.
Introduction Carburizing is the process of infiltrating carbon atoms into the steel surface layer. It also makes the low carbon steel workpiece have the surface layer of high carbon steel, and after quenching and low temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still maintains the toughness of low carbon steel. Plasticity.
The material of the 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 steel surface can be close to high carbon steel. After the workpiece is carburized, it is quenched to obtain high surface hardness, high wear resistance and fatigue strength, and maintain the toughness of the low carbon steel after quenching in the core, so that the workpiece can withstand the impact load. Carburizing processes are widely used in mechanical parts such as gears, axles, camshafts, etc. for aircraft, automobiles and tractors.
The carburizing process can be traced back to 2000 before China. The earliest was carburizing with a solid carburizing medium. Liquid and gas carburizing appeared in the 20th century and is widely used. In the United States in the 1920s, gas drum carburizing was started. In the 1930s, continuous gas carburizing furnaces began to be used industrially. High temperature (960 ~ 1100 ° C) gas carburization in the 1960s was developed. By the 1970s, vacuum carburization and ion carburization occurred.
Principle Carburizing, like other chemical heat treatments, also involves three basic processes.
1 decomposition: decomposition of the carburizing medium produces activated carbon atoms.
2 Adsorption: The activated carbon atoms are absorbed into the surface austenite after being absorbed by the surface of the steel, so that the carbon content in the austenite is increased.
3 Diffusion: When the surface carbon content increases, the concentration of carbon in the heart is different, and the carbon enthalpy on the surface diffuses into the interior. The diffusion rate of carbon in steel depends mainly on the temperature, and is related to the difference between the internal and external concentrations of the infiltrated elements 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 less than 0.25%). After carburizing, quenching must be carried out in order to fully exert the beneficial effect of carburizing. The surface microstructure of the workpiece after carburizing and quenching is mainly high hardness martensite plus retained austenite and a small amount of carbide, and the core structure is low-carbon martensite with good toughness or non-martensitic structure. However, ferrite should be avoided. Generally, the carburized layer has a depth ranging from 0.8 to 1.2 mm and a depth of carburizing up to 2 mm or more. 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 improve 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.
Classification According to different carbon-containing media, carburizing can be divided into solid carburizing, liquid carburizing, gas carburizing and carbonitriding.
Carburizing process 1. Direct quenching low temperature tempering structure and performance characteristics: The grain of steel cannot be refined. The quenching deformation of the workpiece is large, and the amount of retained austenite on the surface of the alloy steel carburized part is large, and the surface hardness is low.
Scope of application: Simple operation and low cost for handling parts with deformation and impact load, suitable for gas carburizing and liquid carburizing processes.
2, pre-cooling direct quenching, low temperature tempering, quenching temperature 800-850 °C structure and performance characteristics: can reduce the workpiece quenching deformation, the amount of retained austenite in the layer can also be slightly reduced, the surface hardness is slightly improved, but the Austrian There is no change in the grain of the body.
Scope of application: Simple operation, small oxidation, decarburization and quenching deformation of workpieces, widely used in various tools made of fine grain steel.
3, a heat quenching, low temperature tempering, quenching temperature 820-850 ° C or 780-810 ° C. Structure and performance characteristics: for the higher core strength requirements, using 820-850 ° C quenching, the heart is low carbon M, surface For those with high hardness, quenching at 780-810 °C can refine the grains. Scope of application: Applicable to carbon steel and low alloy steel workpieces after solid carburizing, gas and liquid carburized coarse grain steel, some workpieces that are not suitable for direct quenching after carburizing and parts that need to be machined after carburizing.
4, carburizing high temperature tempering, one heating and quenching, low temperature tempering, quenching temperature 840-860 °C. Structure and performance characteristics: high temperature tempering causes M and residual A to decompose, carbon and alloying elements in the layer are precipitated as carbides. It is easy to reduce the residual A after cutting and quenching. Scope of application: Mainly used for carburizing workpieces of Cr-Ni alloy
5, secondary quenching low temperature tempering structure and performance characteristics: the first quenching (or normalizing), can eliminate the carburized layer of network carbides and refine the heart tissue (850-870 ° C), the second quenching mainly Improve the layer structure, quenching between Ac1-Ac3 of the material when the performance of the core is not high, and quenching above Ac3 when the core performance is high.
Scope of application: Mainly used for important carburizing parts with high mechanical properties, especially for coarse grain steel. However, after carburizing, two high-temperature heating is required to increase the deformation and oxidative decarburization of the workpiece, and the heat treatment process is complicated.
6. Low temperature tempering structure and performance characteristics of secondary quenching and cold treatment: higher than Ac1 or Ac3 (heart) temperature quenching, more residual A in high alloy surface layer, and cold treatment (-70 °C / -80 °C) to promote A transformation Improve surface hardness and wear resistance.
Scope of application: Mainly used for high-alloy steel workpieces that are not machined after carburizing.
7. Induction heating quenching after carburizing Low temperature tempering structure and performance characteristics: It can refine the infiltration layer and the structure near the infiltration layer. The quenching deformation is small, and the hardened portion is not allowed to be pre-impregnated.
Scope of application: New developments in various gear and shaft carburizing processes
The carburizing process is a very old process, and in China, it can be traced back to 2000. At first it was carburized with a solid carburizing medium. Liquid and gas carburization occurred in the 20th century and is widely used. Later, vacuum carburization and ion carburization occurred. Up to now, the carburizing process still has very important practical value. The reason is that its reasonable design idea is to let the steel surface layer receive the most load (wear, fatigue, mechanical load and chemical corrosion), through the infiltration of carbon. The elements achieve high surface hardness, high wear resistance and fatigue strength and corrosion resistance without the need to treat the entire material by expensive alloying or other complicated 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 toughness of the low-carbon steel after quenching in the core, so that the workpiece can withstand the impact load. Therefore, it fully meets the direction of energy conservation, consumption reduction and sustainable development.
In recent years, a high-concentration carburizing process has occurred, which is different from the conventional process in carburizing in a completely austenite region (temperature at 900-950 ° C, surface carbon content after carburization is 0.85% to 1.05%). The non-uniform austenite state between Ac1 and Accm is carried out, and the carbon concentration on the surface of the layer can be as high as 2% to 4%. As a result, a uniform, dispersed distribution of fine particle carbides can be obtained. The carburizing temperature is reduced to a temperature range of 800 ° C to 860 ° C, which can achieve direct quenching after carburizing of general steel; since the high concentration carburizing layer contains a high amount (20% to 50%) of dispersed carbides, it is displayed It has better wear resistance, corrosion resistance, higher contact and bending fatigue strength, higher impact toughness, lower brittleness and better tempering stability than ordinary carburizing. The process also has the advantages of wide applicability, no special requirements for equipment, high economic efficiency and practical value, and has obtained competitive research and development at home and abroad in recent years.
In order to prevent coarsening of austenite grains during carburization, an appropriate amount of titanium is generally added to the steel to prevent grain growth by forming a grain boundary of the titanium carbonitride particles. The national standard stipulates that the amount of titanium added in the carburized steel is 0.04 to 0.08 wt%. However, recent research has shown that when the titanium content exceeds 0.032%, titanium nitride is precipitated when the carburized steel smelting ingot solidifies. The size of the titanium nitride is on the order of micrometers, which does not prevent the growth of austenite grains. Instead, it is the source of microcracks due to the sharp angle effect of the cubic particles and the discontinuity of the matrix structure. And the crack propagation of the relay station, seriously impairing the toughness and plasticity of the steel. The work also shows that reducing the titanium content to 0.02-0.032% can still effectively control the austenite grain growth, and avoid the formation of harmful titanium nitride particles, so it is recommended to be reasonable. Range of choice
(1) The carbon concentration is too high
1. Causes and hazards: If the carburization is heated rapidly, the temperature is too high, or a new carburizing agent is used when the carburizing is solid, or a strong infiltration agent is too much, the carburization concentration is too high. As the carbon concentration is too high, massive carbides or reticulated carbides appear on the surface of the workpiece. Due to the formation of such a hard and brittle structure, the toughness of the carburized layer is drastically lowered. And high carbon martensite is formed during quenching, and grinding cracks are likely to occur during grinding.
2. Method of prevention
1 can not be heated sharply, it is necessary to use appropriate heating temperature, so that the grain growth of steel is not good. If the grains are coarse during carburizing, the grains should be refined by normalizing or twice quenching after carburizing.
2 Strictly control the uniformity of the furnace temperature, and it should not fluctuate too much. Special attention should be paid to the solid carburization in the reverberatory furnace.
3 When carburizing solids, the carburizing agent should be used in new and old ratios. Preferably, the infiltration agent is 4-7% of BaCO3, and Na2CO3 is not used as the infiltration agent.
(2) The carbon concentration is too low
1. Causes and hazards: If the temperature fluctuates greatly or the infiltration agent is too small, the carbon concentration on the surface will be insufficient. The most desirable carbon concentration is between 0.9 and 1.0%, below 0.8% C, and the parts are prone to wear.
2. Methods of prevention:
1 Carburizing temperature generally adopts 920-940 °C. 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 grain will be coarse.
2 The amount of infiltration agent (BaCO3) should not be less than 4%.
(3) Local carbon deficiency on the surface after carburizing:
1. Causes and hazards: When solid carburizing, charcoal particles are too large or contain impurities such as stones, or the infiltration agent is not uniformly mixed with charcoal, or the workpiece is contacted to cause local carbon-free or carbon-poor. Contaminants on the surface of the workpiece can also cause carbon depletion.
2. Method of prevention
1 Solid carburizing agent must be prepared in proportion and stirred evenly.
2 Do not touch the workpiece of the furnace. When the solid is carburized, the carburizing agent should be compacted, so that the carburization is not caused to collapse and the workpiece is in contact.
3 except the dirt on the surface.
(4) The transition of carburizing concentration is intensified
1. Causes and hazards:
The sudden transition of carburizing concentration is a change in the carbon concentration between the surface and the center, not a uniform transition from high to low, 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 is added little), and the alloying elements such as Cr, Mn, Mo in the steel promote the formation of carbides, resulting in The surface is highly concentrated, the center is low in concentration, and there is no transition layer. The occurrence of this defect causes considerable internal stress in the surface, causing cracks or flaking during the quenching process or during the grinding process.
2. Methods of prevention:
The carburizing agent is old and new according to the prescribed ratio to make the carburization moderate. It is better to use BaCO3 as the infiltration agent because Na2CO3 is relatively sharp.
(5) tempering and cracking during grinding
1. Reasons for the occurrence:
The surface of the carburized layer is softened by grinding, which is called tempering caused by grinding. This is due to the fact that the machining feed rate during grinding is too fast, the hardness of the grinding wheel and the particle size or the rotational speed are not properly selected, or the cooling during the grinding process is insufficient, and such defects are easily generated. This is because the heat during grinding softens the surface. When the tempering defect occurs during grinding, the wear resistance of the part is lowered.
The surface produces a hexagonal crack. This is because the surface of the hard grinding wheel is excessively ground and heated. It is also related to insufficient heat treatment and tempering, and excessive residual internal stress. After etching with acid, any defective part is black, which can be distinguished from the defect. This is the heat tempering that occurs during grinding. The reason why the horse body is transformed into a troostite structure. In fact, the crack can be seen by the naked eye after grinding.
2. Methods of prevention:
1 After quenching, it must be fully tempered or tempered several times to eliminate internal stress.
2 The soft or medium-sized alumina grinding wheel with 40~60 grain size is used, but the grinding feed is not too large.
3 Open the coolant first during grinding and pay attention to the sufficient cooling during the grinding process [2]
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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