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Analysis and Countermeasures of Common Defects in Vacuum Carbonitriding
Vacuum nitrocarburizing is developed on a liquid nitriding substrate, and the salt bath used is a highly toxic cyanide salt. In order to improve the activity of the salt bath and accelerate the process of nitriding and carburizing, air or oxygen is introduced to increase the activity of nitrogen and carbon atoms during the oxidation process. Because cyanide salts caused serious public hazards, it developed into nitrocarburizing with urea as the main component. Although highly toxic cyanide salts are not used, there are still cyanate radicals in the salt bath, and the composition of the salt bath is unstable during use. The application of infiltration technology is limited. The gas nitrocarburizing process is widely used because of its low processing temperature (generally 500-600°C), mainly nitriding and carburizing as a supplement, and the performance after carburizing is more ideal than that of single nitriding or carburizing.
This article analyzes the defects and countermeasures of vacuum carbonitriding:
1. Internal oxidation:
Due to the penetration of oxygen atoms into the inner layer of the metal and the alloying elements, internal oxidation occurs and oxides are formed along the grain boundaries. Under the microscope, it is observed that the black net is an oxide net, and after being etched by nitric acid alcohol solution, the enlarged part is a very fine pearlite and bainite structure, with fine granular carbon and nitrogen compounds, which is a non-martensitic structure. During the co-infiltration process, oxygen atoms gather on the surface of the steel and diffuse along the austenite grain boundary to the inner layer, while alloying elements diffuse from the grain to the grain boundary and the inner layer along the surface, and combine with the austenite grain boundary on the surface , forming oxides of alloying elements. Generally, alloy steels containing Cr and Mn elements are more prone to internal oxidation, which reduces the content of alloying elements in the surrounding austenite, reduces the stability of austenite, and transforms into very fine pearlite and bainite during vacuum quenching, which is non-marine. Tensitic structure, reducing hardness, wear resistance and comprehensive mechanical properties.
Vacuum carbonitriding countermeasures: carbonitriding should not use traditional kerosene with a small gas output for exhausting, but methanol with a large gas output should be used for exhausting; fully dry NH3 gas, remove H2O and properly increase the co-infiltration temperature; if there is enough grinding Quantity, surface shot peening can be carried out to remove the internal oxide layer on the surface and measures such as secondary refining of alloy steel containing W, Mo, V, Co can be used to effectively prevent and avoid internal oxidation.
2. Too much retained austenite:
When the steel is vacuum quenched and cooled, the supercooled austenite transforms into quenched martensite, and a small part of supercooled austenite (retained austenite) cannot transform into martensite, and coexists with martensite at room temperature; quenched martensite After vacuum tempering at different temperatures, the body transforms into different tempered structures to achieve the required organizational properties. Retained austenite can be partially transformed into martensite during vacuum tempering, but the material and process are different or the carbonitride content is too high during co-infiltration and the vacuum quenching heating temperature is too high, and the cooling rate after infiltration is too fast, resulting in carbonitride Insufficient compound precipitation will lead to excessive retention of retained austenite in the service state, reducing hardness, wear resistance, fatigue strength, yield strength, elastic limit and causing insufficient stability of the structure and properties. Therefore, it is easy to cause tissue transformation and volume expansion in the use state, parameter changes, and distortion. For this reason, the content of retained austenite must be strictly controlled in vacuum quenching, and corresponding measures should be taken to eliminate excess retained austenite.
Vacuum carbonitriding countermeasures: control the carbon and nitrogen content, generally the mass fraction is 0.75%~0.95%, and the mass fraction of nitrogen is 0.15%~0.35%. After vacuum carbonitriding and heat preservation, the furnace temperature should not be too high, and the vacuum quenching temperature should be appropriately reduced. Because alloy steel contains a large amount of alloying elements that reduce the martensitic point, excessively high vacuum quenching heating temperature will cause a large amount of carbon and alloying elements in the steel to sneak into high-temperature austenite, and the degree of austenite alloying is high, increasing the stability of austenite , so that the supercooled austenite is not easy to undergo martensitic transformation. Or vacuum tempering at low temperature for a short time after vacuum quenching, followed by cold treatment at minus 60~180°C, the essence is that vacuum quenching continues, and the residual austenite is fully transformed into martensite. The lower the temperature, the more the transformation, which stabilizes the structure and properties of the steel and increases the hardness, wear resistance and mechanical properties.
3. Surface decarburization and oxidation:
Decarburization on the surface of steel parts reduces the amount of carbon on the surface of steel parts due to oxidation. When the oxidation rate is slower than the diffusion rate of carbon to the outer layer of the metal, decarburization occurs; on the contrary, when the oxidation rate is greater than the diffusion rate of carbon to the outer layer of the metal, oxidation occurs and iron oxide scale is formed. The cause of decarburization: the pressure in the furnace is abnormal during vacuum carbonitriding. When the pressure is too low, the air outside the furnace enters the furnace, or the furnace tank is not tightly sealed. When the furnace contains oxygen, CO2, H2O and other oxidizing substances and the iron in the steel reacts chemically to oxidize and decarburize the surface of the steel piece.
The ferrite, pearlite, bainite and other structures formed by oxidation and decarburization during quenching and heating of steel parts are non-martensitic structures, resulting in low hardness, poor wear resistance, and poor fatigue resistance and mechanical properties. It is necessary to strictly prevent and avoid decarburization and oxidation on the surface of steel parts.
Vacuum carbonitriding countermeasures: Before carbonitriding, strictly check whether the equipment is sealed, and maintain positive pressure in the furnace. Strictly prevent the occurrence of negative pressure; NH3 gas in the furnace should be dried to remove H2O; old equipment should be eliminated or transformed, and new equipment should be replaced for vacuum quenching and heating. Controllable atmosphere electric furnace, vacuum electric furnace, and microcomputer control furnace are used. Obtain the best carbonitriding layer depth and microstructure properties, and avoid decarburization and oxidation of the steel surface.
4. Vacuum carburizing surface network compound:
Due to improper control of the carbonitriding concentration, the absorption on the metal surface is greater than the diffusion, and the active carbon and nitrogen atoms accumulate on the metal surface, and the excessive concentration of the carbonitriding layer compound forms a network compound on the surface; when the carbonitride compound expands along the grain boundary, it forms a network and semi-reticular infiltration compounds. The network compound has low toughness and high brittleness, which will greatly reduce the impact resistance and fatigue resistance of the co-infiltration layer, and easily cause stress concentration during vacuum quenching or in use, causing surface cracks on steel parts and early failure of the co-infiltration layer peeling off. The main reason for the formation of network compounds: the carbon and nitrogen potential of the furnace gas is too high, the temperature in the strong infiltration period is too high, the holding time is too long, and high-concentration co-infiltration compounds are formed prematurely.
Vacuum carbonitriding countermeasures: control the supply of carbonitriding agent and reduce the carbonitriding potential in the furnace. Shorten the infiltration time and optimize the infiltration temperature.
5. The vacuum carbonitriding layer is uneven:
Due to the low carbon and nitrogen potential of the furnace gas, the co-infiltration temperature is low. The holding time during the infiltration period is insufficient; the furnace gas is not smooth, forming a “blind area” of the furnace gas, resulting in the phenomenon of “interrupted infiltration”. A large amount of carbon black is deposited on the surface of steel parts, uneven furnace temperature and too dense charging, steel parts contact each other or even surface contact, dirt on the surface and other reasons will lead to uneven carbonitriding layer. Affect product quality and service life, must take measures to avoid.
Vacuum carbonitriding countermeasures: optimize the co-infiltration period and diffusion period, keep the carbon-nitrogen potential within a reasonable range, strictly control the unsaturated hydrocarbons in the co-infiltration agent; reasonably design the fixture to ensure the smooth flow of furnace gas and maintain a certain gap between steel parts. The distance and no contact with each other; the resistance wires are reasonably distributed in the furnace to ensure that the temperature of each part of the furnace is uniform; the dirt on the surface of the steel parts is cleaned before loading the furnace.
6. Improper ratio of hypereutectoid layer + eutectoid layer:
When the ratio of hypereutectoid layer + eutectoid layer is reasonable, the carbonitriding layer has a strong bonding force with the steel substrate, and it has strong peeling resistance and good comprehensive mechanical properties under stress. Conversely, when the ratio of the two is too large or too small, it is easy to cause the carbonitriding layer to peel off easily under the action of external force and the mechanical properties are poor, resulting in early failure of the workpiece. When the ratio of hypereutectoid + eutectoid layer is greater than 3/4 of the total depth of seepage layer, it is too large; when the ratio of hypereutectoid layer + eutectoid layer is less than 1/2 of the total deep layer depth, it is too small. When the proportion of the two is greater than or equal to 1/2 and less than or equal to 3/4, the proportion is reasonable.
Vacuum carbonitriding countermeasures: If the ratio of the two is too large, the ratio of the formula and time of the infiltrating agent during the carbonitriding period and the diffusion period should be adjusted, and the diffusion treatment should be carried out under the protective atmosphere of methanol dilution to meet the technical requirements; If it is small, you can increase the holding time of the strong infiltration period of carbonitriding and supplement infiltration in the active carbonitriding medium with a higher carbonitriding potential in the furnace gas; use a computer to control the carbon potential and fine-grain steel before carbonitriding. Adjust the pretreatment and other measures to meet the technical conditions.
7. Vacuum carburizing layer shallow:
Mainly furnace gas and equipment problems. Due to insufficient exhaust, there is more oxidizing atmosphere in the furnace. Production practice shows that when the content of O2 and CO2 is greater than or equal to 1.0%, it is not easy to form a carbonitriding layer; the co-infiltration agent is poor, contains many impurities, and the decomposition of [C] and [N] atoms is insufficient; the surface of the workpiece is unclean, There are oil stains, rust, and blockage of oil pipes. The furnace tank and co-infiltration tank are not normal, so that the co-infiltration agent does not drip into the furnace and expand into the bottom of the tank, resulting in uneven furnace gas and other reasons, resulting in a shallow co-infiltration layer. The above problems can be found in furnace pressure and exhaust flame.
Vacuum carbonitriding countermeasures: Strictly check the equipment integrity rate before carbonitriding, eliminate equipment failures, clean the surface of the workpiece, and re-infiltrate according to the normal process; select methanol exhaust with large gas production and not easy to form carbon black; ignite exhaust Hole flame, the flame turns from dark purple to golden yellow, and the pressure in the furnace is 150~300Pa, indicating that the C and N potential circulation and production in the furnace are normal; new equipment and new technologies such as microcomputer control of carbon potential can be effectively avoided and eliminated. Carbonitriding defects ensure that the carbonitriding layer of the product has high quality and long life. There are significant technical and economic benefits.
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