Internal oxidation:
Oxygen atoms infiltrate into the inner layer of the metal and internal oxidation of the alloying elements and formation of oxides along the grain boundaries. Under the microscope, the black net was observed as an oxide net. After being etched by the nitric acid alcohol solution, the enlarged part was extremely fine pearlite and bainite structure, and there were fine granular C·N compounds, which belonged to non-martensitic structure. During the co-permeation process, oxygen atoms accumulate on the surface of the steel and diffuse along the austenite grain boundary to the inner layer, while the alloy elements diffuse from the grain inward grain boundary and the inner layer along the surface layer, and combine with the austenite grain boundary at the surface layer. Forming an alloying element oxide. Generally, Cr, Mn, and elemental alloy steels are more prone to internal oxidation, which depletes the content of alloying elements in the surrounding austenite, reduces the austenite stability, and transforms into very fine pearlite and bainite during quenching. The structure of the body reduces hardness, wear resistance and comprehensive mechanical properties.
Countermeasures → Carbonitriding should not be exhausted with kerosene with low gas production. Use methanol with large gas production; exhaust NH3 gas, remove H2O and increase the co-infiltration temperature; if there is enough grinding, surface Shot peening to remove the surface oxide layer and secondary refining with W, M0, V, Co alloy steel and other measures can effectively prevent and avoid internal oxidation.
Excessive amount of retained austenite:When the steel is quenched, the supercooled austenite transforms into quenched martensite, and a small portion of the supercooled austenite (residual austenite) cannot be transformed into martensite, and coexists with martensite at normal temperature; quenched martensite After tempering at different temperatures, it is transformed into different tempered structures to achieve the desired tissue properties. The retained austenite can be partially transformed into martensite during the tempering process, but the C·N content is too high and the quenching heating temperature is too high when the materials and processes are different or co-infiltration, and the cooling rate is too fast after the seepage, resulting in carbon and nitrogen compounds. Insufficient amount of precipitation will result in excess retained austenite remaining in use, reducing hardness, wear resistance, fatigue strength, yield strength, elastic limit and causing insufficient stability of the microstructure. Therefore, it is easy to cause tissue transition and volume expansion in the use state, and parameter changes occur, causing distortion. For this reason, the austenite content of quenching must be strictly controlled, and excessive retained austenite should be removed by corresponding measures.
Countermeasures → Control carbon and nitrogen content, generally containing mass fraction of 0.75% -0.95%, nitrogen content of 0.15% -0.35% is appropriate. After the carbonitriding and heat preservation, the temperature of the furnace should not be too high, and the quenching temperature should be appropriately lowered. Because alloy steel contains a large number of alloying elements that reduce the martensite point, excessively high quenching heating temperature causes the carbon and alloying elements in the steel to sneak into the high-temperature austenite in a large amount, the austenite alloying degree is high, and the austenite stability is increased. It is difficult for martensite transformation to occur in supercooled austenite. Or quenching after a short period of low temperature tempering, and then into the cold 60-180 ° C cold treatment, the essence of quenching continues, prompting the residual austenite to fully transform into martensite. The lower the temperature, the more the amount of transformation, which makes the steel's microstructure properties stable, increasing hardness, wear resistance and mechanical properties.
Surface decarburization and oxidation:The decarburization of the steel surface reduces the carbon content of the steel surface due to oxidation. Decarburization occurs when the oxidation rate is lower than the diffusion rate of carbon to the outer layer of the metal; conversely, when the oxidation rate is higher than the diffusion rate of carbon to the outer layer of the metal, oxidation occurs to form an iron oxide scale. Causes of decarburization: When the C·N co-infiltration, the pressure in the furnace is not normal. When the pressure is too low, the outside air enters the furnace, or the furnace can not be sealed tightly. When the furnace contains oxygen, CO2, H2O and other oxidizing substances and chemical reaction with iron in the steel, the surface of the steel is oxidized and deoxidized.
The structure of iron, pearlite, bainite and other structures formed by oxidative decarburization during quenching of steel is non-martensitic structure, resulting in low hardness, poor wear resistance and inferior fatigue strength and mechanical properties. The decarburization and oxidation of the steel surface must be strictly prevented and avoided.
Countermeasures → Before the CN co-infiltration, strictly check whether the equipment is sealed and maintain a positive pressure in the furnace. Strictly prevent the occurrence of negative pressure; the NH3 gas introduced into the furnace should be dried to remove H2O; the old equipment should be eliminated or modified, and the new equipment should be replaced by quenching heating, using a controlled atmosphere electric furnace, vacuum electric furnace and microcomputer controlled furnace. Obtaining the best C·N co-infiltration depth and microstructure and properties can avoid decarburization and oxidation of steel surfaces.
Surface reticulated compound:Due to the improper control of C·N co-infiltration concentration, the metal surface absorption is greater than diffusion, the active C·N atoms are deposited on the metal surface, and the co-permeation layer compound is concentrated in the surface layer to form a network compound; when the C·N compound expands along the grain boundary, A network of reticulated and semi-networked compounds is formed. The reticular compound has low toughness and large brittleness, which will greatly reduce the impact resistance and fatigue resistance of the co-permeation layer, and easily cause stress concentration during quenching or use, causing surface cracking of the steel part and early failure of the co-exfoliation layer. The main reason for the formation of the network compound is that the C·N potential of the furnace gas is too high, the temperature of the strong infiltration period is too high, the holding time is too long, and a high concentration of the common infiltration compound is formed too early.
Countermeasures → Control the supply of C·N osmotic agent to reduce the C·N potential in the furnace. Shorten the infiltration time and optimize the infiltration temperature.
Carbonitriding layer is not uniform:Due to the low C·N potential of the furnace gas, the co-infiltration temperature is low. Insufficient holding time of infiltration period; poor furnace gas, forming a "blind area" of furnace gas, resulting in "breaking" phenomenon. A large amount of carbon black is deposited on the surface of the steel. The furnace temperature is not uniform and the charging is too dense. The steel parts are in contact with each other or even in contact with each other. The surface is dirty and causes the C·N co-infiltration layer to be uneven. Factors affecting product quality and service life must be avoided.
Countermeasure→Optimize the co-infiltration period and diffusion period, keep the C·N potential in a reasonable range, strictly control the unsaturated hydrocarbon in the co-infiltration agent; reasonably design the fixture to ensure the smooth flow of the furnace gas and maintain a certain distance between the steel parts and not each other. Contact; the resistance wire is distributed reasonably in the furnace to ensure uniform temperature in all parts of the furnace; clean the surface of the steel before cleaning.
Black spotted tissue:In the early stage of carbonitriding, the nitrogen potential of the furnace gas is too high, a large amount of active nitrogen atoms are inhaled in the seepage layer, and a large amount of activated carbon atoms are inhaled in the late stage of C·N co-infiltration, and the carbon concentration is increased, and nitride decomposition and denitrification processes occur, and the atom [N ] It becomes a nitrogen molecule and forms a black spot. It consists of pores of different sizes under the microscope. It is a black spotted structure. It is a non-metallic nitrogen atom that is reduced to a gas molecule to form a "balloon" to produce a dot-like black hole. Due to the degree of supersaturation of the co-permeation layer C·N and the formation of excess nitride product.
Some steels contain a large amount of Si element which is easy to form graphitization. The long-term heating or annealing time of the raw material during high-temperature forging is too long, which promotes the decomposition of Fe3 C→ 3Fe+C in steel. The free carbon which is resolved is graphite. soft. The formation of graphite carbon inclusions remains in the pores, which is black spotted structure, which causes the steel to be both hardened and hardened, and the black spot area becomes a soft spot area, which reduces fatigue strength, hardness, wear resistance and inferior mechanics. Performance, easy to peel off the layer, and the product edge becomes dull and brittle. Measures must be taken to eliminate black spotted tissue.
Countermeasures → Strengthen metallographic examination of raw materials, try not to use materials with graphitization tendency or strictly formulate and implement pre-annealing process before carbonitriding, reduce annealing temperature and shorten annealing time, or use quenching and tempering instead of annealing to avoid Precipitating graphite; selecting steel with lower Si content; appropriately increasing the co-infiltration temperature; reducing the amount of ammonia in the strong infiltration period and diffusion period to ensure product quality.
Banded tissue:High-alloy steel contains a large amount of alloying elements, forming a large number of primary carbides and secondary carbides. Although the ingots are rolled and calendered, the carbide segregation is improved to some extent, but distributed along the rolling direction and along with the diameter of the raw materials or The thickness is increased and serious. Normal C·N co-infiltration and heat treatment cannot be eliminated, and remain in the use state, which significantly reduces the tensile strength, yield strength, plasticity and toughness of the material, resulting in uneven hardness and mechanical properties.
Countermeasures → The banded tissue cannot be saved after it is generated, and only preventive measures can be taken to avoid recurrence. Strengthen the inspection of raw materials into the warehouse, the serious band structure will not be put into production, or the steel forging and forging will ensure the uniformity of the C·N co-infiltration layer and completely avoid the defects of congenital defects of raw materials.
distortion:When the raw material grain size is uneven, the band structure is serious, the hardenability band, the forging flow line is asymmetrical, the large piece, the shape is complicated, the thickness is uneven, the heating equipment is improperly installed, and the carbonitriding heating and quenching temperature are compared. High steel, good thermoplasticity and large internal stress before carbonitriding; easy to be distorted under the action of stress and self-weight. When the co-extrusion furnace is quenched, due to the complexity of the workpiece, the various parts change into different periods, such as thin parts. The quenched martensite is first cooled, and the inner layer of the wall thickness is still supercooled austenite, and the inner layer of the supercooled austenite is continuously cooled to a non-martensitic structure such as bainite, torsite or pearlite. Distortion in tissue specificity results in distortion.
Countermeasures → Strict raw material storage inspection, unqualified materials are not put into production; steel is not too serious for band structure, strip structure is modified by forging and the forged fiber structure is streamlined symmetrically; the workpiece is fully pretreated before final quenching, fully eliminating Organizational stress and cold working stress; improve the design, try to make the shape of the workpiece symmetrical, such as: make the process hole, add thin ribs and rounds instead of sharp corners to reduce stress concentration; The carbide in the steel reaches 1-2 grades; the final quenching adopts measures such as step quenching, austempering and micro-deformation steel, which can effectively avoid heat treatment distortion.
Proportion of over-eutectoid layer + eutectoid layer:When the ratio of the hypereutectoid layer to the eutectoid layer is reasonable, the carbonitriding layer is strongly bonded to the steel matrix, and the anti-flaking property is strong under the stress state and has good comprehensive mechanical properties. Conversely, when the ratio of the two is too large or too small, the carbonitriding layer is easily peeled off under external force and the mechanical properties are inferior, resulting in early failure of the workpiece. When the ratio of hypereutectoid + eutectoid layer is greater than 3/4 of the total depth of the layer, it is too large; when the ratio of the eutectoid layer + the eutectoid layer is less than 1/2 of the total depth of the depth, it is too small. When the ratio of the two is greater than or equal to 1/2 and less than or equal to 3/4, the ratio is reasonable.
Countermeasures → If the proportion of the two is too large, the ratio of C·N co-infiltration period and diffusion period infiltration agent formulation and time should be adjusted, and diffusion treatment should be carried out under the dilution atmosphere of methanol to meet the technical requirements; for those whose ratio is too small, Increasing the heat retention time of CN co-permeability and infiltration in the C·N co-infiltration medium with high carbon and nitrogen potential in the furnace gas; adjusting the pre-treatment before the co-infiltration of the carbon potential and fine grain steel with C·N And other measures to achieve technical conditions.
Co-infiltration layer is shallow:Mainly due to furnace gas and equipment problems. Due to insufficient exhaust, the furnace has a large amount of oxidizing atmosphere.
Production practice shows that when the O2 and CO2 content is greater than or equal to 1. When O%, it is difficult to form a carbonitriding layer; the co-permeation agent is poor, contains many impurities, and the decomposed enthalpy [C], [N] atoms are insufficient; the surface of the workpiece is unclean, oily, rust and tubing are clogged, the furnace tank The co-infiltration tank is not normal, so that the co-infiltration agent does not drip into the furnace and expand into the bottom of the tank, causing uneven furnace gas and the like, resulting in shallow co-infiltration layer. The above problems can be found in furnace pressure and exhaust flame.
Countermeasures → Closely check the equipment integrity rate before carbonitriding, remove equipment failures, clean the surface of the workpiece, re-infiltrate according to the normal process; select methanol exhaust gas with large gas production and not easy to form carbon black; ignite the vent flame, the flame is The dark purple turns golden yellow, the pressure in the furnace is 150-300Pa, which indicates that the C and N potential circulation and production in the furnace are normal; the selection of new equipment and new technologies such as controlling the carbon potential by the microcomputer can effectively avoid and eliminate the carbon co-infiltration defects. Ensure that the carbonitrided layer of the product has high quality and long life. There are significant technical and economic benefits.
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