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The cause of the grinding crack and the process measures to prevent it from occurring.
In the case of cooling hoarding shrinkage, such as the crack in the grinding section, no crack is found to meet the requirements of use, and the technical requirements of carburizing and quenching steel gears (take 20CrMnTi gear as an example) 1) hardness of gear forging after normalizing HB156~207.2) Hardness after carburizing and quenching: tooth surface HRC56~62 core hardness HRC30~46.3) tooth surface carburized layer depth <1.8mm.4) metallographic structure: (1) normalization of gear blank after forging, grain size above 6 The band structure is below grade 3; (2) the tooth surface carburization layer: a certain transition zone is required from the carburized layer to the original structure, and the eutectoid layer (hyperembolic organization + eutectoid organization) is equal to 50%~70 % total carburized layer. After quenching, the structure is non-acicular or fine-needle martensite structure, and no needle-like and reticulated carbides and a large amount of retained austenite are present. The reticulated carbide is below grade 5 and the retained austenite is below grade 5. (3) Heart tissue: After the heart is quenched, it is a low-carbon martensite structure, and the residual fat granules are not allowed to exceed 2 grades. 5) Gear accuracy grade 6 and tooth surface roughness Ra1.6~1.8. Most of the cracked gears occur during the rough grinding process, and some are found after fine grinding.
The crack direction is perpendicular to the tooth direction, and the vertical grinding wheel reciprocates. The cracks are nearly parallel to each other, and the crack is fine. The crack is partially observed by the naked eye, and it is most easily found when reflected by light.
Through production practice, it is found that for the modulus m=6mm, most of the gear cracks are above the pitch circle, and the crack thickness is 0.02~0.03. The crack length is 3~4mm. The tooth surface crack near the tooth end is longer, almost completely from the tooth. Top to the root of the tooth. The relevant data show that the maximum thermal stress is located on the flank of the end face, and the thermal stress is less in the middle of the tooth width. For example, the tooth width is B = 20 mm, and the middle temperature of the tooth width is 1.4 times lower than the end face. When B=200 mm, the difference is obtained. It is 4.35 times, so the tooth end face is more severe than the middle of the tooth width. Some gear teeth also have cracks, the thickness of which is 0.01~ because the surface heat conduction of high carbon alloy steel and carburized alloy steel is slow, and the grinding wheel mainly relies on the sharp mouth of the abrasive grain to grind, such as excessive grinding amount. However, if the grinding wheel is not sharp, the surface of the workpiece is rubbed by the blunt abrasive grains of the grinding wheel, and the volume expands due to strong heat, and the heat is conducted to the inner layer of the workpiece, and the volume is still expanding. In this way, the outer layer of the workpiece is subjected to tensile stress, and when the stress exceeds the material breaking strength, cracks are generated on the surface of the tooth.
Causes of Grinding Cracks The quality of gear heat treatment is an intrinsic factor in causing cracking. The root cause of grinding cracks is grinding heat. Because the temperature of the grinding zone in the moment of contact between the grinding wheel and the tooth surface is very high, even the surface of the tooth is burned. According to the test, when the grinding wheel speed V=18m/S and the grinding depth t=0.05mm, the temperature in the grinding zone reaches 900~1100, and the heating speed C/s is so that the carburized hardened tooth surface is used during grinding. Metal structures will change.
During grinding, the retained austenite in the steel is at 200 ~ 300 ° C, the microstructure is transformed into a secondary quenched structure, and the volume is swollen, forming a large internal stress.
To do this, it is necessary to increase the tempering temperature of the steel as much as possible.
In short, during grinding, the thermal stress and phase transformation stress generated inside the workpiece, together with the tearing stress caused by grinding the workpiece by the grinding wheel, the maximum stress synthesized by the three is consistent with the direction of the tooth length (see), due to this force The effect is that the internal stress opposite to the combined stress is formed on the gear. If the strength of the carburized layer and the structural stress of the metal resist the combined stress generated during grinding, no crack will occur. Otherwise, the gear material is ground. During the process, it will be destroyed, and a crack perpendicular to the direction of the combined stress will occur, and cracking will occur.
The heat and stress generated by the grinding process are the external factors that cause the grinding, such as the hardness of the grinding wheel, the particle size, the type of abrasive, the grinding speed, the coolant, the depth of the knife, the amount of the cutting and cutting, the number of reciprocating times per minute of the grinding wheel and the grinding wheel. Trimming and so on.
Third, the elimination of grinding cracks Our factory production proves that reducing the depth of grinding once, although cracks still occur, but the grinding cracks will become finer, even the naked eye can not see clearly. For example, the material is 20CiMnTi gear, the modulus is m=(6Z=33, straight tooth, tooth width B=60, tooth surface carburization, quenching hardness HRC58~62, tooth surface carburizing depth 0.9~1.3, accuracy is 6 After the relationship between the grinding depth and the crack, it is found that the grinding depth is gradually reduced, and the crack form is a large striate shape, a fine striate shape and a fine striate shape.
Both the grain and the burn have a greater impact. The thicker the wheel is trimmed, the more cracks are formed on the wheel section. Therefore, the heat dissipation during the grinding process is better, the grinding wheel is not easy to block, so that the grinding can be carried out smoothly. The surface of the burn has a large grinding depth of 0.04~0.05mm, and no crack occurs during grinding. If there is crack in the initial stage of grinding, even if the grinding depth is reduced to less than 0.01 mm, the cracking will occur.
Another test found that the gears that have cracked are immediately sent to the heat treatment workshop, placed in the oil tank or low temperature oven, heated to 180~200 C, kept for 8 ~ 12h, and then ground, the crack can be quickly removed. .
When the emulsion is used for cooling, the cooling is intense, the thermal expansion and contraction of the grinding zone is severe, and a large internal stress is generated, so that the high hardness of the tooth surface cracking opportunity is large. The 20 oil coolant is softer than the former and the surface roughness of the grinding is low.
The principle of selecting the grain size of the grinding wheel is that the smaller the particle size, the better, in the case of ensuring the roughness of the tooth surface. Some recommend the use of a white special alumina grinding wheel to grind high-hard steel parts, because the grinding wheel has a large brittleness and is easily broken during grinding to expose a new cutting edge, which is of great significance for avoiding cracking.
Our factory is grinding 20CrMnTi carburized steel quenching gear, using white special alumina grinding wheel with grinding wheel size of 46~60 and hardness KL.
Our factory reduced the circumferential speed of the original grinding wheel by 33m/s to 22 26m/s to reduce the probability of grinding. The peripheral speed of the grinding wheel is related to the size of the machining gear. The same high-speed grinding wheel speed is used to grind the gears of different modulus, and the small modulus gear has a greater tendency to crack. According to the test summary, the 20CrMnTi steel gear is drawn. The curve of the workpiece modulus and the linear speed of the grinding wheel is seen. The lower right part of the curve is the dangerous area where the crack is easy to occur (I area). The upper part of the curve is the 1G safety zone (zone). I8 6 allows cutting depth and grinding crack! 4 and cracks have a great effect. However, if the surface of the grinding wheel is too rough, the surface roughness of the tooth surface will be poor. Therefore, the separation of coarse and fine grinding is not only good for avoiding grinding cracks, but also has a great effect on productivity improvement.
The test of the diamond dressing wheel after the blunt wear and tear proves that the particles broken by the diamond oil sand particles and the binder block the gap of the grinding wheel, so that the contact surface of the grinding wheel and the part is increased in heat, causing the gear tooth surface to burn. Cracks are locally generated.
Experience has shown that when the contact surface between the diamond and the grinding wheel reaches 0.8~1mm, the diamond should be rotated by an angle or removed and reinstalled, otherwise the wear will be intensified.
Properly increasing the low temperature tempering temperature from the conventional 160C to about 250C is beneficial to reduce the grinding stress and eliminate cracks.
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