Cooling to 860 direct oil quenching, and then 180, 120min tempering treatment, the depth of the layer is controlled at 1822mm. After the test, the hardness and the depth of the layer are up to standard, but the length of the common normal length is 007011mm, the internal spline M The value of the distortion is 006008mm, and the flatness is out of tolerance. According to the results of the first test, the modified second test process route is: Forging normalizing machining, tooth processing, spline gas, carburizing, pre-cooling, quenching, low-temperature tempering, shot peening. The carburizing equipment was changed to IPSEN multi-purpose furnace, the carburizing medium was made of acetone, the quenching medium was changed to HoughtonG type quenching oil, and it was mounted by special material rack. After carburizing, the temperature was lowered to 860 quenching. After the furnace was discharged, the tempering treatment was carried out for 180 and 120 minutes. The depth of the layer was 1822 mm. The test results showed that the length of the hairline outside the gear, the M value of the internal spline and the flatness distortion were still out of tolerance. At the same time, in the cold test of two tests, the problem of poor cutting performance and grinding crack of the blank tooth profile was found.
Based on the results of two tests, the characteristics of each hot processing and heat treatment process and possible problems are analyzed in depth. The third test process route is determined as forging secondary normalizing machining tooth processing internal spline carbonitriding precooling quenching low temperature Tempered shot peening plane. In the original process, the normalizing after forging is changed to secondary normalizing, and the normalizing time is changed from the original 180 min to 240 min. The gas carburizing is changed to carbonitriding, and the permeating medium is acetone and ammonia. The quenching temperature after carburizing was reduced to 820, quenched with Houghton G type quenching oil, and the parts were mounted. At the same time, according to the distortion law in the first and second test results, the machining process parameters are corrected. After the test, the test results show that the gear distortion is small, and there is no problem in the machining process. The final machining becomes a simple surface grinding, which simplifies the process and improves the machining efficiency.
Test results and analysis of test results Three measurements of the measured data.
In the test of the influence of normalizing on the distortion of carburized gears, the metallographic structure after forging normalizing was observed. In the first and second tests, granular bainite was present in the normalizing structure, and the ferrite was broken in white. Continued mesh distribution, pearlite is black block, the grain is coarse. This is because the carburized gear billet is coarsened and uneven due to high temperature heating, uneven deformation and high final forging temperature during forging. The low-carbon low-alloy steel formed by the coarse austenite grains has a histological inheritance. When cooling after forging, the coarse grains are easy to obtain unsteady microstructures, such as W and B; It is easy to obtain F P. In the case of normalizing heating, the former will form coarse grains due to tissue inheritance, while the fine parts of FP will obtain smaller grains. When the mixed crystal structure is in normalized air cooling, the coarse grain portion is also likely to form B. Therefore, the cutting performance is deteriorated during machining, and the cutting force becomes large due to the blunt tool wear, which increases the surface residual stress. At the same time, after the normal forging of the gear forging, there is a B-structure formed by the coarse austenite grains. Therefore, in the later carburizing heating, coarse grains are formed due to the tissue, and the coarse grains of this part of the quenching are hardened. The portion larger than the fine grains makes the internal stress distribution in the quenching uneven, resulting in an increase in distortion of the heat treatment of the gear. In response to this situation, the secondary normalizing method of higher temperature and longer time is used to recrystallize the high-temperature austenite by phase transformation hardening to refine the crystal grains and eliminate the effect of tissue inheritance. At the same time, the appearance of reticulated carbon and nitrogen compounds caused by coarse grains during carbonitriding is avoided, and the performance of the gears is ensured.
Effect of carburizing, nitrogen co-infiltration and quenching on gear distortion Low carbon steel can obtain high carbon concentration on the surface by carburizing. After proper quenching and tempering treatment, it can improve surface hardness, wear resistance and fatigue strength. While the heart maintains good toughness and plasticity. However, due to the high carburizing temperature, the austenite grains grow up, but pre-cooled before quenching, but the grains do not shrink. After quenching, the martensite is coarser, the retained austenite is more and the structure is coarse, resulting in resistance. The wearability is reduced and the deformation is large. The carbonitriding is used, the co-infiltration temperature is low, and the grain growth tends to be small. After pre-cooling and quenching, fine nitrogen-containing martensite, retained austenite and carbonitride are obtained, and the crystal grains are fine and the deformation is small. At the same time, when quenching directly after carbonitriding, the temperature difference is less than that of carburizing. Therefore, the thermal stress caused by the temperature difference between the inside and the outside is also small, and the gear distortion is also small. And the wear resistance of nitrogen-containing martensite and a small amount of nitride obtained by heat treatment after gear co-infiltration is better than that of carburized structure, and the core structure is low-carbon martensite and ferrite, which ensures the core of the workpiece is better. High strength and impact toughness.
Metallurgical structure after quenching and tempering after carburizing (400) After the carbonitriding, quenching and tempering of the tooth structure (400) After carbonitriding, quenching during the quenching process, due to the cooling rate of the inner and outer layers of the gear Different temperatures result in inconsistent temperatures, which cause the workpiece to expand and contract differently, resulting in a large thermal stress. At the same time, due to the increase of carbon content on the surface, the surface is prone to martensite transformation, and the surface of the workpiece is first transformed into a martensite hard shell, and the workpiece is fixed in a relatively large size range. When the martensite transforms, the volume is swollen due to the difference in specific volume. At this time, the carburized layer and the center are still in the austenite state, and the first transformed martensite is swollen, and the carburized layer and the center are added. When the martensite is transformed, the expansion is additionally increased, so that the parts cannot be restored to the size before quenching after quenching.
Effect of quenching medium In order to obtain martensite during quenching and cooling, the cooling rate must be greater than VK (critical cooling rate); while rapid cooling will generate huge structural stress and thermal stress, which will cause the workpiece to be deformed and cracked. In order to solve the above contradictions, the ideal quenching medium for steel is cooled. Slow cooling above 650 to reduce thermal stress; fast cooling between 650400 to avoid supercooled austenite decomposition; 400
The following slow cooling passes through the martensite transformation zone to reduce the structural stress generated during martensite transformation to prevent quenching deformation and cracking. The ideal quenching medium cooling curve is an important basis for selecting a cooling medium. The critical cooling rate of alloy steel is small, and a relatively moderate quenching medium can be used.
Therefore, Houghton G type quenching oil was selected in the second and third tests, and the cooling curve was as shown. HoughtonG pattern quenching oil cooling characteristic curve 5 steel ideal quenching cooling curve 224 according to the heat treatment distortion law to adjust the machining process parameters according to the gear heat treatment distortion law to adjust the machining process parameters, so that the heat treated carburizing gear meets the technical requirements. In this test, according to the law that the length of the common line of the gear after heat treatment and the M value of the internal spline become larger, the machining process parameters are appropriately modified, and the effect is obvious.
Conclusion (1) Reasonable normalizing process before carburizing or carbonitriding is a key measure to eliminate abnormal and coarse tissues such as granular bainite, reticular ferrite and massive pearlite. If the heating temperature of the gear blank forging is too high, the microstructure of the gear is abnormal and the grain is coarse. If it is difficult to normalize and refine the structure after a normalizing, the secondary normalizing process can be used to improve. Reduce gear distortion and machining residual stress due to tissue genetics due to coarse grains. At the same time, it can also reduce the reticulated carbon and nitrogen compounds caused by coarse grains during carbonitriding and improve the wear resistance of the gear surface.
(2) For gears with high dimensional accuracy and performance requirements, carbonitriding is used instead of high temperature carburizing. Under the premise of ensuring performance, the high heating temperature is reduced by coarse austenite during quenching. Gear distortion caused by coarse martensite. At the same time, an appropriate reduction in the quenching temperature also reduces the thermal stress distortion caused by the temperature difference.
(3) With the continuous emergence of new quenching media, careful analysis of the quenching and cooling characteristics of the workpiece material, the selection of the appropriate quenching medium is also an effective measure to reduce gear heat treatment distortion.
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