Heat treatment process flow of large gear ring forgings
2022-11-18
Heat treatment process flow of large gear ring forgings
Large gear ring forgings will have great distortion after carburizing and quenching. Through reasonable design and machining and heat treatment process, using the correct correction method and salt quenching, the elliptic distortion of carburized and quenched large ring gear forgings can be controlled within 2mm, the warp and taper distortion can be controlled within 1mm, and the bearing capacity and service life of ring gear forgings can be improved.
The structure of large ring forging is characterized by its thin wall, large diameter to length ratio (outer diameter/tooth width), large carburizing and quenching distortion, irregular and difficult to control, the larger distortion directly affects the product quality and the efficiency of post-sequence processing, resulting in uneven post-sequence processing margin, affecting the depth of effective hardened layer of tooth surface and tooth surface hardness, thus reducing the strength, bearing capacity and fatigue strength of ring teeth. Finally reduce the service life of the gear ring.
1. Processing design
Gear ring forging process: forging - after forging, tempering - rough turning - tempering pretreatment - semi-finishing turning - artificial aging - tooth hobbing - carburizing quenching, tempering - shot blasting - finishing turning - artificial aging - finishing turning - gear grinding - finished product.
2. Pretreatment
If normalizing and high temperature tempering is used for pretreatment, the structure after heat treatment is pearlite and ferrite, and even produces non-equilibrium bainite. Because of uneven air cooling, the uniformity of normalizing structure is poor. Because the cooling uniformity and speed of oil medium are better than that of air, tempering will obtain uniform tempered soxite structure, which can improve or eliminate the original microstructure heterogeneity generated by forging, and improve the uniformity of mechanical properties of the gear ring. The positive heat treatment after forging can improve the forging microstructure, refine the grain, and the tempering pretreatment can uniform the microstructure and reduce the subsequent heat treatment distortion. The combination of the two is very effective for improving the carburized quenching microstructure and distortion.
3. Carburizing furnace
The superposition of carburized ring forging is equivalent to increasing the tooth width and reducing the diameter to length ratio, which is conducive to reducing warpage and elliptic distortion. When cooling after carburizing, the upper and lower end faces of the superimposed gear ring cool relatively fast, and the shrinkage is relatively large, resulting in the waist drum shape feature. Because of the uniform cooling in the furnace before cooling to 650℃, the ring gear forging in the high temperature zone with poor rigidity produces little ellipse and warpage distortion, so it only produces waist drum shape characteristics.
4. Carburizing process
The process route adopts re-heating quenching, which can prevent the grain coarsing caused by long-term carburizing of 20CrMnMo. At the same time, the quenching process can be adjusted by measuring, correcting and detecting the distortion after carburizing. The faster the carburizing temperature rises, the greater the thermal stress will be generated, and the superposition of residual machining stress will produce a large distortion, so it is necessary to step temperature rise. Carburizing must be out of the oven at low temperature. If 760 ℃ is out of the oven, the infiltration layer will produce uneven phase transition, which will produce quenched martensite structure on the secondary surface, increase the specific volume, and the surface is subjected to tensile stress. Especially in winter, when 20CrMnMo steel forgings are placed in the slow cooling pit, the crack probability will increase, and the quenched martensite structure will increase the carburizing distortion. In the later stage of carburizing, 650℃ insulation will make the surface obtain uniform eutectic structure, eliminate stress and prepare for quenching.
5. Correction after carburizing
For saltsalt media, there is a certain proportional relationship between carburizing distortion and quenching distortion. Generally, the quenching elliptic distortion increases by 30% ~ 50% on the basis of carburizing distortion. In a sense, the control of carburizing distortion can effectively control the post-quenching distortion. If the ellipse is found to be large after carburizing, it should be corrected. If the heating temperature of the gear ring is low, such as 280 ℃, the strength of the gear ring is high, and the elastic zone is large at low temperature, making it difficult for plastic deformation to occur. With the increase of temperature, the elastic zone will decrease and the difficulty of correction will decrease. If the heating temperature is too high, the operation is difficult. Practice has proved that the correction effect is better when heating to 550 ℃, the elastic zone is greatly reduced, and plastic deformation can be produced by low stress. Practice has proved that after carburizing and stress removal, the distortion will not rebound after quenching, and the accumulation of quenching distortion can be effectively solved by post-carburizing correction.
6, quenching furnace
The upper and lower face heat of gear ring forging is not balanced, and the upper face heat dissipation is fast during cooling, and the increase is relatively large. See Fig.7 for the schematic diagram of salt-quenching distortion. Distortion is measured after carburizing. The rule of tooth ring loading furnace is that the tooth top circle of the upper end is smaller than the tooth top circle of the lower end, and the pads between the tooth rings are separated. See Fig.8 for quenching loading furnace. The quenching furnace is adjusted according to the distortion after carburizing, and a certain taper value will be generated when the carburizing waist drum features are divided into a single tooth ring. Reasonable use of carburized waist drum shape, can realize the salt quenching cooling difference between the upper and lower end of the taper and carburized waist drum taper offset, to achieve small taper distortion.
7. Quenching and tempering process
Extending the holding time is equal to the disguised phase to increase the quenching temperature and increase the quenching distortion. Therefore, the austenitizing temperature is chosen to hold at 830 ℃ for 4 h. Compared with oil, saltpeter medium use temperature is high, quenching temperature rise is small, graded isothermal quenching makes the surface martensite transformation in the air, cooling slowly, workpiece quenching distortion is small. The melting point of KNO3 + NaNO2 nitrate is 145 ℃, the use temperature of nitrate is 160 ~ 180 ℃, and the cooling ability is strong. When the salt temperature is increased to 200 ~ 220 ℃, and the water content is adjusted to 0.9%, martensite plus a large amount of lower bainite and a very small amount of acicular ferrite will be obtained in the center of the gear ring. Ensure the core performance while producing minimum distortion.
Large gear ring forgings will have great distortion after carburizing and quenching. Through reasonable design and machining and heat treatment process, using the correct correction method and salt quenching, the elliptic distortion of carburized and quenched large ring gear forgings can be controlled within 2mm, the warp and taper distortion can be controlled within 1mm, and the bearing capacity and service life of ring gear forgings can be improved.
The structure of large ring forging is characterized by its thin wall, large diameter to length ratio (outer diameter/tooth width), large carburizing and quenching distortion, irregular and difficult to control, the larger distortion directly affects the product quality and the efficiency of post-sequence processing, resulting in uneven post-sequence processing margin, affecting the depth of effective hardened layer of tooth surface and tooth surface hardness, thus reducing the strength, bearing capacity and fatigue strength of ring teeth. Finally reduce the service life of the gear ring.
1. Processing design
Gear ring forging process: forging - after forging, tempering - rough turning - tempering pretreatment - semi-finishing turning - artificial aging - tooth hobbing - carburizing quenching, tempering - shot blasting - finishing turning - artificial aging - finishing turning - gear grinding - finished product.
2. Pretreatment
If normalizing and high temperature tempering is used for pretreatment, the structure after heat treatment is pearlite and ferrite, and even produces non-equilibrium bainite. Because of uneven air cooling, the uniformity of normalizing structure is poor. Because the cooling uniformity and speed of oil medium are better than that of air, tempering will obtain uniform tempered soxite structure, which can improve or eliminate the original microstructure heterogeneity generated by forging, and improve the uniformity of mechanical properties of the gear ring. The positive heat treatment after forging can improve the forging microstructure, refine the grain, and the tempering pretreatment can uniform the microstructure and reduce the subsequent heat treatment distortion. The combination of the two is very effective for improving the carburized quenching microstructure and distortion.
3. Carburizing furnace
The superposition of carburized ring forging is equivalent to increasing the tooth width and reducing the diameter to length ratio, which is conducive to reducing warpage and elliptic distortion. When cooling after carburizing, the upper and lower end faces of the superimposed gear ring cool relatively fast, and the shrinkage is relatively large, resulting in the waist drum shape feature. Because of the uniform cooling in the furnace before cooling to 650℃, the ring gear forging in the high temperature zone with poor rigidity produces little ellipse and warpage distortion, so it only produces waist drum shape characteristics.
4. Carburizing process
The process route adopts re-heating quenching, which can prevent the grain coarsing caused by long-term carburizing of 20CrMnMo. At the same time, the quenching process can be adjusted by measuring, correcting and detecting the distortion after carburizing. The faster the carburizing temperature rises, the greater the thermal stress will be generated, and the superposition of residual machining stress will produce a large distortion, so it is necessary to step temperature rise. Carburizing must be out of the oven at low temperature. If 760 ℃ is out of the oven, the infiltration layer will produce uneven phase transition, which will produce quenched martensite structure on the secondary surface, increase the specific volume, and the surface is subjected to tensile stress. Especially in winter, when 20CrMnMo steel forgings are placed in the slow cooling pit, the crack probability will increase, and the quenched martensite structure will increase the carburizing distortion. In the later stage of carburizing, 650℃ insulation will make the surface obtain uniform eutectic structure, eliminate stress and prepare for quenching.
5. Correction after carburizing
For saltsalt media, there is a certain proportional relationship between carburizing distortion and quenching distortion. Generally, the quenching elliptic distortion increases by 30% ~ 50% on the basis of carburizing distortion. In a sense, the control of carburizing distortion can effectively control the post-quenching distortion. If the ellipse is found to be large after carburizing, it should be corrected. If the heating temperature of the gear ring is low, such as 280 ℃, the strength of the gear ring is high, and the elastic zone is large at low temperature, making it difficult for plastic deformation to occur. With the increase of temperature, the elastic zone will decrease and the difficulty of correction will decrease. If the heating temperature is too high, the operation is difficult. Practice has proved that the correction effect is better when heating to 550 ℃, the elastic zone is greatly reduced, and plastic deformation can be produced by low stress. Practice has proved that after carburizing and stress removal, the distortion will not rebound after quenching, and the accumulation of quenching distortion can be effectively solved by post-carburizing correction.
6, quenching furnace
The upper and lower face heat of gear ring forging is not balanced, and the upper face heat dissipation is fast during cooling, and the increase is relatively large. See Fig.7 for the schematic diagram of salt-quenching distortion. Distortion is measured after carburizing. The rule of tooth ring loading furnace is that the tooth top circle of the upper end is smaller than the tooth top circle of the lower end, and the pads between the tooth rings are separated. See Fig.8 for quenching loading furnace. The quenching furnace is adjusted according to the distortion after carburizing, and a certain taper value will be generated when the carburizing waist drum features are divided into a single tooth ring. Reasonable use of carburized waist drum shape, can realize the salt quenching cooling difference between the upper and lower end of the taper and carburized waist drum taper offset, to achieve small taper distortion.
7. Quenching and tempering process
Extending the holding time is equal to the disguised phase to increase the quenching temperature and increase the quenching distortion. Therefore, the austenitizing temperature is chosen to hold at 830 ℃ for 4 h. Compared with oil, saltpeter medium use temperature is high, quenching temperature rise is small, graded isothermal quenching makes the surface martensite transformation in the air, cooling slowly, workpiece quenching distortion is small. The melting point of KNO3 + NaNO2 nitrate is 145 ℃, the use temperature of nitrate is 160 ~ 180 ℃, and the cooling ability is strong. When the salt temperature is increased to 200 ~ 220 ℃, and the water content is adjusted to 0.9%, martensite plus a large amount of lower bainite and a very small amount of acicular ferrite will be obtained in the center of the gear ring. Ensure the core performance while producing minimum distortion.
this is forging inspection machine
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