High-precision CNC machine tool gear forging blank-heat treatment process flow

A high-precision CNC machine tool gear uses 40CrNi steel.

(2) Heat treatment technical conditions. After heat treatment, the high-precision gear should meet the level 5 accuracy requirements, the gear hardness is 47-49HRC, the inner L deformation is <0.06mm, the inner L roundness error should be ≤0.015mm, and the keyway width change should be ≤0.015mm.

(3) Processing process. Forging blank - annealing - rough machining - normalizing - high temperature tempering - half rough machining - stress relief annealing - fine machining - vacuum quenching - low temperature tempering - finished product.

(4) Heat processing and heat treatment process specifications

①Annealing. The heating temperature is 800~900℃, the holding time is 2 hours, and the furnace is slowly cooled.

②Normalizing and high temperature tempering. The heating temperature for normalizing is about 860℃, the holding time is 1.5h, and then it is air-cooled out of the furnace; the temperature for high-temperature tempering is about 650℃, the holding time is 2h, and then slowly cooled.

③Stress relief annealing. The heating temperature is 200℃ and the holding time is 24h.

④Vacuum quenching. Heating under a vacuum of 133Pa, heating to 650°C with moderate heat preservation, then raising the temperature to 790°C with heat preservation, and then quenching and cooling (refilling with nitrogen, and cooling with oil under a low vacuum of 66.6Pa).

⑤Low temperature tempering. The heating temperature is 160~180℃, and it is kept warm for 3~4 hours and then cooled in the air. After the above process, the high-precision gear made of 40CrNi steel can meet its technical requirements.

(5) Analysis of heat treatment process of high-precision CNC machine tool gears

①Annealing. Its purpose is to eliminate stress, uniformize the structure, and improve the cutting process performance of the workpiece.

②Normalizing and high temperature tempering. It has two functions: one is to uniformize the structure, refine the grains, improve the structure, and improve the cutting performance of the workpiece; the other is to remove hydrogen in the steel and prevent the occurrence of white spots and hydrogen embrittlement. This process combines the process of refining the product grains with the dehydrogenation of forgings, and considers them as a whole.

③Stress relief annealing. Aimed to further reduce or eliminate residual internal stress formed by cutting.

④Vacuum quenching. The purpose is to achieve smooth quenching of the workpiece, with small deformation of the workpiece, and the obtained workpiece has high dimensional accuracy and good mechanical properties.

⑤Low temperature tempering. It further reduces the residual internal stress caused by the vacuum quenching process, thereby reducing the deformation of the workpiece and ensuring the dimensional stability and high precision requirements of the workpiece.

Key points for implementation of high-precision CNC machine tool gear heat treatment technology

① The forging process should be strictly controlled, and the blank should be repeatedly upset and formed to make the band structure ≤3 levels to ensure a dense structure.

②Normalizing and high temperature tempering. Gears are necessary for high-precision machine tools and must not be omitted. When performing high-temperature tempering operations, be sure to pay attention to the heating temperature and select the upper limit temperature of high-temperature tempering. When cooling after tempering, attention should be paid to slow cooling to eliminate processing stress, stabilize the size of the workpiece, remove hydrogen from the steel piece, and prevent white spots and The purpose of hydrogen embrittlement.

③ Things to note. The specific stress relief annealing here has a heating temperature of only 200°C, but the annealing holding time is as long as 24 hours. The purpose is to reduce the thermal stress generated by the workpiece at a lower temperature, and to reduce residual internal stress due to the low temperature. For this purpose, the holding time must be ④The application of new vacuum heat treatment technology. The heat treatment process of heating and heat preservation in a decompressed space where the ambient pressure is lower than normal atmospheric pressure is called vacuum heat treatment. The role of vacuum heat treatment can be summarized in four points: First, it can prevent metal oxidation. As long as the workpiece is under a vacuum degree of 10 (-1) Pa, the oxidation rate of the metal is extremely slow. When the vacuum degree is <1.33×10 (- 2) Heating metal under a vacuum of Pa, although it is higher than the decomposition pressure of metal oxides, can still obtain a bright surface without oxidation. The second is the purification effect on the surface. During vacuum heat treatment of the metal surface, if the oxygen in the furnace The partial pressure < the decomposition pressure of the oxide, the oxygen generated by the decomposition of the oxide on the metal surface can be discharged out of the furnace by the vacuum pump, and the oil on the metal surface will also be heated and decomposed into hydrogen, water vapor, carbon dioxide, etc., and then be discharged out of the furnace by the vacuum pump, causing The third is degassing. During the vacuum heat treatment process, due to the pressure difference between the inside and outside of the metal parts, the gas dissolved in the metal will diffuse to the metal surface and desorb. The higher the temperature, the greater the degassing effect. Good, fourth, the heating speed is slow. During the air heat treatment, the workpiece mainly relies on radiation heat transfer. The heating speed is slower than the heating in the salt bath furnace and furnace gas, so the heating time needs to be appropriately extended, which reduces the productivity, but the cross-section temperature difference of the workpiece Small, workpiece deformation is smaller than other heating methods.

Nowadays, almost all heat treatment processes can be used for vacuum heat treatment. Vacuum quenching is now widely used in various steels and metal materials such as titanium, nickel, and cobalt-based alloys. After vacuum quenching, the hardness of the steel parts is high and uniform, the surface is smooth, there is no oxidation and decarburization, and the deformation is small. The degassing effect during vacuum heating can also improve the strength, wear resistance, anti-seizure and fatigue strength of steel parts, thereby increasing the service life of steel parts. Therefore, this process should be preferred when conditions permit. Compared with salt bath heating quenching, vacuum quenching has the following characteristics：

① After the workpiece is vacuum quenched, the quenching deformation of the workpiece is small, which provides convenience for quenching precision machine parts.

② Vacuum heating prevents oxidation and decarburization of the workpiece surface, and a bright surface can be obtained after quenching.

③Vacuum quenching can effectively increase the service life of the workpiece. For example, compared with salt bath heating quenching, vacuum quenching can increase the mold life by 30%-40% and has significant economic benefits.

④ Vacuum heating and quenching is pollution-free and environmentally friendly.