Do you know that the lead screw uses high-frequency induction heating equipment for heat treatment?

The screw is one of the important parts of the machine tool. Commonly used machine tool screws mainly include trapezoidal screws and ball screws. Trapezoidal screws are generally used on machine tools. The screw is required to have a certain stiffness and strength, good mechanical properties and high dimensional stability. In order to make the screw achieve mechanical properties, high-frequency induction heating equipment needs to be used to heat treat the screw. Today, I will briefly introduce the process of screw heat treatment.

1. Normalize the fire. The purpose of normalizing is to eliminate the secondary cementite network to facilitate the full progress of spheroidizing annealing. The following points should be noted when implementing the normalizing process.

a. The cooling rate after normalizing and heat preservation must be fast enough, otherwise network carbides will precipitate again. For example, if a bar with a diameter of 90mm is heated at 950-970°C for 5 hours, and then cooled by an electric fan at 10°C, the network carbide will be reduced from the original level 4 to level 3. This is already too high to prevent grinding cracks. It can be seen that natural cooling or blowing cooling is only suitable for situations where the diameter is small and the temperature is low. The effective cooling method is to use a 5% to 10% NaCl aqueous solution or a nitrate salt bath containing 0.3% to 1.0% water at 230 to 350°C. When using salt water for cooling, the water cooling time must be strictly controlled so that it can reheat to about 600°C after the water is discharged (the parts are dark red). If the water cooling time is too short, it is not enough to prevent the carbide network from re-precipitating. If the water cooling time is too long, cracks will easily occur. Generally cooling ls every 4~5mm. The reference time for water cooling of blanks with different diameters is as follows: the time for blanks with a diameter of 90 to 100 mm is about 20 seconds, the time for a diameter of 70 to 80 mm is about 15 seconds, the time for a diameter of 50 to 60 mm is about 10 seconds, and it is naturally air-cooled after water is released.

b. The normalizing heating temperature must be high enough and the holding time long enough so that the network carbide can be fully dissolved. However, if the temperature is too high, the austenite grains will grow and the tendency of wear and cracking will increase. Generally, 950°C is used for 4 to 5 hours of insulation.

c. It should be pointed out that the above-mentioned rapid cooling and normalizing cannot effectively improve the accumulation state of carbides. This can only be controlled during raw material inspection.

2. Isothermal annealing process is isothermal spheroidizing annealing.

a. After isothermal maintenance, the furnace is cooled to <500°C and released, and continues to be air-cooled to room temperature.

b. Compared with general spheroidizing annealing, the structure obtained after isothermal spheroidizing annealing is relatively uniform, the process is easy to control, and the production cycle is shorter. But it should be noted: the heating temperature and holding time must be strictly controlled.

c. The cooling rate must be strictly controlled. After holding at 780°C for 2 to 3 hours, use a speed of 20 to 30°C/h to lower it to 10 to 20°C below the critical cooling temperature of the steel, which is 680°C, and maintain it at this temperature for 5 ~6h (generally 1.5 times the heating time and holding time is appropriate); after long-term room temperature storage, no difference in dimensional stability between cold-treated and non-cold-treated parts has been found. Therefore, this kind of screw can be exempted from cold treatment, but if the hardness requirement is higher than 61HRC and the tempering temperature is not allowed to be increased, cold treatment should be considered.

3. Several issues that should be paid attention to during the implementation of quenching, tempering and aging processes

a. During the quenching and heating process, in order to prevent oxidation of the workpiece when heated, the screw is dip-coated with 20% borax aqueous solution, preheated at 500~600℃ for 1~2h, and heated at 830~850℃.

b. For workpieces requiring level 6 precision, one aging at 180~220℃ (6~8h) can be used; if level 4 or 5 precision is required, two aging times (first time 6~8h, second time 8-12h) ).

From the perspective of stabilizing retained austenite and eliminating and stabilizing residual internal stress, the higher the aging temperature, the longer the aging time, the better the effect, and temperature plays a leading role. In the temperature range of 100 to 320°C, if the aging parameters of the subsequent stage are improperly selected, the stabilizing effect of the retained austenite imparted by the previous stage of aging will be partially destroyed, and destabilization is prone to occur. This must be paid full attention to when formulating the aging process. It should be pointed out that due to natural aging at room temperature, its stability increases with time (that is, the effect of time on aging), which can eliminate doubts about whether the stability of the workpiece will decrease during use.

c. During quenching and cooling, the workpiece moves vertically up and down in oil at 30~70℃, cools to 180~200℃, takes out and straightens while it is hot, and then tempers at 260~320℃ for 4~6 hours.

During tempering or aging above 260°C, the transformation of retained austenite and the relaxation of residual stress are mainly completed within the first few hours, so the effect of extending the aging time too much is not obvious.

d. It should be noted that the residual austenite content after tempering has an important impact on its stability. Under the hardness permitting conditions, increase the tempering temperature as much as possible to obtain the minimum retained austenite content, which is the key to achieving dimensional stability. Experiments show that regardless of whether it has been cold treated before or after tempering, after tempering at 260 to 320°C for 4 hours, the residual austenite content is less than 2%, and no transformation is seen when cooled to -120°C.

Zhengzhou Gou's is a manufacturer specializing in the production of heat treatment equipment. The equipment includes high-frequency induction heating equipment, ultrasonic induction heating equipment, etc. It has the advantages of small size, easy operation, complete functions, and fast heating speed. In terms of quenching, the quenching is uniform and the quenching is The layer depth is much better than other quenching equipment, and it is widely used in the heat treatment of gears, sprockets, and hardware tools.