Key points for each step of heating and annealing the sheet gear on the LC280A lathe

(1) Workpiece name: The sheet gear on the LC280A lathe has a module of m=2mm, a number of teeth of 40, and the material used is 40Cr steel.

(2) Heat treatment technical conditions: The depth of the nitriding layer is 0.15mm, the surface hardness of the gear is 500HVi, the internal L deformation is <0.005mm, the common normal change is <0.006mm, and the noise caused by the gear operation should be <87dB.

(3) Processing process: Forging blank - normalizing - rough machining - quenching and tempering, half fine machining - stress relief annealing - gas nitriding - fine grinding - finished product.

(4) Heat treatment process specifications

① Normalize the fire. The heating temperature is 870~890℃, and after being kept warm for 1-5~2 hours, it is air-cooled after being released.

②Quenching and tempering treatment. The heating temperature is 840~860℃, the holding time is 2 hours, and the oil is quenched; the high temperature tempering temperature is 600-650℃, the holding time is 2 hours, and the oil is cooled after coming out of the furnace.

③Stress relief annealing. The heating temperature is 300~400℃, the holding time is 2~4h, and it is air-cooled when it comes out of the oven.

④Gas nitriding treatment adopts step heating mode and two-stage nitriding process, as shown in Figure 1-113.

(5) Heat treatment process analysis

①Normalize the fire. Its purpose is to uniformize the structure, refine the grains, eliminate forging residual internal stress, and improve the cutting process performance.

②Quenching and tempering treatment. The whole lamella gear (especially the core) can obtain good comprehensive mechanical properties, obtain a fine tempered sorbite structure, and make the gear deformation small.

③Stress relief annealing. Eliminate the internal machining stress generated during machining to ensure the minimum deformation and stable structure during nitriding treatment.

④Gas nitriding treatment. On the premise of ensuring high precision of the workpiece, the workpiece is given the performance requirements of hardness on the surface and toughness on the inside. Even if the surface of the workpiece obtains high hardness, high wear resistance, and high corrosion resistance, the core can still maintain good plastic toughness and other comprehensive mechanical properties. The purpose of the step heating mode is to reduce the thermal stress caused by the temperature difference, thereby reducing the distortion of the workpiece.

The characteristics of the two-stage nitriding process are that the nitriding cycle is shortened, the nitriding speed is accelerated, and the brittleness of the nitrided layer is reduced, but the hardness of the nitrided layer is slightly lower than that of the one-stage method.

(6) Key points for the implementation of heat treatment process technology

①Normalize the fire. It must be noted that the heating temperature should be between 30 and 50°C, and the temperature should be kept for sufficient time to obtain a single, uniform and fine austenite structure, and then be air-cooled out of the furnace in order to transform it into a fine pearlite structure. However, the volume fraction of free ferrite in pearlite after normalizing shall not exceed 5%.

②Quenching and tempering treatment. It has a great influence on the quality of nitriding. If the temperature of the quenching and tempering treatment is too low or the holding time is not enough, free ferrite will exist after the quenching and tempering treatment, which will seriously affect the quality of the nitrided workpiece. That is, the diffusion rate of nitrogen in ferrite is relatively large, and there will be a higher nitrogen concentration after nitriding, which can easily form needle-like nitrides, making the nitrided layer more brittle and easy to peel off. Therefore, free ferrite is not allowed to appear in the surface layer after quenching and tempering treatment. On the contrary, if the quenching temperature is too high, the grains will become coarse after quenching, the nitrides will preferentially extend along the grade boundaries, and a corrugated or network structure will appear after nitriding, which will also increase the brittleness of the nitrided layer.

③Stress relief annealing. Be careful to perform stress relief annealing in a protective atmosphere to avoid oxidation of the workpiece surface and affect the effect of the subsequent gas nitriding process. Return gas nitriding treatment Ammonia protection must be passed during the implementation of the 200°C, 300°C and 400°C step heating modes and the cooling process to prevent negative pressure from being generated and air entering the furnace.