Zhengzhou Gou's talks about the quenching of the large ring gear of the 5.8m tunnel boring machine

(1) The large ring gear of the 5.8m tunnel boring machine is a large-module, low-speed, heavy-duty gear with a module of m=18mm and a diameter of 3m. It is driven by six motors with a power of 150kW and reduced by a planetary reduction box. The rotation speed is 5r/min, the large ring gear drives the cutterhead for excavation. The material used is 35CrMoVA steel.

(2) Heat treatment technical conditions: The hardness of the tooth surface and tooth portion after quenching and tempering treatment should be 235~250HBW.

(3) Processing process: Forging blank - normalizing, ten high temperature tempering - rough machining - quenching and tempering, half fine machining - tooth surface quenching, ten low temperature tempering - fine grinding - finished product.

(4) Heat treatment process specifications

①Normalizing and high temperature tempering. The heating temperature is 850~880°C and the holding time is 4 hours. After coming out of the furnace, it is immediately transferred to a high-temperature tempering furnace that has been preheated to 630~660°C for isothermal maintenance, so that the supercooled austenite undergoes complete pearlite transformation. The holding time is 6 ~8h, then cool to 400℃ at a speed of 30~50℃/h, and then continue cooling to 250~300℃ at a speed of 20~30℃/h. It can be air-cooled out of the furnace.

②Quenching and tempering treatment. The quenching heating temperature range is 840~860℃, the holding time is 4 hours, and it is quenched in oil after coming out of the furnace; then it is subjected to high temperature tempering in the furnace at 600~650℃ for 3~4 hours, and the workpiece is furnace cooled to about 250℃ and air cooled out of the furnace.

③Tooth surface medium frequency induction heating surface quenching. Select or design a special inductor, that is, a single-tooth continuous quenching inductor along the tooth groove, so that the inductor (equipped with a silicon steel sheet magnet) and the tooth are copied, and the large-module gear is continuously medium-frequency induction heated and surface quenched along the tooth groove.

Tempering. Perform low-temperature tempering at 180 to 200°C in a low-temperature tempering furnace, hold for 3 to 4 hours, and air-cool after coming out of the furnace.

(5) Heat treatment process analysis

①Normalizing and high temperature tempering. Its purpose is twofold: a. To refine the grains and improve the structure; b. To remove hydrogen from the steel and prevent the occurrence of white spots and hydrogen embrittlement. This process combines grain refinement and dehydrogenation of forgings into an overall consideration.

②Quenching and tempering treatment. So that the workpiece can obtain good comprehensive mechanical properties as a whole. After quenching and tempering treatment, the tooth surface hardness is 235-250HBW.

③Tooth surface medium frequency induction heating surface quenching. In order to obtain a deeper hardened layer depth (the hardened layer depth is generally 2 to 10mm), it is necessary to reduce the frequency of the induction heating current and use medium frequency (commonly used frequencies are 2500Hz or 8000Hz) induction heating equipment.

④Low temperature tempering. Its purpose is to eliminate residual internal stress, reduce brittleness, and ensure high hardness, high strength and high wear resistance of the workpiece surface.

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

① Due to the poor working conditions of the tunnel boring machine and the complex stress conditions, the raw materials for the large ring gear are strictly required. Vacuum smelting and electroslag refining should be used to reduce non-metallic inclusions in the material; the forged blanks need to undergo ultrasonic flaw detection. In addition, after the tooth surface is quenched, it must undergo magnetic particle inspection to ensure the quality of the workpiece.

② When implementing quenching and tempering treatment, the quenching cooling rate should ensure that most austenite is transformed into martensite, and the volume fraction of ferrite on the surface of the workpiece should not exceed 3% to 5%.

③Due to the extremely large size of the workpiece, medium-frequency induction heating surface quenching should be implemented along the tooth profile to avoid residual tensile stress in the tooth surface quenching transition zone caused by "hat" quenching, which may lead to bending fatigue loads. Broken teeth. Medium-frequency induction equipment has high power, high efficiency, and large current penetration depth, so the wall thickness of its inductor is required to be thicker.