Vacuum heat treatment of stop spring

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The stop spring is one of the important parts in the compensator body of the governor. During the working process, the position of the adjusting screw sleeve is fixed by the stop spring, so as to adjust the oil volume of the engine at medium and low speed. Since the thickness of the part is only 0.5mm and the thickness is extremely thin, it is easy to deform during the heating and cooling process of heat treatment. If the surface of the part is decarburized or increased, it will have a great impact on the mechanical properties of the part. The heat treatment process is of great significance.

The working condition of the stop spring requires that the part should have high elastic limit and fatigue limit, good plasticity, and the highest possible strength and yield ratio. If the hardness is too high, the plasticity and toughness are poor, and brittle fracture is easy to occur; on the contrary, if the hardness is too low, the plasticity and toughness are good, but the elasticity is insufficient. In order to take into account the performance of the above aspects and consider the economy, the technical requirements for the final heat treatment of the stop spring are 430 ~ 500HV10, and the material is stamped from 65Mn-P-T-0.5 steel strip. The main components (mass fraction%) of the steel strip are: 0.62～0.70C, 0.17～0.37Si, 0.90～1.20Mn, ≤0.25Cr, ≤0.25Ni, ≤0.04P , ≤0.04S.

The figure is a schematic diagram of a typical stop spring part in the compensator body of the RQV governor, and its main working surface is the arc surface inside the stop spring.

Process flow:
1. Pre-heat treatment:
Pre-heat treatment adopts vacuum recrystallization annealing (also known as intermediate annealing), which is a method of annealing the parts after blanking in order to ensure the precision of punching, so as to appropriately reduce the hardness of the material and improve the cold plastic deformation ability of the material And effectively prevent cracking during punching, the specific steps can be as follows:

①Install the fixture After the parts are degreasing and cleaned, then install the fixture, discharge the parts sideways and evenly install them in the special mesh tray, and then put the mesh tray with the parts into the material basket. One material basket can hold 30 trays of parts. The furnace loading method is very important to reduce the deformation and uniform hardness of the parts. The parts are installed to half of the height of the mesh plate, and there is a certain gap between the upper and lower layers of the parts to ensure the convection of the atmosphere in the furnace and the uniform heating of the parts during heating, which can also reduce the number of parts. And the deformation of the network disk prolongs the service life of the network disk.

②Recrystallization annealing Recrystallization annealing is carried out in a double-chamber oil quenching gas-cooled vacuum heat treatment furnace, and the annealing temperature is below AC1, that is, (680±2) °C, hold for 90 minutes, then slowly cool to 500 °C and transfer to a cold room filled with nitrogen. After cooling, the furnace is cooled for 30 minutes and then released. The entire processing process above is automatically controlled by a computer program. After annealing, spherical pearlite structure is obtained, and the hardness is 170-190HV10. After recrystallization annealing and then punching, the cold forming work hardening in the blanking process can be eliminated, so as to facilitate punching and forming, avoid cracking of parts in the punching process, and prolong the service life of the die.

2. Final heat treatment process:
The final heat treatment process can be carried out as follows:
①Fixture installation After the parts are degreasing and cleaned, the fixture installation method is the same as that for vacuum recrystallization annealing.

②Vacuum quenching The vacuum quenching is heated by a double-chamber oil quenching and air-cooled vacuum heat treatment furnace. The operator only needs to call the corresponding program in the programmable controller on the automatic control cabinet, and the automatic feeding and discharging are adopted during production. In order to ensure that the parts are heated evenly, first heat the parts to (750±2) °C, keep the temperature here for 35 minutes, then heat up to (830 ± 2) °C, keep the temperature at this temperature for 90 minutes, and the vacuum degree during the whole process is not less than 20Pa , after the heat preservation, the cold room is filled with nitrogen and pressurized to -0.05MPa ~ -0.06MPa, oil cooling quenching requires oil temperature ≤ 60 °C, after quenching and cooling, the fine needle-like martensite structure is obtained, using HVA-10A dimension The hardness is measured with a hardness tester, and the hardness is 785-825HV10.

③Degreasing and cleaning The quenched parts should be removed from the oil stains in the degreasing tank, and cleaned in a hot water tank at a temperature of 90 ℃ ~ 95 ℃, and then dried with compressed air.

④Tempering After the spring is quenched, it must be tempered in time to appropriately reduce the hardness and brittleness, reduce or eliminate internal stress, and improve the elastic limit, plasticity and toughness of the spring, thereby increasing the service life of the spring. The tempering equipment is a well-type tempering furnace, and the quenched parts are heated to (390 ± 10) ℃, and kept for 90 minutes. The hardness is measured with HVA-10A Vickers hardness tester, and the hardness is 453-465HV10.
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