Heat treatment of thermoforming spring
Tip: the heat treatment of thermoforming spring generally adopts quenching and moderate tempering, and the obtained structure is tempered martensite. This kind of microstructure has high elastic limit, yield strength and fatigue strength. Hardness is generally between 40~50HRC, can meet the main performance requirements of the spring. (1) quenching 1) quenching temperature. When the spring heat treatment first must according to the steel critical point correct choice quenching temperature. Most spring steels belong to the subeutectoid steels, and the quenching and tempering are generally adopted in the heat treatment of the quenching hot forming springs, and the structure obtained is tempered martensite. This kind of microstructure has high elastic limit, yield strength and fatigue strength. Hardness is generally between 40~50HRC, can meet the main performance requirements of the spring.
1) quenching temperature. When the spring heat treatment first must according to the steel critical point correct choice quenching temperature. Most spring steel are of hypoeutectoid steel, the quenching temperature shall be the Ac3 + 30 ~ 70 ℃; For hypereutectoid steel, such as 85 steel, the quenching temperature shall be the Accm + 30 ~ 70 ℃. Because of the hot forming spring heating forming, quenching, tempering is continuous, the heating speed, quenching temperature can be appropriately increased to 850 ~ 950 ℃.
Due to the high quality of the spring surface, the surface oxidation and decarbonization and overheating are not allowed when heating. The quenching and heating of springs must be carried out under the condition of protective measures. Most of the quenching and heating of springs are generally carried out in the salt bath furnace. When heating in the air furnace, anti-oxidation and anti-decarbonization measures should be taken. In order to reduce and prevent the spring in the heating process due to the deformation of the weight, the spiral spring in the furnace should be a single layer or a single horizontal, or the use of fixtures, should not be placed vertically, do not stack.
2) quenching and cooling. Since the vast majority of hot-forming springs are made of alloy steel, oil cooling is usually used during quenching. Can be used to prevent deformation, 50 ~ 80 ℃ hot oil quenching. For the spring with large section, water quenching and oil quenching can be adopted to quench the whole section thoroughly. In order to prevent cracking, quenching temperature should be appropriately reduced, the residence time in water should be strictly controlled, and the quenching should be tempered in time.
The quenching cooling mode has a great influence on the spring deformation. When spring quenching, if vertical immersion, the pitch deformation is large, the center line deformation is small; If the horizontal quenching, the deformation of the opposite situation. The spring is inserted into a thin steel tube with the same inner diameter as the spring and filled with small holes, which can be quenched horizontally into the medium to reduce deformation.
(2) the tempering spring shall be tempered in time after quenching to prevent quenching cracks. Tempering temperature is 400 ~ 500 ℃. The holding time in the air furnace shall be calculated as 1.5min /mm, but not less than 30min. In order to make the temperature uniform, should be as far as possible in the air furnace with a fan or nitre furnace temper. The workpiece that has been deformed can be corrected by special fixture when tempering. Spring tempering is generally cooled in water or oil, on the one hand can prevent temper brittleness, on the other hand can form on the surface of the spring compressive stress, is conducive to improve the fatigue strength.