Before changing the heat treatment of the double torsion spring, the rolling surface of the steel is usually made into a spring, so there must be no cracks, wrinkles, hair lines, bubbles, interlayers and pressed scale. Surface decarburization will significantly reduce the fatigue strength of the double torsion spring, so the depth of the decarburization layer should be detected as required.

　　After heat treatment, the double torsion spring gauge should be free of cracks, pitting corrosion and severe quenching deformation observed with the naked eye or under low magnification. Hardness and its average value. Uniformity complies with regulations. In mass production, it is allowed to use a file to sample the hardness, but care must be taken that the position of the file mark must not affect the accuracy of the double torsion spring. Metallographic formation. The structure should be smithsonite or a mixture of smithsonite and sorbite. After the leaf spring is assembled, it usually needs to undergo permanent deformation and static load deformation under the action of working load.

　　In conventional heat treatment, the main cause of part shape changes is the thermal stress and phase change stress generated during heating and quenching. Heating rate. If the speed is too fast, the parts are too large relative to the heating furnace, and the temperature of each part is different, which will cause thermal deformation. Machining residual stress during insulation. It will be released and deformed, and the part's own weight will also cause deformation. During the cooling process, heat is generated due to the different cooling rates of different parts. Parts deform due to stress. Even if the cooling rates are the same, surface cooling is always faster and internal cooling is always slower. Therefore, the phase-changing surface leads to emission from non-phase-changing centers. Plastic deformation. If there is segregation of alloy components or surface decarburization in the material, the more uneven the phase change stress is, the easier it is to cause deformation of the part. Additionally, if the part thickness is not uniform, the cooling rate will be different.

　　Chemical composition and oxidation treatment process of double torsion spring materials. In the heated solution of the double torsion spring, due to the corrosion of the spring surface by the alkali, iron ions are added to the alkali by adding the oxidant nitrous oxide. Sodium can change the oxidation process and form a dense oxide film on the surface of the double torsion spring. This oxide film is mainly magnetic oxidation. It is composed of iron, and the formation time of the oxide film is 30-60 minutes. If the time is extended, the thickness of the oxide film cannot be increased. The cost of oxidation treatment is low, the process formula is simple, and the production efficiency is high. The oxide film has a certain degree of elasticity and basically does not affect the characteristic curvature of the double torsion spring. Therefore, oxidation treatment is widely used as an anti-corrosion and decorative measure for coil springs, spring washers and leaf springs. The quality inspection of springs after oxidation treatment includes appearance inspection and corrosion resistance inspection.

　　Oxidation methods include salt oxidation, alkali-free oxidation and electrolytic oxidation. Alkaline oxidation is a very common method. In the alkaline oxidation method, a double torsion spring is immersed in a sodium hydroxide solution containing oxidizing agent, oxidizing agent and hydrogen at a temperature of about 140°C for a certain period of time. Sodium oxide reacts with iron to form sodium ferrite and sodium ferrite, which then react with each other to form magnetic iron oxide. The thickness of the oxide film is about 0.6~2μm. Although the oxide film can improve the corrosion resistance of the double torsion spring, the oxide film is very thin and porous. The spring has poor protection ability and can only be used for springs working in less corrosive media.