(1) Thermomechanical heat treatment–combining steel deformation strengthening and heat treatment strengthening to further improve the strength and toughness of steel. Thermomechanical heat treatment is divided into high, medium and low temperature. High temperature thermomechanical heat treatment is quenched immediately after deformation occurs in a stable austenite state, and can also be combined with forging or hot rolling, that is, quenched immediately after hot forming. Thermomechanical treatment has been used in the production of automobile leaf springs. (60Si2Mn)
(2) Austempering of springs-Austempering can be used for springs with small diameters or sufficient permeability. It can not only reduce deformation, but also improve strength and toughness. It is best to perform tempering again after austempering. The elastic limit can be increased, and the tempering temperature is the same as the austempering temperature.
(3) Spring relaxation treatment-the spring works under the action of external force for a long time, and the result of stress relaxation will produce a small amount of permanent (plastic) deformation, especially the spring that works at high temperature, the stress relaxation phenomenon is more serious at high temperature. Reduce the accuracy of the spring, which is not allowed for general precision springs. Therefore, this kind of spring should be relaxed after quenching and tempering-pre-load the spring so that its deformation exceeds the deformation that may occur when the spring is working. Then heat it at 20C higher than the working temperature for 8-24h.
(4) Low-temperature carbonitriding-the process of combining tempering and low-temperature carbonitriding (soft nitriding) can significantly improve the fatigue life and corrosion resistance of the spring. This process is mostly used for coil springs.
(5) Shot peening-surface defects such as scratches, folds, oxidative decarburization, etc. tend to become stress concentration places and sources of fatigue fracture during spring operation. If a small steel shot is used to spray the surface of the spring at high speed, it will not only improve the surface quality of the spring, increase the surface strength, and put the surface in a state of compressive stress, thereby increasing the fatigue strength and service life of the spring.
(1) Check the surface for defects such as decarburization and cracks before heat treatment. These surface defects will seriously reduce the fatigue limit of the spring.
(2) The quenching heating should pay special attention to prevent overheating and decarburization, do a good job in salt bath deoxidation, control the furnace gas atmosphere, and strictly control the heating temperature and time
(3) In order to reduce the deformation, the method of loading the spring during heating, the form of fixture and the method of quenching during cooling.
(4) Tempering as soon as possible after quenching, and heating as much as possible. Quick cooling after tempering can prevent temper brittleness and cause surface compressive stress, and improve fatigue strength.
Before heat treatment–
(1) The rolled surface of steel is often the surface after the spring is made, so there should be no cracks, folds, scars, hair lines, bubbles, interlayers, and pressed oxide scales.
(2) Surface decarburization will significantly reduce the fatigue strength of the spring. The depth of the decarburized layer should be inspected as required.
After heat treatment–
(1) There should be no cracks, corrosive pitting and severe quenching deformation when observing the spring watch with the naked eye or low-power magnifying glass.
(2) The hardness and its uniformity meet the requirements. In mass production, it is allowed to use a file to sample the hardness, but it must be noted that the position of the file mark should not affect the final accuracy of the spring.
(3) The metallographic structure should be troostite or a mixed organization of troostite and sorbite.
(4) After the leaf spring is assembled, the permanent deformation and static deflection test under working load are usually carried out.