(1) deformation heat treatment -- the combination of deformation reinforcement and heat treatment enhancement to further improve the strength and toughness of steel.Heat treatment of deformation has high, medium and low temperature.The high temperature deformation heat treatment is to produce the deformation immediately after the austenitic state at a stable distance, or can be combined with forging or hot rolling, that is, quenching immediately after hot forming.The deformation heat treatment has been applied to automobile plate spring production.(60 si2mn)
(2) isothermal quenching of spring -- isothermal quenching can be adopted for springs with small diameter or sufficient permeability. It can not only reduce deformation, but also improve strength and toughness. It is better to conduct another tempering after isothermal quenching, which can increase the elastic limit.
(3) relaxation treatment of spring -- the spring works under the action of external forces for a long time. As a result of stress relaxation, slight permanent (plastic) deformation can be generated. Especially for the spring working at high temperature, stress relaxation is more serious at high temperature, which reduces the precision of the spring.Therefore, this kind of spring should be relaxed after quenching and tempering -- preloading the spring so that its deformation exceeds the possible deformation when the spring is working.Then heat it at a temperature of 20C above the working temperature for 8-24h.
(4) low temperature carbonitriding -- the combination of tempering and low temperature carbonitriding (soft nitriding) can significantly improve the fatigue life and corrosion resistance of the spring.
(5) shot peening treatment -- surface defects such as scratch, folding and oxidized decarburization often become the stress concentration and fatigue fracture source when the spring is working.If the spring surface is sprayed with small steel pellets at high speed, it can not only improve the spring surface quality, increase the surface strength, but also keep the surface in the state of compressive stress, so as to improve the spring fatigue strength and service life.
Operation precautions
(1) check the surface for decarburization, crack and other defects before heat treatment.These surface defects will seriously reduce the spring's fatigue limit.
(2) in quenching and heating, special attention should be paid to prevent overheating and decarburization, do a good job in salt bath deoxidation, control the atmosphere of furnace gas, and strictly control the heating temperature and time
(3) in order to reduce deformation, the installation mode of the spring during heating, the form of clamping device and the quenching cooling method during cooling.
(4) tempering should be done as soon as possible after quenching and heating should be done as far as possible.The rapid cooling after tempering can prevent the brittleness of tempering and cause the surface compressive stress and improve the fatigue strength.
Quality inspection
Before heat treatment --
(1) the rolled surface of steel is usually the surface after making the spring, so there should be no cracks, folds, spots, wrinkles, air bubbles, interlayer and pressed oxide skin.
(2) the surface decarburization can significantly reduce the fatigue strength of spring. The depth of decarburization layer should be tested according to regulations.
After heat treatment --
(1) the naked eye or low-power magnifier shall observe no crack, corrosion pitting and severe quenching deformation of the spring watch.
(2) the hardness and uniformity are in line with the regulations.In mass production, it is allowed to sample hardness with a file, but it must be noted that the position of the file mark should not affect the final precision of the spring.
(3) the metallographic structure shall be a mixture of troxoid or troxoid and sorbite.
(4) after the plate spring is assembled, permanent deformation and static deflection tests under the working load are usually performed.