4. Heat treatment characteristics of typical spring
According to the size of the spring, the forming and heat treatment methods are different.
4.1 cold formed spring
The spring with smaller diameter or thinner thickness should be made of cold drawn spring steel wire or cold-rolled spring steel strip. Steel wire can be made of 60, 70, 65Mn, t7a, T8A and T9A. According to GB / t4357-1989, carbon spring steel wire is divided into three groups: B, C and D. The tensile strength of group B is 1080 ~ 2800mpa, which is used for general low stress spring and other purposes; group C is 1270 ~ 3140 MPa for medium stress spring; group D is 1520 ~ 3240 MPa for high stress spring. In the same group, the smaller the diameter, the higher the strength.
Cold formed spring steel wire can be divided into three types according to different manufacturing processes.
① Isothermal treatment of cold drawn steel wire in lead bath
The main feature of this production process is that the steel wire passes through a certain rapid isothermal cooling process in the cold drawing process, and then it is cold drawn to the required size.
Taking T8A steel as an example, the normalized steel wire is pickled and cold drawn for three times to make the total drawing amount reach 50%. Then the steel wire passes through the heating furnace at the speed of 3.5m/min, heated to the austenitizing temperature of 890-920 ℃, and then isothermal decomposed into sorbite structure in the lead bath of 500-550 ℃, and then the steel wire is repeatedly drawn to the required diameter.
This kind of steel wire is mainly made of 60, 70 and 65Mn, with the highest strength of over 3000 MPa and high plasticity. In order to eliminate the stress and fix the shape of the spring, only stress relief tempering is needed at 200-300 ℃.
The shock absorber spring of motorcycle is made of 55sicra steel wire which is quenched and tempered in lead bath. After forming, it is annealed at 250 ℃ for 2h. The service strength is about 1680mpa and the hardness is about 46-48hrc.
② Oil quenched tempered steel wire
This kind of steel wire refers to oil quenching and tempering after cold drawing to the specified size. The tensile strength of the steel wire is not as good as that of the cold drawn steel wire treated with lead bath isothermal treatment, but its performance is relatively uniform and the fluctuation range of its strength is small, so it is widely used in the manufacture of various power mechanical valve springs. After the steel wire is cold rolled into spring, only stress relief tempering is needed, and no quenching and tempering treatment is needed.
For example, the plunger spring and oil inlet needle valve spring on the vehicle are made of 65Mn steel. The diameter of oil quenched and tempered steel wire is 0.8 ～ 1.5mm, which will be hardened by cold work during cold lap, and has high strength. Therefore, stress relief tempering must be carried out at 200 ～ 250 ℃ for 1H. For 70 steel torsion spring and tension spring, the end of the coil processing, so the second tempering. The first tempering temperature is 200-250 ℃× 1H, and the second tempering temperature is 180-200 ℃× 1H.
The temperature of stress relief tempering should be appropriate. If the temperature is too low, the stress can not be fully eliminated; if the temperature is too high, the tensile strength and elastic limit will be reduced due to tempering softening. Table 1 shows the temperature range of stress relief tempering after several kinds of steel wire cold coil spring, the time is generally 30 ~ 60min, and the performance can not be improved if the time is too long.
Table 1 tempering temperature for stress relief of cold coil spring
Type of spring steel wire stress relief tempering temperature / ℃
Drawing carbon steel wire 230 ～ 260
Oil quenched tempered steel wire 230 ～ 290
Valve spring steel wire 230 ～ 400
CR-V alloy spring steel wire 315 ～ 370
CR Si alloy spring steel wire 425 ～ 455
③ Alloy spring steel wire supplied in annealed condition
The spring made of this kind of steel wire needs quenching and tempering treatment to achieve the required mechanical properties. This kind of steel wire includes 50CrVA steel wire, 60Si2Mn steel wire and 55simnb steel wire.
4.2 hot formed spring
Coil spring or leaf spring whose diameter of steel wire or thickness of spring leaf is more than 10-15mm are usually formed during heating and quenching. The manufacturing process of spring is roughly as follows (taking leaf spring as an example): flat steel cutting → heating to quenching temperature after bending with special fixture clamping direct quenching + medium temperature tempering → shot peening → assembly. At this time, the quenching temperature is 50-80 ℃ higher than usual. After forming, it can be quenched immediately by waste heat. It can also be heated in a salt bath furnace. When the oil is cooled to 100-150 ℃, it can be taken out for intermediate temperature tempering. The tempering temperature is selected according to the performance requirements of the spring, which is generally in the range of 450-500 ℃, and the hardness after tempering is about 38-52hrc. The hardness of leaf spring after tempering is 39-47hrc, that of spiral spring is 44-50hrc, and that of spring subjected to greater shear stress is 47-52hrc.
The maximum operating temperature of sliding nozzle spring of continuous casting machine in a steel company is 500 ℃. The diameter of cylindrical spiral spring is 12.6 mm, the inner diameter is 116 mm, the high temperature hardness is 45-55 HRC, the material is 30W4Cr2VA, the cylinder hollow mandrel (material: Q235) is used to heat and fix the spring, and the b84t true air quenching furnace is used, and the convection preheating at 850 ℃ for 120min is used to improve the uniformity of low-temperature heating, shorten the heating time, reduce the temperature difference of workpiece, so as to reduce the quenching deformation; Then it is heated in vacuum at 1050-1070 ℃× 70min. The heating rate is 10 ℃ / min, and the gas quenching pressure is 5 × 105Pa. The spring is cooled to about 80 ℃ and discharged from the furnace. If the furnace loading is large, the quenching pressure can be increased appropriately. Tempering must be carried out in time in this vacuum furnace with 600 ℃± 10 ℃× 90min. If the vacuum furnace is not used for tempering, the protective atmosphere furnace can be used. Pay attention to the oxidation or decarburization phenomenon, and detect the hardness of 48-51hrc.
The surface quality of spring has a great influence on its service life, because tiny surface defects (such as decarburization, cracks, inclusions, burr and scar) can cause stress concentration and reduce the fatigue strength of spring. In order to improve the fatigue strength of spring, shot peening can be carried out on the spring after quenching and tempering to eliminate the surface defects and stress caused by surface hardening. The test shows that the