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Local induction annealing process of spring material

Spring is a basic part with large quantity and wide range. It is a very key part for the control elements in power machinery, instruments and weapons. Its basic function is to use the elasticity of materials and the structural characteristics of spring, so as to achieve the purposes of cushioning or vibration reduction, control movement or reset.
For carbon spring steel wire, the material brand is IIA, and the drawing requires local annealing. There is no process parameter for local annealing of IIA material specified in any corresponding standard. The local annealing state is achieved through the test to obtain the structure with low hardness and close to the equilibrium state, so as to improve the physical, chemical and mechanical properties of metal or alloy and stabilize the size and shape; Improve the structure and prepare for the next hook process. The purpose of local annealing of this spring is to easily hook and ensure that it can limit the spring pin after assembly.

Determination of local annealing process parameters

IIA spring material belongs to deformation strengthened spring steel, which is supplied in cold deformation (cold drawing or cold rolling) state. After winding the spring, stress relief tempering is carried out to set the spring. This kind of steel wire has high strength and sufficient plasticity and toughness before forming. Only tempering process parameters of 260 ~ 300 ℃ are specified in the standard, and no other heat treatment process parameters are specified. After reading the aviation material manual According to the data of aviation spring steel wire and consulting other relevant units, it is determined that IIA spring material has no annealing parameters. It is suggested to consider using similar materials for test.
Since IIA spring material is made of high-quality carbon steel, which is the upgraded material of carbon steel 65 steel, 70 steel, 65Mn and other spring steel wires, and its comprehensive performance is similar to that of carbon steel 70 steel, it is decided to conduct the test according to the annealing parameters of 70 steel. After consulting relevant standards and referring to the annealing parameters of 70 steel, the annealing test is finally determined at 680 ~ 700 ℃.

Selection of local annealing process

The spring steel wire is thin and small. Combined with the existing production capacity of our company, because the salt bath has the advantages of fast heating speed, uniform heating, and can prevent oxidation and decarburization, it can be used for annealing. Induction heating is directly heated by the induced current inside the parts. It has the characteristics of high heating efficiency and fast heating speed. It can also be used for annealing. Therefore, two methods of salt bath furnace and high frequency induction heating are decided to carry out local annealing test.
(1) Local annealing in salt bath furnace
During local annealing in the salt bath furnace, the heating temperature is high. Due to the small spring, it is difficult for workers to operate. It is inevitable to invade the whole spring working ring into the salt bath, resulting in the annealing of the whole spring, resulting in the loss of elasticity of the spring, unable to play a limiting role, affecting normal work, violating the use regulations of parts, resulting in scrapping of parts and increasing production costs, Therefore, the salt bath furnace heating annealing method cannot be selected.
(2) Induction heating local annealing
Due to the precedent of annealing the thread part of 30CrMnSiNi2A bolt before, the high-frequency machine is used for local induction annealing of the spring. By making appropriate induction heating ring, the heating ring is used to heat the spring locally. With induction heating annealing, there is no decarburization and serious oxidation on the part surface, and the deformation of the part is small, the production efficiency is high, it is easy to realize mechanization and automation, and the processing and manufacturing capacity is effectively widened.

Specific operation method of local annealing

High frequency machine is used for local induction heating annealing at the root of spring parts, and 15mm induction coil is selected × It is made of 15mm hollow copper pipe, the gap of induction coil is about 15mm, and the frequency of high-frequency machine is 2500Hz. The magnetic field in the loop inductor is used for heating and annealing, and the heating temperature is 680 ~ 700 ℃. In actual production, the temperature of induction heating mainly depends on parameters such as voltage, current and heating time. Factories generally use photoelectric pyrometer to measure the heating temperature, and the effect is better. Due to the high speed of induction heating, intermittent heating mode shall be adopted, and heating and stop heating shall be carried out alternately.

In order to reduce the temperature difference between the part surface and the center during penetration heating, the heating speed shall be slow, and the heating power shall be controlled as low as possible to obtain a large current penetration depth. The heating time is determined according to the site conditions. The heating time is changed by changing the relative moving speed of the part and the sensor, subject to the heating penetration of the core of the part. The heating part is shown in the attached figure. After heating and annealing, insert the heating part of the part into the sand to cool it slowly.

20211028105922 17838 - Local induction annealing process of spring material

Schematic diagram of heating annealing area

Test results

After repeated tests and continuous exploration, through the tensile strength test, the strength value is significantly reduced, the local hardness of the spring is significantly reduced, and the fitter is easy to work on the hook. The overall strength of the spring outside the transition zone is not affected, and the heat affected zone is very small, which realizes the characteristics of local hardness reduction. The spring elasticity is within the elastic range, and the local annealing part of the hook can play a limiting role when assembled, Meet the drawing requirements and provide guarantee for subsequent installation. Due to the peptide effect of induction heating, the surface temperature increases rapidly, while the heart temperature changes slowly, and the heart heats up by heat conduction. Therefore, annealing heating needs low power, slow temperature rise and intermittent heating.

Conclusion

The strength of the heated part of IIA spring material decreases obviously after local induction heating annealing, which shows that the local induction heating annealing process of IIA spring material is feasible, and the hardness after annealing is conducive to the efficient and smooth fitter process. At the same time, the annealing process parameters of this kind of spring are determined through experimental exploration, the local annealing process problem of this spring is solved, the technical problems are solved for the branch factory, the parts are processed according to the time node, and the processing capacity of the branch factory is increased. It is the first time to use high-frequency machine for local annealing of spring. It has the advantages of simple operation, short time and good effect. It can be popularized and used continuously.

Author: Jiang Lin

Source: Network Arrangement – China Metal Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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