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Study on local induction heating annealing technology of spring material

Spring is a wide range of basic parts, for power machinery, instruments and weapons in the control elements are very critical parts, its basic function is to use the elasticity of the material and the spring structure characteristics, in order to achieve buffering or vibration reduction, control movement or reset and other purposes.

For the material of carbon spring steel wire, brand for type IIa, partial annealing pattern request, there is no corresponding standard stipulated in the type IIa material partial annealing process parameters, achieve partial annealing condition, through test to get a lower hardness and nearly an equilibrium state of the organization, thus improving metal or alloy of physics, chemistry, mechanical properties, stable size, and shape; The purpose of this spring partial annealing is to make it easy to bend the hook and ensure that the assembly has a limiting effect on the spring pin.

1.Determination of process parameters of local annealing

Type IIa spring material belongs to the deformation strengthening spring steel, with cold deformation (cold drawn or cold rolled) state supply, to eliminate stress tempering after winding spring, spring to finalize the design, the steel wire before the forming of high strength and has enough plasticity and toughness, standard only in the tempering process parameters of 260 ~ 300 ℃, without any other heat treatment process parameters, read the manual of aviation materials, aviation spring wire information and consulting other related units, such as type IIa spring material without annealing parameters, advice may consider using similar material experiment.

As IIa spring material is made of high-quality carbon steel, upgrading material of carbon steel 65, 70, 65Mn, and other spring steel wire, its comprehensive performance is similar to that of carbon steel 70, so it decided to conduct the test according to the annealing parameters of 70 steel. After referring to the relevant standards and referring to the annealing parameters of 70 steel, the annealing test at 680 ~ 700℃ is finally determined.

2.Selection of local annealing process

Spring steel wire is fine and small, combined with our company’s existing production capacity, because the salt bath has fast heating speed, heating uniformity, and can prevent oxidation, decarburization, can be used for annealing. Induction heating is directly heated by the induction current inside the parts, which has the characteristics of high heating efficiency and fast heating speed and can also be used for annealing. So it is decided to adopt two methods of salt bath furnace and induction heating of high frequency machine to carry out local annealing tests.

(1) Salt bath furnace local annealing

Local annealing in the salt bath furnace, the heating temperature is higher, because of the spring, tiny workers is difficult to operate, inevitably will turn the whole spring work world invasion to the salt bath, cause the entire spring annealing, the spring loses elasticity, cannot limit effect, affect the normal work, in violation of the provisions of part use, cause the parts to scrap, increase the cost of production, so can’t choose salt bath furnace annealing method.

(2) Induction heating local annealing

As there is a precedent for annealing the threaded part of 30CrMnSiNi2A bolts, the spring was subjected to local induction annealing with a high frequency machine. By making an appropriate induction heating coil, the spring was partially heated with the heating coil. Induction heating annealing is adopted, the surface of parts is not decarburized, there will be no serious oxidation phenomenon, and the deformation of parts is small, the production efficiency is high, easy to realize mechanization and automation, effectively expand the processing and manufacturing capacity.

3.Specific operation method of local annealing

Local induction heating annealing at the root of spring parts is adopted by the HF machine. The induction coil is made of a 15mm×15mm hollow copper tube. The gap of the induction coil is about 15mm, and the frequency of the HF machine is 2500Hz. The magnetic field inside the coil inductor was used for heating annealing, and the heating temperature was 680 ~ 700℃.In actual production, the temperature of induction heating is mainly controlled by such parameters as voltage, current, and heating time. Generally, a photoelectric pyrometer is used in factories to measure the heating temperature, and the effect is relatively good.Because the induction heating speed is fast, should use the intermittent heating mode, heating, and stop heating alternately.

In order to reduce the temperature difference between the surface and the center of the parts when heating through, the heating speed should be slow, and the heating power should be controlled at the lowest as far as possible, so as to get a larger current penetration depth. The heating time is determined according to the site situation. The heating time is changed by changing the relative moving speed of the part and the inductor. The heating time is based on heating through the part’s heart. The heating part is shown in the attached drawing. After heating annealing, the heating part is inserted into the sand to cool it slowly.

Schematic diagram of heating and annealing area

Schematic diagram of heating and annealing area

4.The test results

Through trial and error, trial and error, by doing tensile strength test, strength decrease, spring and local hardness decrease, fitter to hook is easy to work, and the transition zone spring overall strength is not affected by any heat affected zone is small, implements the local characteristics of lower hardness, the elasticity of spring in the elastic range, partial annealing of the hook, assembled can limit effect, conform to the requirements of the drawings, provide a guarantee for the follow-up of installed. 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 heating, and intermittent heating.


The local induction heating annealing of IIa spring material showed that the strength of the heating part was significantly reduced, indicating that the local induction heating annealing process of IIa spring material was feasible. The hardness after annealing is conducive to the efficient and smooth operation of fitters. The annealing process parameters of this kind of spring were determined by experiment, and the local annealing process was solved, which solved the technical problem for the branch factory, made the parts complete processing according to the time node, and increased the processing capacity of the branch factory. It is the first time to use high frequency machine to carry out local annealing of spring, which is simple to operate, short in time, good in effect, and can be popularized.

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