Zinc Impregnation of Cold Formed Cylindrical Helical Springs: Is It Possible?

Spring is widely used in machinery, instruments, electrical appliances, transportation tools and daily life appliances, etc.

It is a basic part with relatively large influence, among which cylindrical spiral spring is the most widely used.

At present, the surface anti-corrosion treatment of cold formed cylindrical spiral spring is still based on traditional oxidation and zinc plating processes.

Not only is the anti-corrosion capacity limited, it is difficult to adapt to the anti-corrosion requirements of harsh environments such as marine environment, but also because the spring parts are made of high-strength steel wire, there is a serious potential hydrogen embrittlement.

In contrast, the zinc plating technology has the characteristics of excellent corrosion resistance and no hydrogen embrittlement.

Therefore, the feasibility of applying zinc plating technology to the anti-corrosion treatment of cylindrical spiral spring has important practical significance.

Zinc penetration is a solid multi-component thermal diffusion coating processing technology.

It is a chemical heat treatment process that uses the diffusion of zinc (Zn) and alloy elements into the surface of steel components under heating to prepare a Zn Fe alloy protective layer.

The zinc coating has high bonding strength with the base metal, and has excellent high temperature oxidation resistance, corrosion resistance, wear resistance and impact resistance.

Through the research on the size, load, deformation, fatigue strength and steel wire strength change of cold-formed cylindrical spiral spring after galvanizing, this article determines the applicability and scope of the application of the hot galvanizing process for cold formed cylindrical spiral spring, which provides a reference for exerting the anti-corrosion effect of the galvanizing process on spring parts.

1. Spring wire applicability

(1) Steel wire classification

Cold formed cylindrical spiral spring is generally made of cold rolled cold formed spring steel wire, and its general classification is shown in Fig. 1.

Among the spring steel wires mentioned above, except that the stainless steel wire for spring itself has anti-corrosion capacity and no need for anti-corrosion treatment.

The typical common steel wires are: silicon manganese spring steel wire (GB5218-1985), carbon spring steel wire (GB/T4357-2009), and carbon spring steel wire for important purposes (GB/T4358-1995).

(2) Strength change of steel wire before and after zinc penetration

See Table 1 for the selected steel wire and test items.

Table 1 Test conditions for strength change of spring steel wire after hot galvanizing

See Table 2 for the strength change of 70C and 65Mn steel wires before and after zinc penetration.

Table 2 70C, 65Mn Steel Wire（ φ 3mm) before and after zinc penetration

Note: The zinc penetration temperature is 400 ℃.

See Table 3 for hardness change of 60Si2MnA steel wire before and after galvanizing.

Table 3 Hardness Comparison of 60Si2MnA Steel Wire before and after Zinc Permeating (HRC)

Note: The zinc penetration temperature is 400 ℃.

(3) Test analysis

Combined with the test data in Table 2 to Table 3, the analysis is as follows:

First, the tensile strength of 70C-GB/T4357-2009 and 65Mn GB/T4358-1995 steel wire after zinc penetration is lower than that before hot zinc penetration.

This kind of steel wire is cold drawn and strengthened after lead bath isothermal quenching.

Its strengthening mechanism is deformation strengthening, and the isothermal temperature is lower than the hot galvanizing temperature (generally about 400 ℃), so the strength value will inevitably decrease after galvanizing, which indicates that the carbon spring steel wire in the cold drawn strengthened steel wire, the carbon spring steel wire for important purposes, and the spring made of piano steel wire should not use the hot galvanizing process.

Secondly, 60Si2MnA-GB5218-1985 steel wire has been hardened before hot galvanizing, and its hardness has not decreased after hot galvanizing.

60Si2MnA steel wire is a typical representative of cold drawn steel wire.

The strengthening mechanism of the spring made of cold drawn steel wire is to conduct quenching strengthening after the spring is coiled, and its tempering temperature is generally above 400 ℃.

During the hot galvanizing process, the hot galvanizing temperature is not higher than the tempering temperature, which can not reduce the original strength of the steel wire.

Thirdly, the oil quenched and tempered steel wire in the cold drawn steel wire is supplied in the quenched and strengthened state.

Whether the steel wire spring is suitable for the hot galvanizing process shall be determined according to the tempering temperature in the steel wire manufacturing process.

2. Applicability of spring parts

(1) Test piece

According to the applicability results of spring steel wire, 60Si2MnA GB/T5218-1985 steel wire to be hardened is selected as the material of the test piece for the applicability study of spring parts, and different steel wire diameters and sizes are selected as the size specifications.

There are three types of springs, as shown in Table 4.

Table 4 Parameters of Spring Test Pieces

(2) Appearance

The surface color of the spring after zinc coating is gray to silver gray (see Fig. 3 to Fig. 5), which is consistent with the requirement that the zinc coating is gray or silver gray.

In order to ensure the full contact between zinc powder and workpiece, the powder rolling zinc impregnation method is generally adopted, that is, the workpiece and zinc impregnating agent are put into a rotating box (commonly known as the drum) in a certain proportion, and the diffusion zinc impregnation under “dynamic” conditions is realized by heating the drum.

It can be seen that the spring is in a state of constant rotation during zinc plating, so it is necessary to study whether the zinc plating process will lead to spring deformation.

The above test pieces were subject to surface treatment such as galvanization, zinc penetration with core rod (see Fig. 2), zinc penetration without core rod and no treatment, and their appearance comparison is shown in Fig. 3 to 5.

Fig. 2 Cored rod before zinc penetration

Fig. 3 Spring II

(Note: Left 1~3 are not subject to surface treatment; left 4~6 are galvanized parts; left 7~10 are zinc plated parts without core rod, resulting in large deformation; left 10~13 are zinc plated parts with core rod)

Fig. 4 Compression spring

(Note: left 1~2 are not subject to surface treatment; left 3~5 are zinc plated parts; left 6~9 are zinc plated parts without core rod; left 10~12 are zinc plated parts with core rod)

Fig. 5 Locking spring

(Note: left 1-3 are oxidation treated parts; left 4-6 are zinc plated parts; left 7-9 are zinc plated parts without core rod; left 10-13 are zinc plated parts with core rod)

Through size and perpendicularity testing, the spring II with wire diameter of 1mm has a large deformation due to zinc penetration without taking deformation prevention measures, while the zinc penetration of cored rod has no obvious deformation;

The locking spring with a steel wire diameter of 3mm and the compression spring with a steel wire diameter of 6mm without a core rod have no obvious deformation compared with the zinc penetration with a core rod.

The spring is an elastic part.

Although certain deformation will inevitably occur when the spring is in constant motion during the galvanizing process, as long as the deformation does not exceed the elastic deformation range of the spring (such as the locking spring and compression spring above), or measures are taken to control the deformation not to exceed the elastic deformation range of the spring (such as the spring II with cored rod above), when the external force causing the deformation disappears, the spring can quickly return to its normal shape.

Therefore, the deformation-free zinc infiltration treatment of the spring can be realized.

(3) Load

Load test shall be carried out for the springs that are not deformed after zinc penetration.

The load testing equipment for spring II and locking spring is TL-01B spring pressure testing machine, and the load testing equipment for compression spring is CMT5105 universal testing machine due to its large load.

See Table 5 and Table 6 for the load testing results.

Table 5 Load Test Results of Spring II and Locking Spring after Zinc Permeating

Table 6 Load Test Results of Compression Spring after Zinc Permeating

It can be seen that the three spring loads after zinc penetration meet the drawing requirements.

Due to the general rule of zinc impregnation process, the steel wire diameter will increase correspondingly after the spring zinc impregnation, which is equivalent to twice the thickness of the layer, leading to the increase of the spring load.

However, due to the shallow thickness of the infiltration layer, it is generally 30 μ m.

The increase of load is slight and will not cause obvious change of spring load.

(4) Fatigue strength

In actual work, springs are rarely subject to pure static stress.

When the stress changes slowly, or the amplitude of change is small, and the number of times is small, it can be regarded as static stress.

For springs with many times of stress change and large amplitude of change, fatigue strength should be considered.

The above zinc plated and oxidized springs are selected for fatigue test.

The test equipment and parameters are shown in Table 7.

Table 7 Locking Spring Fatigue Test Equipment and Parameters

Of which, 1.5 × 10⁷ times reached the highest level valve spring cycle action times in the reference table of cycle times in Appendix A of GB/T16947-2009 Coil Spring Fatigue Test Specification.

After the test, the zinc plated spring and the oxidized spring did not appear fatigue cracks and breaks.

The reduction of height and load is shown in Table 8.

The reduction is not only slight, but also has no significant difference, indicating that the zinc plating process will not lead to a significant reduction in the fatigue strength of the spring.

Table 8 Spring Height and Load Drop Value after Fatigue Test

3. Conclusion

(1) There is a hidden danger of reducing the strength of steel wire after zinc impregnation of cold drawn lead bath quenched steel wire springs, which is not suitable for hot zinc impregnation process.

(2) As long as the correct process is selected, zinc penetration will not reduce the performance of cold drawn steel wire springs except for the increase of steel wire diameter.