Quality inspection of spring surface treatment

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The spring surface is treated for quality inspection, mainly including appearance inspection, corrosion resistance inspection, protective layer bonding force inspection and hydrogen embrittlement inspection, and the thickness of the protective layer should be inspected if necessary.

The above items will be checked for each type of spring according to its use conditions and different surface treatment methods. Generally, the inspection items and methods are specified in the part product drawing or corresponding technical standards.

(1) Appearance inspection

Appearance inspection is the most basic and most commonly used inspection item. Springs with unqualified surface treatment and appearance do not need to undergo the following inspection items.

After surface treatment, the spring should be bright and beautiful, with normal color, without defects such as deposits, spots, bubbles, shedding, deep scratches, and yin and yang surfaces. However, minor defects that do not damage the appearance and do not reduce the corrosion resistance (such as saponification or passivation liquid marks, etc.) are allowed.

Before oiling (or varnishing) oxidation and phosphating treatment, the appearance of the oxide film and phosphating film should be inspected. After oiling (or varnishing), it is also necessary to inspect the appearance of the finished product.

Based on appearance and surface quality, springs can be classified as qualified, repaired, and scrap. Repaired springs require the removal of the unqualified protective layer and reprocessing. However, springs that have been repeatedly repaired and failed or are excessively corroded must be scrapped.

2)Corrosion Resistance Test

The corrosion resistance test of springs is the most important test item for the quality of spring surface treatment. Currently, the most commonly used test item for spring products is the neutral salt spray test. The salt spray test can be applied to springs with various surface treatment methods. The specific method is: conduct it according to the requirements of GB/T 10125-2012. After the salt spray test, the coating is divided into four grades according to the time it takes for red rust to appear on the coating, ranging from 120 to 1000 hours.

However, salt spray testing requires a relatively long test cycle, so there are relatively quick test methods for evaluating the corrosion resistance of spring surfaces, primarily the immersion method and the drip method.

The drip method typically uses a 2% copper sulfate solution. During testing, clean the spring surface with alcohol. Several drops of copper sulfate solution are added while a stopwatch is run. A spring is considered acceptable if no copper spots appear on the oxide film (blackening) after 20 seconds, and if no copper spots appear on the phosphate film after more than 3 minutes.

The soaking solution is generally a 2% copper sulfate solution or a 3% salt solution.For oxide film, soak it in a 2% copper sulfate solution at room temperature for 20 seconds, then remove it and rinse the surface with water. If there is no red contact point, it is qualified. If it is soaked in a 3% salt solution for 15 minutes, remove it and it is qualified if there is no rust.

For phosphate coatings, soak the spring in a 2% copper sulfate solution. After holding at room temperature for 3 minutes, remove the spring and rinse the surface with water. If there are no red contact spots, the spring passes. If soaked in a 3% salt solution for 2 hours, remove the spring and if there are no rust marks, the spring passes. Alternatively, soak the spring in a 3% salt solution for 15 minutes, then remove and rinse with water. Allow the spring to air dry for 30 minutes. If no yellow rust appears, the spring passes.

In addition to the conventional inspection methods mentioned above, since automobiles and mechanical products that use a large number of springs have relatively high quality requirements for springs, some large multinational companies have special test items for some important springs, such as damp heat tests (tests under specified temperature and humidity conditions), cyclic corrosion tests (tests under periodic cyclic conditions of temperature, humidity and salt spray), and gravel impact tests (for automobile suspension springs with higher requirements). Spring manufacturers need to equip themselves with corresponding testing equipment and technical capabilities according to user needs.

3)Adhesion test

The adhesion of the protective layer (also known as bonding strength) refers to the degree of firmness of the bonding between the coating or coating and the substrate or intermediate layer, also known as the adhesion test.

There are also various test methods for testing adhesion. For non-electrolytic zinc-rich metal coatings, there are the tape test (according to GB/T 5270-2005) and the cross-cut method (see Section 4.3 of this chapter). There are also the following test methods:

1) Coating Adhesion Test: After treating the spring at 220°C for 30 minutes, immediately place it in room temperature water. If the coating shows no blistering or cracking, the test is acceptable.

2) For springs with smaller diameters, use the same steel wire as the spring and sample the surface along the groove. For diameters d < 1mm, wrap the steel wire sample around a shaft that is three times the diameter (= 3d). For diameters d ≥ 1mm, wrap the copper wire sample around a shaft of the same diameter, completing 5-10 turns. No peeling or shedding should occur.

3) For springs with larger diameters, use a file to file along the protective coating at a 45° angle, or use a steel needle (or blade) to cross-cut the coating and observe for peeling or shedding at the intersections or filing points. 4) For galvanized or chrome-plated springs, to determine the bonding strength of the coating, sample the spring and heat it in a resistance furnace to 180-200°C for 1 hour to see if the coating bulges or falls off.

(4) Hydrogen embrittlement test

Spring materials are very strong and deform severely during forming, so they are particularly sensitive to hydrogen embrittlement (especially small coil springs or leaf springs with a small curvature radius). In actual production, large numbers of springs are often scrapped due to hydrogen embrittlement.

The method for checking springs for hydrogen embrittlement is relatively simple. For coil springs, apply an appropriate strong compressive (or tensile, or torsional) load. For leaf springs, apply force and deformation, depending on the specific situation. After a certain period of time (e.g., 24 hours), observe whether there are cracks or breaks. If the spring is broken, clamp the broken spring in a vise, use the pliers to grasp the protruding part, and bend it forcefully. Hydrogen-embrittled springs often break in a very brittle manner within a bend angle of 45° to 90°.

(5) Plating (coating) thickness inspection

To ensure the corrosion resistance of the protective layer, the plating (coating) layer should be of sufficient thickness. The thickness of the plating layer can be checked with a micrometer, plug gauge, thread ring gauge, etc. It can also be measured by chemical methods or metallographic methods.

The chemical method for checking coating thickness is often the drop method. To measure, a solution is dripped onto the measurement area using a pipette. After the solution remains for 1 minute, it is wiped off with cotton wool. A second drop is then added until the base metal is exposed. Finally, the coating thickness is calculated based on the total number of drops.

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