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Failure of a critical application (automotive suspension, medical, marine or industrial) can be catastrophic. It could result in costly downtime, safety hazards or even significant warranty costs. Conversely precision stainless steel coil springs are designed to endure years of cyclic load application, corrosive environment and a range of ambient temperatures that can vary widely. What is the secret? There are four secrets of the long life of precision stainless steel coil springs.
1. Alloy Selection for Environmental Compatibility
The first fundamental aspect to consider for long-life applications is selecting the proper stainless steel alloy. Grades 302 and 304 stainless steel coil springs will exhibit good general corrosion resistance suitable for food processing equipment, moderately humid environments, and indoors. If an application involves salt, or other corrosives (like on a boat at sea, or in chemical environments), 316 stainless steel (containing 2-3% Molybdenum) must be used. High-stress applications, in both space and in terrestrial fields, are well-suited to 17-7 PH (precipitation hardening stainless steel).
Hongsheng Spring works diligently with customers to provide the best alloys based on temperature, corrosive exposures, humidity, and mechanical stress applied to the springs. When an incorrect alloy, even a top quality one, is selected (for instance, 304 on a salt water environment vs. a sweet water environment where 304 will exhibit pitting), the spring life will be considerably reduced. This tailored application is the first "secret."
2. Precision Coiling and Stress Management
The way the spring is wound is critical for its fatigue life. An old, improperly calibrated machine can produce a spring with inadequate coil pitch, surface defects, or residual tensile stresses, which significantly accelerate the growth of cracks.
Hongsheng Spring's precisely controlled CNC coiling machines ensure that critical dimensions remain within tolerances (down to ±0.05). Subsequent to coiling, the springs undergo stress relieving heat treatment processes to alleviate the tensile stresses induced by the coiling operation. For higher cycle life applications, our springs can be subjected to shot peening which results in compressive stresses on the spring surface-enabling them to last between 2 and 3 times longer than unpeened springs in the same application, as applied load will tend to alleviate compressive stress rather than tension in a fatigue cycle.
3. Surface Finish and Electropolishing
As cracks originate from flaws in the spring surface (like a tool mark or scratch, or an inclusion, or a rough surface), a spring with a highly finished, defect-free surface will outlive its more roughly finished counterparts. Hongsheng Spring's electropolishing processes can create microscopic surface roughness down to an average Ra of 0.04 microns or less, removing small defects and burrs and improving the uniformity and chromic oxide layer responsible for corrosion resistance. The dramatically lower rate of fatigue failure comes from reducing the sites at which a crack can initiate; these features are often removed by electropolishing.
4. Proper Heat Treatment and Passivation
It is important to note that stainless steel coil springs rely on a cold-work hardening effect rather than a quench-and-temper process for strength; however, appropriate heat treatment remains important. The key here is not about inducing strength, but eliminating residual stress through stress relief annealing. If the stress-relief is not carried out correctly, it will exacerbate any remaining tensile stresses induced from coiling, again speeding up fatigue failure. The last one is proper passivation-the removal of any free iron contamination from the surface of the stainless steel coil springs. Free iron acts as galvanic sites and can initiate rust and corrosion. Passivating with nitric or citric acid, to ASTM A967 standards, removes any contamination and reinforces the passive chromium oxide layer inherent in stainless steel, resulting in ultimate corrosion resistance.
Summary
Using the appropriate alloy for the application (302, 304, 316, and 17-7 PH stainless steel coil springs), precision manufacturing techniques (proper coiling and shot peening), surface finishing methods (electropolishing), and proper passivation, precision stainless steel coil springs will out-live equipment itself. Hongsheng Spring can custom engineer your long life stainless steel coil springs to suit the specific application.