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When it comes to demanding applications such as in automotive suspension systems, medical equipment, marine environments or industrial machines, springs undergo millions of repeated cycles of loading and unloading. The failure of a spring due to fatigue can lead to costly warranty claims, equipment downtime and potential safety issues. The fatigue resistance of stainless steel coil springs is far superior to those of regular carbon steel, particularly in high humidity and corrosive environments. There are four main reasons why stainless steel coil springs perform exceedingly well under cyclic loading and throughout their long service lives.
1. Material Composition and Corrosion Resistance
Failure in fatigue tends to start at surface defects. Rust pitting, or corrosion, on carbon steel springs hastens crack initiation and once a corrosion pit has formed, it becomes a stress raiser, which dramatically decreases the fatigue life of the spring. In stainless steel coil springs (made with materials 302, 304, 316, etc.), the inherent corrosion resistance requires no additional coating or plating.
The chrome content in the stainless steel forms a passive oxide layer on the surface of the spring. This layer remains intact even in saltwater, chemical, and humid environments. This prevents pitting which would ultimately become initiation sites for fatigue cracks. Thus for application in medical environments (sterilization), marine settings or for exterior mechanism, stainless steel coil springs hold their surfaces over many years, and fatigue life increases proportionally.
2. Optimized Heat Treatment and Stress Relief
The fatigue life of a spring does not solely depend on the material of which it is constructed, but equally as much on heat treatment and residual stress control. Stainless steel coil springs must be austenized, quenched, and tempered appropriately to the correct tensile strength while maintaining flexibility. Incorrect heat treatment would introduce residual tensile stress on the surface of the spring where a fatigue crack is likely to start.
At Hongsheng Spring, stainless steel coil springs are properly heat treated in atmosphere controlled ovens using very specific heating profiles. Following the coiling of the springs, they are annealed to relieve stresses induced by the forming process and, where appropriate, in high cycle applications, they are shot peened. Shot peening works by bombarding the surface of the spring with small, spherical shot in order to induce beneficial residual compressive stresses. This combined heat treatment and surface enhancement can at least double or triple the fatigue life of a stainless steel coil spring.
3. Surface Finish and Defect Control
Fatigue cracks nearly always originate at the surface. A scratch or flaw in tooling, decarb, or inclusions will all become initiation sites. A manufacturer of quality stainless steel coil springs will minimize all such defects on the surface of the spring.
Hongsheng Spring manufactures all stainless steel coil springs using precision CNC coilers and polished dies to minimize surface marks. We electropolish critical springs to remove a layer of surface material from all imperfections in order to minimize initiation sites and enhance passive oxide layer. Electropolished stainless steel coil springs, due to minimal initiation sites will be well above that limit. Each lot is inspected using automated visual systems in order to guarantee a flawless surface before the springs are shipped. This attention to detail assures the fatigue life will be limited only by material properties.
4. Application Specific Design for Fatigue Life
The fatigue resistance of a stainless steel coil spring is as much dependent on the physical design of the spring. A sharp stress raiser at any point on the spring, for example, is less likely to cause a fatigue failure in a well-designed spring than one without the stress raiser even if made of the same material and heat treated to the same profile. Important considerations in spring design are wire diameter, coil count, pitch, and end type and size, as well as residual stresses during application.
Hongsheng Spring can assist in the design process for any of our stainless steel coil springs to maximize the fatigue life for each specific application. We can take into account all stress levels and predict cycles using Goodman or Gerber diagrams, as well as recommends shot peening or preset where it may benefit the spring. Design of springs is such that when application necessitates infinite life like that found in automotive springs and industrial parts, operation is well within the fatigue limit of the material. We also offer FEA for complex part geometries before the tool is cut.
Summary
The fatigue performance of stainless steel coil springs is a direct result of the use of corrosive resistant materials, optimized heat treatment and shot peening for beneficial residual compressive stresses, strict quality control of surface finish through electropolishing options, and careful application specific design to minimize residual stress. Hongsheng Spring can produce stainless steel coil springs that excel in automotive, medical, marine, and industrial applications. Call today for your fatigue critical spring requirements.