Materialien & Verarbeitung

Passivation is a chemical treatment (usually in nitric or citric acid) that removes 'free iron' from the surface of the stainless steel ring and enhances the protective chromium-oxide layer. For medical applications, this is critical to prevent 'rust' spots and ensure biocompatibility. The process follows ASTM A967 standards. In a spiral ring, which has multiple layers in close contact, passivation must be performed carefully to ensure the acid reaches the surfaces between the turns. If free iron remains trapped between turns, it can lead to localized galvanic corrosion. Post-passivation, the rings are rinsed in deionized water to ensure no chemical residue remains, which could cause adverse reactions in a clinical setting.

AISI 316 stainless steel is preferred for marine environments due to its molybdenum content ($2-3\%$), which enhances resistance to pitting and crevice corrosion in chloride-rich sea water. Unlike AISI 302, 316 is significantly more stable against corrosion but has a lower tensile strength ($10-15\%$ less). This means the thrust capacity of a 316 ring will be lower than a 302 equivalent. For subsea sensors, the non-magnetic property of 316 (permeability $μ_r < 1.02$) is often essential. Engineers must account for the lower yield strength in their $P_r$ and $P_g$ calculations and may compensate by using a slightly thicker material or a deeper groove to maintain the required safety factor.

Beryllium Copper (typically Alloy 25) is used for retaining rings requiring high electrical conductivity and non-magnetic behavior. It has a high fatigue strength and can be hardened after forming to levels approaching carbon steel ($S_u \approx 1300$ MPa). In electronics, it serves a dual purpose: mechanically retaining components and providing an EMI/RFI shielding path. Its low modulus ($E \approx 128$ GPa) allows for easier installation over shafts without permanent deformation compared to stainless steel.

'Blueing' is a low-temperature heat treatment ($260-370^\circ C$) that produces a thin, protective magnetite ($Fe_3O_4$) layer on carbon steel. Beyond the aesthetic appeal, this process acts as a mild stress relief and provides a base for oil-dip corrosion inhibitors. For spiral rings, which are wound from flat wire, blueing helps stabilize the diameter and prevents the 'springback' that can occur if the material was only cold-worked. It is a cost-effective alternative to more expensive coatings for indoor industrial machinery.

While 302 stainless steel has higher tensile strength due to its higher carbon content and work-hardening rate, 316 stainless steel contains 2-3% Molybdenum. This addition significantly improves resistance to 'pitting' and 'crevice corrosion' in chloride-rich marine environments. In spiral retaining rings, where the overlapping turns create natural crevices, 302 would likely fail due to localized corrosion. 316 is the standard for subsea sensors and offshore oil platform equipment, even though the allowable thrust load is approximately $15\%$ lower than 302.

Carbon steels like SAE 1070 undergo a ductile-to-brittle transition (DBTT) at low temperatures. In cryogenic applications (below $-40^\circ C$), the fracture toughness $K_{Ic}$ drops precipitously, making the ring prone to shattering under impact or even during installation. For these applications, austenitic stainless steels like 302 or 316, or nickel alloys, must be used as they retain their face-centered cubic (FCC) structure and toughness down to absolute zero.

Elgiloy is specified for spiral retaining rings in extreme environments like downhole drilling or medical heart valves. It offers a unique combination of extremely high fatigue strength and resistance to virtually all corrosive media. Its modulus $E = 200$ GPa is similar to steel, but its endurance limit is significantly higher. The material is typically provided in a cold-worked and aged condition. For biomedical use, its biocompatibility and non-magnetic properties make it ideal for MRI-compatible surgical instruments.

After the coiling process, multi-turn wave springs contain significant residual stresses from the cold-forming of the waves and the helix. Stress relieving, typically performed at temperatures between $315^\circ C$ and $480^\circ C$ for stainless steels, allows for the redistribution of these internal stresses. This stabilization prevents 'growth' or 'shrinkage' of the spring dimensions during service and improves fatigue life by reducing the peak internal tension. For precision medical implants, vacuum stress relieving is used to ensure no oxidation layer forms on the surface.

A286 (UNS S66286) is an iron-base superalloy used when a combination of high strength and corrosion resistance is needed at temperatures up to $538^\circ C$ ($1000^\circ F$). While not as strong as Inconel X-750 at the extreme end, it is more cost-effective and provides better oxidation resistance than 17-7PH. It is frequently used in jet engine exhaust assemblies and turbocharger seals where the thermal cycling would cause standard stainless steels to fatigue rapidly due to thermal creep.

High-carbon steels like SAE 1070 are extremely susceptible to hydrogen embrittlement during acid pickling or electroplating processes. Atomic hydrogen migrates into the grain boundaries, causing brittle fracture at stresses well below the yield point. For wave springs, which have high surface-area-to-volume ratios and are under constant tension at work height, this is fatal. To mitigate this, springs must be baked within 1-4 hours after plating at $190-220^\circ C$ for at least 8 to 24 hours to drive out the hydrogen. Failure to do so in automotive braking systems can lead to sudden, catastrophic component failure.

Inconel X-750 (UNS N07750) is a nickel-chromium alloy made precipitation-hardenable by additions of Aluminum and Titanium. In subsea environments, it is chosen for its immunity to chloride-ion stress corrosion cracking and its ability to maintain mechanical properties in sour gas ($H_2S$) environments. The material must be heat treated per NACE MR0175 standards to ensure a maximum hardness of 35 HRC, preventing hydrogen embrittlement. Its modulus of elasticity $E \approx 213$ GPa must be used in all spring rate calculations, which is slightly higher than carbon steel.

17-7PH is a precipitation-hardening stainless steel that provides significantly higher tensile strength and better relaxation resistance than 302 or 316. In the CH900 condition (cold reduced and then aged at $900^\circ F$), it can withstand operating temperatures up to $343^\circ C$ ($650^\circ F$) with minimal load loss. Standard 302 stainless relies purely on work hardening, which begins to recover (anneal) at much lower temperatures, leading to a rapid loss of spring force (stress relaxation). For aerospace fuel injectors, 17-7PH is the baseline for maintaining constant pressure across the flight envelope.

Spiral retaining rings are produced by coiling pre-tempered flat wire on its edge. This results in a circumferential grain flow that follows the curvature of the ring. Stamped snap rings are punched from flat sheet, meaning the grain flow is linear across the ring. The circumferential grain flow in spiral rings is superior for resisting radial cracks and provides higher fatigue strength and 'toughness' under impact. In a stamped ring, the grain ends are exposed on the edges, which can act as initiation points for cracks. Coiled rings also eliminate the 'burr' associated with stamping, reducing the risk of stress concentrations without requiring secondary deburring.

Black Oxide (MIL-DTL-13924) is a conversion coating that provides a uniform black appearance and some corrosion resistance when oiled. Its primary advantage is that it adds virtually no thickness (0.1-0.2 μm) and does not cause hydrogen embrittlement. Zinc Phosphate (MIL-DTL-16232) is a heavier coating that provides better corrosion protection and acts as a lubricant base. However, the phosphate process involves acid pickling which introduces hydrogen, necessitating a baking cycle. For high-precision spiral rings with tight groove clearances, Black Oxide is often preferred to maintain dimensional integrity, whereas Zinc Phosphate is chosen for outdoor or heavy-duty industrial applications.

Beryllium Copper (CuBe) Alloy 25 (UNS C17200) is the highest-strength copper-based alloy. It is used for spiral rings in explosive environments (non-sparking) and in sensitive electronics or MRI machines (non-magnetic, μ < 1.001). CuBe rings can be age-hardened to achieve tensile strengths up to 1400 MPa, comparable to many steels. However, its Modulus of Elasticity $E$ is lower ($≈$ 125 GPa compared to 200 GPa for steel), meaning CuBe rings provide less radial tension for the same thickness. Design engineers must also account for the toxicity of beryllium dust during any subsequent machining, though the finished ring is safe for use.

Coiling a spiral ring involves significant cold working, which introduces high residual tensile stresses on the outer diameter and compressive stresses on the inner diameter. Without stress relieving (typically at 350-400°C for carbon steel), these residual stresses can lead to 'springback' or dimensional instability over time. More importantly, residual tensile stresses on the surface significantly lower the fatigue threshold and increase the risk of stress-corrosion cracking. Stress relieving 'stabilizes' the ring, ensuring that the installation tension and diameter remain within tolerance during the service life of the component.

316 Stainless Steel is an austenitic alloy with high molybdenum content, providing superior resistance to pitting and SCC in chloride-rich marine environments. However, it cannot be heat treated to high hardness, resulting in lower thrust capacities. 17-7PH is a precipitation-hardening stainless steel with much higher tensile strength but is more susceptible to SCC if not properly over-aged. For critical marine applications where load is high, 17-7PH in the CH900 condition is often used but must be carefully monitored. For the absolute best SCC resistance without compromising too much strength, nitrogen-strengthened alloys like Nitronic 50 or specialized Inconel grades are preferred.

Passivation (per ASTM A967) is a chemical treatment using nitric or citric acid to remove 'free iron' from the surface of the wave spring. For 302 or 316 stainless steel, this process enhances the protective chromium-oxide layer. While passivation does not inherently change the bulk mechanical properties, it prevents the formation of pit corrosion sites. In cyclic applications, pits act as stress concentrators ($K_t$) that significantly accelerate fatigue crack initiation. By eliminating these sites, passivation ensures that the fatigue life calculated using $S_{alt} = \frac{S_{max} - S_{min}}{2}$ remains valid in corrosive environments, preventing premature failure due to corrosion-fatigue interaction.

Elgiloy (complying with ASTM F1058) is chosen for medical implants due to its extreme biocompatibility, fatigue resistance, and corrosion resistance. Metallurgically, it is a cobalt-based alloy that achieves its properties through a combination of cold work and age hardening. It is non-magnetic, which is vital for MRI compatibility. The fatigue strength of Elgiloy is roughly 30% higher than 316L stainless steel, allowing for smaller, thinner wave springs in applications like cardiac valves or orthopedic implants. The aging process (typically 5 hours at 480°C) optimizes the precipitate distribution, providing a yield strength exceeding 1900 MPa.

A286 is an iron-base superalloy that maintains high strength and, crucially, high toughness at cryogenic temperatures down to -196°C (77K). Unlike standard carbon steels or some martensitic stainless steels which undergo a ductile-to-brittle transition (DBT), A286 remains austenitic. Its coefficient of thermal expansion is also relatively stable. In cryogenic spring design, the increase in Modulus of Elasticity $E$ at low temperatures must be calculated; $E$ can increase by 5-10%, meaning the spring rate $k$ will be higher than at room temperature. A286's precipitation-hardened state (aged at 1300-1400F) ensures it can withstand the high stresses required in compact cryogenic seals.