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Beryllium Copper (typically Alloy 25, UNS C17200) is used for wave springs in electronics due to its excellent electrical conductivity and non-magnetic properties. Beyond conductivity, it can be heat-treated to reach tensile strengths up to 200 ksi, comparable to some steels. This allows the spring to function as both a high-force mechanical contact and an EMI/RFI shield. In medical or scientific equipment like MRI machines, its non-magnetic nature is essential. The material is typically aged at $600^{\circ}F$ to achieve its maximum spring properties. However, designers must be cautious of its cost and the environmental regulations regarding beryllium handling during the manufacturing phase.

Passivation is a chemical treatment (usually with nitric or citric acid) that removes free iron from the surface of stainless steel rings and enhances the formation of a protective chromium-oxide layer. During the coiling and handling process, microscopic particles of carbon steel from the tooling can become embedded in the surface. If not removed, these particles will rust, leading to localized pitting corrosion even in stainless steel. The process is typically governed by ASTM A967 or AMS 2700. For medical or food-grade applications, passivation is mandatory to ensure biocompatibility and prevent contamination. Without passivation, the 'stainless' property is compromised at the microscopic level, which can lead to premature failure in corrosive environments.

Stress relieving is a thermal process performed after coiling to stabilize the spring's geometry. During cold coiling, significant residual stresses are introduced into the material. Without stress relieving, these stresses can cause the spring to relax or 'creep' over time, leading to a loss of free height and reduced load at the working height. For carbon steel, stress relieving occurs at $450^{\circ}F-500^{\circ}F$. For 17-7PH, the precipitation hardening at $900^{\circ}F$ also acts as a stress relief. This process 'sets' the waves, ensuring that the spring rate $k$ remains constant throughout its service life. Skipping this step in a high-precision application will result in 'load loss' and potential assembly loosening.

SAE 1070 carbon steel is unsuitable for exhaust systems because its mechanical properties degrade rapidly above $250^{\circ}F$, and it lacks oxidation resistance. A286 is an iron-base superalloy (UNS S66286) designed for continuous service up to $1300^{\circ}F$. A286 provides high tensile strength and maintains a stable modulus of elasticity at elevated temperatures. Unlike carbon steel, A286 is precipitation-hardenable (solution treated and aged) to achieve a hardness of 35-42 HRC. In exhaust environments, A286 forms a protective oxide layer that prevents further corrosion. For designers, using A286 means the thrust capacity calculated at room temperature will remain relatively consistent during the thermal cycles of the engine operation.

In medical imaging (like MRI), wave springs are often exposed to cryogenic temperatures. As temperature decreases, the Young's modulus $E$ of 302 Stainless Steel increases. For example, at $-320^{\circ}F$ (liquid nitrogen), $E$ can increase by approximately 5-10% compared to room temperature. Since the spring rate $k$ is directly proportional to $E$ ($k \propto E \cdot b \cdot t^3$), the spring will become significantly stiffer in cryogenic states. Furthermore, 302 SS may undergo a partial martensitic transformation at low temperatures, which can slightly increase its magnetic permeability. For MRI applications, non-magnetic materials or specifically processed 316 Stainless Steel are often used to avoid image distortion caused by magnetic field interference.

Inconel X-750 (UNS N07750) is the industry standard for spiral retaining rings in subsea environments where hydrogen-induced stress cracking (HISC) and chloride stress corrosion cracking are prevalent. This nickel-chromium alloy is precipitation-hardened and maintains its mechanical properties in cryogenic temperatures up to $1300^{\circ}F$. For subsea use, it is typically processed to meet NACE MR0175 standards. The modulus of elasticity for Inconel X-750 is $31 \times 10^6$ psi, which is higher than standard 302 stainless steel, allowing for higher retaining force. The heat treatment involves a solution anneal followed by age hardening to achieve a hardness of 32-42 HRC, providing a balance of high yield strength and enough ductility to withstand the installation stresses of being expanded over a shaft.

17-7PH (Condition CH900) is preferred in aerospace due to its exceptional strength-to-weight ratio and superior corrosion resistance. Unlike SAE 1070 carbon steel, which requires an oil quench and temper (RC 40-52), 17-7PH is work-hardened (Condition C) and then precipitation-hardened at $900^{\circ}F$ ($482^{\circ}C$) for one hour. This process results in a high tensile strength (up to 240 ksi) and excellent fatigue resistance. For applications involving temperatures up to $650^{\circ}F$, 17-7PH maintains its elastic modulus $E$ much better than carbon steel, which begins to lose structural integrity and suffer from creep above $250^{\circ}F$. Furthermore, 17-7PH is resistant to hydrogen embrittlement, a common failure mode for plated carbon steel springs.

Spiral retaining rings are coiled from flat wire, which inherently has a smooth, rolled surface finish on the top and bottom. However, the 'edges' of the wire can have micro-slitting marks or burrs. These surface imperfections act as stress concentrators (notches). Under cyclic axial loads, fatigue cracks initiate at these edge defects. Specifying a 'vibratory deburr' or 'tumble' finish is standard to smooth these edges. For high-cycle applications, a surface finish of 16-32 micro-inches RA is targeted. Additionally, a smooth finish reduces the friction during installation and removal, preventing 'galling' of the shaft or bore surface.

Phosphor Bronze (typically Alloy C51000) is used primarily for its 'non-sparking' properties. In environments with volatile gases or dust (ATEX zones), a steel ring striking a steel housing during installation or failure could generate a spark and trigger an explosion. Phosphor Bronze is also non-magnetic and provides good corrosion resistance. However, its tensile strength is significantly lower than carbon steel (approx. 90-110 ksi). Engineers must compensate for this by designing wider radial walls or using the material only in 'low-load' positioning applications. Its lower $E$ ($16 imes 10^6$ psi) also means it has less 'grip' on the groove, necessitating careful RPM limit calculations.

Beta-C Titanium is used when an extreme strength-to-weight ratio and corrosion resistance are required. It can be heat-treated to a tensile strength of 180-200 ksi while being 40% lighter than steel. Its Modulus of Elasticity ($E \approx 15 imes 10^6$ psi) is roughly half that of steel. This lower $E$ is actually an advantage for retaining rings because it allows for greater elastic deflection during installation without reaching the yield point. Beta-C also offers exceptional resistance to 'Hot Salt Stress Corrosion Cracking,' a critical failure mode in jet engine environments where rings are exposed to sea salt and temperatures up to $600^{\circ}F$.

302 Stainless Steel is metastable and can transform from austenite to martensite when cold-worked (during the coiling process) or when exposed to cryogenic temperatures. This transformation causes a volume expansion (approx. 4%). For precision retaining rings used in aerospace liquid oxygen ($LOX$) systems, this can cause the ring to 'grow' and lose its grip on the groove. 'Cryogenic Treatment' (soaking at $-320^{\circ}F$) is used to force this transformation to completion during manufacturing. After the transformation, the ring is stress-relieved. This ensures that the ring's dimensions remain stable during actual cryogenic service, preventing the ring from expanding and failing to retain the assembly.

For high-carbon steel rings (SAE 1070-1090), 'Austempering' involves quenching the ring from the austenitic temperature into a salt bath held at $550-750^{\circ}F$ to produce a Bainitic structure. This results in superior toughness, higher ductility, and minimal distortion, which is critical for maintaining the tight 'flatness' tolerances of spiral rings. 'Martempering' (or Marquenching) involves quenching to just above the martensite start ($M_s$) temperature, then air cooling to form Martensite, followed by tempering. While Martempering provides higher hardness, Austempering is generally preferred for spiral rings because it significantly reduces the risk of 'quench cracking' and yields a ring that can withstand greater 'winding' stresses during installation without permanent deformation.

In MRI applications, components must be non-magnetic to avoid distorting the magnetic field and to prevent the 'projectile effect' where parts are pulled toward the magnet. Standard carbon steels and 400-series stainless steels are ferromagnetic and unsuitable. 316 Stainless Steel is generally non-magnetic (permeability $\mu < 1.01$), but cold-working can induce magnetism through martensite formation. Therefore, for 'Zero-Magnetic' requirements, materials like Beryllium Copper (CuBe) or MP35N are preferred. CuBe (alloy 25) offers excellent spring properties and remains completely non-magnetic regardless of cold work or heat treatment, making it the industry standard for MRI-compatible wave springs.

At $1000^{\circ}F$, Inconel X-750 is the superior choice for wave springs due to its high nickel content (70% min) and the formation of a stable $Cr_2O_3$ protective oxide layer. It maintains its spring properties and resists creep through $\gamma'$ precipitation. A286 (an iron-base superalloy) is also suitable up to $1000^{\circ}F$ but is more prone to oxidation and loss of modulus at the upper end of that range. The modulus of elasticity $E$ for Inconel X-750 drops from $31 imes 10^6$ psi at room temperature to approximately $25 imes 10^6$ psi at $1000^{\circ}F$, a factor that must be included in the rate calculation ($k \propto E$). A286 is often chosen for its cost-effectiveness when the extreme corrosion resistance of Inconel is not required.

Pre-setting, or 'removing the set,' involves compressing the wave spring to its solid height (or a height lower than its operating height) during manufacturing. This process intentionally exceeds the elastic limit of the material at the highest-stress locations (wave peaks). This induces beneficial residual stresses in the opposite direction of the service load. For a carbon steel spring (SAE 1070), pre-setting increases the apparent yield strength and allows the spring to operate at higher loads without further plastic deformation (relaxation) during service. Without pre-setting, a spring might lose 5-10% of its free height upon its first compression in the field, leading to a loss of critical preload in the assembly.

Elgiloy (conforming to AMS 5833) provides a significantly higher fatigue limit and modulus of elasticity compared to 316 Stainless Steel. While 316 is biocompatible, its yield strength is relatively low (approx. 35-45 ksi annealed), limiting its load-bearing capacity in miniature wave springs. Elgiloy can be age-hardened to reach tensile strengths over 280 ksi, allowing for thinner cross-sections and smaller device footprints. Furthermore, Elgiloy's resistance to pitting and crevice corrosion in chloride-rich environments (like human body fluids) is superior to 316, reducing the risk of stress corrosion cracking (SCC) and metal ion release over long-term implantation.

17-7PH (AISI 631) in the CH900 condition offers superior fatigue resistance and corrosion protection compared to SAE 1070. The CH900 state is achieved through cold reduction followed by precipitation hardening at $900^{\circ}F$ ($482^{\circ}C$), resulting in a martensitic structure with finely dispersed aluminum-rich precipitates. This yields a typical tensile strength of 240-265 ksi. Unlike SAE 1070, which is prone to hydrogen embrittlement during plating, 17-7PH is inherently corrosion-resistant and maintains high elastic modulus ($E \approx 29 imes 10^6$ psi) stability at temperatures up to $600^{\circ}F$ ($315^{\circ}C$), whereas SAE 1070 begins to lose structural integrity and suffer from creep above $250^{\circ}F$.

A286 (UNS S66286) is an iron-base superalloy used when high strength and oxidation resistance are needed at temperatures up to $1300^{∘}F$ ($704^{∘}C$). While Inconel X-750 is superior in extreme corrosion environments (like sour gas), A286 is often specified in aerospace engine components because it is more cost-effective and easier to machine while providing similar mechanical properties at moderately high temperatures. A286 is precipitation-hardened, and its coefficient of thermal expansion is closer to that of standard alloy steels, which helps maintain 'tightness' in the groove during thermal cycling. In jet engine bearing retainers, A286 provides the necessary creep resistance and fatigue strength required for flight safety.

Passivation is a chemical treatment in a nitric or citric acid bath that removes 'free iron' from the surface of the stainless steel and enhances the formation of a dense, protective chromium-oxide ($Cr_2O_3$) layer. For $17-7PH$ CH900 rings, which contain both chromium and aluminum, passivation is vital after the precipitation hardening process. During heat treatment, the surface can become slightly depleted of chromium, or contaminants from the furnace can be embedded. Passivation ensures that the ring achieves its full corrosion resistance potential. In medical and aerospace standards (e.g., AMS 2700), passivation is a mandatory step to prevent premature 'staining' or 'rusting' in humid environments.

Beryllium Copper (Alloy 25 / UNS C17200) is used for spiral retaining rings when high electrical conductivity and non-magnetic properties are required, such as in EMI/RFI shielding or MRI machines. CuBe can be heat-treated to reach a tensile strength of $160-200$ ksi, comparable to some steels. The material's modulus $E \approx 19 \times 10^6$ psi is lower than steel, which allows for easier installation into deep grooves without permanent deformation. However, processing must be handled carefully due to the toxicity of beryllium dust (though the solid alloy is safe). In electronics, these rings are often 'Gold' or 'Silver' plated to further enhance surface conductivity and prevent oxidation.