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'Oil Tempered' SAE 1070 carbon steel is processed by heating to the austenitic range, quenching in oil, and then tempering to the desired hardness (typically HRC 45-52). This process results in a very fine tempered martensite structure, providing an excellent balance of high yield strength and toughness. For spiral retaining rings in heavy machinery, this material is preferred over 'Cold Drawn' wire because it has lower internal stresses and better dimensional stability. However, it must be protected from corrosion via phosphate coating or zinc plating. It is also critical to avoid 're-tempering' during any subsequent coating processes to maintain the structural integrity of the ring.

While both are austenitic stainless steels, $316$ Stainless Steel contains $2-3\%$ Molybdenum ($Mo$), which significantly enhances its resistance to 'pitting' and 'crevice corrosion' in chloride-rich environments (like seawater). $302$ Stainless Steel is more susceptible to localized attack. However, $302$ can be cold-worked to higher tensile strengths ($>200$ ksi) than $316$ ($>160$ ksi), meaning a $302$ ring can hold higher thrust loads for the same size. For marine applications, if the load is moderate, $316$ is preferred. If high load is required, a $302$ ring with a 'Passivation' treatment (per ASTM A967) or a 'Black Oxide' finish may be used, though $17-7PH$ is often the better compromise for both strength and corrosion resistance.

In the production of wave springs, 'Stress Relieving' is a low-temperature thermal treatment used after coiling to stabilize dimensions and reduce internal stresses induced by cold working. For $302/316$ stainless steel, this typically occurs at $600^{∘}F$ to $750^{∘}F$. It increases the elastic limit without significantly changing the hardness. 'Full Annealing' involves heating the material above its critical temperature ($>1900^{∘}F$) to create a fully austenitic structure, which completely softens the material. Annealing is never used after the spring is formed because it would destroy the spring tempered properties. Designers must specify stress relief to prevent 'creep' or 'set' during the first several cycles of the spring's operation in the field.

MP35N (a Cobalt-Nickel-Chrome-Molybdenum alloy) is selected for wave springs in medical implants, such as heart valves or vascular stents, due to its unmatched biocompatibility and extreme fatigue life. MP35N offers a tensile strength exceeding $250$ ksi and is virtually immune to all forms of corrosion in physiological fluids (saline). The material's high modulus of elasticity ($E \approx 234$ GPa) allows for very thin, low-profile wave springs that can still provide high clamping forces. Furthermore, it is non-magnetic, making it MRI-safe. The processing of MP35N requires cold working followed by aging at $1000^{∘}F$ ($538^{∘}C$) to achieve the required spring properties, making it one of the most high-performance and expensive materials in the industry.

High-carbon steels like SAE 1070 to 1090 are susceptible to hydrogen embrittlement during acid pickling and electroplating processes (e.g., zinc or cadmium plating). Atomic hydrogen ($H$) diffuses into the crystal lattice, accumulating at grain boundaries and dislocations, which reduces ductility and leads to unpredictable brittle fracture under static load. To mitigate this, a 'Baking' process is mandatory. The springs must be baked at $375^{∘}F \pm 25^{∘}F$ ($190^{∘}C$) for at least $4$ to $24$ hours within $1$ hour of plating. Failure to bake results in 'delayed fracture', where the wave spring snaps hours or days after installation in a bore. In high-vibration automotive environments, this is a catastrophic failure mode.

Inconel X-750 (UNS N07750) is the industry standard for subsea oil and gas applications due to its exceptional resistance to chloride-ion stress corrosion cracking (SCC) and hydrogen embrittlement. The material is typically heat-treated to the NACE MR0175 standard to ensure performance in sour gas ($H_2S$) environments. The hardening process involves solution annealing followed by precipitation aging, which creates $\gamma'$ ($Ni_3(Al, Ti)$) precipitates. These precipitates pin dislocations, providing a high yield strength at temperatures ranging from cryogenic to $1300^{∘}F$ ($704^{∘}C$). When designing wave springs for deep-water valves, the lower shear modulus $G$ and Young's Modulus $E \approx 31 \times 10^6$ psi must be used in the rate calculations to ensure the spring provides sufficient sealing force at $10,000$ psi ambient pressure.

$17-7PH$ (AISI 631) in the CH900 condition offers a superior combination of high strength and corrosion resistance compared to $302$ Stainless Steel. $17-7PH$ undergoes a precipitation hardening process (Condition C to CH900) which yields a tensile strength up to $240$ ksi. This allows for higher stress levels ($σ_{all} \approx 0.75 \cdot σ_{uts}$) and a smaller footprint for the same load. Additionally, $17-7PH$ exhibits better relaxation resistance at temperatures up to $650^{∘}F$ ($343^{∘}C$). In contrast, $302$ Stainless Steel, while cheaper and highly ductile, is limited to approximately $550^{∘}F$ ($288^{∘}C$) and has lower fatigue limits. For mission-critical aerospace components, the phase transformation in $17-7PH$ provides the dimensional stability required for tight-tolerance wave spring applications.

Cryogenic treatment (cooling the material to $-300^°F$ for 24+ hours) is sometimes used on high-carbon steel or certain stainless steel rings to ensure the complete transformation of retained austenite into martensite. Retained austenite is unstable and can transform over time or under stress at room temperature, causing the ring to expand or contract slightly. For high-precision applications, such as retaining rings used in aerospace guidance systems, cryogenic treatment ensures 'dimensional stability'—the ring will not change size over years of service. It also slightly increases the hardness and wear resistance of the material by promoting the precipitation of fine eta-carbides.

A-286 should be specified when the application requires high strength at elevated temperatures (up to $1000^°F$) or when high strength is needed in a truly non-magnetic material. 316 Stainless Steel is highly corrosion-resistant but has relatively low yield strength, making it prone to 'taking a set' during installation or failing under high thrust loads. A-286 is an age-hardenable austenitic stainless steel that achieves yield strengths of 100-120 ksi through precipitation of the gamma-prime phase. This makes it ideal for aerospace turbine components where a retaining ring must maintain its integrity under both high heat and high centrifugal loads.

Black Oxide (per MIL-DTL-13924) is a conversion coating formed by a chemical reaction with the iron in the steel. Unlike plating, it does not change the dimensions of the ring significantly (it adds less than 0.5 μm), which is critical for maintaining the precise fit in the groove. While it provides only minimal corrosion resistance (it must be supplemented with oil or wax), its primary benefit in engineering is the elimination of 'hydrogen embrittlement' risks associated with electroplating. It also provides a non-reflective surface and can help retain lubricants, reducing friction during the installation of the ring into the groove.

Elgiloy is a 'super-alloy' known for its extreme resistance to Hydrogen Sulfide ($H_2S$) induced stress corrosion cracking, which is a major failure mode in 'sour' oil wells. It offers high strength and a high modulus of elasticity. For spiral retaining rings in subsea connectors, Elgiloy provides the necessary 'cling' to the groove while remaining nearly immune to the corrosive effects of seawater and sour gas. The processing involves a complex heat treatment: cold work followed by aging at $900^°F$. This results in a material with a fatigue limit far exceeding that of 17-7PH or Inconel 718 in corrosive environments.

Medical devices often require biocompatibility and resistance to sterilization processes like autoclaving or chemical wipe-downs. 302 Stainless Steel (per ASTM A313) provides excellent corrosion resistance and can be cold-worked to high tensile strengths (up to 250 ksi for small diameters). Unlike carbon steel, it does not require a secondary plating (like zinc or chrome), which could flake off and contaminate a sterile field. Furthermore, 302 is non-magnetic in the annealed state and only slightly magnetic after cold working, making it suitable for certain MRI-adjacent equipment where carbon steel rings would cause image artifacts or be physically pulled by the magnetic field.

Nitriding is a thermo-chemical process that diffuses nitrogen into the surface of the spring material, usually carbon steels or alloy steels like 4140, to create a hard, wear-resistant 'white layer' and a diffusion zone. For wave springs subjected to high-cycle fatigue, nitriding introduces beneficial compressive residual stresses on the surface. These stresses counteract the tensile stresses experienced during deflection, effectively increasing the fatigue limit. The formula for the modified fatigue strength is $\sigma_{e'} = \sigma_e + \sigma_{comp}$, where $\sigma_{comp}$ is the magnitude of the compressive stress. However, if the nitrided layer is too brittle, it can crack under high strain, acting as a stress riser that leads to core failure; therefore, the nitriding depth must be carefully controlled to roughly 10% of the wire thickness.

316 Stainless Steel is selected for its superior corrosion resistance (due to 2-3% Molybdenum content) compared to 302/304. However, 316 has a lower work-hardening rate, meaning it requires significantly more cold reduction to reach the same tensile strength required for spring applications. For a wave spring, this means the flat wire must be precisely cold-rolled to a 'Spring Temper' (typically Full Hard or Extra Hard). If the tensile strength is too low, the spring will suffer from excessive relaxation. Engineers must specify a minimum tensile strength (e.g., 185,000 psi) rather than just the alloy grade to ensure the wave spring can support the design load without plastic deformation.

A-286 (ASTM A453) is an iron-base superalloy that maintains exceptional ductility and impact strength at cryogenic temperatures (down to $-423^°F$). Unlike standard 300-series stainless steels, A-286 does not undergo a phase transformation to martensite when cold worked or cooled, which prevents the material from becoming magnetic and brittle. For wave springs in LNG valves or aerospace liquid oxygen systems, A-286 provides a stable spring rate and high fatigue resistance. The material is typically aged at $1325^°F$ to precipitate the Ni3(Al, Ti) phase, resulting in a yield strength of approximately 100 ksi at room temperature, which actually increases at cryogenic temperatures.

SAE 1070 carbon steel is commonly used for cost-effective wave springs, but the cold-forming process introduces significant residual tensile stresses at the wave crests. Without a proper stress-relief heat treatment (typically $600-650^°F$ for 30 minutes), these residual stresses combine with operational loads to exceed the material's yield strength, leading to 'set' or premature fatigue failure. Furthermore, if the spring is electroplated for corrosion resistance (e.g., zinc plating), it is highly susceptible to hydrogen embrittlement. Atomic hydrogen can migrate into the high-stress areas of the grain boundaries, causing sudden, brittle fracture. A baking cycle at $375^°F$ for at least 4 hours immediately following plating is mandatory to drive out the hydrogen.

17-7PH stainless steel in the CH900 condition offers excellent high-strength properties up to $650^°F$ ($343^°C$). It is precipitation-hardened, achieving a high tensile strength through a combination of cold reduction and aging. However, for temperatures exceeding $700^°F$, Inconel X-750 is required due to its superior resistance to relaxation (creep). Inconel X-750 is a nickel-chromium alloy made precipitation-hardenable by additions of Al and Ti. While 17-7PH may show a load loss of over 10% after 100 hours at $750^°F$, Inconel X-750 typically maintains 95% of its initial load. The processing for X-750 involves a solution anneal followed by double aging (No. 1 Temper), which optimizes the gamma-prime phase for maximum creep-rupture life.

Oil tempering (quenching and tempering) SAE 1070 carbon steel results in a fine-grained martensitic structure that offers an excellent balance of toughness and ductility. For heavy-duty spiral rings, this process ensures that the material can withstand the high strain of installation without cracking. Compared to 'Hard Drawn' wire, oil-tempered wire has more uniform mechanical properties and lower residual stress, which leads to better dimensional stability of the ring over time. This is particularly important for large-diameter rings used in heavy equipment where the axial loads and potential impact forces are high.

Cadmium plating was historically the standard for aerospace due to its lubricity and excellent corrosion resistance. However, due to its high toxicity and environmental regulations (REACH/RoHS), Zinc-Nickel (Zn-Ni) has become the preferred alternative. Zn-Ni provides comparable or superior salt-spray resistance ($>1000$ hours) and exhibits better galvanic compatibility with aluminum housings. Crucially, Zn-Ni plating processes generally involve less hydrogen evolution, though high-strength carbon steel rings ($>40$ HRC) still require a mandatory de-embrittlement bake to prevent delayed brittle fracture.

Passivation (per ASTM A967) is a chemical treatment in nitric or citric acid that removes 'tramp' iron and other surface contaminants from the stainless steel. This process enhances the formation of a thin, protective chromium-oxide layer. Without passivation, microscopic iron particles from the manufacturing tooling can embed in the ring surface and cause 'bloom' or localized pitting in corrosive environments. For 316 Stainless Steel, which contains molybdenum for better chloride resistance, passivation is critical for subsea applications to prevent crevice corrosion within the groove where stagnant water can accumulate.