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Carbon steel (SAE 1070-1090) and Stainless Steel (AISI 302 and 316) are the most prevalent materials for general and corrosion-resistant industrial environments.

Austempering is a heat-treating process that results in a Bainite microstructure, whereas Martempering produces Tempered Martensite. For spiral retaining rings, which are thin and prone to distortion, Austempering offers several advantages: (1) Reduced distortion because the transformation occurs at a constant temperature above the Martensite start ($M_s$) point. (2) Increased toughness and 'ductile-to-brittle' transition resistance at a given hardness (typically HRC 45-52). (3) Superior fatigue life. Because retaining rings must be expanded/contracted during installation, the higher ductility of the Bainitic structure prevents cracking during the assembly process. Martempering, while effective for larger cross-sections, often leaves residual stresses that can lead to 'quench cracking' in the thin, multi-turn geometry of spiral rings.

MP35N (a nickel-cobalt base alloy) is the gold standard for subsea wave springs due to its immunity to Hydrogen Induced Stress Cracking (HISC) and Sulfide Stress Cracking (SSC) in 'sour' environments containing $H_2S$. MP35N achieves ultra-high strength (up to $300$ ksi) through work hardening and aging. Unlike 17-7PH, which may be susceptible to embrittlement in certain electrolytic subsea conditions, MP35N maintains its ductility and fatigue resistance. The alloy's high Modulus of Elasticity ($33.8 \times 10^6$ psi) allows for very high spring rates in compact envelopes. For a subsea valve actuator with a 25-year service life requirement, the material's resistance to chloride-induced SCC and its massive cathodic protection compatibility make it the only viable choice despite its extreme cost.

While 302 Stainless Steel offers a higher tensile strength due to its higher carbon content and work-hardening rate, 316 Stainless Steel is preferred in environments where molybdenum is required to resist pitting and crevice corrosion. 316 contains 2-3% molybdenum, which significantly improves resistance to chlorides (e.g., seawater, de-icing salts). In medical or chemical processing applications, the risk of Stress Corrosion Cracking (SCC) is higher with 302. Therefore, if the application involves immersion in saline solutions or harsh chemicals, 316 is the correct metallurgical choice. The engineer must compensate for the roughly 10-15% lower thrust capacity of 316 by either increasing the ring thickness or using a deeper groove.

A286 (iron-base superalloy) is chosen for wave springs operating between $700^{\circ}F$ and $1000^{\circ}F$ due to its resistance to stress relaxation. Stress relaxation is the time-dependent decrease in load under a constant deflection, primarily driven by dislocation climb and grain boundary sliding. For A286, the precipitation of $\gamma'$ ($Ni_3Ti$) particles during the aging process ($1325^{\circ}F$ for 16 hours) pins dislocations, reducing the relaxation rate. If a wave spring is designed for a $100$ lb preload, exposure to $900^{\circ}F$ for 1000 hours might result in a 5-10% loss in load. Engineers must over-design the initial preload using the Arrhenius equation to predict the 'end-of-life' load, ensuring the system remains functional despite the inevitable loss of spring force over time.

SAE 1070 carbon steel is the standard for general-purpose retaining rings due to its high carbon content and cost-effectiveness, providing a yield strength around $160$-$200$ ksi after oil quenching and tempering. However, SAE 1070 becomes brittle at cryogenic temperatures and loses strength rapidly above $250^{\circ}F$. In contrast, Inconel X-750 (AMS 5699) is a nickel-chromium alloy that remains ductile at temperatures as low as $-300^{\circ}F$ and maintains mechanical integrity up to $1300^{\circ}F$. Inconel X-750's precipitation hardening involves the formation of $\gamma'$ phase ($Ni_3(Al, Ti)$), which provides excellent creep-rupture strength. For subsea or aerospace applications where corrosion and temperature extremes coincide, Inconel X-750 is the mandatory selection despite the significant cost premium over carbon steel.

17-7PH CH900 (Condition C, precipitation hardened to 900°F) is a semi-austenitic precipitation-hardening stainless steel that offers significantly higher tensile strength and fatigue resistance compared to 302 or 316 stainless steels. While 302 gains strength through cold working, 17-7PH undergoes a phase transformation and subsequent aging process that yields a typical tensile strength of $200$-$230$ ksi and an elastic modulus $E$ of approximately $28.5 \times 10^6$ psi. This allows for higher stress levels in smaller envelopes. Furthermore, 17-7PH exhibits superior dimensional stability during heat treatment and better relaxation resistance at operating temperatures up to $650^{\circ}F$ ($343^{\circ}C$), making it ideal for precision medical and aerospace actuators.

15-7 MO (PH 15-7 Mo) is a semi-austenitic precipitation-hardening stainless steel. By replacing 2% of the Chromium in 17-7PH with Molybdenum, it achieves higher strength and better resistance to softening at elevated temperatures. In the CH900 condition, it can reach a UTS of over 1700 MPa. For retaining rings, this translates to the highest possible thrust load capacity per unit of thickness. Its resistance to 'set' and its high fatigue limit make it the preferred choice for heavy-duty retaining rings in landing gear and high-pressure hydraulic cylinders where space is at a premium and the environment is mildly corrosive.

The edge-winding process used to create spiral rings induces significant residual tensile stresses on the outer diameter and compressive stresses on the inner diameter. Stress relieving (typically at $350^∘C$ to $450^∘C$ for 30-60 minutes) is critical to stabilize the ring's dimensions. Without this, the ring may 'relax' over time, changing its free diameter and losing its grip on the shaft or bore. Additionally, stress relieving reduces the peak residual stresses that would otherwise add to the operational stresses, thereby increasing the fatigue threshold. This is especially important for rings used in high-vibration aerospace connectors.

Black oxide (MIL-DTL-13924) provides minimal corrosion resistance (mostly for aesthetics and light oil retention) and has negligible impact on fatigue. Zinc Phosphate (heavy phosphate), however, provides a porous crystalline structure that holds much more corrosion-inhibiting oil, offering significantly better salt spray resistance. However, the phosphating process involves an acid pickling step which, like plating, can induce hydrogen embrittlement. Furthermore, for high-cycle fatigue applications, the crystalline structure of phosphate can act as a micro-abrasive if the ring is subjected to vibration. For maximum fatigue life in mildly corrosive environments, a simple oil-dipped finish or switching to 302SS is often preferred.

Beryllium Copper (Alloy 25) should be specified in three scenarios: 1) Non-magnetic requirements, such as in MRI machines or sensitive electronic sensors where steel would interfere with magnetic fields; 2) Non-sparking environments, such as oil refineries or explosive handling, where a steel ring striking a surface could ignite vapors; and 3) High electrical conductivity requirements. CuBe2 has a tensile strength comparable to alloy steel (up to 1400 MPa) but with a much lower modulus ($E \approx 130$ GPa). This lower modulus must be accounted for in the design, as it will result in lower grip pressure and lower rotational speed limits compared to a steel ring of the same dimensions.

A286 (ASTM A638) is an iron-base superalloy that maintains high strength and oxidation resistance up to $700^∘C$. For retaining rings in turbines, it is superior to stainless steels because it does not lose its 'spring temper' at operating temperatures. The material is precipitation-hardened to achieve a yield strength of approximately 700-1000 MPa. Its coefficient of thermal expansion is also closely matched to many nickel-based turbine alloys, which minimizes the risk of the ring losing its grip due to differential thermal expansion. Processing involves solution treating at $980^∘C$ followed by aging at $720^∘C$ for 16 hours to precipitate the $\gamma'$ phase.

Edge-winding (also known as 'No-Tooling-Cost' coiling) involves winding a pre-tempered flat wire on its edge. This process preserves the grain flow of the material along the circumference of the spring, which significantly enhances fatigue life and load consistency. In contrast, stamping involves punching the spring from a sheet, which results in 'cross-grain' orientation at various points around the circle, creating weak spots susceptible to premature failure. Furthermore, edge-winding eliminates the scrap associated with stamping and allows for the easy production of multi-turn Crest-to-Crest springs with shim ends, which provide a $360^∘$ contact surface for more uniform load distribution.

Cryogenic treatment of 302 stainless steel wave springs, involving immersion in liquid nitrogen ($-196^∘C$), promotes the transformation of retained austenite to martensite. This transformation increases the hardness and wear resistance of the wave peaks. More importantly for precision optics or cryo-valves, it improves dimensional stability by relieving internal stresses caused by the edge-winding process. Without cryo-processing, the spring might undergo subtle 'walking' or diameter changes when cycled between ambient and cryogenic temperatures, which would compromise the axial preload on sensitive lens assemblies.

Elgiloy is the gold standard for implantable medical wave springs due to its biocompatibility, extreme fatigue resistance, and non-magnetic properties. It exhibits a high UTS of up to 2600 MPa after cold work and aging. Its modulus of elasticity ($E = 190$ GPa) is stable across a wide temperature range. In medical pumps, Elgiloy's resistance to pitting and crevice corrosion in saline environments (body fluids) outperforms 316L stainless steel. The processing involves a cold-reduction of approx 85% followed by aging at $500^∘C$ for 5 hours. This creates a dense dislocation network that prevents the initiation of micro-cracks during the millions of cycles required for heart-assist devices.

High-carbon steel like SAE 1070 is highly susceptible to hydrogen embrittlement (HE) during acid pickling and electroplating processes (e.g., zinc or nickel plating). Atomic hydrogen diffuses into the grain boundaries, particularly in the high-stress areas at the wave peaks, leading to brittle fracture under load. To mitigate this, a 'bake-out' process is mandatory. The springs must be placed in an oven at $190^∘C$ to $220^∘C$ within 1 to 4 hours of plating. The baking duration (typically 8 to 24 hours) allows the hydrogen to diffuse out of the steel matrix. For critical aerospace components, mechanical plating or organic coatings are often substituted to eliminate the risk of HE entirely.

At operating temperatures of $350^∘C$, 17-7PH stainless steel begins to suffer from over-aging and a significant reduction in yield strength, leading to stress relaxation (creep). Inconel X-750 (AMS 5699) is the superior choice for subsea O&G due to its exceptional resistance to chloride-induced stress corrosion cracking (SCC) and high-temperature stability. X-750 undergoes precipitation hardening through the formation of $\gamma'$ phase ($Ni_3(Al, Ti)$), which maintains its mechanical properties up to $700^∘C$. The spring must be heat treated to the No.1 temper (HT) to optimize for relaxation resistance. Furthermore, X-750's lower modulus ($E \approx 213$ GPa) compared to 17-7PH ($E \approx 200$ GPa at temp) requires a slight adjustment in the number of turns to maintain identical load characteristics.

Vibro-deburring (or tumbling) is used to remove any sharp edges or 'slivers' from the coiling process. While primarily for safety and ease of installation, it also improves the performance of the ring by creating a uniform radius on all edges, which reduces stress concentrations. For fatigue-critical applications, controlled tumbling can induce a small amount of compressive residual stress on the surface, similar to a mild shot-peening effect. This helps retard the initiation of surface cracks. However, excessive tumbling must be avoided as it can cause 'edge-rounding' to the point where the ring's contact area in the groove is reduced, thereby lowering the effective thrust capacity.

SAE 1070 carbon steel is the most economical choice for retaining rings but lacks inherent corrosion resistance. In high-humidity automotive applications (e.g., under-hood or suspension components), it will oxidize rapidly, forming iron oxide ($Fe_2O_3$), which leads to a reduction in the effective cross-section and premature failure. Common protection methods include zinc phosphate coating ('Oil and Phosphate') or zinc flake coating (e.g., Magni or Geomet). However, these coatings can add thickness, potentially interfering with groove fit. If the application involves salt spray, the transition to 302 stainless or 17-7PH is usually necessary to ensure the 10-15 year service life required by modern OEMs.

Elgiloy should be specified when a combination of extreme corrosion resistance, high fatigue strength, and high-temperature stability (up to $450^{\circ}C$) is required. It is particularly resistant to sulfide stress cracking (SSC), making it a favorite for both aerospace and sour-gas oil applications. Its Modulus of Elasticity is approximately $200$ GPa, similar to steel, but it maintains its spring properties under much harsher conditions. In aerospace, Elgiloy is used in fuel systems and engine controls where failure is catastrophic. The material is typically aged at $480^{\circ}C$ for 5 hours after coiling to achieve its full mechanical properties, providing a yield strength that can exceed $250$ ksi.