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For 300-series stainless steels, cryogenic treatment (cooling to $-196°C$) is used to complete the transformation of retained austenite into martensite. In spiral rings, this process increases the hardness and dimensional stability. For rings used in cryogenic valves (e.g., Liquid Oxygen or LNG), this treatment ensures that the ring does not undergo a phase transformation in service, which could cause a change in volume and loss of 'cling' or groove tension. It also improves the wear resistance of the ring edges, which is beneficial in applications where the ring might see slight axial oscillations.

MP35N (a Cobalt-Nickel-Chrome-Moly alloy) is selected for the most demanding subsea environments because it offers an unparalleled combination of ultra-high strength (UTS up to $2000$ MPa) and exceptional resistance to Hydrogen Induced Stress Cracking (HISC) and Sulfide Stress Cracking (SSC). In the presence of cathodic protection systems and $H_2S$, standard stainless steels like 17-4PH or even 316 can fail. MP35N's face-centered cubic structure is highly stable. It also has a high modulus of elasticity ($E imes 233$ GPa), allowing for very high 'cling' forces in retaining rings that must withstand extreme pressures and corrosive 'sour' gas.

Quench and Temper (Q&T) produces a tempered martensite structure, which is hard but can be prone to micro-cracking and brittleness if not controlled. Austempering involves quenching the spring into a salt bath held just above the martensite start ($M_s$) temperature, typically around 300°C, and holding it until the austenite transforms into 'Bainite.' This acicular ferrite and cementite structure provides higher impact toughness and greater ductility for a given hardness level. For wave springs, which experience complex bending stresses, a bainitic structure reduces the sensitivity to surface notches and enhances the fatigue life, particularly in dynamic automotive clutch assemblies.

Beryllium Copper (C17200) offers a unique combination of high mechanical strength (approaching that of steel) and excellent electrical conductivity (approx. $20\%$-$25\%$ IACS). In EMI/RFI shielding, the wave spring acts as both a mechanical loader and an electrical contact. The low modulus of elasticity ($E \approx 125 \text{ GPa}$) allows for a more compliant spring compared to stainless steel, which is useful for accommodating large tolerances in electronic enclosures. Furthermore, CuBe is non-magnetic, which is essential for MRI medical equipment and sensitive avionics where magnetic interference could distort signals. Its corrosion resistance in marine environments also makes it suitable for outdoor communications hardware.

'Spring Temper' Inconel X-750 achieves its strength primarily through cold work, which provides very high static load capacity but lower thermal stability and potentially lower fatigue life due to the high density of internal defects and residual stresses. 'No. 1 Temper' involves a solution heat treatment at approximately 1150°C followed by a double-aging cycle. This process produces a more stable microstructure with optimized grain size and discrete $\gamma'$ precipitation. For high-cycle fatigue, the No. 1 Temper provides better resistance to crack initiation. While the ultimate tensile strength (UTS) might be lower than spring temper, the endurance limit and stability at temperature are superior, which is critical for wave springs used in aerospace seals.

High-carbon steels like SAE 1070 to 1090 with hardness exceeding HRC 40 are highly susceptible to Hydrogen Embrittlement (HE). During the acid pickling or electroplating process, atomic hydrogen diffuses into the steel lattice and accumulates at grain boundaries and stress concentrators (the peaks of the waves). Under axial load, these hydrogen atoms promote brittle fracture. To mitigate this, a baking process is mandatory within 4 hours of plating. According to ASTM B633 or similar standards, the springs must be baked at 190°C to 220°C for 8 to 24 hours depending on the section thickness and strength level. Failure to bake results in sudden, catastrophic fracture during the first compression cycle or during static hold.

At 550°C, A286 (an iron-base superalloy) reaches its upper limit for effective spring performance. It offers good corrosion resistance but suffers from accelerated creep and load loss ($>15\%$) at sustained temperatures above 500°C. Inconel X-750, a nickel-chromium alloy, is precipitation-hardened with aluminum and titanium to form $\gamma'$ phase precipitates. These precipitates pin dislocations, providing superior resistance to stress relaxation. At 550°C, X-750 typically exhibits less than $5\%$ load loss over 1,000 hours. For cryogenic applications, A286 remains ductile, while X-750 is preferred for its high-temperature oxidation resistance and structural stability up to 700°C (with appropriate #1 temper heat treatment).

Passivation (ASTM A967) is a critical post-manufacturing process for stainless steel rings. During coiling and handling, particles of 'tramp iron' from the tooling can become embedded in the surface of the ring. These particles act as initiation sites for corrosion (pitting). Passivation involves immersing the rings in a nitric or citric acid bath which dissolves the free iron and enhances the formation of a thin, protective chromium-oxide layer. For medical or aerospace components, passivation ensures that the ring remains 'stainless' and prevents premature failure due to localized galvanic corrosion cell formation.

Type 316 Stainless Steel is used when superior corrosion resistance to chlorides (seawater) is required, due to its 2-3% Molybdenum content. However, 316 cannot be hardened by heat treatment; it only gains strength through cold working. Consequently, a 316 spiral ring will have a lower thrust capacity and lower 'cling' force than a 17-7PH ring of the same dimensions. Designers must often use a heavier cross-section to achieve the required mechanical performance. In subsea instrumentation, 316 is often the minimum requirement, though for high-pressure housings, alloys like Monel K-500 or Inconel 718 may be substituted if the stress levels exceed 316's limits.

Black Oxide (MIL-DTL-13924) is a conversion coating formed by a chemical reaction with the surface of the carbon steel. Unlike electroplating, it does not change the dimensions of the ring significantly (typically $< 0.00001$ inches) and, most importantly, does not introduce Hydrogen Embrittlement. While it provides only marginal corrosion resistance (rated for approx. 24-96 hours in salt spray with oil), it is excellent for internal mechanical assemblies where the ring is submerged in oil. It also provides a non-reflective surface and increases the lubricity of the ring, which aids in installation and reduces the risk of galling in the groove.

Beryllium Copper (Alloy 25) is selected for spiral retaining rings when non-magnetic properties or high electrical conductivity are required. It also maintains its ductility and strength at cryogenic temperatures, unlike many carbon steels that become brittle. CuBe2 is heat-treated to an 'AT' or 'HT' temper to achieve tensile strengths up to $200$ ksi. Its lower Modulus of Elasticity ($E \approx 18.5 \times 10^6$ PSI) compared to steel ($30 \times 10^6$ PSI) means the ring will have less 'cling' force for a given deflection, which must be compensated for by increasing the wire thickness or the initial interference fit during design.

17-7PH (Type 631) is the industry standard because it combines high yield strength, excellent fatigue properties, and good corrosion resistance with minimal distortion during heat treatment. The manufacturing process involves coiling the material in 'Condition C' (cold reduced) and then performing a precipitation hardening heat treatment at $900^\circ F$ (Condition CH900). This aging process increases the hardness to HRC 41-48. Unlike 300-series stainless steels, which are too soft, or 400-series, which are brittle and prone to corrosion, 17-7PH provides the 'spring back' needed for the ring to snap into the groove and stay there under high centrifugal or axial loads.

A-286 is an iron-base superalloy designed for high-strength applications at temperatures up to $1300^\circ F$. Unlike carbon steels that lose all structural integrity above $400^\circ F$, A-286 maintains a high yield strength and oxidation resistance. In jet engines, wave springs are used to maintain axial tension on bearings or seals. The heat treatment involves a solution anneal followed by precipitation hardening at $1325^\circ F$. The resulting microstructure contains $Ni_3Ti$ precipitates which block dislocation movement. When designing with A-286, engineers must use a reduced Modulus of Elasticity ($E \approx 23 \times 10^6$ PSI at $1000^\circ F$) in their rate calculations to ensure the required preload is achieved at operating temperatures.

Austenitic stainless steels like 316 exhibit a high work-hardening rate during the flat-wire rolling and subsequent coiling process. As the wire is shaped into the wave profile, the localized plastic deformation increases the hardness and yield strength but reduces ductility. For very thin sections (e.g., $t < 0.005$ inches), the material can become brittle, leading to micro-cracking at the wave peaks. Process control involves monitoring the 'Springback' angle during coiling, which is a function of the ratio $E/E_{tan}$ (where $E_{tan}$ is the tangent modulus). If the work hardening is inconsistent across the coil, the resulting wave heights will vary, leading to 'cocking' of the spring when under load.

Elgiloy is a 'super-alloy' used in extremely corrosive environments where resistance to sulfide stress cracking (SSC) and stress corrosion cracking (SCC) is mandatory, complying with NACE MR0175 standards. Its metallurgy provides a unique combination of ultra-high strength (up to $300$ ksi tensile) and excellent corrosion resistance in sour gas ($H_2S$) and chloride-rich seawater. The alloy is work-hardened and then aged at $900^\circ F$-$1000^\circ F$. In wave springs, Elgiloy provides a virtually flat relaxation curve at temperatures up to $850^\circ F$. This is critical for subsea valves where the spring must maintain a constant sealing force over a 25-year design life without maintenance access.

High-carbon steels like SAE 1070-1090 are susceptible to Hydrogen Embrittlement (HE) during the acid pickling or electroplating process (e.g., zinc or cadmium plating). Atomic hydrogen diffuses into the grain boundaries of the steel, especially under high tensile stress areas like the wave crests. This leads to brittle fracture at loads significantly below the yield strength. To mitigate this, springs must undergo a 'baking' process immediately after plating—typically at $375^\circ F \pm 25^\circ F$ for at least 4 to 24 hours depending on the part thickness and hardness. Failure to bake within 1-4 hours of plating often results in irreversible damage. For critical aerospace components, mechanical plating or vacuum deposition is preferred to avoid hydrogen exposure entirely.

17-7PH Stainless Steel (Condition CH900) is a semi-austenitic precipitation-hardening alloy that offers significantly higher strength and better fatigue resistance than Type 302. After cold-working to Condition C, it is aged at $900^\circ F$ to reach Condition CH900, resulting in a typical tensile strength of $240$-$265$ ksi. Type 302 depends solely on cold reduction for its $160$-$190$ ksi strength. In medical applications where miniature wave springs are subject to repeated sterilization (autoclaving), 17-7PH is preferred because its higher elastic limit prevents permanent deformation during high-strain cycles. Furthermore, 17-7PH exhibits superior dimensional stability during heat treatment compared to 300-series steels which may warp.

The process of edge-coiling a spiral ring involves significant plastic deformation, which leaves high residual tensile stresses on the outer edges and compressive stresses on the inner edges. Without stress relieving, these residual stresses can lead to 'warping' or dimensional instability over time. Stress relieving carbon steel at $650^{\circ}F$ to $750^{\circ}F$ for 30 minutes allows for the relaxation of these internal stresses without significantly reducing the hardness achieved during the previous heat treatment. This ensures that the ring maintains its free diameter and flatness, which are critical for proper seating in the groove and for achieving the calculated 'cling' force.

Beryllium Copper (UNS C17200) is used for its excellent electrical conductivity and high strength. It can operate effectively up to approximately $400^{\circ}F$ ($204^{\circ}C$). Beyond this temperature, the material begins to over-age, leading to a rapid loss of tensile strength and elastic modulus. In electrical connectors, the ring serves as a retention device that must also withstand thermal cycling. Its thermal expansion coefficient ($α \approx 9.4 · 10^{-6} / ^{\circ}F$) must be matched with the housing material to prevent loosening at high temperatures. Unlike steel, BeCu is non-sparking and non-magnetic, making it ideal for volatile aerospace environments.

Black oxide (MIL-DTL-13924) is a conversion coating that provides minimal corrosion resistance (mostly for aesthetics and oil retention) but does not change the dimensions of the ring or introduce hydrogen embrittlement. Zinc plating (ASTM B633) provides significantly better corrosion protection through sacrificial anode behavior. However, zinc plating carries a high risk of hydrogen embrittlement and adds thickness ($0.0002$ to $0.0005$ inches), which can interfere with the fit in precision grooves. Furthermore, zinc plating can 'flake' under the high-stress coiling and installation of a spiral ring. For high-fatigue applications, black oxide with a rust-preventative oil is often preferred to avoid the risk of embrittlement-induced cracking.