Materiali e lavorazione

Black Oxide (MIL-DTL-13924) is a conversion coating that provides a dark appearance and a small degree of corrosion resistance, primarily intended for indoor applications or where the ring will be submerged in oil. Zinc Phosphate (heavy) provides better corrosion protection and acts as a substrate for lubricants. For spiral rings, the coating must be thin enough to not interfere with the ring's ability to flex during installation. Carbon steel rings (SAE 1070-1090) must be carefully processed to avoid hydrogen embrittlement if these coatings involve an acid pickling step.

Beryllium Copper (Alloy 25, UNS C17200) is specified for its unique combination of high strength (comparable to steel), excellent electrical conductivity, and non-sparking properties. It is also non-magnetic, making it ideal for MRI medical equipment or sensitive electronic sensors. BeCu rings are typically age-hardened at $600^{\circ}F$ to achieve their full spring properties. Additionally, BeCu offers excellent corrosion resistance in marine environments. However, due to the toxicity of beryllium dust during manufacturing and the high material cost, its use is restricted to applications where these specific properties are mandatory.

MP35N is a cobalt-nickel-chromium-molybdenum alloy that provides an extraordinary combination of ultra-high strength (up to 300 ksi), excellent toughness, and superb corrosion resistance. It is specifically selected for the most demanding medical implants (due to biocompatibility) and subsea applications where exposure to hydrogen sulfide ($H_2S$) and high pressures is expected. Its modulus $E$ is approximately $33.5 \times 10^6$ psi. Due to its high cost and specialized processing requirements, it is only used when 17-7PH or Inconel X-750 cannot meet the fatigue life or corrosive environment requirements of the system.

Most wave springs are manufactured using an 'edge-winding' process (No-Tooling-Charge method), where flat wire is coiled on edge. This process preserves the grain flow of the material along the longitudinal axis of the wire, which is ideal for spring performance. In contrast, punching springs from flat sheet results in transverse grain orientation and significant material waste. Edge-winding allows for the use of pre-tempered or cold-reduced wire, which possesses higher tensile strength and better fatigue life. The metallurgical integrity of the edge-wound spring is superior because the rolling process induces beneficial compressive residual stresses on the outer edges of the wire.

Carbon steel springs (SAE 1070-1090) are susceptible to hydrogen embrittlement during acid cleaning or electroplating processes (e.g., zinc plating). Atomic hydrogen diffuses into the crystal lattice, leading to brittle fracture under static load. To mitigate this risk, springs must undergo a 'baking' process immediately after plating, typically at $375^{\circ}F \pm 25^{\circ}F$ ($190^{\circ}C$) for at least 4 to 24 hours, depending on the material's hardness and thickness. For critical aerospace or safety-critical applications, mechanical plating or alternative coatings like zinc-flake (Geomet) are preferred because they do not involve the electrolytic generation of hydrogen.

Inconel X-750 (UNS N07750) is a nickel-chromium superalloy utilized for wave springs in extreme environments, such as subsea oil and gas valves or aircraft engines. Its primary advantage is its resistance to relaxation and creep at temperatures up to $1300^{\circ}F$ ($704^{\circ}C$). The material is precipitation-hardened, and for spring applications, it is typically supplied in the 'No. 1 Temper' or 'Spring Temper' followed by age hardening. In subsea environments, its resistance to chloride-induced stress corrosion cracking (SCC) makes it indispensable, though designers must account for its higher density and lower modulus ($E \approx 31 \times 10^6$ psi) compared to carbon steel when calculating spring rates.

17-7PH (UNS S17700) is a precipitation-hardening stainless steel that offers a superior combination of high strength and corrosion resistance. In the CH900 condition (cold rolled and aged at $900^{\circ}F$), it achieves a much higher elastic limit than 302 stainless steel. This allows for thinner cross-sections to achieve the same load, which is critical for miniaturized medical or aerospace components. Furthermore, 17-7PH exhibits significantly less 'set' or relaxation over time when held at elevated temperatures, maintaining its load-carrying capacity up to approximately $650^{\circ}F$ ($343^{\circ}C$), whereas 302 begins to lose spring properties significantly above $400^{\circ}F$.

Spiral rings are coiled from wire that is already in a 'spring temper' or 'hard drawn' state. This means the material already possesses its high tensile strength before the coiling process. The coiling machine must exert enough force to plastically deform the wire into a circle, while accounting for 'springback', where the ring diameter increases slightly after being released from the coiling mandrel. This cold-working process further increases the yield strength on the outer fiber of the ring, but if the wire is too hard, it can develop micro-cracks on the inner diameter during coiling, leading to failure during installation.

Oil-dipping provides only temporary 'shelf-life' corrosion protection and is unsuitable for outdoor or humid environments. Zinc-Phosphate coating (per MIL-DTL-16232) provides a porous crystalline structure that holds oil more effectively, offering significantly better salt-spray resistance (often $72+$ hours). In automotive drivetrain applications, zinc-phosphate is preferred because it prevents rust during shipping and assembly while remaining compatible with transmission fluids. For even higher protection, mechanical zinc plating or organic coatings are used, though they increase the ring's thickness and may affect fitment in precision grooves.

Beryllium Copper (typically Alloy 25) is specified when an application requires a combination of high strength, non-magnetic properties, and high electrical conductivity. Unlike steel, CuBe is 'non-sparking', making it mandatory in explosive environments (e.g., oil refineries or grain silos). It can be age-hardened to achieve tensile strengths comparable to some steels ($160-200$ ksi). Furthermore, its excellent thermal conductivity helps dissipate heat in high-speed bearing housings. However, its high cost and the toxicity of beryllium dust during machining (not coiling) are factors that limit its use to specialized aerospace and industrial cases.

After coiling, SAE 1070 carbon steel spiral rings are 'oil-quenched and tempered' or 'austempered' to achieve a hardness typically between 45-52 HRC. The tempering process (heating to $700-900^{\circ}F$) is critical to transform the brittle martensite into tempered martensite, which provides the necessary toughness and ductility. Without proper tempering, the rings would snap during installation when expanded over a shaft. The final hardness must be tightly controlled: too high leads to brittleness, while too low leads to 'set' and loss of retention force.

AISI 302 is the standard stainless steel for spiral rings, offering high tensile strength due to its ability to be severely cold-worked. AISI 316 contains $2-3\%$ molybdenum, providing superior resistance to chlorides and pitting, making it essential for marine or chemical processing. However, 316 has approximately $10-15\%$ lower tensile strength than 302 for the same reduction, meaning a 316 ring will have a lower thrust capacity. In subsea applications, the trade-off for corrosion resistance usually justifies the need for a thicker 316 ring or a deeper groove to compensate for the lower shear strength.

Passivation is a critical chemical process (per ASTM A967) that removes free iron from the surface of 17-7PH wave springs, enhancing the protective chromium-oxide layer. In medical instruments, this prevents localized pitting and 'tea-staining' during repeated autoclave sterilization cycles. The process involves immersion in nitric or citric acid baths. For 17-7PH, passivation is performed after the CH900 heat treatment to ensure any surface contaminants introduced during the coiling or aging process are removed. This ensures biocompatibility and long-term reliability in sensitive surgical environments.

At cryogenic temperatures (e.g., $-320^{\circ}F$), 302/304 stainless steel can undergo a partial martensitic transformation, which may increase magnetism and brittleness. A286 (AMS 5525) is an iron-base superalloy that remains fully austenitic and ductile at cryogenic temperatures. A286 provides higher yield strength than 300-series stainless through precipitation hardening, making it suitable for high-load cryogenic valves. While 302 is more cost-effective and common for general industrial use, A286 is the technical standard for LH2 (Liquid Hydrogen) or LNG (Liquefied Natural Gas) systems where toughness and non-magnetic properties are paramount.

Carbon steel wave springs (e.g., SAE 1070-1090) are susceptible to Hydrogen Embrittlement (HE) during acid pickling or electroplating processes. Atomic hydrogen migrates into the grain boundaries of the high-strength martensitic structure, leading to catastrophic brittle fracture under static load. Mitigation involves 'baking' the springs immediately after plating (typically within 1 to 4 hours) at approximately $375^{\circ}F$ ($190^{\circ}C$) for 4 to 24 hours depending on the coating thickness and material hardness. For critical subsea or automotive applications, many engineers specify mechanical zinc plating or stainless steel alternatives to eliminate the risk of HE entirely.

Inconel X-750 (AMS 5699) is a nickel-chromium alloy specified for wave springs operating in environments exceeding $700^{\circ}F$ ($371^{\circ}C$), where standard stainless steels would undergo rapid stress relaxation. The material is typically precipitation hardened after coiling. For high-temperature service, the #1 Temper (heat treated at $1350^{\circ}F$ for 16 hours) is preferred over the spring temper to optimize creep resistance. In gas turbine seals, Inconel X-750 wave springs maintain their preload despite thermal cycling, preventing bypass leakage. The design must account for a lower Modulus of Elasticity ($E ≈ 31 imes 10^6$ psi) compared to carbon steel, requiring slightly different geometry to match load requirements.

17-7PH (Type 631) stainless steel is a precipitation-hardening alloy that offers an excellent combination of high strength, fatigue resistance, and corrosion resistance. In the CH900 condition, the material is cold-reduced to Condition C and then age-hardened at $900^{\circ}F$ ($482^{\circ}C$) for one hour. This process transforms the martensitic structure and precipitates aluminum-rich intermetallic compounds, raising the tensile strength significantly (up to 240-265 ksi). This high elastic modulus and yield strength allow for thinner cross-sections in wave springs, facilitating miniaturization in medical devices and aerospace sensors while maintaining stable spring rates.

Cryogenic treatment (cooling to $-300^{\circ}F$ using liquid nitrogen) is sometimes employed to ensure the complete transformation of retained austenite into martensite in high-carbon steels. Retained austenite is unstable and can transform over time at room temperature, causing a slight volume expansion. For precision spiral rings in high-accuracy optical or navigation systems, this dimensional drift can be problematic. Cryogenic processing ensures a stable microstructure, maximizing the hardness and ensuring that the ring's free diameter $D_s$ remains constant over the tool's lifecycle.

A286 is an iron-base superalloy that provides high strength and oxidation resistance at temperatures up to $1000^{\circ}F$ ($538^{\circ}C$). It is particularly valuable in the hot sections of gas turbines where standard stainless steels would lose their temper and relax. A286 is hardened by the precipitation of the $\gamma'$ phase during a long aging cycle (e.g., $1325^{\circ}F$ for 16 hours). For spiral rings, A286 offers a stable coefficient of thermal expansion $\alpha$ that closely matches the turbine housing materials, minimizing the change in 'cling' or clearance as the engine transitions from cold-start to full-power temperatures.

Passivation (per ASTM A967) involves treating the ring in a nitric or citric acid bath to remove free iron from the surface and enhance the protective chromium-oxide layer. For 17-7PH, this is typically performed after the CH900 heat treatment. The heat treatment itself creates a light discoloration (oxide scale), which must be removed before passivation to ensure a clean surface. In aerospace, this process is critical because even microscopic iron contaminants can initiate localized galvanic corrosion, leading to pitting and eventual fatigue failure in flight-critical hydraulic actuators.