Materiale & Verwerking

Traditional oil quenching and tempering of high-carbon steel creates a martensitic microstructure which is highly susceptible to hydrogen embrittlement, especially during electroplating. Austempering involves an isothermal transformation to bainite. The resulting lower bainite structure offers a superior combination of ductility and toughness at high hardness levels (HRC 45-50). Because bainite is less sensitive to the interstitial hydrogen pressure that causes 'delayed fracture,' austempered wave springs exhibit a much lower failure rate in applications where hydrogen is introduced during acid cleaning or zinc plating processes. However, a post-plating bake (e.g., 190°C for 4-24 hours) remains mandatory.

17-7PH CH900 (Condition C, then precipitation hardened at 900F) is an excellent general-purpose material with high tensile strength ($≈$ 1380 MPa), but its maximum operating temperature is limited to approximately 340°C (650°F). Above this, it suffers from rapid stress relaxation. For subsea valve actuators at 300°C, 17-7PH is near its limit. Inconel X-750 (Ni-Cr alloy) is the superior choice for high-temperature stability up to 700°C. It maintains its elastic modulus $E$ and resists creep far better. While X-750 is more expensive and harder to form, its resistance to chloride-induced stress corrosion cracking (SCC) makes it the standard for high-reliability subsea oil and gas environments.

Material selection involves a balance between environmental corrosion and mechanical load limits.

Carbon Steel (SAE 1070-1090):
- Usage: Standard industrial assemblies with oil or grease protection.
- Tradeoff: Extremely high yield strength and thrust capacity. Susceptible to rapid oxidation and rust in humid or chemical environments.

316 Stainless Steel:
- Usage: Marine, chemical processing, medical, and high-temperature environments.
- Chemical Resistance: Elevated nickel content and the addition of Molybdenum ($2-3\%$) provide high resistance to chloride pitting, crevice corrosion, and organic acids.
- Strength Tradeoff: 316 stainless steel is non-hardenable by heat treatment (unlike 17-7PH) and relies purely on cold working for its mechanical strength. It has a significantly lower yield strength than carbon steel (approximately $35-45\%$ lower thrust capacity), meaning a thicker ring or deeper groove must be designed to achieve equivalent safety margins.

Standard coiled compression springs use round wire, whereas premium wave springs utilize flat wire rolled precisely from high-quality round wire.

Engineering Merits of Flat Wire Rolling:
1. Uniform Cross-Section: Precision rolling mills control the width-to-thickness ratio with tolerances tighter than $\pm 0.005\text{ mm}$. Any variation in material thickness ($t$) has a cubic effect ($t^3$) on the spring rate ($k$);
$$\Delta k \approx 3 \cdot \frac{\Delta t}{t}$$
Therefore, absolute thickness consistency is paramount.
2. Grain Orientation: Rolling aligns the metal grain structure along the longitudinal axis of the flat wire, maximizing the active tensile and compressive stresses the wire can handle when coiled into waves.
3. No Dynamic Twist: The rectangular cross-section prevents the wire from twisting out-of-plane during coiling, ensuring the wave crests remain perfectly parallel to the mating surfaces, maintaining uniform axial loading.

Aerospace and petrochemical applications demand accurate Spring Rates under high operating temperatures.

17-7 PH Stainless Steel Limits:
- Maximum Operating Temp: $650^\circ F$ ($343^\circ C$)
- Performance Characteristics: High load-bearing capacity, cost-effective, but experiences severe stress relaxation (load-loss) above $650^\circ F$ due to thermal micro-structural creep.

Inconel X-750 (Nickel-Chromium Alloy) Limits:
- Maximum Operating Temp: $1300^\circ F$ ($704^\circ C$)
- Heat Treatment: Precipitation hardened via solution annealing and age hardening to optimize creep-rupture strength.
- Performance: Possesses exceptional resistance to oxidation and creep. At temperatures between $650^\circ F$ and $1000^\circ F$, Inconel X-750 maintains its elastic modulus ($E_t = E_0 \cdot [1 - \alpha \Delta T]$) far superior to standard stainless steels, experiencing less than 5% load loss over extended cyclic exposure.

17-7 PH (AISI 631) is a semi-austenitic precipitation-hardening stainless steel widely chosen for its high fatigue strength and corrosion resistance.

Processing and Metallurgy Sequence:
1. Cold Winding (Condition C): Flat wire is rolled and coiled cold from the annealed condition. Cold reduction achieves severe deformation, transforming the austenite matrix into high-strength cold-worked martensite.
2. Heat Treatment / Aging (Condition CH900): After forming the wave spring, the parts are subjected to thermal aging at $900^\circ F$ ($482^\circ C$) for 1 hour, followed by air cooling.
3. Precipitation Hardening: At this aging temperature, fine sub-microscopic intermetallic compounds of Aluminum ($Ni_3Al$) precipitate within the martensitic matrix. This restricts dislocation movement and raises the yield strength up to $1700\text{ MPa}$ and tensile strength up to $1900\text{ MPa}$.
4. Dimensional Stability: The CH900 treatment stress-relieves the cold-coiled spring while optimizing fatigue limits and mitigating dimensional drift in high-load operating cycles.

The commonly used materials for Multi Turn Wave Springs (Multi-Turn Wave Springs) mainly include spring steel and stainless steel. Specifically, for spring steel, grades like 72A, 65Mn, 60Si2Mn, 50CrVA, etc. are often used. While for stainless steel, SS304, SS316, 17-7PH, and other grades are commonly employed. These materials possess excellent elasticity and toughness, meeting the requirements of various applications.

Additionally, depending on specific application needs, other materials such as high-temperature alloys like Inconel 718, Inconel X-750, etc. may also be used to adapt to high-temperature environments or special working conditions.