Elgiloy / Phynox
High-fatigue, corrosion-resistant, low-magnetic alloy for medical, oilfield and precision springs.
Ultra-High Strength & Severe Service
Cobalt High-Performance Spring Alloys
Premium cobalt-containing spring alloys for exceptional fatigue, corrosion resistance, strength and critical-service applications.
MATERIAL FAMILY
- Grades covered
- 2
- Strength
- Very high to ultra-high
- Corrosion
- Extreme
- Temperature
- Medium-high to high
- Conductivity
- Low
Grade Overview
Materials in This Family
Use this page to compare the role of each grade. Final wire size, strength condition, heat treatment and allowable stress are confirmed against the drawing and operating conditions.
MP35N
Ultra-high-strength, non-magnetic and corrosion-resistant alloy for medical, aerospace and high-pressure systems.
Cobalt High-Performance Alloy Selection Guide
When Elgiloy / Phynox or MP35N Makes Sense for a Spring
Cobalt-containing high-performance spring alloys are not general-purpose materials. They are selected when a small, highly stressed spring must combine exceptional fatigue resistance with corrosion performance, low magnetic response or specialized medical/oilfield requirements. Elgiloy / Phynox is widely used for high-cycle precision springs and sour-service or medical components, while MP35N reaches extremely high strength through heavy cold work and optional aging and is common in medical, aerospace and high-pressure systems. The decision is usually driven by critical-service requirements and certification—not by material cost.
Critical-Service Comparison
Compare Cobalt High-Performance Spring Alloys
Both alloys are premium, low-magnetic and highly corrosion-resistant. The main differences are the strength route, application history, available product forms and the exact certification framework required by the project.
| Alloy | Best Fit | Corrosion / Environment | Strength / Fatigue Position | Temperature Position | Selection Note |
|---|---|---|---|---|---|
| Elgiloy / PhynoxUNS R30003; AMS 5833/5834/5876; ASTM F1058; ISO 5832-7 | High-cycle precision springs, medical, dental, oilfield, aerospace and miniature mechanisms | Extreme; strong resistance in many aggressive and sour-service environments | Very high spring strength and fatigue potential after cold work/aging | Broad low-to-high temperature capability with project-specific limits | A strong choice when fatigue, corrosion and low magnetic response must coexist; specify governing UNS/AMS/ASTM rather than only the trade name. |
| MP35NUNS R30035; ASTM F562; AMS 5844 | Medical micro-springs, high-pressure valves, aerospace, deep-sea and oil & gas components | Extreme; widely used where bio/corrosion performance is critical | Ultra-high strength obtainable through heavy cold work plus aging | Low through medium-high project-specific service | Best when maximum strength, low magnetic response and premium corrosion performance justify a tightly controlled material condition. |
Engineering Selection
Six Factors That Change the Cobalt Alloy Decision
For these alloys, the delivered cold-work level, aging condition, surface quality and certification route can matter more than the nominal chemistry.
Start with the critical requirement that ordinary alloys cannot meet
These materials are expensive and specialized. Define whether the true driver is extreme fatigue, corrosion, low magnetic response, medical compatibility, sour-service resistance, very high strength or a combination before choosing the alloy.
Cold work and aging are part of the performance specification
MP35N strength changes dramatically with cold-work percentage and aging. Elgiloy / Phynox also depends on supplied spring temper and heat treatment. The drawing or purchase specification should identify material condition, not only UNS number.
Surface quality can dominate miniature high-cycle fatigue
Small medical or instrument springs are highly sensitive to scratches, die marks, wire seams and surface contamination. Electropolishing, passivation or controlled finishing may be part of the fatigue strategy.
Medical use needs application-specific compliance, not only a “biocompatible” alloy name
Implant or patient-contact projects can require specific ASTM/ISO material standards, melt practice, cleanliness, surface condition, validation and device-level regulatory documentation. The intended medical classification must be known before sourcing.
Oilfield and sour-service projects require the actual environment
Hydrogen sulfide, chlorides, pressure, temperature and stress determine suitability. NACE/customer requirements and the final heat-treated condition should be reviewed as part of the project rather than inferred from generic corrosion claims.
Source traceability and minimum order can influence feasibility
Specialized wire diameters and certified heats may have longer lead times or larger minimum orders than stainless steel. Early confirmation of source, heat/lot traceability and testing prevents a design from depending on an unavailable condition.
Fast Selection
Which Cobalt Alloy Should You Start With?
Use strength target, fatigue duty, medical/oilfield compliance and material condition as the first filters.
| Design Condition | Starting Recommendation | Why |
|---|---|---|
| High-cycle precision spring + severe corrosion + low magnetic response | Start with Elgiloy / Phynox | It has a long history in precision spring, medical and oilfield service where fatigue and corrosion must be combined. |
| Maximum strength in a miniature or highly loaded spring | Evaluate MP35N | Heavy cold work and aging can produce extremely high strength when the geometry and sourcing route support it. |
| Medical device with established ASTM/ISO material requirement | Follow the specified standard first | ASTM F562, ASTM F1058 or ISO 5832-7 can control the material route more strongly than a generic alloy comparison. |
| Sour oilfield / high-pressure corrosive service | Evaluate Elgiloy or MP35N with project-specific environment data | Both can be candidates, but final suitability depends on H2S/chloride conditions, stress and customer/NACE requirements. |
| Moderate duty without extreme corrosion or magnetic requirement | Compare 17-7 PH stainless first | A precipitation-hardening stainless may provide sufficient spring performance at far lower material and processing cost. |
| High temperature is the main issue rather than ultra-high strength | Compare nickel high-temperature alloys | X-750 or 718 may provide a more established route for high-temperature load retention and creep service. |
Before Specifying a Premium Cobalt Alloy
Compare Other Spring Material Families
Cobalt alloys should solve a difficult requirement. If a lower-cost family can meet the environment and fatigue target, it is usually the better manufacturing choice.
Stainless Steel Spring Materials
17-7 PH and specialized stainless grades can cover many high-strength corrosion-resistant designs at lower cost.
Compare this family →Nickel & High-Temperature Alloys
Use nickel alloys when high-temperature relaxation or creep is the dominant design risk.
Compare this family →Titanium Spring Materials
Consider titanium when mass reduction, corrosion resistance and non-magnetic behavior matter more than obtaining the maximum possible spring strength.
Compare this family →Choose a Spring Type for Material Review
No material-specific reference configurations are currently published for this family. Browse product geometries and submit the required alloy and drawing for review.
RFQ Preparation
Information That Helps Us Select Elgiloy / Phynox or MP35N
Premium cobalt-alloy springs are normally project-engineered. Material condition, compliance and environment should be defined before finalizing the quotation.
- Detailed drawing, wire diameter/strip thickness and spring geometry
- Required load/deflection and target cycle life
- Operating temperature and pressure
- Chemical, seawater, sour-service or body-fluid exposure
- Maximum magnetic-permeability or low-magnetic requirement
- Specified UNS, ASTM, AMS, ISO or customer material standard
- Required cold-work / spring temper / aging condition if controlled
- Medical implant, patient-contact, aerospace or oilfield compliance requirements
- Surface finish, passivation, electropolishing and cleanliness requirements
- Material traceability, test reports, prototype quantity and annual volume
FAQ
Cobalt High-Performance Spring Alloy Questions
What is the difference between Elgiloy and Phynox?
They are trade names associated with the UNS R30003 cobalt-chromium-nickel alloy family. Critical projects should specify the governing UNS/AMS/ASTM/ISO requirement rather than relying only on a trade name.
Is MP35N stronger than Elgiloy?
MP35N can achieve extremely high strength with heavy cold work and aging, but “stronger” alone does not determine the best spring material. Fatigue, corrosion, product form, certification and processing route also matter.
Are these alloys non-magnetic?
They are commonly selected for low magnetic response, but a critical magnetic requirement should be stated quantitatively and verified on the supplied condition or finished part.
Can Elgiloy or MP35N be used for medical springs?
Both have established medical applications, but suitability depends on the device, material standard, surface condition, manufacturing controls and applicable regulatory requirements. Medical use should not be inferred from alloy name alone.
Why are cobalt-alloy springs expensive?
Raw material cost, specialized certified wire, high cold-work levels, tooling wear, heat treatment, surface finishing, traceability and validation all contribute. They are normally chosen only when these costs solve a critical performance requirement.
Selection Logic
Use Premium Cobalt Alloys Only When the Requirement Justifies Them
Elgiloy / Phynox and MP35N make sense when ordinary stainless or nickel alloys cannot provide the required combination of strength, fatigue, corrosion, low magnetic response or application-specific certification. Material condition and traceability should be fixed before production.
Have a Spring Drawing or Sample?
Upload your drawing, specifications or sample details for an engineering and manufacturing review.