Orthopaedic Cobalt-Chrome Alloys
The implant cobalt alloys are Co-28Cr-6Mo in two forms — F75 investment-cast and F1537 wrought — plus the Co-Cr-Ni-Mo spring alloys MP35N, Elgiloy and Phynox and the Co-Cr-W-Ni wire alloy F90. Cast F75 yields about 450 MPa; hot-worked F1537 reaches 700–830 MPa with 12 % elongation, and the cold-worked and aged spring alloys exceed 1,800 MPa tensile. The chromium oxide film makes every one of them inert in body fluid, and the carbide-hardened matrix (30–45 HRC) wears an order of magnitude slower than titanium against polyethylene.
Popular orthopaedic Co-Cr grades
Compare in the explorer| Grade | Yield, MPa | Tensile, MPa | Density, g/cm³ | Machinability | Weldability | Best for |
|---|---|---|---|---|---|---|
| F75 | 510 | 720 | 8.3 | Not recommended | Poor | Cast and printed joint components, dental frames |
| F90 | 460 | 1,000 | 9.13 | Poor | Very good | Wire, tube and cold-formed sheet |
| Elgiloy | 2,000 | 2,200 | 8.3 | Not recommended | Poor | Watch mainsprings and guidewires |
| Phynox | 1,850 | 2,050 | 8.3 | Not recommended | Poor | Stent and spring wire, European drawings |
| F1537 Alloy 2 | 950 | 1,280 | 8.3 | Not recommended | Poor | Strongest F1537 alloy |
| F1537 Alloy 3 | 930 | 1,280 | 8.3 | Not recommended | Poor | Dispersion-strengthened implant grade |
| F1537 Alloy 1 | 610 | 1,000 | 8.3 | Not recommended | Good | Most weldable F1537 alloy |
All 7 orthopaedic Co-Cr grades
Choose orthopaedic Co-Cr for
Metal bearing surfaces in hip and knee joints, dental crowns, bridges and partial-denture frameworks, spinal rods and trauma plates that need more strength than titanium, pacemaker leads and stent wire, and any implant part that must be hard, polishable to a mirror and inert for decades.
Look elsewhere when
The part bonds to bone — titanium Grade 23 has a modulus closer to bone and half the weight; the patient is cobalt- or nickel-sensitive (ceramic or titanium); the hardware is a screw or plate that is cheaper in 316LVM; or a hard wear surface is wanted outside the body, where Stellite 6 does the job without the medical pedigree.
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Orthopaedic Co-Cr by application
- Joint bearing surfaces
- F75 cast or printed for knee condyles and hip heads; ASTM F1537 Alloy 2 when the surface is machined from wrought bar. Both polish below Ra 0.05 µm and hold it against polyethylene.
- Stems, rods and plates
- ASTM F1537 Alloy 1 forged or machined from bar: 700–830 MPa yield with the fatigue strength titanium cannot match in a thin section.
- Wire, leads and springs
- F90 for formed wire and tubing; MP35N, Elgiloy and Phynox cold-drawn and aged for pacing conductors, guidewires and stents at 1,800 MPa or more.
Working with orthopaedic Co-Cr
- Machining
- Cutting speeds run at 10–20 % of stainless with rigid setups, sharp coated carbide and a heavy constant feed — the alloy work-hardens the instant a tool dwells. Machining cost usually exceeds material cost, so cast or print near-net shape and machine only the fits and bearing faces.
- Casting and printing
- F75 is investment-cast or laser powder-bed printed and then HIPed to close porosity, solution-treated and ground. Printing is now the routine route for patient-specific implants and dental frameworks.
- Heat treatment
- F75 castings are solution-annealed or HIPed; F1537 is used hot-worked, warm-worked or annealed with no further hardening. MP35N, Elgiloy and Phynox gain their strength from 40–60 % cold reduction followed by ageing at about 540 °C.
- Finishing
- Bearing surfaces are polished to a mirror and passivated in nitric or citric acid; dental frameworks are electropolished and porcelain-bonded. No coating is needed for corrosion.
Other cobalt-chrome alloy series
Orthopaedic Cobalt-Chrome Alloys — frequently asked
What is orthopaedic Co-Cr?
The implant cobalt alloys are Co-28Cr-6Mo in two forms — F75 investment-cast and F1537 wrought — plus the Co-Cr-Ni-Mo spring alloys MP35N, Elgiloy and Phynox and the Co-Cr-W-Ni wire alloy F90. Cast F75 yields about 450 MPa; hot-worked F1537 reaches 700–830 MPa with 12 % elongation, and the cold-worked and aged spring alloys exceed 1,800 MPa tensile. The chromium oxide film makes every one of them inert in body fluid, and the carbide-hardened matrix (30–45 HRC) wears an order of magnitude slower than titanium against polyethylene.
Which grade of orthopaedic Co-Cr should I use?
F75 for anything investment-cast or laser-printed to shape: femoral knee components, hip heads, dental frameworks. ASTM F1537 Alloy 1 (low carbon) for machined and forged stems, plates and rods where fatigue strength governs; ASTM F1537 Alloy 2 (high carbon) for bearing surfaces that need the extra carbide wear resistance; ASTM F1537 Alloy 3 is the dispersion-strengthened variant for the highest strength. F90 for wire, tubing and thin sheet that will be cold-formed. MP35N for pacing leads, springs and fasteners that must survive hundreds of millions of cycles; Elgiloy and Phynox are the same class of alloy in watch-spring and guidewire form.
Which grade of orthopaedic Co-Cr is the strongest?
Elgiloy has the highest yield strength in the series at 2,000 MPa; Phynox follows at 1,850 MPa. Values are for each grade's default condition.
Which grade of orthopaedic Co-Cr is easiest to machine?
F90 (machinability poor, 16/100), then F1537 Alloy 1 (14/100).
F75 or F1537?
Same Co-28Cr-6Mo chemistry, different route. F75 is the casting (and printing) grade: it takes any shape and yields about 450 MPa. F1537 is the wrought bar: hot-worked material yields 700 MPa or more with better fatigue life, so it is machined or forged into stems, rods and plates. Where the shape is complex, cast F75; where the loads are high and the shape is simple, machine F1537.
What is MP35N and why is it in the implant group?
A 35 % cobalt, 35 % nickel, 20 % chromium, 10 % molybdenum alloy that is cold-worked and aged to 1,800–2,000 MPa tensile — the strongest corrosion-resistant metal in common use. It is non-magnetic, immune to chloride cracking and biocompatible, which is why pacemaker leads, orthopaedic cables and aerospace fasteners use it. Elgiloy and Phynox are its Co-Cr-Ni-Fe cousins, made as spring wire and strip.