ASTM B265 Gr2 Gr5 Titanium Bar Medical Implants

ASTM B265 Gr2 Gr5 Titanium Bar Medical Implants

An implant is only as good as the bar it was machined from. A hip stem, a dental fixture or a bone plate starts as titanium round bar, and the grade, the interstitial content and the internal soundness of that bar decide how the finished device behaves after ten years in the body. These bars are Gr2 and Gr5 to ASTM B265, 3 mm to 200 mm diameter, annealed and tested, with the traceability that a medical supply chain requires.

Why Titanium Sits in the Body

Titanium forms a stable oxide film within milliseconds of being exposed to air, and that film does not dissolve in body fluid. The metal releases no ions at a rate that the body reacts to, so tissue grows against it instead of forming a fibrous capsule. Its modulus, about 105 to 114 GPa, is closer to bone than stainless steel or cobalt chrome, which reduces the stress shielding that causes bone to resorb around a stiff implant. Titanium is also non magnetic, so a patient can be scanned after surgery.

Bar Data

Standard Gr5 or ELI

Standard Gr5 is the general purpose alloy. For implants the specification is usually Gr5 ELI, extra low interstitial, where oxygen is held to about 0.13 percent and iron and nitrogen are tightly controlled. Lower interstitials raise ductility and toughness at a small cost in strength, which matters for a component that will be fatigue loaded in a corrosive environment. Where a device is regulated as an implant, the applicable standard is ASTM F136 rather than B265, and the certificate should name it.

Surface, Fatigue and Cleanliness

Fatigue cracks in titanium start at the surface, so bar for implants is turned or ground to remove the alpha case and any tool marks, then polished on the finished part. Cutting fluids, lapping compounds and blasting media must be dedicated to titanium; a fragment of iron embedded in the surface becomes a corrosion pit and a crack starter. Parts are passivated and cleaned to the applicable medical standard, packed in cleanroom bags, and sterilised by the device maker rather than the material supplier.

Traceability Through the Supply Chain

Each bar carries a heat number that links back to the melt, the chemistry and the mechanical test results. A device maker has to be able to show that link years later, so the certificate, the packing list and the marking on the bar all have to agree. Where the bar is cut, the cut piece should be re-marked before it leaves the saw, otherwise the chain breaks at the first operation.

Ordering and Packing

Quote by diameter, length, grade, condition and standard, for example 25 mm Gr5 ELI ASTM F136 annealed, ground finish, 3000 mm random length. Bars are packed in plywood cases with end protection and a document pocket carrying the mill certificate.

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