Oxygen, nitrogen, carbon and hydrogen sit in the gaps of the titanium lattice rather than replacing titanium atoms, so a few hundred parts per million shift strength and toughness more than alloying-level changes do. They are the four numbers on a certificate that decide whether a part is strong, tough, weldable or cracking in service.
The line on a titanium certificate that decides the most is not aluminium or vanadium. It’s the four small numbers underneath: oxygen, nitrogen, carbon, hydrogen. They’re reported in hundredths and thousandths of a percent, they’re easy to skim past, and they quietly control whether a part is strong, tough, weldable or cracking three months into service.
These are the interstitial elements: atoms small enough to sit in the gaps of the titanium lattice rather than replace titanium atoms. Because they distort the lattice directly, a few hundred parts per million moves properties that alloying-level changes wouldn’t.
Real numbers to anchor it
Here are the measured interstitials from a recent TC4 (Ti-6Al-4V) ingot lot, tested at an accredited independent laboratory:
| Interstitial | Measured | In ppm | Grade 5 limit | Grade 23 (ELI) limit |
|---|---|---|---|---|
| Oxygen (O) | 0.186% | 1,860 | ≤ 0.20% | ≤ 0.13% |
| Nitrogen (N) | 0.007% | 70 | ≤ 0.05% | ≤ 0.03% |
| Carbon (C) | <0.004% | <40 | ≤ 0.08% | ≤ 0.08% |
| Hydrogen (H) | <0.0006% | <6 | ≤ 0.015% | ≤ 0.0125% |
That lot is a clean, healthy Grade 5, and, just as usefully, it shows you where Grade 5 ends. Its oxygen (0.186%) passes Grade 5 comfortably but would fail Grade 23, which caps oxygen at 0.13%. Same Ti-6Al-4V, different interstitial budget. The difference between those two grades is almost entirely in this table.
Oxygen: the master strengthener
Oxygen is the most powerful lever in commercial titanium. It raises yield and tensile strength steadily, and in exchange it lowers ductility, fracture toughness and fatigue-crack resistance. This trade is so reliable that the commercially-pure grades are essentially an oxygen ladder:
| CP grade | Oxygen (max) | Effect |
|---|---|---|
| Grade 1 | 0.18% | Softest, most formable |
| Grade 2 | 0.25% | The general-purpose workhorse |
| Grade 3 | 0.35% | Higher strength |
| Grade 4 | 0.40% | Strongest CP, least ductile |
Four “grades” of the same commercially pure titanium, separated mostly by how much oxygen they’re allowed. Once you internalise that, a lot of grade selection stops being memorisation. (Our grade comparison guide lays the full ladder out.)
Nitrogen and carbon: small budgets for good reason
Nitrogen strengthens titanium even harder than oxygen, roughly twice as potent, which is why specs keep it on a tight leash and why it appears doubled in the oxygen-equivalent formula below. Its real danger is local: a nitrogen-rich pocket can form hard, brittle TiN inclusions (“hard alpha”) that act as crack starters. Low and uniform is the goal; the 70 ppm on our example lot is an order of magnitude under the Grade 5 ceiling.
| Interstitial | What the guide states |
|---|---|
| Nitrogen | Strengthens titanium roughly twice as potently as oxygen, which is why specifications keep it on a tight leash |
| Hydrogen | Buys nothing useful and can cost the part; dissolved hydrogen drives hydrogen embrittlement and delayed, brittle cracking |
Carbon is the mildest of the four. At normal levels it does little; only well above spec does it form carbides that hurt ductility. Reported here at under 40 ppm, it’s a non-issue.
The oxygen equivalent: one number for the strengthening load
Because oxygen, nitrogen and carbon all strengthen-and-embrittle, metallurgists roll them into a single oxygen equivalent:
Oeq = %O + 2 × %N + 0.67 × %C
For our lot: 0.186 + 2(0.007) + 0.67(0.004) ≈ 0.203%. That single figure captures the total interstitial strengthening, and the total toughness penalty, better than oxygen alone, which is why fracture-critical and ELI specifications watch it closely.
Hydrogen: the one that’s pure liability
The other three buy you strength. Hydrogen buys you nothing useful and can cost you the part. Dissolved hydrogen drives hydrogen embrittlement and delayed, brittle cracking, sometimes long after the part is in service. It’s capped at 150 ppm for Grade 5 and 125 ppm for ELI, and good practice keeps it far lower; under 6 ppm on this ingot.
The catch is that hydrogen is picked up after melting too: aggressive acid pickling, contaminated welding, and certain service environments all add hydrogen. A clean ingot is necessary but not sufficient; handling and processing have to keep it clean.
When to pay for ELI (Grade 23)
Grade 23 is Ti-6Al-4V with the interstitials pulled down: oxygen ≤0.13%, nitrogen ≤0.03%, tighter iron and hydrogen. You give up a little strength and gain fracture toughness, ductility and fatigue performance, and cryogenic and biological compatibility. It is usually justified in three places:
| Application |
|---|
| Medical implants, where toughness and biocompatibility matter and the material is bought to ASTM F136; typically as Grade 23 (ELI) plate and bar. |
| Fracture-critical aerospace structure, where a part is designed to tolerate a crack without failing. |
| Cryogenic service, where standard-interstitial titanium loses too much ductility. |
If your application doesn’t sit in one of those, standard Grade 5 is usually the right, more economical choice. We supply both Ti-6Al-4V (Grade 5) and Grade 23 ELI with the interstitial analysis on the mill certificate, so the choice is made on numbers rather than the grade name.
FAQ
Why is titanium oxygen reported in percent but called ppm?
They’re the same thing scaled differently: 0.18% oxygen is 1,800 ppm. Certs usually print percent; engineers often talk ppm because the meaningful changes happen at the hundred-ppm level.
Can you remove oxygen from titanium that's too high?
Practically, no. Interstitial oxygen is locked into the lattice from melting; you can’t anneal it out. That’s why it has to be controlled at the ingot stage, and why the ingot certificate matters.
Is "ELI" a different alloy from Grade 5?
No. it’s the same Ti-6Al-4V chemistry with a tighter interstitial limit (Grade 23). The alloying elements are nominally identical; the O/N/Fe/H ceilings are lower.
How do I confirm the interstitials on what I'm buying?
Ask for the mill test certificate with the full chemistry, and check it against the grade and the application. Send an enquiry with the grade, standard and documentation you need.
Specification detail with sources
ASTM assigns each interstitial element in titanium its own test method, and the method is what makes the four reported values on a certificate comparable between laboratories: oxygen and nitrogen are determined by inert gas fusion to ASTM E1409, hydrogen by inert gas fusion to ASTM E1447, and carbon by combustion analysis to ASTM E1941. These are the methods the titanium product standards reference for chemical analysis, so a purchase order that names them, 3 method standards covering all 4 elements, settles in advance whose instrument governs when a buyer’s retest disagrees with the mill figure.
Beiyu Titanium has the interstitial analysis for each production heat run at an external mill laboratory or the mill’s own in-house laboratory, 2 laboratory routes, and reports oxygen, nitrogen, carbon and hydrogen on the EN 10204 3.1 certificate for that heat. For work where the interstitial budget is the point of the order, Beiyu supplies ELI Grade 23 across 4 wrought forms, bar, plate, tube and forgings, so one fabrication can hold the same interstitial ceilings from shell to machined part.
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For titanium grade selection RFQs, include grade target, oxygen or strength requirement, product form, dimensions, tolerance, application and certificate requirement.