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Ti Grade 4 / UNS R50700 / AMS 4901 / AMS 4921

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Ti Grade 4 — commercially pure titanium, high oxygen.

Properties: The commercially pure titanium grades have an excellent strength-to-density ratio and good corrosion resistance. These properties make commercially pure titanium suitable for the manufacture of components in weight-saving structures carrying low mass forces, and also for components requiring high corrosion resistance. In addition, because of the low thermal expansion of titanium, thermal stresses in titanium structures are lower than in other metallic materials. Together with this property, titanium components are increasingly becoming the material of choice in a widening range of sectors. The materials are also widely used in the medical sector because of their exceptional biocompatibility.​‌​​‌​

Application areas: chemical industry, aerospace industry, medical applications.

Machinability: Because Ti Grade 4 offers more mechanical strength and hardness than Ti Grade 2, it can be more difficult to machine. It can nevertheless be machined with suitable process conditions and techniques.​‌​​‌​

Machining: It can be processed by operations such as milling, turning and drilling. Because of the hardness and high melting point of titanium, machining at low speed is recommended. Cutting tools should generally be a hard alloy or carbide, and cutting fluids should be used to prevent the titanium from overheating.

Welding: It can be welded by the TIG (Tungsten Inert Gas) and MIG (Metal Inert Gas) methods. Using shielding gases such as argon during welding prevents oxidation.​‌​​‌​

Cold forming: It is suitable for cold forming operations and can be shaped by processes such as plasma cutting and bending.

Hot forming: It is also suitable for hot forming, but oxidation of the titanium must be prevented at high temperatures and carefully monitored.​‌​​‌​

What Titanium Grade 4 Is — and Why “Commercially Pure” Is a Misleading Phrase​‌​​‌​

Titanium Grade 4 (UNS R50700 / W.Nr. 3.7065 / DIN designation Ti 4, often sold as Ti99.5) is the strongest of the four unalloyed (commercially pure, CP) titanium grades. And there is only one thing you need to understand about it: Grades 1, 2, 3 and 4 are the same metal. None of them contains an alloying addition — no chromium, no nickel, no aluminium, no vanadium. The only difference between them is the deliberately different ceiling placed on the interstitial elements: oxygen, iron, nitrogen and carbon.

Put differently, the alloying element in Grade 4 is oxygen. Oxygen occupies interstitial sites in the HCP (α) titanium lattice, locks slip planes and raises strength — at the cost of ductility, fracture toughness and cold formability. The oxygen ceiling in Grade 1 is 0.18 %; in Grade 4 it is 0.40 %. That single change lifts the minimum tensile strength from 240 MPa to 550 MPa, roughly 2.3×. The identity of the metal does not change and its corrosion behaviour is effectively unchanged; only the mechanical behaviour moves.​‌​​‌​

The commercial consequence in one sentence: Grade 4 is CP titanium that delivers about 1.75× the strength of Grade 2 while giving up nothing in corrosion resistance — and it pays for that in ductility, formability, supply chain and available product forms. That last item is the most valuable information on this page and almost no distributor sheet prints it: Grade 4 has no pipe specification, no tube specification and no casting specification.

Rp0.2 also has an UPPER limit — and almost nobody prints it

ASTM B265 and B348 define the yield strength of Grade 4 as a BAND of 483–655 MPa. The lower bound is what you expect; the upper bound is the line most datasheets quietly drop. The reason follows directly from the logic above: what pushes yield up is oxygen, and a very high yield means unacceptably low toughness and formability. A heat measuring Rp0.2 = 690 MPa is not “better” — it is out of specification and must be rejected. On the buying side this is one of the rare cases where the instinct “higher numbers are good” works against you.​‌​​‌​

The same logic applies to cold work. Grade 4 can be cold drawn above 950 MPa, but that is not the annealed specification condition and cannot be compared with ASTM B265 / B348 minimums. Cold-worked Grade 4 wire or bar is bought to a company specification — see the “forms with no standard” section below.

Standards by Product Form​‌​​‌​

The Grade 4 standards map is an abridged version of the Grade 2 map — and the missing entries directly affect purchasing. The table below follows current ASTM scope texts; rows where the grade is absent from a specification are marked explicitly.​‌​​‌​

A subtlety worth knowing about AMS 4901: AMS does not name the grade the way ASTM does. It names it by guaranteed yield strength — the formal title of AMS 4901 is “Titanium Sheet, Strip and Plate, Commercially Pure, Annealed, 70.0 ksi (485 MPa) Yield Strength”. The same logic gives AMS-T-9047 CP-70 on the bar side. So the words “Grade 4” may never appear on an AMS certificate, and the buyer wrongly concludes the material is something else. It is the same metal under a different naming system.​‌​​‌​

ASME Code Acceptance and Maximum Code Temperatures

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Two separate questions must not be conflated here. First: “has ASME adopted the material specification?” Second: “has ASME published a design stress for this grade?” The answer to the first is yes, to the second unverified — and in code work it is the second that decides.

Non-code use: outside the pressure codes Grade 4 is used widely and without difficulty. The limitation is administrative, not metallurgical — present it as a listing gap, not a material defect.

Product Forms With NO Standard — the Commercially Valuable Section​‌​​‌​

This is the section your sales engineers should memorise. Grade 4 enquiries most often stall here, and an honest answer is worth far more than a lost order.

These gaps all come from one commercial fact: Grade 4 belongs to the strength market, not the corrosion market — and there Ti-6Al-4V and Ti-3Al-2.5V already offer better answers. The real market for Grade 4 is sheet, bar, wire, forgings and implants.

Chemical Composition​‌​​‌​

ASTM B265 / B348 / B381 / B863, Grade 4 (R50700), weight %: N ≤0.05 · C ≤0.08 · H ≤0.015 · Fe ≤0.50 · O ≤0.40 · residuals 0.1 each, 0.4 total · Ti balance. There are no alloying additions.

Mechanical Properties​‌​​‌​

Physical Properties

Heat Treatment and Thermal Stability

Start with the most important negative fact: Grade 4 cannot be hardened by heat treatment. Unalloyed titanium has no precipitation hardening, takes no benefit from martensitic transformation, and there is no solution-treat-and-age (STA) route. Strength comes from only two places: interstitial content (grade selection) and cold work. If a datasheet prints “solution treated and aged” for Grade 4, that sheet has put Ti-6Al-4V data in the wrong place.​‌​​‌​

The good news on thermal stability: Grade 4 has no damaging phase window. There is no sigma phase, no ordering embrittlement, no 475 °C embrittlement, no carbide sensitisation. The thermal risk in titanium is not phase transformation but contamination — pickup of oxygen, nitrogen and hydrogen. That is why everything discussed in titanium heat treatment is furnace atmosphere and cleanliness.​‌​​‌​

Welding

What governs titanium welding is not technique but environment. Molten and hot titanium absorbs oxygen, nitrogen and hydrogen from air without saturating, and every interstitial picked up embrittles the weld irreversibly. Grade 4 is the most sensitive grade of the family in this respect, because its chemistry already sits at the top of the interstitial ceiling: every extra ppm of oxygen eats into an already narrow ductility margin.​‌​​‌​

Post-weld strength: a Grade 4 weld approaches base-metal strength, but grain growth and reduced HAZ ductility are normal. An ERTi-2 weld is deliberately softer than the base metal — a considered choice, not a defect.​‌​​‌​

Machining

Titanium is not difficult because it is hard; it is difficult because it does not carry heat away and because it is chemically active. Thermal conductivity is about a third of steel — nearly all the heat generated in the cutting zone stays at the tool tip instead of in the chip. Hot titanium also reacts with carbide and wears the tool chemically. On top of that comes the low modulus: the workpiece springs away from the tool.​‌​​‌​

CP titanium machines more easily than Ti-6Al-4V but presents a different problem: it is gummy. Grade 4 is the best machining CP grade because its higher strength gives cleaner chip breaking — Grades 1 and 2 are stickier.

Corrosion — Where It Excels, and WHERE IT FAILS

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First, the single most important sentence: the corrosion resistance of Grade 4 is effectively identical to Grades 1, 2 and 3. Interstitial content changes mechanical behaviour; it does not change the passive film. So everything below applies to all CP titanium grades. The claim “Grade 4 is stronger so it corrodes less” is false and must not be made in a sales conversation.​‌​​‌​

Titanium’s resistance comes from a spontaneously formed TiO₂ passive film that repairs itself within seconds in the presence of ppm levels of oxygen or moisture. Every strength and every weakness of titanium reduces to one question: does the environment let that film re-form?

Where it excels​‌​​‌​

Seawater: negligible corrosion to 260 °C (500 °F). No pitting, no attack even under biofouling. A condenser tube exposed for 16 years to polluted seawater was slightly discoloured with no evidence of corrosion.
Erosion-corrosion: outstanding — it withstands flow velocities of 30 m/s, decisive in the pump and condenser duties that eliminate copper alloys and stainless steels.
Oxidising environments: nitric acid, oxidising chlorides, chlorinated water, hypochlorite, chlorine dioxide, chlorate, perchlorate — full resistance. Fatigue: titanium suffers no significant loss of fatigue strength in seawater.

WHERE IT FAILS — this section is mandatory​‌​​‌​

The honest summary your page should carry: titanium is the material of oxidising and chloride-bearing environments, and there it is unmatched. It is not the material of reducing acids, fluoride or anhydrous environments. Those two sentences prevent the great majority of titanium sales errors.​‌​​‌​

Frequently Asked Questions

We use Grade 2. Does moving to Grade 4 make sense?​‌​​‌​

There is one honest criterion: is the part governed by thickness or by corrosion?
If strength sets the wall thickness, Grade 4 delivers a real gain: yield rises from 275 MPa to 483 MPa, i.e. 75 % higher. You can reduce section, cut weight and sometimes drop a size step. For load-bearing implants, diving and defence hardware, fasteners and structural brackets the switch makes sense.
If corrosion allowance, expandability or forming sets the thickness, the switch is pointless and harmful. The corrosion resistance of the two grades is identical — Grade 4 does not last longer in any environment. The prices you pay are real: elongation drops from 20 % to 15 %, bend radii grow, deep drawing gets harder, and the margin for error in welding narrows.
And the argument is usually settled by supply: Grade 2 is in stock worldwide in every form; Grade 4 is a niche material with long lead times, a higher price per kilogram, and no pipe, tube or casting forms at all. Choose Grade 4 when strength is genuinely required and the form is sheet, bar, wire or forging. If you are on the fence, do not skip Grade 3: it is often the compromise you actually want and it is easier to find.

I need Grade 4 pipe or heat-exchanger tube. Which specification do I order to?​‌​​‌​

Short answer: there isn’t one. And saying so is far better than taking an order you cannot fill.
None of the ASTM specifications for seamless pipe (B861), welded pipe (B862) or heat-exchanger tube (B338) lists Grade 4 in its grade table. B861 enumerates thirty-four grades, B862 thirty-three, B338 twenty-eight — Grade 4 is in none of the three. Some older cross-reference tables print “B338 Gr 4”; the current scope texts do not support it.
There are four real options. (1) If the duty is corrosion-driven — which for a heat exchanger it almost always is — move to Grade 2 tube; the corrosion resistance is identical, expandability is far better, and price and delivery are not comparable. (2) If pressure is genuinely high, look at Grade 9 (Ti-3Al-2.5V): that alloy exists precisely for the “high-strength titanium pipe” requirement and is within the B861/B862 scope. (3) For small quantities, have bored/machined pipe made from B348 Gr 4 bar — chemistry conforms to B348, tolerances and mechanicals are by agreement, and the certificate cannot say “B861”. (4) If strength is the requirement and corrosion is secondary, Ti Grade 5 ELI pipe is within the B861/B862 scope.
Never write “ASTM B861 Gr 4” in a written quotation. No such line exists and it will be rejected at third-party inspection.

Is Grade 4 good enough for an implant, or should I buy Ti-6Al-4V ELI?​‌​​‌​

Both are implant materials and the choice depends on what you are making — there is no “better” one.
What speaks for Grade 4 is that it is unalloyed. It contains no aluminium and no vanadium. The long-running debate about vanadium release is the main line of criticism aimed at Ti-6Al-4V ELI; Grade 4 sits entirely outside it. The specification route is clean too: ASTM F67 and ISO 5832-2. Dental implant roots, bone screws and plates are Grade 4’s largest market — and in dental implants Grade 4 is effectively the standard.
What speaks for Ti-6Al-4V ELI is mechanical. Yield 759 MPa (483 MPa for Grade 4), tensile 828 MPa (550 MPa), and — the real point — markedly better fracture toughness and fatigue behaviour. For load-bearing implants that will see millions of cycles over years — hip stems, spinal instrumentation, intramedullary nails — the choice is usually ELI, via ASTM F136 and ISO 5832-3.
The practical dividing line: if the section can be generous and metallurgical simplicity is wanted, Grade 4; if section is constrained and cyclic load is high, Grade 23 (ELI). Do not position Grade 4 as “the cheap implant titanium” — it is a different engineering choice, not a lower one.

Is Grade 4 really harder to weld than Grade 2?​‌​​‌​

The process is the same; the margin for error is narrower.
Both grades are welded with the same equipment, the same gas and the same process (GTAW). Neither needs preheat or a special interpass regime. The difference is chemical margin: the oxygen ceiling of Grade 2 is 0.25 %, that of Grade 4 is 0.40 %. Grade 4 therefore starts at the top of its interstitial budget; every additional oxygen and nitrogen atom picked up from air during welding can push ductility below acceptable levels in Grade 4 even at a level Grade 2 would tolerate. Elongation is already working against a 15 % minimum.
In practice this means three things. First, trailing shield and back purge are not negotiable; a set-up that “gets by” on Grade 2 will not do on Grade 4. Second, the colour acceptance criterion must be applied more strictly — a blue bead on Grade 4 is a clear reject. Third, filler selection must be deliberate: matching ERTi-4 preserves strength but also moves the weld toward the brittle side, while one-step-softer ERTi-2 makes the weld more ductile than the base metal and reduces cracking risk. Where strength is not critical, ERTi-2 is often the better engineering decision — but it means the joint will be weaker than the parent metal, and that must be agreed at order stage.

Common Datasheet Errors and Purchasing Traps​‌​​‌​

1. “ASTM B338 Grade 4 heat-exchanger tube” — WRONG. The current B338 scope enumerates 28 grades and Grade 4 is not among them. Some older secondary listings say otherwise; the scope text governs.
2. “ASTM B861 / B862 Grade 4 pipe” — WRONG. Grade 4 is absent from the grade list of both specifications. Grades 1, 2, 2H, 3, 5, 7, 9, 11–29 and 33–38 are there; 4 is skipped. This is a deliberate scope decision, not a typographical omission.
3. “ASTM B367 Grade C-4 casting” — WRONG. The B367 grade list is C-2, C-3, C-5, C-7, C-8, C-9, C-12, C-16, C-17, C-38. There is no C-4.
4. “Grade 4 is hardened by heat treatment / solution treated and aged” — WRONG. There is no STA route in unalloyed titanium. Strength comes from interstitial content and cold work. Such a line is a sign that Ti-6Al-4V data has migrated onto the wrong page.
5. Dropping the upper bound of the yield band. ASTM Grade 4 yield is the band 483–655 MPa. Most sheets print only “≥483 MPa”. A heat exceeding 655 MPa is out of specification — it is not “stronger, therefore better”.
6. Hardness scale confusion. Grade 4 is typically ~100 HRB, roughly 20 HRC. A reader who takes “100” for HRC imagines a material that does not exist. Always print the scale.
7. Confusing β transus with α transus. For Grade 4 the β transus is ~935–963 °C and the α transus ~891–918 °C; one source gives a single value of 949 °C. These depend on the grade’s oxygen content and are not the same as Grade 2’s — there is no single “titanium β transus”.
8. Thermal conductivity conflict. Published values scatter across 17.2 / 18 / 22 W/m·K, depending on measurement temperature and which CP grade the source meant. Do not publish a single exact figure; give the order of magnitude.
9. “Titanium is immune to corrosion” — DANGEROUSLY WRONG. Titanium fails in hydrofluoric acid, in free fluoride, in dry chlorine, in red fuming nitric acid and in anhydrous methanol, and it has concentration and temperature thresholds in reducing acids (~7 % HCl, ~5 % H₂SO₄ at room temperature). Never put that sentence in a quotation.
10. Treating wet and dry chlorine as the same. Titanium is excellent in wet chlorine; dry chlorine attacks it and can ignite it. Thresholds: ~1 % water at room temperature, ~1.5 % at 199 °C. The difference between two states of the same gas is critical.
11. Using AMS and ASTM interchangeably. AMS 4901 and AMS 4921 identify the grade by 70 ksi yield, not by the words “Grade 4”, and their acceptance testing, sampling and traceability requirements differ from ASTM. A plate certified to ASTM B265 is not automatically compliant with AMS 4901.
12. “Grade 4 is ASME approved” — incomplete and misleading. SB-265 and SB-348 do include Grade 4 as a material specification; but code design requires a published allowable stress in ASME II Part D, and such a listing for Grade 4 could not be verified in this research. The Titanium Association’s code presentation speaks of 17 approved titanium grades for VIII-1 and its charts show Gr 1, 2, 2H, 12, 28; Grade 4 is never mentioned. For code work, do not offer Grade 4 without confirmation from the current II-D edition.
13. Taking “Ti99.5” for a purity guarantee. It is a naming convention from the DIN tradition (the Ti 1 / Ti 2 / Ti 3 / Ti 4 sequence), not an analysis result. What the specification guarantees is interstitial ceilings, not a percentage purity.
14. Believing “F67 = purer Grade 4”. The Grade 4 chemical limits in ASTM F67 are identical to B265/B348. What F67 adds is microstructure, grain size, surface and traceability requirements. It tightens inspection, not chemistry.
15. Confusing ISO 5832-2 with ISO 5832-3. 5832-2 is unalloyed titanium (the ISO counterpart for Grade 4); 5832-3 is Ti-6Al-4V. If you see “ISO 5832-3” on a Grade 4 datasheet, that sheet has merged data from two different materials.
16. An offer of titanium covered electrodes. Titanium is not welded by SMAW and no such AWS classification exists. A supplier offering them is selling either another material or a fiction.
17. Ignoring the chip fire risk. Fine titanium chips and grinding dust are flammable and are not extinguished by water. It is the item almost never printed on a datasheet and the one with the most concrete consequence on the shop floor.

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