Difference Between Grade 2、 Grade 4 Titanium

1. Chemical Composition: The Root of Differences

The primary variable between Grade 2 and Grade 4 is the oxygen content, which is tightly controlled in CP titanium to tailor performance. Other impurities (iron, carbon, nitrogen, hydrogen) are also limited but play a smaller role in distinguishing the two grades.

Component	Grade 2 Titanium (ASTM B265 Standard)	Grade 4 Titanium (ASTM B265 Standard)	Key Impact of the Difference
Titanium (Ti)	Balance (>99%)	Balance (>99%)	Base metal; consistent across both grades.
Oxygen (O)	Maximum 0.25%	Maximum 0.40%	Higher oxygen in Grade 4 increases strength but reduces ductility.
Iron (Fe)	Maximum 0.30%	Maximum 0.50%	Trace impurity; slightly higher in Grade 4 but has minimal effect compared to oxygen.
Carbon (C)	Maximum 0.08%	Maximum 0.08%	Uniform limit; no impact on performance differences.
Nitrogen (N)	Maximum 0.05%	Maximum 0.05%	Uniform limit; no impact on performance differences.
Hydrogen (H)	Maximum 0.015%	Maximum 0.015%	Strictly limited in both to prevent hydrogen embrittlement.

In short, Grade 4 has ~60% more oxygen than Grade 2-this is the single most important factor driving their performance gap.

2. Mechanical Properties: Strength vs. Ductility

Oxygen acts as a "strengthening impurity" in CP titanium: it distorts the titanium crystal lattice, hindering dislocation movement (the main cause of plastic deformation). This leads to a clear trade-off between strength and ductility for Grade 2 vs. Grade 4:

Mechanical Property	Grade 2 Titanium (Annealed State)	Grade 4 Titanium (Annealed State)	Key Comparison
Tensile Strength	370 – 480 MPa	620 – 790 MPa	Grade 4 is ~50–65% stronger than Grade 2.
Yield Strength (0.2% Offset)	275 – 345 MPa	485 – 620 MPa	Grade 4 has ~75–80% higher yield strength than Grade 2.
Ductility (% Elongation)	20 – 25%	10 – 15%	Grade 2 is twice as ductile as Grade 4; it bends more easily before breaking.
Brinell Hardness (HBW)	110 – 150 HBW	180 – 220 HBW	Grade 4 is ~30–45% harder than Grade 2, making it more resistant to scratches/indentation.
Fatigue Strength	~170 MPa (10⁷ cycles)	~280 MPa (10⁷ cycles)	Grade 4's higher strength translates to better resistance to repeated loading.

Notably, neither grade can be strengthened by heat treatment (unlike alloyed titanium like Grade 5/Ti-6Al-4V). Their properties are fixed by their impurity content, so annealing only softens them slightly without altering the strength-ductility balance.

3. Processing & Fabrication Characteristics

The strength and ductility differences directly affect how Grade 2 and Grade 4 are manufactured and shaped:

Grade 2 Titanium

Formability: Excellent cold and hot formability. Its high ductility allows it to be bent, rolled, stamped, or drawn into complex shapes (e.g., thin-walled tubing, large sheets) without cracking.

Weldability: Superior to Grade 4. Low oxygen content minimizes brittle phase formation in weld zones, resulting in strong, ductile welds. It is often welded using TIG (tungsten inert gas) or MIG (metal inert gas) methods without pre-heat or post-weld heat treatment.

Machinability: Moderate. Its lower hardness means cutting tools wear less quickly than with Grade 4, but it still requires sharp tools and proper cooling to avoid work hardening.

Grade 4 Titanium

Formability: Poor compared to Grade 2. Its low ductility limits cold forming-sharp bends or deep draws often cause cracking. Hot forming (at ~600–800°C) is possible but requires careful temperature control and slower processing.

Weldability: Fair, but less reliable than Grade 2. Higher oxygen content increases the risk of brittle "alpha-case" formation in welds (a hard, brittle layer of oxygen-enriched titanium). Post-weld heat treatment (e.g., annealing at 650–700°C) is often needed to restore ductility to weld zones.

Machinability: Challenging. Its higher hardness and strength cause faster tool wear. Machining requires high-speed steel or carbide tools, low cutting speeds, and ample coolant to prevent overheating and work hardening.

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4. Corrosion Resistance

Both Grade 2 and Grade 4 exhibit excellent corrosion resistance in most environments, thanks to their ability to form a dense, self-healing titanium dioxide (TiO₂) film. However, Grade 2 has a slight edge in extreme conditions:

Grade 2: Performs marginally better in mild corrosive environments (e.g., pure water, dilute acids, atmospheric humidity) because its lower oxygen content creates a more uniform TiO₂ film.

Grade 4: Corrosion resistance is still strong (superior to stainless steel in seawater, for example), but its higher oxygen content can lead to minor localized pitting in highly concentrated chloride solutions (e.g., >200°C saltwater) if the TiO₂ film is damaged.

For most industrial and consumer applications, the corrosion resistance difference is negligible-both grades outperform common metals like steel or aluminum.

5. Typical Applications

Their unique property balances make Grade 2 and Grade 4 suited for distinct use cases:

Grade 2 Titanium (The "Workhorse" CP Grade)

Used where formability, weldability, and ductility are prioritized over maximum strength:

Chemical processing: Thin-walled tubing, tanks, and heat exchanger tubes for handling dilute acids, alkalis, or food-grade fluids.

Medical devices: Non-implantable tools (e.g., surgical forceps), flexible medical tubing (e.g., catheters), and dental trays.

Marine/aerospace: Lightweight structural components (e.g., aircraft ducting, marine handrails) that require welding and bending.

Consumer goods: Architectural panels, jewelry, and sports equipment (e.g., bicycle frames) where aesthetics and formability matter.

Grade 4 Titanium (The "High-Strength" CP Grade)

Used where maximum strength and hardness are critical, and formability is less important:

Industrial equipment: High-pressure vessels, valve bodies, and fasteners (e.g., bolts, nuts) for heavy-duty machinery.

Aerospace: Structural brackets, engine components (e.g., low-temperature ducting), and aircraft landing gear parts (non-critical load zones).

Medical implants: Short-term bone fixation devices (e.g., temporary pins) where strength is needed, but ductility is less critical (and long-term biocompatibility is not a concern).

Oil/gas: Downhole tools and wellhead components that withstand high pressure and mechanical stress in harsh drilling environments.