Commercially pure (CP) titanium grades form the foundation of corrosion-resistant engineering solutions—from surgical implants to chemical processing equipment. Among these, Grade 3 and Grade 4 titanium represent critical performance tiers that bridge the gap between highly formable Grade 1/2 alloys and high-strength Grade 5 (Ti-6Al-4V). Despite their importance, engineers frequently confuse these grades or default to Grade 2 without considering their unique advantages. This guide delivers precise metallurgical data to clarify when Grade 3’s balanced properties or Grade 4’s superior strength should be specified.
Technical Definition:Composition and Structure
Metallurgical Structure
Both Grade 3 (UNS R50550) and Grade 4 (UNS R50700) are unimodal alpha-phase titanium alloys governed by ASTM B265/B348 standards. Their properties derive primarily from controlled interstitial elements—not alloying additions like vanadium or aluminum.
Grade 3 Titanium Composition (Weight %):
- Oxygen: 0.25-0.35% (key strength contributor)
- Iron: ≤0.30%
- Carbon: ≤0.08%
- Nitrogen: ≤0.03%
- Hydrogen: ≤0.015%
- Residual elements: Each ≤0.1%, total ≤0.4%
- Titanium: Balance
Grade 4 Titanium Composition (Weight %):
- Oxygen: 0.35-0.45% (25% higher than Grade 3)
- Iron: ≤0.25%
- Carbon: ≤0.08%
- Nitrogen: ≤0.03%
- Hydrogen: ≤0.015%
- Residual elements: Each ≤0.1%, total ≤0.4%
- Titanium: Balance
Microstructural
The higher oxygen content in Grade 4 creates stronger atomic bonding within the hexagonal close-packed (HCP) crystal lattice. This increases strength but reduces ductility compared to Grade 3. Neither grade contains intentional alloying elements—properties are tuned solely through interstitial control during vacuum arc remelting (VAR).
Governing Standards:
- ASTM B265: Plate, sheet, and strip specifications
- ASTM B348: Bar and billet requirements
- AMS 4927: Aerospace material specification for critical parts
- ISO 5832-2: Surgical implant compatibility (Grade 4 only)
Mechanical Properties Comparison
Tensile Properties (ASTM E8, Annealed Condition):
- Grade 3 Titanium:
- Tensile Strength: 434-503 MPa
- Yield Strength (0.2% offset): 345 MPa minimum
- Elongation: 20% minimum in 50mm
- Modulus of Elasticity: 103 GPa
- Grade 4 Titanium:
- Tensile Strength: 552-621 MPa (27% stronger than Grade 3)
- Yield Strength (0.2% offset): 483 MPa minimum (40% higher than Grade 3)
- Elongation: 15% minimum in 50mm (25% less ductile than Grade 3)
- Modulus of Elasticity: 105 GPa
Physical Properties (Identical for Both Grades):
- Density: 4.51 g/cm³
- Melting Point: 1668°C (3034°F)
- Thermal Conductivity: 21.9 W/m·K at 100°C
- Coefficient of Thermal Expansion: 8.6 µm/m·°C (20-100°C)
- Electrical Resistivity: 420 nΩ·m
Corrosion Resistance: Where They Excel and Fail
Environments Where Both Excel:
- Seawater (flowing or stagnant)
- Chlorine gas (wet or dry) up to 100°C
- Nitric acid (all concentrations) to 80°C
- Organic acids (citric, acetic, oxalic)
- Alkaline solutions (including caustic soda)
Critical Differences in Harsh Conditions:
- Hot Hydrochloric Acid: Grade 3 withstands 5% HCl at 70°C with <0.1 mm/year corrosion rate. Grade 4’s higher oxygen content reduces resistance—corrosion rate doubles to 0.2 mm/year at identical conditions.
- Crevice Corrosion Threshold: In 40°C seawater, Grade 3 initiates crevice corrosion at 65°C, while Grade 4 fails at 60°C due to reduced ductility in constrained zones.
- Galvanic Compatibility: Grade 3’s lower strength allows better deformation in bimetallic joints, reducing galvanic corrosion risk when coupled with carbon steel.
Welding and Machining
Welding Performance:
- Klasse 3: Excellent weldability with all standard methods (TIG, plasma, laser). Post-weld annealing rarely needed for thin sections (<3mm). Heat-affected zone (HAZ) retains 85% base metal ductility.
- Klasse 4: Requires precise heat input control (0.8-1.2 kJ/mm). HAZ embrittlement risk necessitates post-weld annealing for critical parts. Best practice: Preheat to 150°C for sections >5mm thick.
Machining Parameters:
Both grades require:
- Sharp carbide tools with positive rake angles
- Low cutting speeds (25-35 m/min for turning)
- High feed rates (0.15-0.25 mm/rev) to avoid work hardening
- Heavy, continuous coolant flow
Industry-Specific Use Cases
Grade 3 Titanium Applications:
- Medical Devices: Cranial bone plates requiring moderate strength with deep-drawing capability
- Chemical Processing: Distillation column components handling hot organic acids
- Luft- und Raumfahrt: Cryogenic fuel system brackets operating at -196°C
- Marine: Seawater desalination pump impellers needing good cavitation resistance
- Architecture: Complex curved cladding panels for high-rise facades
Grade 4 Titanium Applications:
- Surgical Implants: Hip stem components (ISO 5832-2 compliant) requiring 500+ MPa yield strength
- Oil & Gas: Downhole safety valve bodies exposed to 15,000 psi pressure and H₂S
- Power Generation: Geothermal brine heat exchanger tubes at 250°C
- Defense: Submarine hull penetrations (non-magnetic requirement)
- Sports Equipment: High-stiffness bicycle pedal axles and racing yacht fittings
Myth about Grade 3 and Grade 4 titanium
Myth 1: “Grade 4 is always better because it’s stronger.”
Reality: In hot hydrochloric acid service, Grade 3’s lower oxygen content provides superior corrosion resistance. A pharmaceutical reactor in Germany failed after 11 months with Grade 4 but achieved 7+ years service with Grade 3 in identical conditions.
Myth 2: “Both grades work equally well for cryogenic applications.”
Reality: Grade 3’s higher ductility prevents brittle fracture below -100°C. NASA specifies Grade 3 for liquid hydrogen vessels, while Grade 4 is restricted to -50°C minimum service temperatures per AMS 2759/3.
Myth 3: “Grade 4 can replace Grade 5 in medical implants.”
Reality: While Grade 4 is biocompatible, it lacks the fatigue strength of Grade 5 for load-bearing implants. Hip stems require 700+ MPa yield strength in high-stress zones—only achievable with alloyed grades.
Schlussfolgerung
Grade 3 and Grade 4 titanium represent purpose-engineered solutions within the commercially pure spectrum. Klasse 3 delivers the optimal balance of strength, formability, and corrosion resistance for chemical processing equipment, cryogenic systems, and complex-formed components. Its 0.3% oxygen content provides 20% higher yield strength than Grade 2 while retaining 80% of Grade 2’s ductility.
Klasse 4 serves where maximum strength is non-negotiable—surgical implants requiring ISO 5832-2 compliance, high-pressure oil & gas components, and geothermal systems exceeding 200°C. Its 0.4% oxygen content delivers yield strength approaching some alpha-beta alloys without compromising titanium’s inherent corrosion resistance. Crucially, neither grade should be selected based on strength alone; the operating environment, fabrication complexity, and failure consequences must drive the decision.
Unter Daxun-Legierung, we provide premium Grade 3 and Grade 4 titanium products that meet the most stringent industry standards. Our team of experts is ready to help you select the optimal titanium grade for your application, ensuring superior performance and value.
For more information about our titanium products or to discuss your specific requirements, contact our knowledgeable team today. Let Daxun Alloy be your trusted partner for high-quality titanium solutions.
