1. What are the characteristics of grade 4 titanium?
Chemical Composition: It consists of over 99% pure titanium, with strictly controlled impurity elements. The most critical impurity is oxygen, with a maximum limit of 0.40%-the highest among all CP titanium grades. Other impurities include iron (max 0.50%), carbon (max 0.10%), nitrogen (max 0.05%), and hydrogen (max 0.015%). This higher oxygen content is the primary driver of its superior strength compared to Grades 1–3.
Microstructure: At room temperature, it has a uniform hexagonal close-packed (HCP) alpha (α) phase structure with no phase transformations. This structure ensures good ductility while enabling significant strength enhancement via oxygen solid-solution strengthening. It also avoids brittle transition risks even at low temperatures (down to -200°C).
Corrosion Resistance: It retains the excellent corrosion resistance inherent to CP titanium. A stable titanium dioxide (TiO₂) passive film forms rapidly on its surface, providing protection against seawater, freshwater, humid atmospheres, dilute hydrochloric acid, and dilute sulfuric acid. However, its corrosion resistance in strongly oxidizing acids (e.g., concentrated nitric acid) is slightly lower than Grade 3 due to its higher oxygen content-though it still outperforms most common metals (e.g., stainless steel 304).
Physical Properties: It has a density of approximately 4.51 g/cm³ (only 56% of steel's density), offering significant weight-saving benefits. Its melting point is 1668°C (3034°F), consistent with other CP titanium grades. At 25°C, its thermal conductivity is 17 W/m·K and electrical resistivity is 420 nΩ·m-these values are nearly identical to Grade 3, with only minor fluctuations from impurity differences.
Processability: It can be fabricated using conventional metalworking processes, including cold rolling, forging, and welding. However, its higher strength requires greater force during cold forming, and stress relief annealing (typically at 500–600°C for 1–2 hours) is often needed post-forming to eliminate internal stresses. Welding performance is good, but post-weld heat treatment is recommended to restore plasticity in the heat-affected zone and prevent brittleness.
2. What is the mechanical strength of grade 4 titanium?
Tensile Strength: Its maximum stress before fracture ranges from 750 to 850 MPa, approximately 15% higher than Grade 3 (650–750 MPa). This makes it ideal for structural parts subjected to moderate loads (e.g., pressure vessels, fasteners).
Strength-Toughness Balance: Despite its high strength, it retains 10–15% elongation, providing enough plastic deformation capacity to avoid brittle fracture under impact or vibrational loads. This balance is a key advantage over some high-strength alloys that sacrifice toughness for strength.
3. What is the yield strength of grade 4 titanium?
Typical Values and Standard Requirements:
Per the ASTM B265 standard, the minimum 0.2% offset yield strength for Grade 4 titanium is 550 MPa-the highest minimum requirement among all CP titanium grades.
In actual production, typical yield strength ranges from 650 MPa to 750 MPa, depending on processing conditions. For fully cold-worked (cold reduction ≥50%) products, yield strength can even exceed 750 MPa.
Key Factors Influencing Yield Strength:
Oxygen Content: Oxygen is the most impactful strengthening element for Grade 4 titanium. Its maximum oxygen limit of 0.40% (higher than Grade 3's 0.35%) enhances yield strength significantly through solid-solution strengthening, which restricts dislocation movement in the crystal structure.
Degree of Cold Work: Cold working (e.g., cold rolling, drawing) creates dislocation accumulation in the metal, increasing yield strength. For example, fully hard Grade 4 titanium (high cold work) has a yield strength of over 750 MPa, while annealed Grade 4 (heated at 500–600°C for 1–2 hours to relieve stress) has a yield strength of around 650 MPa.
Temperature: At low temperatures (-200°C to room temperature), yield strength increases slightly while maintaining good ductility. At elevated temperatures (above 300°C), yield strength decreases with rising temperature-at 400°C, it drops to approximately 60% of its room-temperature value (400–450 MPa), making it unsuitable for long-term load-bearing applications at high temperatures.
