Grade 2 titanium (commercially pure titanium, UNS R50400) is defined by strict oxygen content limits in accordance with international standards such as ASTM B348 (for titanium and titanium alloy bars) and ASTM B265 (for titanium and titanium alloy sheets/plates).
The maximum oxygen content of Grade 2 titanium is 0.25% by weight.
This limit is a key differentiator from lower-purity grades (e.g., Grade 1 has a max oxygen content of 0.18%) and higher-strength grades (e.g., Grade 3 has a max oxygen content of 0.35%). Oxygen is intentionally controlled as an interstitial alloying element to balance mechanical properties and processability.
Oxygen acts as a strengthening interstitial element in commercially pure titanium, but its content has a trade-off effect on strength and toughness:
Impact on Strength
Within the standard limit (≤0.25 wt%), an increase in oxygen content leads to a significant rise in the strength of Grade 2 titanium. This is because oxygen atoms occupy interstitial sites in the hexagonal close-packed (HCP) crystal structure of titanium, causing lattice distortion and impeding the movement of dislocations. As a result, both tensile strength and yield strength increase proportionally with oxygen content. For example, Grade 2 with oxygen content at the upper limit (0.25%) typically has a yield strength of ~275 MPa, while a Grade 2 sample with lower oxygen content (e.g., 0.15%) may have a yield strength of ~240 MPa.
Impact on Toughness
The increase in oxygen content is accompanied by a reduction in the toughness and ductility of Grade 2 titanium. Excessive oxygen (approaching or exceeding the 0.25% limit) increases the brittleness of the material, as the distorted lattice structure makes it harder for the material to deform plastically before fracture. This manifests as a decrease in elongation at break and reduction of area. For instance, Grade 2 with 0.25% oxygen has an elongation of ~20–25%, whereas a lower-oxygen variant may have an elongation of ~28–30%. Beyond the standard limit, oxygen can even cause embrittlement, severely compromising the material's resistance to impact and fatigue.
The mechanical properties of Grade 2 titanium are temperature-dependent, with its performance varying significantly across cryogenic, ambient, and elevated temperature ranges. Below are the typical mechanical property data (in accordance with ASTM standards) for Grade 2 titanium at common temperatures:
Key Notes on Temperature-Related Performance:
Cryogenic Temperatures (-270°C to -196°C): Grade 2 titanium exhibits excellent cryogenic performance-its strength increases significantly compared to ambient temperatures, while ductility is maintained at a reasonable level. This makes it suitable for low-temperature applications such as liquid gas storage tanks.
Ambient Temperature (25°C): The material achieves a balanced combination of strength and ductility, which is the basis for its wide use in chemical processing, marine engineering, and medical devices.
Elevated Temperatures (100°C to 400°C): Strength gradually decreases with rising temperature, but ductility increases slightly. Grade 2 titanium can be used continuously at temperatures up to 315°C; above 400°C, its oxidation resistance and mechanical stability decline sharply, limiting its long-term service in high-temperature environments.
