Grade 4 titanium is the strongest commercially pure (CP) titanium grade within the unalloyed titanium family. Unlike alloyed titanium (e.g., Grade 5 Ti-6Al-4V), it contains no intentionally added alloying elements-its performance is derived from titanium's inherent properties, modified only by controlled levels of trace impurities.
Its key advantage lies in balancing exceptional corrosion resistance and superior strength compared to other CP titanium grades (Grades 1–3). It maintains resistance to harsh environments such as seawater, acidic solutions, and chlorinated media, while offering the highest mechanical strength among unalloyed titanium variants. This makes it ideal for applications requiring both durability and corrosion protection, including:
Chemical processing: Corrosion-resistant pipes, valves, and tanks.
Marine engineering: Structural components exposed to saltwater.
Medical industry: Orthopedic implants (e.g., hip stems) and dental fixtures, where biocompatibility and strength are critical.
Grade 4 titanium has a minimum titanium purity of 99.0%, with strict limits on impurity elements to ensure consistent performance. The following table outlines its typical chemical composition, compliant with ASTM B265 (the standard for titanium plate, sheet, and strip):
Oxygen is the most impactful impurity here: Grade 4 titanium has the highest oxygen content among CP grades (up to 0.40%). This controlled oxygen addition is intentional-it significantly boosts the material's strength without compromising its core corrosion resistance.
The hardness of Grade 4 titanium varies based on its heat treatment state (e.g., fully annealed, cold-worked) and the testing standard used. Below are typical values for its most common supply state: fully annealed Grade 4 titanium.
Brinell Hardness (HB)
Typical range: 180–220 HB
Testing standard: ASTM E10
Methodology: Uses a 10 mm diameter steel ball indenter with a 3000 kgf (kilogram-force) load.
Rockwell Hardness
Rockwell B (HRB): 85–90 HRB
Testing standard: ASTM E18
Methodology: Uses a 1/16 inch diameter steel ball indenter with a 100 kgf load.
Rockwell C (HRC): Not recommended for Grade 4 titanium. Its hardness is too low for accurate HRC measurements (typically <25 HRC).
Vickers Hardness (HV)
Typical range: 190–230 HV
Testing standard: ASTM E92
Methodology: Uses a diamond pyramid indenter with a 1 kgf load, ideal for measuring relatively soft metals like CP titanium.
Effect of Cold Working: If Grade 4 titanium undergoes cold working (e.g., rolling, forging without post-processing annealing), its hardness can increase by 15–25%. For example, Vickers hardness may rise to ~280 HV, but this improvement comes with a trade-off: reduced ductility.
