Ti-99.8, or ASTM Grade 4, is the highest-strength member of the commercially pure titanium family. It carries the greatest allowable oxygen and iron content of the four CP grades, and since those interstitial elements are what strengthen titanium's crystal structure, Grade 4 also delivers the highest tensile and yield strength of the series — approaching the lower end of what alloyed grades like Ti-6Al-4V can achieve, without any deliberate alloying additions. That strength comes with a trade-off familiar across the CP series: Grade 4 is the least ductile and least cold-formable of the four grades, though it remains far more workable than most alpha-beta or beta titanium alloys.

Because it offers a meaningful strength bump over Grades 1 through 3 while remaining unalloyed, weldable, and fully corrosion-resistant, Grade 4 is often specified where designers want more mechanical performance than lower CP grades provide but don't need the added strength — or added machining difficulty and cost — of an alloyed titanium. It's a common choice for airframe fittings, fasteners, and surgical instruments, and like every CP grade it relies on titanium's naturally passive oxide film for excellent resistance to seawater and industrial chemical environments.

Grade 4 shares the same underlying machining behavior found across the titanium family. Its thermal conductivity is only a fraction of steel's, so cutting heat concentrates at the tool edge instead of leaving with the chip, pushing tool-tip temperatures up quickly even at cutting speeds that would be conservative for steel. Once hot, titanium reacts chemically with common tool materials, encouraging adhesion and edge buildup rather than a clean, continuous shear.

Grade 4's low modulus of elasticity means the workpiece deflects under cutting force more than steel does, so a genuinely rigid setup — short overhang, solid clamping — is necessary to avoid chatter and hold dimensional accuracy. Titanium also work-hardens wherever a tool rubs instead of shearing cleanly, so feed rate needs to stay high enough to consistently cut into fresh material; a light, hesitant feed just glazes the surface and shortens tool life on the next pass.

Because Grade 4 carries the highest oxygen content and strength of the CP series, it's the most demanding of the four to machine, requiring somewhat lower cutting speeds than Grades 1 through 3 — though it's still considerably easier to cut than alpha-beta or beta titanium alloys, since it remains single-phase and unalloyed. Sharp, positive-rake tooling, rigid workholding, adequate feed, and generous coolant are the fundamentals for acceptable tool life and finish.