Commercially Pure Titanium

Commercially pure titanium is unalloyed metal in which the oxygen content — 0.18 % in Grade 1 rising to 0.40 % in Grade 4 — sets the strength: yield climbs from about 170 MPa to 480 MPa across the four grades while formability and weldability fall. Grades 7, 11, 16 and 17 add 0.05–0.25 % palladium to the same base for reducing acids and hot brines. All of them are single-phase alpha, cannot be hardened by heat treatment, and carry the best corrosion resistance of any titanium — the reason they fill chemical plants, desalination trains and heat exchangers rather than airframes.

Grade 2 unless there is a reason not to: about 275 MPa yield, the widest choice of sheet, plate, bar and tube, the ASTM B265 / B338 standard for process equipment and the easiest grade to weld. Grade 1 when the part must be deep-drawn, spun or bent tight (plate heat-exchanger plates, expansion bellows) and 170 MPa yield is enough. Grade 3 and Grade 4 carry more oxygen for about 380 and 480 MPa yield — Grade 4 is the CP grade for bone screws, dental implants and surgical instrument bodies under ASTM F67 where Grade 5 would be more than the design needs. Grade 7 is Grade 2 with palladium for hydrochloric, sulphuric and phosphoric acid and crevice-prone brine above 80 °C; Grade 16 does most of the same job with a quarter of the palladium, and Grade 11 is the palladium version of Grade 1 for the formable equivalent.

Ti Grade 35 has the highest yield strength in the series at 920 MPa; VT14 follows at 850 MPa. Values are for each grade's default condition.

Grade 11 (machinability fair, 45/100), then C-1 (42/100).

No. These grades are strengthened by cold work and softened by annealing, not by heat treatment; the conditions are listed on each grade page.

Grade 7 is Grade 2 with 0.12–0.25 % palladium, which shifts the corrosion potential so the oxide film repairs itself in reducing acids and in hot chloride crevices — hydrochloric acid, wet chlorine, brine above about 80 °C — where Grade 2 pits under deposits and gaskets. Mechanical properties, welding and forming are identical. Grade 7 costs several times as much per kilogram because of the palladium; Grade 16 with 0.04–0.08 % Pd or Grade 12 with Mo and Ni covers most of the same crevice-corrosion duty for less.

Yes — CP titanium and Ti-6Al-4V weld together by TIG with either ERTi-2 or ERTi-5 filler under full inert shielding; the joint strength is set by the Grade 2 side (about 345 MPa tensile), so design the weld on that basis. Stress-relieve the assembly at 480–595 °C if the Grade 5 part is machined afterwards. What you cannot do is buy the Grade 2 strength back by heat treatment — CP grades gain strength only from oxygen content and cold work.