In addition to recrystallization, there are also changes in composition, quantity and morphology of a phase and beta phase in the annealing process of nearly a titanium alloy and a+ beta titanium alloy.For metallurgical plant delivery, the main requirements are stable and plastic structure.Annealing temperature is generally selected below the phase transition point of a+ beta/beta at 120 ~ 200℃. Air cooling is also used for cooling.However, for annealing before final use, the influence of annealing process on microstructure and mechanical properties must be determined by test.
The time required for stress removal annealing depends on the thickness of the workpiece, the small residual stress, the annealing temperature used and the degree of stress removal desired.Consider the thickness of the workpiece can refer to the case of stainless steel.As for the selected temperature and the degree of hope to eliminate stress generally through the experiment to determine.Effects of heating temperature and holding time on residual stress of TC4 titanium tube and titanium alloy.Obviously the heating temperature is high and the time required is short.
Air cooling is generally used to eliminate stress annealing.For large size and complex shape of the parts can also be used furnace cold.Stress relief annealing system of titanium alloy.Within the recommended temperature and time range, the above principles shall be specified by test.It should be noted that stress relief annealing of titanium welds is often considered in the same way as subsequent heat treatment.Stress relief annealing of cold forming parts is carried out at the same time as hot orthopedic process.
The purpose of full annealing of titanium plate and titanium alloy is to obtain stable, plastic or corresponding microstructure with certain comprehensive properties.Recrystallization occurs mainly during this process, which is also known as recrystallization annealing.There are also changes in the composition, morphology, and number of a and beta phases.Most a and a+ titanium alloys are used in a fully annealed state.The two-phase zone of all a type titanium alloy is very small.Annealing temperature is generally selected below the phase transition point of a+ beta/beta at 120 ~ 200℃.Too high temperature will cause unnecessary oxidation and grain growth, too low temperature recrystallization is not complete.The cooling speed has little effect on the microstructure and properties of this kind of alloy.
As for metastable type titanium alloy, complete annealing is solid solution treatment.The annealing temperature of metallurgical plants before delivery is generally 80 ~ 100℃ above the phase transition point of a+ beta/beta.Within the recommended range of full annealing processes, the specific process shall be determined according to the material's processing history, actual chemical composition, and the equipment used by the test.In order to avoid unnecessary oxidation, the lowest temperature and shortest time should be used when selecting the process on the premise of meeting the performance requirements.
In recent years, the beta annealing process has emerged.A titanium alloy and a+ beta titanium alloy are heated in the beta phase zone and air-cooled to precipitate acicular (or flake) a on the thick beta grains.This kind of structure corresponds to high fracture toughness, creep resistance and notch sensitivity, but reduces the plasticity index such as reduction of section.Where this performance is emphasized, it may be used as appropriate.In addition, some alloys, such as TC9 and tc6 titanium alloys, are often treated with a second annealing or isothermal treatment higher than the service temperature when it is required to improve the performance stability of the alloys working for a long time at the service temperature.