The heat treatment of titanium alloys is a process that changes the phase composition and organizational structure of the alloy by heating, heat preservation, and cooling, thereby improving its performance. The important basis for determining the process parameters of heat treatment of titanium alloys is the transformation temperature of phase B. The heat treatment of alloys can be divided into annealing, solution treatment (quenching), and aging treatment.
The purpose of annealing is to homogenize the organization and phase composition, obtain uniform performance, improve plasticity, and eliminate the internal stress formed during pressure processing, welding, machining, etc. Complete annealing is heating above the recrystallization start temperature but below the transformation temperature of phase B. The annealing temperature is usually selected below the transformation temperature of phase B by 120 ~ 200C. For stress elimination, incomplete annealing is generally selected below the recrystallization start temperature by 150 ~ 250C. The annealing temperature of plates is lower than that of bars and forgings. Annealing of titanium alloys is generally carried out under non-vacuum conditions, but sometimes vacuum annealing is required to remove hydrogen or prevent pollution from harmful gases such as oxygen and nitrogen. The vacuum annealing temperature is generally 500 ~ 750C. To stabilize the structure and performance of heat-resistant titanium alloys, double annealing or isothermal annealing can be used. Double annealing consists of two-stage heating and intermediate air cooling, that is, heating and air cooling at 20 ~ 160C below the transformation temperature of phase B, and then heating and air cooling at 300 ~ 450C below the phase transformation temperature. The difference between isothermal annealing and double annealing is that the cooling speed between the two heatings is different. Double annealing uses air cooling. Isothermal annealing is transferred from a high-temperature furnace to a low-temperature furnace for insulation and cooling with the furnace.
