The main types of titanium alloy annealing
The main types of titanium alloy annealing include stress relief annealing, simple annealing, isothermal annealing, double annealing, recrystallization annealing and vacuum annealing.
(1) Stress relief annealing
In order to eliminate the internal stress caused by plastic deformation, welding and other processes, the workpiece is heated to the recrystallization temperature below the annealing process. The de-stress annealing temperature is low, which belongs to the annealing without recrystallization, and the recovery occurs during the annealing process.
(2) Simple annealing
An annealing process in which the workpiece is heated to a temperature slightly below the starting temperature of recrystallization in order to eliminate residual stress. This annealing method is commonly used in metallurgical products.
(3) recrystallization annealing
Annealing process in which the workpiece is heated above the recrystallization temperature. Recrystallization mainly occurs in this annealing process, so that the work hardening is eliminated, the microstructure is stable and the plasticity is improved. The annealing temperature is between the recrystallization temperature and the phase transition temperature.
(4) Vacuum annealing
The hydrogen content of the surface layer of titanium alloy is reduced to a safe concentration and the possibility of hydrogen embrittlement is eliminated. In addition, the residual stress is reduced and the mechanical properties and serviceability of the alloy are guaranteed.
The purpose and significance of the above four kinds of heat treatment are very clear, but the concept of isothermal annealing and double annealing is often easily confused. Isothermal annealing and double annealing are only suitable for α+β alloy.
(5) Isothermal annealing
Isothermal annealing: The workpiece is heated to a temperature high enough for recrystallization to occur, then cooled to a temperature at which the β-phase is highly stable (which is generally lower than the recrystallization temperature), held at this temperature, and subsequently cooled in air.
Compared with simple annealing, in the second stage of insulation is to make β phase decomposition, β phase in a stable state, the properties and structure of titanium alloy stability, titanium alloy has relatively high molding, thermal stability and durable strength. Therefore, isothermal annealing is suitable for α+β alloy with high β content.
(6) Double annealing
The difference between double annealing and isothermal annealing is that after the first stage of double annealing, the alloy is cooled in air to room temperature, after which the alloy is reheated to the temperature of the second stage (which is lower than the temperature of the first stage).
The advantage of dual annealing is that part of the metastable phase can be retained after the first annealing, and the β phase can be fully decomposed after the second annealing, which leads to the strengthening effect, and can improve the shape, fracture toughness and microstructure stability of α+β titanium alloy.
