Titanium alloy heat treatment-annealing

[ Information Release:Admin | Times:2019-11-15 | Hits:1518 ]

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 an annealing process below the recrystallization temperature. The stress-relief annealing temperature is lower, which belongs to the annealing without recrystallization, and recovery occurs during the annealing process.

(2) Simple annealing

In order to eliminate residual stress, the workpiece is heated to an annealing process slightly below the recrystallization start temperature. This kind of annealing method is a common one when metallurgical products leave the factory.

(3) Recrystallization annealing

An annealing process performed by heating the workpiece above the recrystallization temperature. In this annealing, recrystallization mainly occurs, so that work hardening is eliminated, the structure is stable, and the plasticity is improved. The annealing temperature is between the recrystallization temperature and the phase transition temperature.

(4) Vacuum annealing

Reduce the hydrogen content of the titanium alloy surface layer to a safe concentration and eliminate the possibility of hydrogen embrittlement. In addition, reduce residual stress and ensure the mechanical properties and serviceability of the alloy.

The purpose and significance of the above four heat treatments are very clear, but the concepts of isothermal annealing and double annealing are often easily confused. Isothermal annealing and double annealing are only applicable to α+β alloys.

(5) Isothermal annealing

Isothermal annealing: The workpiece is heated to a higher temperature sufficient for recrystallization, and then cooled to a temperature where the β phase has high stability (this temperature is generally lower than the recrystallization temperature), kept at this temperature, and then cooled in air.

Compared with simple annealing, the visual observation of heat preservation in the second stage is to fully decompose the β phase, make the β phase in a stable state, stabilize the properties and structure of the titanium alloy, and make the titanium alloy have a relatively high shaping and heat. Stability and lasting strength. Therefore, isothermal annealing is suitable for α+β alloys with high β content.

(6) Double annealing

The difference between double annealing and isothermal annealing is that in the first stage after double annealing, the alloy is cooled to room temperature in the air, and then the alloy is reheated to the temperature of the second stage (this temperature is lower than the temperature of the first stage).

The advantage of double annealing is that part of the metastable phase can be retained after the first annealing, and it can be fully decomposed during the second annealing, that is, the β phase is decomposed, causing a strengthening effect, which can improve the shaping of the α+β titanium alloy, Fracture toughness and organizational stability.