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Titanium alloy heat treatment - annealing

[ Info Diffusion : Admin | Times : 2018-09-26 | Hits : 54 ]

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 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 recovers during the annealing process.

(2) Simple annealing

To eliminate residual stress, the workpiece is heated to an annealing process that is slightly below the recrystallization starting temperature. This type of annealing is a common type of metallurgical product when it leaves the factory.

(3) Recrystallization annealing

The annealing process is performed by heating the workpiece to a temperature above the recrystallization temperature. In this annealing, recrystallization mainly occurs, so that work hardening is eliminated, the structure is stabilized, 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 titanium alloy surface layer is reduced to a safe concentration, eliminating the possibility of hydrogen embrittlement. In addition, the residual stress is reduced and the mechanical properties and performance of the alloy are ensured.

The purpose and significance of the above four heat treatments are clear, but the concepts of isothermal annealing and double annealing are often confused. Isothermal annealing and double annealing are only suitable for α+β type alloys.

(5) Isothermal annealing

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

Compared with simple annealing, the visual observation of the second stage of heat preservation is to fully decompose the β phase, to make the β phase in a stable state, to stabilize the properties and structure of the titanium alloy, and to make the titanium alloy have a relatively high shape and heat. Stability and durability. Therefore, isothermal annealing is suitable for α+β type alloys with higher 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 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 a part of the metastable phase can be retained after the first annealing, and can be fully decomposed after 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 tissue stability.