Heat Treating of Titanium and Titanium Alloys

Heat Treatment Purposes of Titanium and Titanium Alloys

Heat treating titanium and titanium alloys serves several purposes:

• Stress relieving: Reduces residual stresses from fabrication.
• Annealing: Optimizes ductility, machinability, and dimensional and structural stability.
• Solution treating and aging: Increases strength.
• Special properties optimization: Enhances fracture toughness, fatigue strength, and high-temperature creep strength.

Types of Heat Treatment

Various heat treatment processes are used to achieve specific mechanical properties:

• Stress relieving
• Annealing: Single, duplex, beta, and recrystallization annealing
• Solution treating and aging

These treatments also help prevent chemical attacks in corrosive environments, prevent distortion, and condition the metal for forming and fabricating operations.

Alloy Types and Heat Treatment Response

The response of titanium alloys to heat treatment depends on their composition and the effects of alloying elements on the α-β crystal transformation. Not all heat-treating cycles apply to all titanium alloys as they are designed for different purposes.

Effects of Alloying Elements

Unalloyed titanium is allotropic, with its structure changing from hexagonal close-packed (α phase) to body-centered cubic (β phase) at 885°C (1625°F). Alloying elements are classified as α stabilizers or β stabilizers based on their effects on this transformation.

• Alpha stabilizers: Oxygen, aluminum (raise α-to-β transformation temperature)
• Beta stabilizers: Manganese, chromium, iron, molybdenum, vanadium, niobium (lower α-to-β transformation temperature)

Alloy Types and Heat Treatment

Titanium alloys are classified based on their alloying elements:

• Alpha and Near-Alpha Alloys: Can be stress relieved and annealed, but high strength cannot be developed by heat treatment.
• Beta Alloys: Metastable β alloys that strengthen upon exposure to elevated temperatures.
• Alpha-Beta Alloys: Most versatile and common, comprising both α and β phases at room temperature.

Stress Relieving

Titanium alloys can be stress relieved without affecting strength or ductility. This process decreases residual stresses from various manufacturing steps, helping maintain shape stability and eliminating unfavorable conditions like the Bauschinger effect.

Annealing

Annealing increases fracture toughness, ductility, dimensional and thermal stability, and creep resistance. Common annealing treatments include:

• Mill annealing
• Duplex annealing
• Recrystallization annealing
• Beta annealing

Common Annealing Treatments

Solution Treating and Aging

Solution treating and aging provide a wide range of strength levels in α-β or β alloys. Heating to the solution-treating temperature produces a higher ratio of β phase, which is maintained by quenching and decomposes upon aging for high strength.

Summary

Heat treating titanium and its alloys optimizes various mechanical properties based on the specific needs of the application. The selection of appropriate heat treatment methods and conditions depends on the alloy composition and desired properties. Proper understanding and application of these treatments ensure enhanced performance and reliability of titanium components in various industries.