Heat treatment of titanium and titanium alloys is a key process for controlling their microstructure and properties. Because titanium reacts readily with oxygen, nitrogen, and hydrogen at high temperatures and undergoes a phase transformation (α⇌β, with a transformation temperature of approximately 882°C), its heat treatment presents distinct challenges.
Primary Heat Treatment Processes:
1. Annealing
Most commonly used; objectives: stress relief, softening, microstructural stabilization, and improved ductility.
1) Stress-relief annealing: 450–650°C, 1–4 hours, All titanium alloys Eliminates residual stresses from machining/welding; no phase transformation occurs
2) Recrystallization annealing: 680–800°C (for α+β alloys below Tβ); after cold working; to relieve work hardening and restore ductility
3) Double annealing: High-temperature stage + low-temperature stage (e.g., Gr5: 950°C/2 h air cooling + 530°C/4 h air cooling); α+β alloy; Refines the microstructure, improving creep resistance and fracture toughness
4) Isothermal annealing: After heating, the material is rapidly cooled to a temperature below Tβ and held there; α+β alloy ; to obtain a more uniform microstructure and reduce internal stresses
5) β annealing (β transformation): Above Tβ (e.g., Gr5: 1050°C), air-cooled or furnace-cooled α+β alloy ; produces a Widmanstätten structure (basket-weave pattern), improves fracture toughness, and reduces fatigue crack propagation rate
2. Solution Treatment and Aging (STA)
The primary strengthening method, suitable for α+β and metastable β alloys.
Principle: After high-temperature solution treatment, the alloy is rapidly cooled (water quenching/oil quenching), trapping β-stabilizing elements in supersaturated α' (martensite) or metastable β. During aging, fine, uniformly distributed α phases or intermetallic compounds precipitate.
3. Chemical Heat Treatment (Surface Hardening)
Oxygen diffusion: Increases surface hardness and wear resistance
Nitriding: Forms a TiN hardening layer
Boron diffusion: Extremely high hardness, but high brittleness
Ion implantation: Improves surface wear and corrosion resistance
Chemical heat treatment of titanium must be performed in a vacuum or protective atmosphere to prevent the formation of a surface contamination layer (α-embrittlement layer).
Key Control Factors in Heat Treatment
1. Heating Atmosphere and Contamination Prevention
Titanium behaves like a "chemical beast" at high temperatures:
>400°C: Significant oxygen absorption, forming a hard and brittle α layer (Case/α-case)
>500°C: Significant nitrogen absorption
>300°C: Hydrogen absorption leading to hydrogen embrittlement
Measures:
Vacuum heat treatment (vacuum level ≤ 10⁻³ Pa; for higher temperatures, ≤ 10⁻⁴ Pa)
Inert gas protection (high-purity argon; ensure dew point < -50°C)
Coating protection (e.g., specialized anti-oxidation coatings such as Brycoat)
Pickling to remove the α-embrittlement layer (HF + HNO₃ mixed acid)
2. Temperature Control: Precise Control of Tβ
The solution treatment temperature for α+β alloys must be below Tβ (typically 30–80°C lower); otherwise, β grains will grow rapidly, forming a coarse Widmanstätten structure, leading to "β brittleness."
Tβ varies with composition: approximately 995°C for TC4 and 1000°C for TC11; adjustments must be made based on specific composition.
3. Cooling Rate
Annealing: Typically air-cooled or furnace-cooled
Solution Treatment: Water quenching (to prevent β-phase decomposition and achieve a supersaturated microstructure)
When the cross-sectional thickness exceeds 25 mm, the cooling rate at the core may be insufficient; quenching hardenability must be considered
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