Heat Treating of Titanium and Titanium Alloys

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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.

Alloy	Designed For
Ti-5Al-2Sn-2Zr-4Mo-4Cr, Ti-6Al-2Sn-4Zr-6Mo	Strength in heavy sections
Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-5Zr-0.5Mo-0.2Si	Creep resistance
Ti-6Al-2Nb-1Ta-1Mo, Ti-6Al-4V	Resistance to stress corrosion in aqueous salt solutions, high fracture toughness
Ti-5Al-2.5Sn, Ti-2.5Cu	Weldability
Ti-6Al-6V-2Sn, Ti-6Al-4V, Ti-10V-2Fe-3Al	High strength at low-to-moderate temperatures

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

Treatment	Description
Mill Annealing	General-purpose treatment, not a full anneal.
Duplex Annealing	Improves creep resistance or fracture toughness.
Recrystallization Annealing	Heats alloy into upper α-β range, held, and cooled slowly.
Beta Annealing	Heats above β transus for improved fracture toughness.

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.

Alloy	Solution Temperature [°C]	Solution Time [h]	Cooling Rate	Aging Temperature [°C]	Aging Time [h]
α or near-α alloys
Ti-8Al-1Mo-1V	980-1010	1	Oil or water	565-595	-
Ti-2.5Cu (IMI 230)	795-815	0.5-1	Air or water	390-410 (Step 1), 465-485 (Step 2)	8-24 (Step 1), 8 (Step 2)
Ti-6Al-2Sn-4Zr-2Mo	955-980	1	Air	595	8
Ti-6Al-5Zr-0.5Mo-0.2Si (IMI 685)	1040-1060	0.5-1	Oil	540-560	24
Ti-5.5Al-3.5Sn-3Zr-1Nb-0.3Mo-0.3Si (IMI 829)	1040-1060	0.5-1	Air or oil	615-635	2
Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.3Si (IMI 834)	1020	2	Oil	625	2
α-β alloys
Ti-6Al-4V	955-970	1	Water	480-595	4-8
Ti-6Al-6V-2Sn (Cu+Fe)	885-910	1	Water	480-595	4-8
Ti-6Al-2Sn-4Zr-6Mo	845-890	1	Air	580-605	4-8
Ti-4Al-4Mo-2Sn-0.5Si (IMI 550)	890-910	0.5-1	Air	490-510	24
Ti-4Al-4Mo-4Sn-0.5Si (IMI 551)	890-910	0.5-1	Air	490-510	24
Ti-5Al-2Sn-2Zr-4Mo-4Cr	845-870	1	Air	580-605	4-8
Ti-6Al-2Sn-2Zr-2Mo-2Cr-0.25Si	870-925	1	Water	480-595	4-8
β or near-β alloys
Ti-13V-11Cr-3Al	775-800	0.25-1	Air or water	425-480	4-100
Ti-11.5Mo-6Zr-4.5Sn (Beta III)	690-790	0.125-1	Air or water	480-595	8-32
Ti-3Al-8V-6Cr-4Mo-4Zr (Beta C)	815-925	1	Water	455-540	8-24
Ti-10V-2Fe-3Al	760-780	1	Water	495-525	8
Ti-15V-3Al-3Cr-3Sn	790-815	0.25	Air	510-595	8-24

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.