3.7.4 Heat Treating Titanium Alloys

Titanium alloys are heat treated to achieve the following:

• Stress relieving, to reduce residual stresses developed during fabrication.
• Annealing, to achieve an optimum combination of ductility, machinability, dimensional stability and structural stability.
• Solution treating and aging, to increase strength.
• Combinations of processes are employed to optimize properties and gain other advantages such as:
  • Fracture toughness
  • Fatigue strength
  • High temperature creep strength
  • Resistance to preferential chemical attack
  • Prevent distortion
  • Condition the forging for subsequent forming and fabricating operations.

There are three principal types of heat treatment.

1. Stress Relieving Titanium alloys can be stress relieved without adversely affecting strength or ductility. The process for forgings takes place at 595 to 705^°C (1100 to 1300^°F) for a period of one to two hours, followed by air cooling. It decrease undesirable residual stresses that may result during forging processes.
2. Annealing Mill annealing, which is usually applied to forging bar stock, is not a full anneal, and may leave traces of cold or warm working in some products. Duplex and triplex annealing are used to improve creep resistance and fracture toughness.
3. Solution Treatment and Aging This process consists of heating to a specified temperature for the alloy, quenching at a controlled rate in either oil, air or water, and aging. Aging consists of reheating to a temperature between 425 and 650^°C (800 to 1200^°F) for approximately two hours. This process develops higher strengths than are achievable by the other processes.