Heat treatment of titanium fabrications is not normally necessary. Annealing may be necessary following severe cold work if restoration of ductility or improved machinability are desired. A stress relief treatment is sometimes employed following severe forming or welding to avoid cracking or distortion due to high residual stresses, or to improve fatigue resistance. Cleanliness of titanium parts to be heat treated is important because of the sensitivity of titanium to contamination at elevated temperatures. Titanium fabrications should be cleaned carefully prior to heating, using nonchlorinated solvents or a detergent wash, followed by a thorough water rinse. Handling following cleaning should be minimized to avoid potential surface contamination.
Most titanium grades are typically stress-relieved at about 1000°F (538°C) for 45 minutes and annealed at 1300°F (704°C) for two hours. A slightly higher stress relief temperature [1100°F (593°C), 2 hrs.] and annealing temperature [1450°F (788°C), 4 hrs.] are appropriate for the Grade 5 alloy. Air cooling is generally acceptable.
Although no special furnace equipment or protective atmosphere is required for titanium, a slightly oxidizing atmosphere is recommended to prevent pickup of hydrogen. Direct flame impingement for extended periods, leading to temperatures in excess of 1200°F (649°C), should be avoided because of the potential for contamination and embrittlement. Hydrogen or cracked ammonia atmospheres, also, should never be used, because their use would lead to excessive hydrogen pick-up, and embrittlement.
If a scale removal treatment, following a high temperature (1200°F; 649°C) anneal is not feasible, a vacuum or inert gas (dry argon or helium) atmosphere is recommended. Superficial surface discoloration, caused by annealing below 1200°F (649°C), may be removed by acid pickling in a 35% nitric acid – 5% hydrofluoric acid bath at 125°F (52°C). However, if long heating times or temperatures above 1200°F (649°C) have been used, a molten caustic bath or mechanical descaling treatment, followed by nitrichydrofluoric acid pickling, is necessary to remove scale.
