Annealing Conditions
Annealing conditions should be set according to the processing method and use of the titanium products. The annealed structure is generally a uniform equiaxed structure. Because titanium is easily oxidized and an oxide film forms at high temperatures in a trace-oxygen-containing atmosphere (visible as tempering colors), the annealing of cold-rolled titanium coils must be carried out in a vacuum annealing furnace. Depending on the application, annealing is sometimes performed in an atmospheric annealing furnace followed by pickling with alkali molten salt to remove surface oxide scale.
Before Vacuum Annealing
Before vacuum annealing, the cold-rolled coil must be washed to completely remove rolling oil and other grease, generally using alkali washing for degreasing.
Vacuum Annealing Process
The washed cold-rolled coils are heated after vacuum exhaust in the vacuum annealing furnace. The temperature rise must be carried out in stages to ease the temperature difference between the inside and outside of the coils. After the vacuum is exhausted for a certain period, the furnace is backfilled with a protective gas and stirred with a forced fan to produce convection heat conduction and uniform temperature. The vacuum exhaust state is maintained at 400-500 deg C to obtain a cleaner surface. The annealing temperature is 600-650 deg C with a holding time of 5-10 hours. Including heating and cooling time, the processing time of one furnace is about 10-50 hours.
Atmospheric Annealing
The temperature of atmospheric annealing is 700-800 deg C with a short holding time. A catenary-type annealing furnace can produce coils with good flatness and can also reduce the leveling load in the finishing process. After atmospheric annealing, alkali molten salt at about 500 deg C is used to remove the oxide scale, followed by pickling. The final returned materials must be flattened or shaped as needed, then edged, stripped, and plated, and shipped out of the factory after surface quality inspection.
Frequently Asked Questions
Why must titanium be annealed in vacuum? Titanium oxidizes easily at high temperatures; vacuum annealing prevents oxide film formation and tempering colors.
What is the vacuum annealing temperature? 600-650 deg C, held for 5-10 hours.
How is atmospheric annealing finished? t 700-800 deg C, followed by alkali molten salt treatment and pickling to remove oxide scale.
Why must the coil be degreased before vacuum annealing? Cold-rolled coils carry rolling oil and grease; alkali washing removes them completely so the annealed surface stays clean and free of contamination.
Why is the furnace temperature raised in stages? Staged heating eases the temperature difference between the inside and outside of the coil, preventing distortion and uneven structure in large coils.
