Heat treating titanium and titanium alloys
The heat treatments for individual titanium materials differ in terms of temperature and duration. While the temperature range for annealing and stress relieving is relatively large, narrower temperature ranges need to be observed when heat treating titanium alloys to achieve specified properties.
Annealing is a heat treatment aimed either at reversing work hardening brought about by forming or age hardening, or at stabilizing the material in question. Annealing generally takes place at above recrystallization temperature.
Stress relieving is a heat treatment aimed at reducing residual stresses without significantly modifying the microstructure. Stress relieving is generally performed below recrystallization temperature and may be required e.g. after welding, forming at temperatures below 650°C or localized forming, machining (in particular grinding) or leveling.
The parameters for annealing and stress relieving commercially pure titanium materials (3.7025, 3.7035, 3.7055 and 3.7065) and titanium alloys TiAl6V4 (3.7165) and TiAl6V6Sn2 (3.7175) are listed in the table.
The holding times indicated for stress relieving apply to wall thicknesses up to 10 mm. Beyond this, holding times should be increased by approx. 15 min for each further 10 mm of thickness.
In all heat treatments it is important to remember that titanium and its alloys react readily with hydrogen, oxygen and nitrogen. It must therefore be ensured that the heat-transfer media used by the available heat-treatment equipment do not cause inadmissible reactions with the material. This applies in particular to the inadmissible absorption of hydrogen from the furnace atmosphere.
Hydrogen absorption begins at temperatures of 500°C upwards. At temperatures above 700°C, oxygen and nitrogen lead increasingly to the formation of scale layers, with oxygen also diffusing into the workpiece surface (diffusion zone). These chemical reactions lead to reduced toughness and thermal stability of the titanium materials.
In contrast to oxygen and nitrogen, hydrogen can be largely removed by vacuum annealing under suitable conditions.
In addition to protective atmosphere (inert gases only) and vacuum furnaces, heat treatment can also be carried out in electric or gas-fired air furnaces. Gas-fired furnaces should be set to an excess air content of around 10 - 15%, and direct contact of the part with the gas flame should be avoided due to the risk of hydrogen absorption or localized overheating.
For each batch, the heat treatment equipment should only be charged with parts of the same type and roughly the same thickness, as the holding time depends on part thickness. Parts must be dry (i.e. all dirt and grease removed) to prevent decomposition products from diffusing into the material. Any steel fixtures which come into contact with the titanium must be largely free of scale due to the risk of chemical reactions between titanium and iron oxide at high heat treatment temperatures.
Furthermore, with certain metals (e.g. nickel and copper) titanium forms low-melting phases which could cause localized melting at the temperatures normally used for heat treatment. Contact with these metals (e.g. charging racks) must therefore be avoided.
The scale layers formed during heat treatment have to be removed by mechanical descaling methods such as sand blasting, grinding or brushing and subsequent pickling. Slight discoloring can be removed by pickling alone.
An aqueous solution of
- 20 vol.-% HNO3 (65% nitric acid) and
- 2 vol.-% HF (40% hydrofluoric acid)
has proved successful for pickling.
In this treatment it is essential that not just the surface oxide layer is removed but also the oxygen-enriched diffusion zone beneath it, as this has a negative impact on machining characteristics, i.e. the edge life of turning and milling tools.
Once the scale and oxide layers have been removed, checks should always be carried out on hydrogen content as there is a risk of hydrogen absorption during annealing and pickling.
For further details on the heat treatment of titanium and titanium alloys, please refer to DIN 65084.
Deutsche Titan, Nov. 2000