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Commercially pure titanium, the
ductile, low-alloy alpha and unaged beta titanium alloys can be cold
formed within certain limits. The amount of cold forming either in
bending or stretching is a function of the tensile elongation of the
material. Tensile elongation and bend data for the various grades of
titanium sheet and plate can be found in ASTM Specification B265.
Heating titanium increases its
formability, reduces springback, and permits maximum deformation with
minimum annealing between forming operations. Mild warm forming of most
grades of titanium is carried out at 204-316¡C (400-600¡F) while more
severe forming is done at 482-788¡C (900-1450¡F). Heated forming dies
or radiant heaters are occasionally used for low temperature forming
while electric furnaces with air atmospheres are the most suitable for
heating to higher temperatures. Gas fired furnaces are acceptable if
flame impingement is avoided and the atmosphere is slightly oxidizing.
Any hot forming and/or annealing
of titanium products in air at temperatures above approximately 590-620¡C
(1100-1150¡F) produces a visible surface oxide scale and diffused-in
oxygen layer (alpha case) that may require removal on fatigue- and/or
fracture-critical components. Oxide scale removal can be achieved
mechanically (i.e., grit-blasting or grinding) or by chemical descale
treatment (i.e., molten hot alkaline salt descale). This is generally
followed by pickling in HF-HNO3 acid solutions, machining or
grinding to ensure total alpha case removal, where required. These acid
pickle solutions are typically maintained in the 5:1 to 10:1 volume %
HNO3 to HF ratio (as stock acids) to minimize hydrogen pickup
depending on alloy type. |