Titanium and Titanium Alloys - Hot Working

Background

Techniques for press and hammer forging of titanium are essentially the same as those for low alloy steels, bearing in mind the low thermal conductivity of titanium and its susceptibility to contamination by hydrogen when heated in reducing atmospheres. Electric preheating furnaces are ideal, but oil or gas fired furnaces are often used, provided that excess oxygen is maintained in the atmosphere and time at temperature is kept to a minimum consistent with uniform heating. In oil or gas fired furnaces, direct contact between flame and workpiece should be avoided.

As a rough guide, 1 hour per 50 mm of section thickness should be allowed. It is preferable to have forging equipment waiting for the metal rather than metal soaking at high temperatures awaiting its turn at the forge. Good handling methods and plant layout minimise contamination by reducing the number of re-heats necessary. Preheating of tools and keeping the contact time as short as possible reduces surface chilling and possible cracking.

There is an optimum rate of working depending upon the alloy being forged, the geometry of the workpiece and the temperature employed. Excessive heavy working can give rise to build-up of heat in the centre of the piece, resulting in grain growth or a possible phase change causing loss of ductility in the finished forging. On the other hand, repeated light blows or attempts to continue forging at too low a temperature may promote cracking and should be avoided. Moreover, a large number of re-heats with only a small amount of deformation between heats is also detrimental since it leads to coarsening of the microstructure and consequently poor mechanical properties.

Source: Materials Information Service – The Selection and Use of Titanium, A design Guide

For more information on this source please visit The Institute of Materials, Minerals and Mining.