Page 31 - ITA Journal 3-2018

Oxidation will be evident visually as discolouration as may be seen from Figure 1.
Weld made with 500 ppm oxygen in Weld made with 70 ppm oxygen in Weld made with 12 ppm oxygen in purge gas purge gas purge gas
Fig.1: It can be generally assumed that for titanium and other refractory and reactive alloys a 20 ppm level is necessary if oxidation is to be avoided.
Discolouration itself may be acceptable and may be removed mechani- cally after welding but a more signi cant result of oxygen contamina- tion is the effect on mechanical properties. Tensile strength is often increased but at the expense of loss of ductility. There can also be a dramatic reduction in corrosion resistance. Since the primary uses of these alloys are in applications where strength and corrosion resistance are mandatory, oxidation is clearly unacceptable.
Protection is thus essential and this is achieved by surrounding the joint with an inert gas such as argon or helium. However, the gas shield associated with a standard GTAW torch is inadequate, offering insuf cient cover. Specialised purging equipment has evolved over the past 25 years.
Trailing Shields®
The extra coverage provided by trailing shields not only protects the fusion zone, it also provides inert gas protection to all the hot adjacent metal. Figure 2.
Technical Papers
Fig.2: It can be generally assumed that for titanium and other refractory and reactive alloys a 20 ppm level is necessary if oxidation is to be avoided.
Custom made trailing shields are available commercially, but these are inevitably expensive and have limited use. The shields manufactured under the Argweld®11 name have in-built exibility.
For curved shapes, eg pipes, tubes and cylindrical vessels, trailing shields are available to suit a speci c diameter. They can be con gured for either internal or external welding and there are versions for mech- anised welding.
ITAtube Journal No3/October 2018
31