Page 19 - ITAtube Journal 1 2019

but they are expensive. Welding procedures need to be carefully developed and stringently applied to avoid expensive waste, rework or risk of service failure.
Successful fusion joining tech- niques have evolved1 since the alloys were first used in engi- neering applications. The major- ity of metallurgical problems, even considering dissimilar metal welding, have been resolved and filler materials are readily availa- ble. However, their reactive nature make it essential to continue to address the requirement for thor- ough pre-cleaning and particularly oxidation at the high tempera- tures involved in arc welding.
Cleanliness
Weld repair of plant that has been in service presents the most diffi- cult situation for welding reactive and refractory alloys. The equip- ment is usually dirty and may have process scale on the surface. In most cases, the repairs must be done outdoors where the work area and conditions are not optimal for welding reactive metals - the environment usually is dirty. High humidity and wind also interfere with the welding process.
Furthermore, plant shutdowns that require equipment repair usually result in pressure to complete the work quickly. There is little merit however in cutting corners. Repairs that must be made to par- tial-penetration welds or where crevices exist pose a special problem because of contaminants in crevices.
Poorly prepared surfaces can result in weld contamination and lead to a premature failure and a repeat of the weld repair. Overall, expect to spend more time preparing to
weld than actually welding when work is undertaken on site.
Even under the cleaner condi- tions prevailing in most factory environments however there is still a need to prepare surfaces by removing all contamination prior to welding. Published information on cleaning techniques is availa- ble 2 - 8 but the basic principles are to abrade surfaces to remove any corrosion products and other debris then degrease and finally dry. Prepared surfaces should then be protected by covering until any joints are to be made.
Protection Against Oxidation
Reactive metals have a strong affinity for oxygen at the tem- peratures prevailing in fusion welding and when combined form very stable compounds. Refractory metals exhibit many of the same characteristics.
Exposed pipe in readiness for freezing operation
Technical Papers
Huntingdon Fusion Techniques
Stukeley Meadow SA16 0BU Carms UK
Tel: +44 1 554 836 836 Fax: +44 1 554 836 837
hft@huntingdonfusion.com www.huntingdonfusion.com
Accu-Freeze-PAF-PHO-50C-Frozen-Clam-on-Pipe
ITAtube Journal No1/February 2019
19