The local intense TIG welding arc provides a process ideally suited to the control of weld pool formation in pipe butt joint. Weld pool control can be further enhanced by low frequency or thermal pulsing of the current sequentially between high and low level. Filler wire may be added independently to the molten pool and can be used to improve the formation of the penetration bead in the pipe root. The advantageous features of TIG welding have been applied over many years and the process has become the logical choice and most favoured system for root run control.