TWI Industrial Member Report Summary 1014/2012
By R Zhou and M Gittos
Background
This report summarises an investigation into the feasibility of using a buffer layer approach for welding of corrosion resistant alloy (CRA) clad/lined pipes, as an alternative to the current approach of using CRA weld metal. The concept of the buffer layer approach is illustrated in Figure 1.
Previous work (Zhou and Holmes, 2010) described the disadvantages associated with the current industrial practice of using CRA weld metal for circumferential welds in clad/lined pipes. The main issue for the use of nickel alloy consumables, is that the resulting weld metal might under-match the strength of parent materials at high operating temperatures (above 140°C) for high-strength-grade pipes (X65 and above) (Farrar et al, 1997). The buffer layer approach has the potential to overcome this issue by enabling the use a carbon steel filler metal to overmatch the pipe properties (Figure 1a). It also has the added advantage of lower consumable cost, particularly for thick-walled pipes.
This work (Zhou and Holmes, 2010) also included bead-on-plate trials using selected buffer layer filler metals. The results showed that deposition of the pure Fe weld metal on alloy 625 substrates using the surface tension transfer® (STT®) process was feasible and further experimental work was suggested on representative butt welded joints.
Work reported here focused on butt weld experiments and optimisation of existing welding parameters based on earlier results. This was followed by an assessment of the effectiveness of this approach, to evaluate the overall feasibility of the buffer layer method.
Objective
Examine the feasibility of employing a pure iron buffer layer plus carbon steel fill technique to butt welds in clad/lined pipes, as an alternative to the current approach of using CRA weld metal throughout.