TWI Industrial Member Report Summary 981/2011
By Runlin Zhou
Background
Subsea pipelines used to transport corrosive products are often made of carbon steel internally clad/lined with a layer of corrosion resistant alloy (CRA). The CRA cladding/lining is approximately 2-3mm thick, either in stainless steel or nickel alloy. The carbon steel substrate provides the required strength and the CRA cladding/lining provides the adequate corrosion resistance to the product being transported. The dissimilar metals that are present through the thickness of the pipe wall bring certain challenges to welding of clad/lined pipes, because welding of such pipes is usually carried out from the outside, using a single-sided welding technique (Farrar et al, 1997).
The current approach adopted for single-sided welding of CRA clad/lined pipes is to use a matching strength or over-matching CRA filler metal which also matches or exceeds the corrosion resistance of the cladding, eg a highly alloyed nickel filler. This approach presents a major issue in terms of filler metal selection for high strength pipes (X65 or above) clad/lined with nickel alloys (alloy 625/825). It was reported that such nickel alloy weld metal may under-match the clad/lined pipe material at operating temperatures above 140°C (Farrar et al, 1997). To overcome this issue, a buffer layer approach may be adopted. This technique involves depositing the root and hot pass with CRA weld metal followed by a buffer layer of special low-carbon filler metal. Then the joint is completed using matching/over-matching strength carbon steel fillers.
The current industrial application of the buffer layer technique utilises the manual metal arc (MMA) welding process. An existing sub-sea pipeline was constructed successfully using such technique. However, the use of the MMA process is considered unsuitable for current pipe-laying practices, because the weld quality achieved is highly dependant on welder skills, and the deposition rate is much lower than the mechanised metal inert gas/metal active gas (MIG/MAG) welding process (most common for offshore lay-barges).
Therefore, there is a strong industrial need to improve the existing practice by switching to a mechanised wire process for the buffer layer deposition. Such improvements will include better and more consistent weld quality, higher welding speed (comparable to current industrial practices for fill-pass deposition in clad/lined pipes, around 500mm/min), and good applicability to narrow-gap welding. Hence, this two-year Core Research Project (CRP) is looking into the feasibility of depositing pure Fe (low-carbon)/pure Ni (ERNi-1) solid wires as a buffer layer, using advanced MIG/MAG processes (eg Lincoln Electric STT®) and the TOPTIG (developed by Air Liquide) welding process.
Objectives
- Review and select suitable welding processes for mechanised buffer layer deposition on CRA clad pipe.
- Carry out preliminary bead-on-plate trials for welding parameter development for the buffer layer weld metal.