# Metallurgical Aspects in the Welding of Clad Pipelines--A Global Outlook
## 1 Introduction  
Clad or lined pipes represent cheap alternative to pipes fully made of corrosion-resistant alloys (CRAs), which are very expensive materials including stainless steel or nickel-based alloys. As a substrate, a carbon steel is used. Clad pipes are widely used in refineries and petrochemical industries, fertilizer and chemical industry, power generation, environmental technology, seawater desalination, pulp and paper manufacturing, the food industry, and shipbuilding [1]. Concerning the oil and gas industry, clad and lined pipes, including the pipe-in-pipe method, have already been installed in the North Sea and the Norwegian Sea. These are often made of carbon steel with an inner clad or liner to protect the carbon steel against hydrogen embrittlement and sulfide stress corrosion cracking. The clad and liner are most frequently made of austenitic stainless steel, often 316L, or a Ni-based superalloy, typically Inconel 625. Clad pipes are mainly produced by hot roll bonding, explosion bonding, and weld overlay. Hot roll bonding is by far the most widely adopted production method [2]. Nickel-based superalloys also frequently used in the aerospace industry [3]. There are different methods of producing clad pipe, e.g., via a mechanical inner liner. This technology is a cost-effective alternative to more expensive options such as solid CRAs or metallurgically bonded clad pipe. The principles are the same as for metallurgically bonded clad pipes, but involve being connected to the external carbon steel pipe through a mechanical bond. Recent results [4] claimed that the liner separation is of minor importance and the use of the established acceptance criterion derived for plain carbon steel pipes can be justified to apply to CRA-lined pipelines. Both clad pipe and lined pipes can be laid by reeling. Although lined pipes are a cheaper solution than clad pipes, the welding technology for the former case may be more challenging. Since the welding solutions for reelable CRA-lined pipe may be considered as key enabling technologies for future exploitation of deep-water oil and gas reserves, it will not be further addressed in the present work. Welding represents a very complicated joining process by the melting and subsequent solidification of metal forming a hermetic and strong joint. During the process, all four states of matter are present and interact. The most widely used methods are gas tungsten arc welding (GTAW) and gas metal arc welding (GMAW). In the case of GTAW, filler wire is added externally to the weld pool where the tungsten electrode is nonconsumable, while in GMAW, filler wire is added directly since it is a consumable electrode. Both are very well suited to automated processes [5; 6; 7]. The welding of clad pipelines may encounter several challenges, specifically when damage or leakage occur and the repair of clad pipes is required. Repair of a pipeline implies replacement of the damaged part with a new pipe section to be joined at the seabed via remote control. Such subsea operations are very expensive, which makes the robustness of the joining operation of crucial importance. This led to the development of the welded sleeve concept for pipe repair, which was the only technique where acceptable robustness could be achieved [8]. In a repair situation, the welded sleeve technique involves the installation of an oversized pipe segment (sleeve) over the joint by threading the pipe ends through the sleeve and performing a subsea fillet weld between the pipe and sleeve on each end [9]. This technique is fully remotely controlled and has been proven by simulating conditions in a hyperbaric chamber to work down to at least 2500 m depth, corresponding to 250 bar pressure conditions, for the welding process. However, the sleeve solution is not acceptable for clad pipes due to the direct exposure to the environment, as it may suffer from corrosion and/or hydrogen embrittlement during service [10]. A comprehensive review is available for further studies [11] with cohesive zone numerical modeling [12].