Hydrocarbon–metal reactions during metal arc welding under oil (MAW-UO)

Abstract

Metal arc welding under oil is a new, revolutionary process for the repairs of pipelines which should minimise the need of hot tapping repairs and the construction of bypass lines. The process uses an automated metal arc welding set-up with a continuous wire feed. This process was developed for the in situ internal repairs of in service cross-country pipelines, tanks and vessels by using a smart pig with a modified mechanical design that allows for joint preparation and welding. The unique characteristics found in the under oil arc are discussed in detail. The large range of temperatures that exists between the arc at the order of 10 000°C (18 000°F) and the surrounding oil (ambient) allows for complex reactions between the oil hydrocarbon chains and the steel weld metal. These reactions may considerably impact the weld microstructure. Details of the dissociation of hydrocarbon chains and ionisation of the resulting species must be known to better control the process. In addition, vaporisation of the oil will form oil vapour bubble to surround the welding arc. The oil vapour bubble will also be high in decomposed products that can significantly affect the weld metal composition. Gas chromatography–mass spectrometry was used to investigate the behaviour of a welding arc under oil and the carbon pick-up. Analyses were conducted on oil samples with different welding exposure times to validate the arc behaviour mechanistic model and the gas chromatography–mass spectrometry analysis results.