Activated tungsten inert gas (TIG) welding, which involves the deposition of an inorganic powder on the metal surface prior welding, improves TIG welding performances in terms of weld bead penetration. Most common commercial fluxes contain halides and oxides. The present paper focuses on fluxes made of oxides. It appears that oxides have two different effects: one on the fusion zone chemistry and the other on the electric arc behaviour. First, oxygen reverses the Marangoni convection movements that become centripetal, contributing thus to increase 'D' penetration. On the other hand, oxides may cause an increase in the energy flux density transferred by the arc to the metal. The characterisation of the arc plasma by optical emission spectroscopy suggests that oxides do not increase the arc temperature. It is established that the oxide effects by Marangoni convection appear for a critical value of the welding energy, whereas the oxide effects on the electrical arc exist irrespective of the welding energy value.

The mechanisms of the inorganic powders, named activating flux and used in ATIG welding process, are not well identified. Oxides and fluorides usually compose activating fluxes. The present paper studies the fluoride activating effect. The analysis of the arc energy influence on the weld bead geometry allows determining the mechanisms that occur during ATIG welding with fluorides. It seems that fluoride activating effect could depends on the fluoride lattice energy and/ or the ionic radius of the element. Fluorides act on the arc physics only. An arc energy densification is observed. Furthermore, an arc temperature increase, find out with optical emission spectrometry, characterises the fluoride activating effect. The fluorides do not affect the weld bead surface chemistry. Marangoni convection movements stay centrifugal as in TIG welding and do not favour a penetration increase.

The use of an inorganic powder, termed activating flux, is a technique that can be used to improve the tungsten inert gas welding process. The activated tunsten inert gas (ATIG) process greatly increases single pass depth penetration. In the present work the activating effect of different oxides on 304L stainless steel was studied, to determine the main relationships between the oxide propertie(s) and the weld bead geometry variation and consequently to determine the mechanism(s) occurring during the ATIG welding process. The results show that oxide melting point T m , the liquid state (T b 2T m ), the lattice energy E L , and the quantity of oxygen that can be liberated seem to play an important role, even though T m is the main property. Moreover, for some oxides, the hygrometry rate is another property that can increase the activating effect.

The aim of this paper is to review the challenges of onshore and offshore welding operations in Arctic extreme environments in terms of: Health, Safety and Environment,Equipment exposure to cold environment,Logistics,Welding performance requirements: weld quality and productivity. It lists the key elements to ensure successful onshore and offshore welding operations meeting both project productivity and quality targets in extreme cold areas. Qualification of equipments either in cold chamber or in extreme cold conditions is a pre-requisite. It must be ensured that batteries, materials (metallic and non metallic), fluids (hydraulic and engine oils …) are suitable for the working conditions. Due to logistics issues, preventive maintenance on all equipments is very important before starting any operation. Work organization is highly impacted by workers exposure to very low temperature. High performance protective clothes are helpful but harsh external conditions mainly promote the use of automated, reliable and high deposition rate welding processes. Finally, welding quality is a major concern to avoid pipeline leak or failure in very sensitive environments with difficult access for emergency repairs. It relies on qualified welding procedures, a limited number of manual operations, narrow ranges of welding heat inputs, strictly controlled preheating conditions and real-time monitoring of welding parameters during operations. The main contributions of this paper are: to list the most important parameters to make welding operations successful in Arctic extreme environments and to reduce logistics issues.to highlight the interest to use fully automated, versatile and compact (limited coating cut backs lengths) welding equipments in terms of welding crew requirements and operation time.to evidence how good low temperature properties (in particular impact properties) can be achieved on QT seamless and TMACC seam welded pipes throughout optimization of welding thermal cycles.

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