**1. Introduction**

Narrow gap welding (NGW) is a technique used to weld thick joints with the aim of reducing the molten metal deposited volume, ultimately decreasing distortion and residual stresses caused by thermal stresses developed during welding. One of the drawbacks associated with this technique is sidewall penetration caused by the arc wandering during welding. This problem is so critical in heavy duty welding that models to predict it are found in the literature [1].

Various NGW variants were developed to overcome such drawbacks: rotating arc, swing arc and wave-shaped wire systems. Another reason to use these modified systems in narrow gap welding is to improve the wettability of the sidewall by the welding pool and to better distribute the heat across the weld to mitigate distortions. A common setback for these systems is their expense and the use of highly trained personnel in their operation, which considerably increases the costs of manufacturing.

Not only are arc-modified processes employed in NGW, but also increased-deposition processed are successfully used, e.g., tandem gas metal arc welding (T-GMAW) [2] and twin GMAW [3]. However, these processes rely on increased deposition, while they increase the amount of heat transferred to the substrate, which might cause increased distortion or residual stresses. To mitigate distortion and other heat-induced detrimental effects, cold metal transfer (CMT) was developed, in which the wire feed motion is reversed at a time synchronized with the pulse current to optimize the metal transfer. A review of the applications of CMT emphasizing the low dilution and the effect of post weld heat treatments (PWHT) in various materials can be found in [4].

One alternative to these sophisticated welding processes is the cold wire gas metal arc welding (CW-GMAW), which consists of the standard gas metal arc welding (GMAW) with an extra cold wire (non-energized) fed into the arc-welding pool system. This process increases the deposition of the welds while maintaining the nominal heat input for the same welding parameters in standard GMAW. Previous research [5] has shown that the feeding of cold wire causes a slight increase in current without a corresponding increase in penetration. A feature that distinguishes GMAW and CW-GMAW is the reduced dilution caused by cold wire feed rates, which decreases penetration and melting of the base metal. Ultimately, the two processes differ by range of admissible parameters, since the cold wire can stabilize metal transfer [6]. Moreover, this difference in dilution can be linked to the thermal signature, as reported in [7], showing that cold wire welds result in lower heat-induced distortion compared to standard GMAW welds.

Previous research [8] demonstrates the possibility of welding U-grooves using CW-GMAW with pure carbon dioxide as shielding gas, and this method was primarily developed to be applied in shipbuilding. The feasibility of this process to weld a 5 mm wide groove was recently studied on ASTM A131 grade A steel, revealing that, due to the pinning of the arc to the cold wire, the sidewall erosion was considerably reduced in comparison to welds manufactured with standard GMAW [9]. Subsequent research [10] studied the effect of the CW-GMAW on process-induced residual stresses, concluding that welds manufactured by CW-GMAW have lower levels of residual stresses. This decrease in residual stresses might explain the improvement in fatigue life reported in [11].

The present work reports the further development on the work of prior research [9], and reports preliminary results regarding the application of CW-GMAW to weld high strength pipeline steels in narrow gap configuration. The feasibility of NGW employing CW-GMAW was assessed by comparing the severity of sidewall erosion in the welds and the presence of defects in the root pass. The results point to the general feasibility of NGW using CW-GMAW pipeline applications, and illuminate possible future work to avoid certain defects found during welding. The arc attachment to sidewalls can be ascribed to the arc shortest electron path according to Zhang et al. [12].
