# Characteristics of GMAW Narrow Gap Welding on the Armor Steel of Combat Vehicles
## 1 Introduction  
Narrow gap welding (NGW) refers to butt welding applied to a narrow groove between two thick plates, where the gap between the two plates is of Type I, or a similar type. Butt welding is considered NGW when the groove angle is less than 20\({}^{\circ}\). Through NGW, high productivity and quality can be achieved and thermal deformation and residual stress can be minimized; it can also be used in any welding position, including vertical upward and downward positions. Owing to such advantages, NGW is frequently applied to nuclear power plants where numerous welding works on piping and equipment are required. In particular, since stress corrosion cracking (SCC) accidents are frequently caused by the residual stress from welding in many nuclear power plants, maintaining the soundness of device welding, including the welding of pipes in nuclear power plants, is becoming one of the most critical safety issues to ensure the safe operation of nuclear power plants [1; 2; 3; 4]. Generally, the welding method for the piping of a nuclear power plant requires a large level angle and multiple welding passes. Therefore, the size of the welding part proportionally increases and, consequently, the residual stress and the possibility of welding defects also increase. To resolve these issues, studies on NGW are currently focused on the welding process for piping and machinery in nuclear power plants. In addition, studies on NGW are also focused on developing new equipment that enables welding in narrow spaces and on applications of new welding processes [5; 6; 7; 8; 9]. Likewise, NGW has the advantage of ensuring high-quality welds with low welding deformation and residual stress while, at the same time, enabling higher productivity than the existing thick plate welding method through an efficient, narrow welding joint.  
This study focused on NGW for MIL-STD-12560-grade armor steel, which is used for caterpillar-type and wheeled armored combat vehicles. Combat vehicles, such as tanks, use materials with excellent impact toughness (armor steel, MIL-STD-12560) in order to ensure the safety and survival of the crew. Depending on the position of the crew and the structural importance, 30-150 mm thick armor steel is applied. In the welding joint design for armor steel, the size and shape of the level angle are determined within the level angle range (45-55\({}^{\circ}\)) of the joint, as determined in accordance with MIL-HDBK-21. To ensure the strength of the welding part, using a wide level angle is usuallypreferred, based on experimental data. In particular, the welding design for combat vehicle structures is applied rather conservatively, wherein the bevel angle is excessively wide, or the shape of the welding joint is almost precisely determined to be as verified, since protecting the structure is directly related to the life and death of the crew. In this case, as the thickness of the material increases, issues arise, such as welding deformation and residual stress due to the increased number of welding passes, which are required by the welding bevel angle. When residual stress occurs in the welding part, cold cracking occurs due to the high equivalent carbon content and susceptibility of cold cracking. Moreover, the impact toughness of the welding part decreases due to the excessive welding heat input, and the productivity of the process decreases due to the high number of welding passes. To resolve this, the present study is focused on the characteristics of NGW parts in armor steel in order to apply NGW to the welding part of armor steel used for combat vehicles. Therefore, the mechanical properties and low-temperature impact toughness of the existing welding part and the NGW part were comparatively evaluated; furthermore, the residual stress was experimentally and analytically investigated. Finally, a ballistic test was conducted on a NGW part to confirm the protection performance, which is required for the welding part of armored vehicle.