Keywords: Buried Arc Gas Metal Arc Welding, GMAW-B, DH 36 steel, EH 36 steel
Introduction: Shipyards have historically used submerged arc welding (SAW), gas metal arc welding (GMAW), and flux cored arc welding (FCAW) to fabricate butt joints in thick steel plate. For butt joints in steel ½ in. or thicker, beveling of the plates is typically required along with two or more passes. Frequently, two-side welding is required with back gouging and grinding prior to welding the second side. Hybrid laser arc welding (HLAW) enables butt joints in thick steel plate to be produced in a single pass at travel speeds much higher than legacy processes, but high capital expenditures and stringent fit-up are required.
Advances in buried arc gas metal arc welding (GMAW-B) technology has made this process variant a viable alternative to current processes used for welding butt joints in thick steel plate. GMAW-B uses a single filler wire and power source waveform technology to enable butt joints in thick steel plate to be welded in a single pass with minimal plate prep. GMAW-B has been promoted in the past, but erratic behavior and difficulty “tuning” the process have limited its use. Improvements across the spectrum of this technology have made this process variant a potential high-productivity, high-quality method for joining thick steel plates with lower distortion. Because GMAW-B is still in the early stages of commercialization, process and business case data were developed to support potential implementation into the U.S. shipbuilding industry.
Experimental Procedures: The National Shipbuilding Research Program – Advanced Shipbuilding Enterprise funded a 2019 Welding Technology Panel Project to evaluate GMAW-B for shipyard applications. EWI led the project with guidance from several shipyards, the Naval Surface Warfare Center Carderock Division, and OTC DAIHEN. Two flat position butt joint applications were selected for investigation: 3/8-in. EH 36 steel plate welded with ER70S wire, and ½-in. DH 36 steel plate welded with ER70S wire. An OTC DAIHEN GMAW-B system was setup on a side beam. A copper backing bar was designed and built for placement against the underside of the joint. Tests required for NAVSEA procedure qualification of legacy processes were identified for evaluation of weldments produced with the best developed procedures, along with additional tests selected by the project team.
Trials were performed to develop single-sided single-pass GMAW-B procedures for each application that enabled the NAVSEA procedure qualification test requirements of legacy processes to be met. Procedures were developed for 3/8-in. EH 36 steel plate and ½-in. DH 36 steel plate using butt joints with no root opening. Weldments produced using the best developed procedures, were subjected to nondestructive evaluation and mechanical testing. Nondestructive testing included visual, magnetic particle, radiographic, and ultrasonic testing, while mechanical testing included transverse tensile specimens, face bend specimens, and root bend specimens. Although not required by NAVSEA for qualifying legacy processes for the selected applications, impact testing was performed for select weldments. High speed data acquisition was used to calculate heat input.
Trials were also performed to evaluate root opening tolerance and the ability to use procedures developed with copper backing for weldments with ceramic backing.
Results and Discussion: EWI reviewed NAVSEA Technical Publication S9074-AQ-GIB-010/248 to determine tests required for procedure qualification of legacy processes. Because the joint designs used with GMAW-B for the selected applications are not listed in MIL-STD-22D, special NAVSEA procedure qualification approval will be required. For this project, weldments produced with the best developed procedures were subjected to the tests required for procedure qualification of legacy processes.
A single-sided single-pass procedure was developed for welding square groove butt joints in 3/8-in. EH 36 steel plate using no root opening and copper backing. A weldment produced with this procedure met the NAVSEA procedure qualification test requirements for legacy processes. The travel speed for this procedure was 18 ipm and the heat input was 74 kJ/in. Based on trials with a similar GMAW-B procedure, the root opening tolerance with copper backing is expected to be 0 to 1/8 in. for this application.
A single-sided single-pass procedure was developed for welding butt joints in ½-in. DH 36 steel plate using no root opening and copper backing. A weldment produced with this procedure met the NAVSEA procedure qualification test requirements for legacy processes. Weld-metal Charpy V-Notch (CVN) specimens and heat affected zone (HAZ) CVN specimens were prepared and tested at -20°C (-4°F). The weld metal CVN specimens met the requirements for the ER70S-3 wire; the HAZ CVN specimens met the requirements for DH 36 plate. Although not required for complete penetration welds, a joint preparation that consisted of a 5/16-in. root face and 90-deg included angle was used so that the reinforcement requirement for visual testing could be met in the as-welded condition. The travel speed was 18 ipm, and heat input was 71 kJ/in. A second weldment produced using this procedure and ceramic backing met the nondestructive testing requirements for legacy processes.
Heat input and productivity data for weldments produced with the best identified procedures for both applications will be compared to data available for weldments produced with shipyard qualified SAW procedures.
Conclusions: GMAW-B is emerging as a viable alternative to current processes used for welding butt joints in thick steel plate. Single-sided single-pass GMAW-B procedures developed for 3/8-in. EH 36 steel and ½-in. DH 36 steel met the NAVSEA procedure qualification test requirements for legacy arc welding processes. Travel speed for these procedures was 18 ipm. The project results indicate that both copper backing and ceramic backing can be used, and that the process has tolerance to root opening variations. The ability to use the GMAW-B process with both copper and ceramic backing, the process’s expected tolerance to root opening variations, and the ability to use the process with both fixed and portable mechanized and automated equipment make it a candidate for use in panel lines, during ship erection, and onboard the ship.
Acknowledgements: The data in this presentation was developed in a project funded by the National Shipbuilding Research Program – Advanced Shipbuilding Enterprise (NSRP-ASE). OTC DAIHEN provided the GMAW-B system used for this work.