Performance Evaluation of Alternating Current Square Waveform Submerged Arc Welding as a Candidate for Fabrication of Thick Welds in 2.25Cr-1Mo Heat-Resistant Steel

Abstract

The paper evaluates the performance of alternating current (AC) square waveform submerged arc welding (SAW) as a candidate technology for manufacturing thick welds for high-pressure vessels. A new mathematical formulation for calculating melting efficiency in square waveform arc welding is presented. The melting efficiency and the heat consumption are presented as a mathematical model of welding parameters, namely welding current, welding speed, current frequency, and electrode negativity (EN) ratio. The proposed approach is demonstrated through the welding of 2.25Cr-1Mo heat-resistant steel performed over a wide range of welding parameters. The investigation provides deeper insights into the interplay between process parameter, total heat consumption, and melting efficiency. The effect on flux consumption is also explained. The melting efficiency is inversely proportional to flux consumption. The welding heat does not necessarily promote the plate melting. Improper use of welding heat may lead to decreased melting efficiency and increased unwanted melting and consumption of welding flux. Compared to the conventional direct current (DC) power sources, the AC square waveform welding achieves almost the same order of melting efficiency with added advantages of better weld bead shape and flux consumption in a desirable range. The two additional parameters (frequency and EN ratio) of the AC square waveform power source provide more freedom to fine-tune the process and thereby efficiently use welding heat. The results of this investigation will be advantageous to the designers and fabricators of high-pressure vessels using AC square waveform welding.

Parametric Models and Process Map for Alternating Current Square-Waveform Welding of Heat-Resistant Steel

“…Contrary to the conventional SAW, the penetration is less affected by the welding speed, which agrees with the previous results on square waveform SAW. The square form welding can maintain the penetration at higher speeds [18]. The increase in the EN ratio brings in the same changes observed when polarity changes from direct positive to direct negative, such as increased deposition rate and decrease in penetration, dilution, and melting efficiency.…”

“…The square AC waveform is recommended compared to the positive DC electrode (DCEP) for improved toughness in low-temperature environments. Square alternating current waveforms can maintain penetration at higher welding speeds [18]. In combination with DCEP (lead wire), the trailing wire with the AC square waveform in tandem welding improves the deposition rate.…”

“…The theoretical cross-sectional area is calculated as per the following relation [25] where 𝑣 ̂ is the specific volume of weld metal in mm 3 /gram, 𝑞 𝑒𝑞 is the heat required to melt a unit mass of metal, and 𝐻 𝑎 is the heat input per unit length in J/mm. The 𝑞 𝑒𝑞 for AC square-waveform welding of 2.25Cr-1 Mo heat-resistant steel is obtained from the literature [18].…”

“…Square waveform welding has not been extensively investigated for process models as it is one of the latest technologies for SAW. The main areas of investigation for square-waveform welding have been limited to performance evaluation [18], weld bead shape prediction [19], signal processing [20], and thermal modelling [21]. Studies on process models and maps available for conventional submerged arc welding [22] are not reported for square-waveform arc welding.…”

Metallurgical Characterization of Penetration Shape Change in Workpiece Vibration-Assisted Tandem-Pulsed Gas Metal Arc Welding

“…The metallurgical notch occurs at the conjunction of two separate instantaneous weld-pools. The instantaneous weld pools are the results of the varying arcing phenomenon because of the physical separation of the arcs (as in the present case) or instantaneous polarity reversal as in the case of a square sine wave [ 30 ]. On the other hand, both the specimens (w/ and w/o) exhibit typical WM and HAZ microstructure of the low carbon steel.…”