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This investigation aims to make clear the relationships among geometric features, macrostructures, microstructures, and mechanical performances of the weld bead in the gas metal arc welding (GMAW) process for ASTM A606 type IV weathering steel. The results indicate that with the increase of welding heat input, the geometry of the top reinforcement and penetration increases proportionally until reaching the peak value, and then declines with the further increase of welding heat input; while the values of bottom reinforcement increase continuously with the increase of welding heat input. The average ultimate tensile strength (F), maximum displacement (L), and failure energy (E) of the welded joint produced by the moderate welding heat input are 10.27 kN, 1.88 mm, and 13.55 J, respectively, which are obviously larger than those obtained by the insufficient welding heat input (F: 8.33 kN, L: 1.20 mm, and E: 7.10 J). Although the mechanical properties of the welds achieved by the overlarge welding heat inputs present comparable levels (F: 9.73 kN, L: 1.86 mm, and E: 13.51 J), their standard deviation is much higher. Acicular ferrite, grain boundary ferrite, side plate ferrite, Widmanstätten ferrite, and pearlite are noticed in the fusion zone (FZ), while the coarse grain heat-affected zone (CGHAZ) is composed of upper and lower bainite, grain boundary ferrite, Widmanstӓtten ferrite, polygonal ferrite, and pearlite. As the welding heat input increases, the pre-austenite grain width in the FZ and CGHAZ decreases. The descending order of the microhardness distributed in the welded joints is fusion line, CGHAZ, FZ, fine grain heat-affected zone (FGHAZ), and base metal. In addition, the microhardness values of FZ and CGHAZ decline with the rise of the welding heat input. Nonetheless, no apparent links are found between the microhardness levels of the FGHAZ and welding heat input.

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Electrode degradation in continuous resistance spot welding process of BH 220 steel was evaluated by electrode life test, and weldability tests were conducted by geometry features measurement, mechanical property analysis, and electrode diameter measurement with 88 or 176 weld intervals. The analysis of weld geometry shows that the HAZ width, nugget diameter, and nugget area tend to decrease rapidly, while the nugget height tends to firstly increase with the weld repetitions till the welding heat input becomes too small to form the valid nugget. The peak load shows a rapid downward trend, while failure energy and maximum displacement slightly increase till the 176th weld and then decrease. The cavities and pores in the nugget mainly appear after the 176th spot weld. The electrode diameter increases during welding. The reason for the increase in electrode diameter may be that the contact area between the electrode and BH 220 steel sheets becomes smaller in the welding process, which causes the continuous sticking phenomenon between the electrode and the BH 220 steel sheets. In the absence of alloying, the edge of the electrode is geometrically deformed, while Cu-Zn-Fe alloying occurs in the area in contact with the BH 220 steel sheet.