Undermatched Welding of Ultra-High-Strength Steel S1100 with Metal-Cored Wire: Influence of Welding Positions on Mechanical Properties

Ultra-high-strength steel significantly contributes to lightweight design. However, it is essential that excellent mechanical properties are maintained after the applied welding process in terms of structural integrity, strength and ductility. This study investigates the microstructural and mechanical properties of metal active gas-welded X-joints of 20-mm-thick S1100 thermomechanically rolled plates with undermatched alform 960 L-MC filler metal. Welding was carried out fully automated in order to obtain uniform properties of the welds for different heat inputs in flat (PA) and vertical up (PF) positions. The mechanical properties of weldments were characterized by hardness, impact and tensile tests and complemented by microstructural analysis using optical and scanning electron microscopy. Different heat inputs due to altered welding positions caused changes in prior austenite grain size and orientation in the weld metal (WM) microstructure which is presented and discussed in detail. The microstructures of the WM consist of ferrite, while the heat-affected zone (HAZ), which was exposed to several thermal cycles, is dominated by bainite and martensite-austenite (M/A) constituents. While in the HAZ the top layer shows fresh martensite of high hardness, the HAZ of filler and root passes was tempered showing lower hardness values. Transverse tensile tests always failed in the weld metal not fulfilling base materials strength requirements. Although the weld metal microstructure consisted of acicular ferrite in PA and PF position, PF weld has lower toughness.