Welding of S1100 Ultra high-Strength Steel Plates with Matching Metal-Cored Filler Wire: Microstructure, Residual Stresses, and Mechanical Properties

This study investigates the relationship between the microstructure and the mechanical properties and the residual stresses of thermomechanically rolled, butt-welded S1100MC ultra high-strength steel plates. Matching metal-cored filler wires are used for metal active gas welding of the plates in horizontal and vertical-up positions. The effect of the heat input on the microstructures of both the weld metal (WM) and the heat-affected zone (HAZ) is studied in detail. The microstructure of the WM consists of different types of ferrite, but the microstructure of the HAZ predominantly consists of fresh martensite at the top and bottom areas. Structure and size of the prior austenite grain boundaries are dependent on the welding positions and affect the impact toughness of the welds. HAZ of the weld produced in vertical-up position is under intensive tensile residual stresses next to the fusion line. Even with matching filler wires, the strength of the welds is insufficient, and fracture of the weld metal occurs because the extended cooling time leads to the formation of acicular ferrite instead of martensite.