Effect of heat input on the microstructure and impact toughness of submerged arc weld metal for high-performance weathering steel

Abstract

This study aims to elucidate the appropriate heat input (E j ) range for submerged arc welding (SAW) of high-performance weathering steel. Generally, by increasing E j , the welding efficiency can be improved, but the toughness of the weld metal may be deteriorated. Therefore, SAW was employed to produce the weld microstructure under varying E j from 20 to 50 kJ cm−1. The Charpy V-notch impact tests were conducted at −40 °C, and the weld microstructures were characterized by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD). The results indicate that the weld microstructures consist of polygonal ferrite (PF), acicular ferrite (AF), granular bainitic ferrite (GBF), and martensite/austenite (M/A) constituents under each E j . With the increase in E j , the proportion of PF increases, while AF and GBF are coarsened, and the area fraction (f M/A) and mean size (dM/A) of M/A constituents increase monotonically. Further, the fraction (f MTA>15°) of high-angle grain boundaries (HAGBs) with the misorientation tolerance angles (MTAs) greater than 15° is reduced, while the mean equivalent diameter (MEDMTA≥15°) of ferrite grains with HAGBs increases. Accordingly, with the increase in E j , the impact toughness of weld degrades from 128.4 to 47.6 J. The higher degree of micro-strain concentration caused by the increase in M/A size and area leads to the formation of larger microcracks under small plastic deformation, while the reduced HAGBs have a lower inhibition effect on crack propagation. Finally, the impact toughness decreases with the increase of E j . Overall, the findings suggest that the E j of SAW should not exceed 40 kJ cm−1 in the construction of high-performance weathering steel.