Study of Submerged Arc Weld Metal and Heat-Affected Zone Microstructures of a Plain Carbon Steel

A detailed study on the microstructure of submerged arc (SA) weld metal and the heat-affected zone of a 1.2- cm (0.5-in.) thick plain carbon steel plate was carried out using transmission electron microscopy. The various subzone microstructure observed in the HAZ of a SA weld are spheroidized, partially transformed, grain-refined and grain-coarsened. The grain-coarsened area exhibits predominantly Widmanstatten ferrite with pearlite, while the other subzones of HAZ reveal polygonal ferrite and pearlite. Depending on the number, size and distribution of inclusions, the weld metal microstructure varies. With a larger number of inclusions, grain boundary ferrite and, in absence of inclusion, either side plate with pearlite or cementite along the boundaries of side plates are observed. It is noticed that a limited number of larger size inclusions favor the formation of acicular ferrite. Because of the prevalence of varying cooling rates in weld metal, a wide range of microsturctures, such as periodic pearlite, grain boundary ferrite with pearlite, and side plate with cementite along the side plate boundaries, are observed.