Microstructural Characterization of Thick Walled Ultra High Strength Steel S1100 Welded in Different Weld Positions

Using ultra high strength structural steels enables not only light weight constructions, by reducing sheet thickness and thereby weight and filler material but also contributes to achieving current challenges in different fields of technology. Due to their accurately tuned chemical composition and precisely controlled thermo-mechanical processes during the production, a very efficient microstructure with respect to strength and ductility is maintained. Various strengthening mechanisms are applied to achieve the desired strength, ductility, toughness and fatigue properties.
Joining processes for these steels are of particular importance in many fields of application. Welding in the horizontal (PA) and vertical (PF) position play an important role in production or large structural components such as cranes, crane booms, concrete pumps, pressure vessels, oil/gas transportation pipes, shipbuilding and offshore industries. However, different welding positions require different parameters with heat input summarizing the most important ones. This study focuses on the microstructural changes depending on heat input of MAG weldments of S1100 using matching metal cored filler material. Welding process was performed fully automatic to accomplish uniform weldments in PA and PF positions with different heat input values according to their own characteristics. The effects of heat input on weld metals and heat affected zones (HAZ) were investigated in detail. The microstructures of the fusion zone obtained consisted of ferrite forms and the heat affected zone was predominately martensite and tempered martensite. Prior austenite grain boundaries (PAG) structure differs according to applied welding positions.