Electron Backscattered Diffraction Characterization of S900 HSLA Steel Welded Joints and Evolution of Mechanical Properties

In welding of high-strength steels by conventional methods, it is essential to preserve the strength and ductility close to the base material. The primary focus of this research was to investigate the electron backscattered diffraction (EBSD) characteristics in the coarse grain heat-affected zone (CGHAZ) of S900 base material and correlate them with the changes of mechanical properties of the welded joints. For this purpose, the S900 high-strength low alloy steel was welded with different heat inputs. The thermal cycles were obtained using the Simufact welding simulation software. Scanning electron microscope and EBSD analyses were used to evaluate the microstructure, and tensile tests were used to assess the mechanical properties. The results showed that reducing the welding heat input changed the failure region from the CGHAZ to the base material. The depression of heat input can decrease the deleterious effect of the welding process and lead to achieving superior mechanical properties for the welded joints. Reducing prior austenite grain size in CGHAZ, forming less granular bainite, and maintaining the dislocation density high enough in CGHAZ were the main reasons for improving the mechanical properties of the joints welded at lower heat input.