Effects of Nb Addition on the Microstructure and Mechanical Properties in Friction Welded Joints of Non-Quenched and Tempered Steels

In this study, the effects of Nb addition on the microstructure and mechanical properties of friction welded joints of non-quenched and tempered (NQT) steels were investigated. For this purpose, NQT steels with different Nb content were bonded with high carbon S45C steel by friction welding. The microstructures of welded joints were characterized by optical microscopy (OM), scanning electron microscopy (SEM), electron back-scattered diffraction (EBSD), and transmission electron microscopy (TEM). For evaluating the mechanical properties of welded joints, the tensile, Charpy U-notch impact, and Vickers hardness testing were performed. There is no fusion zone in the friction welded-joints, and thermomechanically affected zone (TMAZ) and heat-affected zone (HAZ) are observed as the distance from the bond line between NQT steels and S45C steel increases. The microstructures in TMAZ and HAZ of S45C steel consist of grain boundary ferrite (GBF) and pearlite. NQT steels consist of bainite and GBF in TMAZ and HAZ, and the effective grain sizes in TMAZ is larger than those in HAZ. Nb addition contributed to fine precipitation of MC carbide in NQT steel, making TMAZ and HAZ structure fine and finally enhancing mechanical properties in friction welded-joint.