Post weld heat treatment of hypereutectoid hadfield steel: Characterization and control of microstructure, phase equilibrium, mechanical properties and fracture mode of welding joint

In this study, the effect of temperature and time of austempering process on microstructural evolutions, phase equilibrium, mechanical properties and fracture mode of weld metal in Hadfield steel joints were evaluated. For this purpose, the austempering process was carried out at 500, 600 and 700 °C for 15 and 30 min on Hadfield steel weld joints, which were prepared using shielded metal arc welding (SMAW) method. Optical Microscope (OM), Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM) as well as energy Dispersive Spectroscopy (EDS) and X-ray Diffraction spectroscopy (XRD) were used to evaluate microstructural and phase evolutions of the weld metals. Mechanical properties of the weld joints were examined using tensile, Charpy and Vickers microhardness tests. After tensile and Charpy tests, fracture mode of the fractured surfaces was also determined by SEM. Results of the study showed that increasing the time and temperature of the austempering process leads to the increased yield strength, tensile strength, microhardness, proeutectoid (Fe, Mn)3C carbide content and reduction of austenite grain size, plastic deformation capability, fracture energy as well as reduction of the ductile fracture tendency in the weld metal of Hadfield welding joints. According to the results of this study, the most optimal austempering heat treatment cycle which yields the suitable combination of microstructure, phase equilibrium, mechanical properties and fracture mode in the weld metal of Hadfield welding joint relates to the 600 °C and 30 min cycle.