Mechanical and microstructural behavior of C-Mn steel weld deposits with varying titanium contents

Abstract

The effect of micro-alloying on the microstructure of C-Mn steel weld metals has been extensively studied in the past and the effect of titanium, in particular, has motivated several investigations, due to its essential contribution to the occurrence of acicular ferrite. However, there are cases where a substantial amount of acicular ferrite present in the last pass is not directly related to higher impact energy, since other parameters such as the proportion of the transformed phases by reheating, occurrence of micro-phases and non-metallic inclusions also influence the toughness of weld metal. Based on this scenario, the current work complements previous investigation and proposes an alternative methodology for evaluation of the behaviour of microstructure and impact toughness of C-Mn steel weld metals containing varying titanium contents, in the range of 6 and 255 ppm, based on comparison with the microstructural characteristics observed at the Charpy-V notch, where impact toughness was measured.The weld metal microstructure was characterized at the top bead and at the mid-thickness by Optical (OM), Electron Microscopy (SEM) and electron backscattered diffraction (EBSD). Vickers microhardness tests were performed at the same positions where metallographic examination was done. Charpy-V impact tests employing test samples removed from the center line and at mid-thickness of the weld deposits were carried out in order to obtain test temperatures corresponding to impact energy of 100 joules.The results evidenced that the increase of titanium content promoted a substantial amount of acicular ferrite and the refinement of microstructure at the top bead. At the Charpy-V notch location, the same behavior was observed, the difference being relative to the lower amount of acicular ferrite. Optimum impact toughness was attained for 28 ppm Ti, although the metallographic examination revealed that the higher amount of refined acicular ferrite presenting a lower effective grain size and increased frequency of high angle boundaries was observed for 255 ppm Ti. From these findings, a methodology involving OM, SEM and EBSD for an adequate characterization of the microstructure at the Charpy-V notch location is applied, in order to evaluate the influence of reheating, microstructure, effective grain size, frequency of high angle boundaries, presence of MA constituents and inclusions. The analysis revealed that the behavior of impact toughness is associated with the presence of the M-A constituents occurring at the Charpy-V notch for 255 ppm Ti.The obtained results indicate the need for a more stringent procedure for an explanation of the behaviour of the impact properties of C-Mn steel weld metals, due to the role of the thermal cycle build-up of multi-layer weldments. Keywords: Microstructure, Mechanical properties, Weld metal, Titanium, Acicular ferrite