Effect of acicular ferrite on cracking sensibility in the weld metal of Q690+Q550 high strength steels

Abstract

High strength steels Q690+Q550 were welded using three different welding wires without preheating by gas shielded arc welding process. Microstructural characteristics of weld metals and nucleation mechanism of acicular ferrite were investigated by means of scanning electron microscope (SEM), energy dispersive spectrum (EDS) and transmission electron microscope (TEM). The straight Y-groove test and impact test were employed to assess cracking resistance and impact toughness of joints. Results indicated that the addition of Ti and Cr in the MK·G60-1 weld metal suppressed the formation of pro-eutectoid ferrite and ferrite side plate and contributed to the formation of acicular ferrite. The inclusions composed of (Ti, Mn) oxides and Mn, Cu sulfides became nucleation sites of primary acicular ferrite. Secondary acicular ferrites were nucleated on some pre-existing ferrite laths. Impact toughness and cracking resistance were enhanced with the increase of acicular ferrite in the weld metal. © 2011 ISIJ.