Effect of weld metal microstructures on cold crack susceptibility of FCAW weld metal

The effect of microstructures on weld metal cold cracking has been studied using flux-cored are welding (FCAW) wires with a 600 MPa strength level. Two FCAW wires were prepared by controlling the Ni content to allow a sufficient variation in weld microstructures, but with little change in the weld metal strength. The microstructural analysis showed that there was a significant difference in the proportions of the microstructural constituents of the weld microstructure. The 1.5%Ni wire resulted in a weld microstructure with a lower grain boundary ferrite (GF) content associated with an increased proportion of acicular ferrite (AF) than the 0% Ni wire. The GF contents of 0%Ni and 1.5%Ni weld metal were measured to be 19% and 6%, respectively. The cold crack susceptibility of these two FCAW wires was evaluated using the gapped bead-on-plate (G-BOP) test at the two different levels of diffusible hydrogen content As a result of the G-BOP tests, it was demonstrated that the 1.5%Ni wire had better resistance to cold cracking than the 0% Ni wire because its weld deposit had a lower GF phase content. This implies that, in addition to the hydrogen control approach, microstructural modification can be pursued to develop new welding consumables with an improved resistance to cold cracking. In the discussion, the detrimental effect of GF against cold cracking is addressed based on the microstructural characteristics of cold cracks in ferritic weld metal.