Microstructure homogenisation by adapting the melting behaviour of flux-cored wires in GMAW

Abstract

The composition of filling-alloyed flux-cored wires is achieved by adjusting the filling components without changing the wire material. A disadvantage of this type is, that due to the high alloy concentrations in the filling, microscale segregation can occur in the melt pool. The ‘theory-of-two-melts’ is cited in the literature as the reason for this. This theory states that, due to the typical separate melting behaviour of filling-alloyed flux-cored wire, there are droplets of different compositions (melt A and melt B) which do not homogenise in the melt pool and cause these microscale segregations. To test this theory, the melting behaviour of filling-alloyed flux-cored wires and the homogeneity of the individual droplets were investigated in this work by using the indicator material P92 (9% chromium steel). For this purpose, the liquid melting droplets were separated and then analysed metallographically via optical microscope and chemically using energy dispersive X-ray spectroscopy. Based on this, the melting behaviour was then optimised by adjusting the wire temperature. For this purpose, two different methods (conductive heating before and after current contact nozzle – CCN) for wire preheating were analysed. On the one hand, the results show no deviations in the chemical composition between the individual drops. On the other hand, chromium-rich micro segregations could already be detected directly in the droplets. The ‘two-melt’ theory as the cause of the microscale inhomogeneities can therefore not be confirmed. However, it was shown that an increase in the wire temperature, regardless of the preheating system, leads to a reduction of the chromium-rich inhomogeneities in the droplets by up to 95%. This effect was strongest in the presence of a melting behaviour similar to solid wire, which could be realised by increasing the contact tip working distance to 70 mm.