Influence of weld overlaying methods on microstructure and chemical composition of Inconel 625 boiler pipe coatings

The aim of this work was to identify the impact of the welding method of boiler pipes made from P235GH and 16Mo3 steels on the microstructure and chemical composition of coatings. The investigations were carried out on a boiler pipes weld overlaid by Inconel 625 welded under various conditions (cold metal transfer (CMT), gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW)). The microstructural examinations were performed on longitudinal cross-sectioned samples. The qualitative and quantitative chemical composition analyses on metallograpic samples were determined on SEM by means of energy dispersive spectrometry (EDS). The investigations showed that regardless of welding method the overlays consisted of the following microstructural zones: fusion zone, partially mixed zone, partially melted zone and heat-affected zone. The content of Fe, observed in all deposited coatings, was higher than the concentration of Fe in the Inconel 625 wire, however, it was within the basic technological requirement for the Fe content (< 5 wt.%). The Fe content was clearly higher in the partially mixed zone than in the fusion zone and decreased with the distance from the interface towards the coating surface. Close to the coating surface the highest Fe content was identified in the weld overlays that exhibited the least uniform fusion boundary and the least smooth outer surface. The most uniform external surface was identified in the coating deposited by the GMAW and subsequently fused by the GTAW method.