Effect of Filler Metal Type on the Microstructure of Welded Joints of HP Heat-Resistant Steel Modified by Niobium

In this research, HP heat-resistant steel modified by niobium was welded using the GTAW process and ER NiCr-3, ER NiCrMo-3, and MF filler rods. The microstructure of the base metal, weld metals, heat-affected zone, and interface between the base and weld metals was studied using optical and electron microscopes (FESEM) equipped with an EDS point analysis system. The microstructure of HP steel base metal with an austenitic matrix and chromium-rich and niobium-rich precipitates on the grain boundary was observed. All three weld metal microstructures have a fully austenitic matrix that is dendritically solidified. In all weld metal produced by three different weld wires, precipitates rich in chromium (M23C6 carbide) and precipitates rich in niobium and titanium (MC carbide) were observed. The microstructural studies showed that the interfaces between the base metal and weld metals are fully continuous and free of cracks and voids. An unmixed zone (non-homogeneous zone) with a large width was formed at the interface between the base metal and the ERNiCr-3 weld metal. Any unmixed zone was not observed at the interface of the base metal and weld metals of the ERNiCrMo-3 and MF steel. Epitaxial growth was observed at the interface between the base metal and weld metals of ER NiCrMo-3 and MF steel.