Study of Weldability for Aluminide Coated Steels through A-TIG Welding Process

ABSTRACT To achieve higher efficiency in high-temperature environments, 9Cr-1Mo steels (also known as P91 steels) are promising candidates in power plants, petrochemical industries, heat exchangers, and nuclear applications. To enhance the performance against harsh environments, such as high-temperature oxidation and corrosion during operations, adding aluminum oxide (Al2O3)/iron aluminide (FeAl) coating through the aluminizing process has been found promising. Such coatings have been considered reliable for resistance to liquid-metal corrosion against flowing lead-lithium liquid breeders and against tritium permeation in blanket modules of fusion reactors. However, the welding process for aluminized coated steels is a challenge and is scarcely reported. The aluminum-rich coating at the top may alter the metallurgy of weld metal and subsequently the mechanical properties after welding. To investigate the same, preliminary bead-on-plate trials with a novel activated-tungsten inert gas welding process has been attempted for the first time. Bead-on-plate welding samples were prepared (with the same welding parameters) with five different fluxes, such as iron (III) oxide (Fe2O3), nickel (II) oxide (NiO), copper (II) oxide (CuO), cobalt tetroxide (Co3O4), and Al2O3, for aluminized coated 9Cr-1Mo steel. The resultant weld metal microstructure was evaluated using optical and electron microscopy, elemental analysis using energy-dispersive X-ray spectroscopy, phase analysis using X-ray diffraction, and Vickers hardness testing. It was observed that the presence of oxide-type fluxes results in arc constriction, which thereby affects the depth of penetration. The weld metal prepared with Fe2O3 flux demonstrates a superior depth-to-width ratio compared to other fluxes. An analysis for delta ferrite was also carried out and was observed to be present in Fe2O3 flux welds only. A detailed analysis of the weld metal under different activated fluxes is presented in the paper. The outcome of this work presents the feasibility of the welds of aluminized 9Cr steels as relevant for fusion reactor applications.