CHEMISTRY and Crystallographic Evaluation of NI-BASED ALLOY and STEEL Dissimilar Interface

Dissimilar metal welds have large applications in various industrial sectors like oil & gas and nuclear for both joining and overlay. In this sense, the fusion boundary microstructure plays an important role to ensure the integrity of weld overlays. The present investigation aims to evaluate the differences in chemical composition and crystallography of microstructure observed at the interface of dissimilar welds produced with the Inconel 625 superalloy and ASTM A516 Gr 60 steel from the solidification process of the molten pool. The weld overlays were performed in a single layer on steel substrate using TIG cold wire feed process. The metallographic analyses were carried out in samples taken from the cross section of the coatings deposited. The microstructural characterization was performed using the techniques of light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and electron backscattered diffraction (EBSD). Hardness and bond strength tests along the interface were performed to investigate possible effects on the mechanical property. Simulations were also performed using the Thermo-calc program to assist in understanding the solidification process. The microstructure of the interface features, in general, a well-defined interface. In the weld metal was observed the presence of a narrow clear zone along the interface. Type II boundary parallels the fusion boundary typically observed in dissimilar welds were not found in this work. Through chemical analysis of EDS was able to map the chemical composition of the clear zone, which has a transition in terms of chemical composition between steel and alloy Inconel 625. Based on martensite crystallographic texture measured by EBSD was possible to calculate the austenite orientation at high temperature and compare with Ni-fcc grain orientation indicating there is a crystallographic relationship.