Microstructural and Mechanical Characterization of the Transition Zone of 9%Ni Steel coated with Inconel 625^®

Abstract:

Carbon steels are coated with Inconel 625^® to comply the pre-salt project requirements. However, this generates micro-regions such as the unmixed zone (UMZ) and a partially mixed zone (PMZ), with the formation related to the dissimilarity between the base metal and the addition metal. The purpose of this article is the analyze of the effect of dilution, PMZ and UMZ and the reheating of these micro-regions on the pullout resistance of 9% Ni steel coated with the Inconel 625® alloy. The UMZ has similar crystallographic directions to the heat affected zone (HAZ) and is BCC equal to HAZ. The PMZ presents more heterogeneous crystallographic directions when compared to the weld metal and is FCC equal to the weld metal. The PMZ nano-hardness is similar to HAZ and up to 42.5% greater than the weld metal. UMZ's nano-hardness is up to 14.9% less than PMZ, up to 48.2% less than weld metal and up to 15% less than HAZ. All specimens had crack propagation in the weld metal and comply the requirement of 140 MPa minimum for shear stress. The dilution presents a correlation with the shear stress.

Key-words:
9% Ni Steel, Inconel 625^®; Unmixed Zone; Partially Mixed Zone; Coating