Estudo das zonas parcialmente misturadas de aços 9Ni soldados com a liga Inconel 625 sob influência da energia de soldagem

Abstract

9% wt Ni steels have been widely used since the 1960s as construction materials for Liquefied Natural Gas (LNG) storage tanks; and currently, they have been applied in liquid CO2 reinjection systems owing to their low ductile-fragile transition temperatures (DBTT) for cryogenic situations - up to -196°C and pressures up to 500 bar; and, therefore, have good impact toughness. The necessity for good mechanical properties in critical points of those systems implies welding 9Ni steels with materials resistant to harsh environments, such as nickel alloys. The welding of dissimilar materials causes the formation of a zone adjacent to the fusion line, a few microns long, called the Partially Mixed Zone, which has a chemical composition gradient and high hardenability, with high chances of formation of high-hardness martensite, in case of being base metal rich. Moreover, this zone has different solidification modes due to this chemical composition and temperature gradient from the welding process. In view of this complexity, the present work aimed to evaluate the mechanical, microstructural and microchemical properties of the interfaces of welded joints of ASTM A333 Gr.8 (9Ni) steel with Inconel 625 alloy, by the GMAW process, under the influence of heat input. The SEMBSE images confirm the Partially Mixed Zones’ existence and the microhardness changes that occured between the HAZ and FZ of the analyzed regions. In addition, martensite formation was observed, along with planar and cellular solidification mode regions in the PMZ's. The EDS mappings confirm the segregation of Ni, Cr, Mo and Nb elements to the interface, justifying the increase in local hardenability. The Ni element was determinant for the microstructural relationship of the PMZ's, since it can affect the local hardenability and stabilize the austenite phase, implying the decrease of hardness values in the region. The conditions of lower (0,7 kJ/mm) and intermediate (1,5 kJ/mm) welding heat input presented the most detrimental conditions for the use of these materials - due to the presence of more significant extensions of PMZ, as well as higher values of microhardness, leading to loss of toughness. The condition of higher welding energy (2,5 kJ/mm) had the lowest values of extension and microhardness of the PMZ.