Estudo de liga de níquel alternativa na união de tubos de aço API 5L X65 revestidos internamente com Inconel 625

Abstract

Most recent technologies for drilling, extraction and transportation of oil from the pre-salt layer require increasingly resistant materials and optimized processes, with reduced costs and high productivity in order to produce joints for pipes/risers. The launch of rigid pipes is one of the most crucial steps and can have big influence over the design and manufacturing of the rigid pipes. Among the several launch methods used by the offshore industry, the Reel Lay method is perceived as an attractive alternative provided that pipe production takes place onshore, resulting in faster launch operations. However, this launch method requires better mechanical properties, including yield strength of the weld metals. Currently, the main challenge being faced is to assure that the nickel alloy being used as weld metal results in high yield strength weld beads, making it possible to use lighter pipes with thinner walls, which ultimately brings economic and operational benefits. The goal of this work is to study the alternative nickel alloy called Inconel 680, designed for the production of welded joints on cladded API 5L X65 pipes, in order to develop parameters of welding for obtaining without defect welds and acceptable initial mechanical properties for the application on rigid pipe weldment designs. The results of this work were obtained through joint welding of API 5L X65 steel pipes internally clad with Inconel 625, using the Inconel 680 alloy as weld metal, using the GMAW welding process. Thermodynamic simulations were used to assess the metallurgical behavior of the alloy. For the metallurgical evaluation of the weld, several samples were taken along the bead’s cross section and analyzed through Scanning Electron Microscopy techniques. In addition, many mechanical tests have been applied: Vickers hardness tests, microhardness maps, Charpy impact tests, bending tests as well as rectangular and all weld tensile tests results were produced to evaluate yield strength and ultimate tensile strength values. Finally, fracture toughness testing was utilized for the construction of J-R curves. Simulation results have shown that the alloy has a wide solidification temperature range and the formation of deleterius precipitation phases was observed, i.e., δ, η, Laves and P phases, presenting hot cracking susceptible behavior. Through metallurgical analysis, solidification cracks and Nb and Mo rich precipitates were identified, which contributes to validate simulation results. The results of this study have show that Inconel 680 alloy filler metal provides a potencial solution for welding API 5L X65 pipes internally clad with Inconel 625 in terms of producing defect free welds and meeting the base metal overmatching requirement for reeling applications in the oil and gas industry. The filler metal Inconel 680 besides excellent results of mechanical properties, such as hardness, impact toughness, yield strength and tensile strength. The J-R curve generated for the regions of the weld metal and the fusion line will serve as a larger critical study, together with the initial design assumptions, to verify the total applicability in the use of weld joints of submarine pipelines on oil and gas industry.