Metodologia para análise metalográfica da região do ponto triplo de tubos mecanicamente revestidos - MLP

Abstract

The use of lined pipes, especially Mechanically Lined Pipe (MLP), in rigid risers and pipelines for ultra-deepwater applications has grown significantly in recent decades. These applications demand a material that can withstand corrosive environments at high pressures and temperatures. The MLP manufacturing process consists of inserting a liner pipe with a highly corrosion-resistant alloy (CRA), usually Nickel 625 alloy, into a carbon steel support pipe, generally API-X65QT. Subsequently, this assembly is subjected to hydrostatic expansion to provide the necessary mechanical bond, and finally, the ends of these pipes are welded together (WO). When performing the transition welding, a region defined as the triple point (TP) is created, which is the metallurgical union between the carbon steel pipe, the liner pipe, and the weld. This region (TPZ) is potentially the most susceptible to defect growth and, therefore, must be subjected to high quality control. The Petrobras technical specification I-ET-0000.00- 0000-219-P9U-001 recommends maximum values for discontinuity height and gap in the TP region, and that the TPZ region shall be essentially free of carbides, nitrides, and intermetallic phases, limiting their content to 1% in a 400x field of view. In this work, a well-defined metallographic characterization methodology was developed to meet the quality required in MLP manufacturing standards. Different microscopic techniques (OM and SEM) and four different chemical etching techniques were used to count second-phase elements such as carbides, nitrides, and intermetallic phases. There was a good correlation in the second-phase count between the two techniques used (OM and SEM); however, SEM provided greater precision in the identification and quantification of the phases. For measuring the gap between the liner and the carbon steel and other discontinuities in the TPZ region, the best option was to work with a polished sample only. And for second-phase counting, the most accurate chemical etching method on Nickel 625 alloy MLPs was oxalic electrolytic etching. The study concludes that this methodology is promising for inspecting the TPZ region in MLPs.