Hyper Duplex Stainless Steels (HDSS) are dual phase, ferritic-austenitic materials with a remarkable yield strength (≥ 700 MPa) and corrosion resistance (PREN/W > 49). It has been developed as an alternative to super-duplex stainless steels, where higher mechanical and corrosion performance is required. Unfortunately, such highly alloyed materials are susceptible to brittle intermetallic phase formation, such as the sigma phase. Understanding the intermetallic formation is essential to obtain its optimal properties and define advantages and limitations for a welding specification. Overlay experiments on three-layered HDSS deposited over a carbon steel plate using 1.1kJ/mm and 1.65kJ/mm shown a clear austenite/ferrite phase ratio difference between the first and last layer. The last deposited layer has a larger ferrite volume fraction and chromium nitride presence. However, no sigma phase was found on the overlay conditions. A sigma phase kinetic model was developed using Thermocalc Prisma, experimentally adjusted, and validated by physical simulation in a series of isothermal heat treatment tests. The additivity rule was used to calculate continuous-cooling-transformation (CCT) curves from the adjusted temperature-time-transformation (TTT) curves. The kinetic model predicts no sigma precipitation for cooling rates faster than 4°C/s. Physical simulation with controlled cooling rates validated the model. Also, the thermal history analysis of the overlay experiments has shown no sigma was expected due to the total time and temperature transformation and cooling rates not reaching the calculated CCT.
High strength steel pipes internally clad with Ni superalloys are components widely used in the oil and gas industry, with distinguished application in the production of rigid steel risers. The joining of clad pipes is carried out through a girth welding process, in which the joint is totally filled with a filler material similar to the internal clad. However, depending on mechanical strength of the pipe substrate, an undermatching condition is created. Thus, a critical weaker region results in the welded joint, where loads and plastic deformations will occur preferentially. In view of this problematic, this work aims to apply an alternative approach as a method of joining clad steel pipes, whereby an overmatching condition is achieved in the joint. This technique is carried out through a double-sided weld, which enables the use of steel filler wire in the joint filling externally, preserving the internal cladding through an inside diameter weld with a CRA filler wire. A C-Mn API X-65 steel pipe, metallurgically cladded with Inconel 625 was used in the study. For joining the pipes, modern welding techniques as GMAW CCC (Controlled Short-Circuit) and GTAW with dynamic wire feeding were applied. The developments showcased weld beads with excellent surface appearance and without welding defects, fact verified by internal videoscopy and cross section macrography. Energy Dispersive X-ray Spectometry (EDS) indicated low iron dilutions, around 2,7% in weight, on the internal weld bead surface. The double-sided welding methodology produces favorable conditions to reach joint properties equalization, enabling the employment of steel filler wire in joint filling, in addition to preserving the internal clad and its corrosion properties.
ResumoEste trabalho analisa a tenacidade e dureza do metal de solda e ZAC da solda circunferencial de tubos X65, X70 e X80 para serviço ácido. Os tubos de elevada resistência, fabricados para serviço ácido, apresentam grande desafio para a soldagem. Os consumíveis e processos devem ser capazes de produzir uma junta soldada de alta resistência, baixa dureza e elevada tenacidade. O objetivo deste estudo foi identificar problemas de amolecimento ou fragilização da ZAC e verificar se a elevada resistência não prejudica a tenacidade e dureza do metal de solda. A metodologia incluiu a soldagem circunferencial e ensaios de dureza e CTOD. Os resultados mostraram que o metal de solda e a ZAC do X65 e X70, são adequadas para o emprego em dutos terrestres. Observou-se um amolecimento na ZAC do X70 e X80 que, de acordo com a literatura, não é crítico para projetos baseados em tensão. Para o X80, o CTOD da ZAC ficou abaixo do valor mínimo desejado, indicando que pode ter ocorrido a formação de zonas frágeis localizadas e resultando na necessidade de estudos complementares para ajustar os parâmetros de soldagem. Os resultados deste estudo servem como base para uma futura aplicação destes materiais em campo. Palavras-chave: Soldagem circunferencial; X70; X80; Serviço ácido. EFFECT OF GIRTH WELDING AT THE WELD METAL AND HAZ OF X65MS, X70MS AND X80MS PIPES AbstractThis paper analyses the HAZ and weld metal toughness and hardness of sour service X65, X70 and X80 girth welds. The high strength steel pipes produced for sour service introduce a big challenge for welding. The process and consumables applied should be able to produce a high strength, high toughness and low hardness welded joint. The purpose of this study was identify HAZ softening and embrittlement and whether the high strength does not affect the weld metal hardness and toughness. The methodology includes pipe girth welding, CTOD and hardness tests. Results showed that HAZ and weld metal of X65 and X70 pipes are suitable for pipelines. The X70 and X80 HAZ presented softening that, according to the literature, is not a critical point for stress based design. The X80 HAZ presented CTOD below the desired value. It indicates that local brittles zones could be present and new tests should be done in order to define better welding parameters.
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