Evaluation of Microstructure and Mechanical Properties of Seamless Steel Pipes API 5L Type Obtained by Different Processes of Heat Treatments

© 2017 Materials Research. 2017; 20(2): 514-522 DOI: http://dx.doi.org/10.1590/1980-5373-MR-2016-0545 Evaluation of Microstructure and Mechanical Properties of Seamless Steel Pipes API 5L Type Obtained by Different Processes of Heat Treatments Leonardo Barbosa Godefroida*, Betânia Mendes Senaa, Vicente Braz da Trindade Filhoa a REDEMAT, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil Received: July 18, 2016; Revised: December 12, 2016; Accepted: February 02, 2017 This research presents the influence of manufacturing processes and heat treatments on the resulting microstructures and mechanical properties of an API 5L PSL2 seamless steel pipe. Three different conditions are considered – as rolled, normalized and quenched and tempered - to obtain the grades X42R, X42N and X70Q, respectively. Scanning electron microscopy techniques was used to characterize the resultant microstructures. Tensile, hardness, impact, fracture toughness (J integral) and fatigue crack growth tests (da/dN x ΔK) were used to study the materials mechanical behavior. The results show the possibility of achieving API grades for a seamless pipeline steel, through suitable heat treatments. The microstructural modifications and mechanical properties changes observed showed a remarkable structure-properties relationship of the steel, and attempt to a proper selection as required by the structural design. The quenching and tempering process increased tensile mechanical properties and fracture toughness, but combined to a significant decrease in fatigue crack growth resistance. Keywords: Seamless steel pipes; API 5L line pipe; Fracture toughness; Fatigue crack growth 1. Introduction High strength low alloys steels (HSLA) have been used for the production of pipes for more than 30 years. However, the alloy design of pipeline grades is being continuously modified and the process technology optimized because of increasing demand of high strength-toughness combination requirement of pipeline steels1-11. This demand is related to the increase of oil and gas world production and the consumption of their products. To achieve this demand, it is necessary that the pipes have large diameters and work under high internal pressures in order to increase the transportation efficiency, and avoid the use of very high wall thicknesses in order to reduce its weight and to decrease the project cost. The microalloyed steels are obtained by thermomechanical processing (in case of welded pipes) or heat treatment, e.g. quenching and tempering heat treatment (in case of seamless pipes), and have a characteristically microstructure consisting of well-selected phases and refined grain sizes. The American Petroleum Institute (API) provides standards for pipe that are suitable for use in conveying gas, water, and oil in both the oil and natural gas industries. The API 5L 12 specification describes the requirements of chemical composition, tensile test characteristics and impact toughness behavior. The property requirements of steel vary depending on the particular application and operating conditions. The basic requirements, however, are high mechanical strength together with superior toughness at low temperature and excellent weldability. It is also important that steels should exhibit superior corrosion resistance. * e-mail: Cracks can nucleate and propagate in pipelines during its operation, while the structure can be subjected to operational static overloads or cyclic loads, and a catastrophic failure can occur. In this context, the knowledge about the resistance to fracture of steels is very important, to provide information for pipeline design and materials selection during construction and predict the operational life of pipeline. To evaluate fracture properties of pipeline steels, various laboratoryscale testing methods have been studied since 1980s. Among them, Charpy V-notch impact test (CVN) and drop-weight tear test (DWTT) are most widely used to characterize the resistance to static loads, while S-N curves and ε-N curves represent the fatigue behavior of the steel. However, these methods are not based on fracture mechanics concepts to evaluate fracture toughness and fatigue resistance, and their data have large deviations because they largely depend on the specimen size and geometry. Little information is available on fracture toughness 13-19 and fatigue crack growth resistance20-22, mainly on near-threshold fatigue crack growth behavior. It is also important to note that the publications are mainly concentrated on welded microalloyed steel pipes, obtained from thermomechanical operations (controlled rolling), with a characteristically complex microstructure. Regardless of the manufacturing process, the technical literature assigns an improvement of mechanical properties to the strict control of chemical composition (especially inclusion content and shape control, presence of microalloyed elements), lower volume fraction of M-A constituent, smaller effective ferritic grain size, presence of acicular ferrite, refinement of the martensitic structure 14,18,23-31. Evaluation of Microstructure and Mechanical Properties of Seamless Steel Pipes API 5L Type Obtained by Different Processes of Heat Treatments Conventional heat treatments applied to seamless steel pipes result in special microstructures that allow greater mechanical resistance and fracture toughness than thermomechanical processing applied to welded steel pipes, showing the advantage of manufacturing seamless steel pipes when compared to welded steel pipes. Consequently, it can observe that seamless steel pipes do not require high grades as welded pipes because intermediate grades for the seamless pipes have similar mechanical properties to the high grades for the welded pipes. There is a huge advantage in the manufacturing process of seamless steel pipes, because from a relatively simple steel, as the steel in this research, and with the assistance of conventional heat treatment processes, it is possible to achieve intermediate grades, unlike welded pipes that require more elaborate and rigorous chemical composition in addition to a specific thermomechanical processing with many processing parameters such as reheating temperature, percentage reduction, deformation temperature, cooling rate, and coiling temperature. Unfortunately, much of what is studied about seamless steels is not published due to industrial restrictions and information security, but engineers and researchers in the Vallourec Group (one of the most important world producer of seamless steel pipes) has access to this information and mention the advantages of this route for the production of pipes in their papers 32-34. Therefore, the present research was carried out to evaluate the behavior of an API 5L PSL2 seamless pipeline steel manufactured by one of the Vallourec steel plants in Brazil. Three different processing routes were adopted – hot rolling, normalizing heat treat (...truncated)