Marcelo Torres Piza Paes

Study of microstructural evolution of friction taper plug welded joints of C–Mn steels

Abstract This paper describes the microstructural evolution of friction taper plug welded joints of C–Mn steels. Experimental and numerical analyses included calculations based on Calphad and continuous cooling transformation curves, and characterisation techniques. The studied friction taper plug welded joint contains three macroregions: plug material, thermomechanically affected zone (TMAZ) and base material. The thermomechanical conditions imposed in the studied friction taper plug welded joint precluded the formation of a heat affected zone. However, seven subregions were identified within the TMAZ region and details are discussed. The interface zone is found in the TMAZ region, where the most relevant phase transformations take place. It is suggested that the phase transformations in TMAZ region depend on local conditions, such as chemical composition, deformation rate, thermal history and the previous thermomechanical history of the parent materials.

Microstructure evaluation of UNS S32205 duplex stainless steel friction stir welds

UNS S32205 duplex stainless steel welds were performed by friction stir welding (FSW). Advancing and retreating sides showed distinct characteristics in the welded joint. The advancing side shows the strongest grain refinement which is corroborated by microhardness measurements. The microstructure characterization was carried out by optical, scanning and transmission electron microscopy. The thermomechanically affected zone displays austenite islands deformed in a ferrite matrix. The stir zone (SZ) showed a fine recrystallized microstructure providing an outstanding increase of hardness associated with better corrosion performance. Transmission electron microscopy and corrosion tests have corroborated the absence of intermetallic phases on welded joints.

Duplex Al-based thermal spray coatings for corrosion protection in high temperature refinery applications

The application of thermal spray coatings has been effective in preventing corrosion of steel and iron products. It has been used in a wide range of applications spreading from the petroleum to the food industry. In this work, the performance and effectiveness of a two-layered aluminum-based thermal spray coating applied to an ASTM A387 G11 steel was evaluated. The coating structure was comprised of an inner Al-Fe-Cr layer and an outer layer of aluminum. Coated samples were tested in the reactor zone of a fluid catalytic cracking unit (FCCU) of a petrochemical plant for 2.5 years. The reactor zone temperature was about 793 K (520 °C) and the environment was a mixed gas containing sulfur, oxygen and carbon. Laboratory-scale tests were also conducted on the coated samples in order to gain a better understanding of the corrosive effect of the gaseous species present in the FCCU atmosphere. Porosity present in the thermal spray coatings allowed the penetration of the atmosphere corrodents, which instigated intergranular corrosion of the steel substrate. The presence of an inner Al-Fe-Cr layer did not prevent coating spallation, which further contributed to the internal corrosion process.

Corrosion resistance of a steel under an oxidizing atmosphere in a fluid catalytic cracking regenerator

In the present work, the corrosion resistance of an ASTM A 387 G11 steel was evaluated under two conditions: an oxidizing atmosphere in a fluid catalytic cracking regenerator of a petroleum processing unit and a simulated atmosphere in the laboratory, at temperatures of 650 °C and 700 °C. The characterization of the phases present in the oxidized layer was carried out by X-ray diffraction (XRD), optical microscopy (OM) and scanning electron microscopy (SEM) with X-ray energy dispersive analysis (EDS). Severe corrosion was observed after exposure to both the real and simulated conditions, with formation of several iron oxides (Fe2O3, Fe3O4 and FeO) in the product scale layer, as well as a slight inner oxidation and sulfidation of chromium in the substrate. Internal nitridation of the silicon and the manganese was observed only in the real condition, probably related to the long-term exposure inside the regenerator.

Rotation Control Performance of a Friction Welding Repair System Using a Neural Network

This paper presents the rotation control performance of a friction welding repair system. The control parameters are adjusted by a neural network (NN). This control is capable of supporting the wide variations in torque that occur in this type of repair process. The rotation system consists of a hydraulic motor whose speed is controlled by a proportional directional valve. For this, a NN was developed with various structures composed of one, three, four and six neurons having activation functions of the following type: linear, quadratic, sigmoid and step. A theoretical plant was developed that simulates the behavior of the rotation system of a friction drive unity. This plant was used in the adjustment of the weights of the NNs by means of a genetic algorithm (GA). This plant was also used to evaluate the performance of NNs in the rotation control after their adjustment. The choice of the most suitable NN was made taking into consideration not only their control performance, but also the number of parameters to be adjusted, because the higher the number of these parameters, the greater the difficulty faced by the GA in the adjustment. Having selected the NN, its performance was compared to that of a PID controller. It was observed that the NN that reached the best performance was of one neuron with a linear activation function and its performance was higher than that of the PID controller.

Friction stir welding of steels for the oil and gas industry

Friction stir welding (FSW) has become a viable and important manufacturing alternative in several industries. This solid-state welding process offers considerable improvements in the mechanical properties of the joint. At the beginning it was developed as a joining alternative for light Al and Mg alloys and it eventually evolved to higher melting temperature alloys, such as steels, stainless steels, titanium alloys and Ni-based alloys. Most of these alloy systems are widely used in the oil and gas industry, where dissimilar joining among them is not uncommon. Conventional arc welding processes and more recently hybrid combinations with laser welding are widespread or under development to be used in these industries. However, metallurgical issues associated with the solidification process and hydrogen embrittlement impair the weldability of most of these high melting temperature alloys. Therefore, FSW is an interesting alternative to overcome some of the challenges associated with the similar and dissimilar joining of structural steels, stainless steels, and Ni-based alloys, especially for circumferential joining of pipelines. Recent developments on weldability studies of pipeline steels seeking the technology deployment will be presented ranging from parameters development, microstructural characterization, and fracture toughness to process robustness evaluation.

Fracture Toughness of X-60/Alloy 625 Welded Joints

Line pipes internally clad by nickel alloy are being considered as steel catenary risers when exploitation of corrosive oils is envisaged and corrosion-fatigue failures are possible. The presence of the Ni alloy as internal clad requires a matching filler metal and, consequently, the girth welds are dissimilar joints. Although these types of welds are widely used in the petroleum industry their fracture toughness are not well documented. Of particular concern is the formation of hard zones in HAZ which are usually associated with low fracture toughness. Joints of API X-60 pipes welded using NiCrMo-3 consumables were CTOD tested at 0°C. Results consistently revealed high fracture toughness values for the weld metal. The HAZ-FL, on the other hand, presented lower values and they were significantly dispersed. Microstructural and fractographic analyses revealed that low values of toughness are associated with local brittle zones (LBZ) mainly IRCGHAZ. The fracture of the specimen that presented the lowest toughness initiated in a partially mixed zone containing hard martensite.Copyright

Evaluation of Abrasive Wear of Polymeric Materials Using Single-Pass Pendulum Scratching

Scratching abrasion due to rubbing against the sediment layer is an important degradation mechanism of flexible cable in deep water oil and natural gas exploitation. The present study was initiated to gain relevant data on the wear behaviour of some commercial materials used to externally protect these cables. So, Comparison tests were carried out using the single-point scratching technique, which consists of a sharp point mounted at the extremity of a pendulum. The energy dissipated during the scratching is used to evaluate the relative scratch resistance. The results showed, that the contact geometry strongly affects the specific scratching energy. Using SEM imaging, it was found, that these changes were related to the operating wear mechanisms. The observed wear mechanisms are also compared with those observed on some cables in deep water operations.© 2002 ASME