Materials Science Forum Vol. 758

Abstract: Welding is a complex process where localized and intensive heat is imposed to a piece promoting mechanical and metallurgical changes. Phenomenological aspects of welding process involve couplings among different physical processes and its description is unusually complex. Basically, three couplings are essential: thermal, phase transformation and mechanical phenomena. Welding processes can generate residual stress due to the thermal gradient imposed to the workpiece in association to geometric restrictions. The presence of tensile residual stresses can be especially dangerous to mechanical components submitted to fatigue loadings. The present work regards on study the residual stress in welded superduplex stainless steel pipes using experimental and a numerical analysis. A parametric nonlinear elastoplastic model based on finite element method is used for the evaluation of residual stress in superduplex steel welding. The developed model takes into account the coupling between mechanical and thermal fields and the temperature dependency of the thermomechanical properties. Thermocouples are used to measure the temperature evolution during welding stages. Instrumented hole drilling technique is used for the evaluation of the residual stress after welding process. Experimental data is used to calibrate the numerical model. The methodology is applied to evaluate the behavior of two-pass girth welding (TIG for root pass and SMAW for finishing) in 4 inch diameter seamless tubes of superduplex stainless steel UNS32750. The result shows a good agreement between numerical experimental results. The proposed methodology can be used in complex geometries as a powerful tool to study and adjust welding parameters to minimize the residual stresses on welded mechanical components.

Abstract: Welding is a fabrication process widely used in several industrial areas. The welding of metallic alloys presents some basic characteristics as the presence of a localized intensive heat input that promotes mechanical and metallurgical changes. Different from conventional welding processes, where macroscopic fusion is observed, friction welding is a solid state welding process where the joint is produced by the relative rotational and/or translational motion of two pieces under the action of compressive forces producing heat and plastic strain on the friction surfaces. Friction Stir Welding (FSW) process has received much attention for its special characteristics, like the high quality of the joints. Although there are several experimental works on the subject, numerical modeling is not well stated, as the process is very complex involving the coupling of several non-linear phenomena. In this contribution a tridimensional finite element model is presented to study the temperature distribution in plates welded by the FSW process. A weld heat source is proposed to represent the heat generated during the process. The heat source model considers several contributions present in the process as the friction between the tool and the piece and the plastic power associated to the plastic strain developed. Numerical results show that the model is in close agreement with experimental results, indicating that the model is capable of capturing the main characteristics of the process. The proposed model can be used to predict important process characteristics, like the TAZ (Thermal Affected Zone), as a function of the welding parameters.

Abstract: The present work is part of a wide research program which the main goal is the development of welding procedures for chain and accessories for application in mooring systems of oil platforms. In the specific case of the work in subject, the development of different covered electrodes formulations is discussed for obtaining high mechanical strength and impact toughness, of the order of 860 MPa and 50 joules at –20°C, respectively. Welded joints using the developed electrodes were prepared for evaluation of the mechanical properties, using preheat of 200°C, direct current, flat position and heat input of 1.5 kJ/mm. After welding, tensile, impact Charpy-V and hardness tests were performed in specimens removed integrally from the weld metal, both in as welded and heat treated conditions. The post weld heat treatment (PWHT) was conducted at 600°C for 1, 2 and 3 hours. The results shows that the obtained weld metals have mechanical properties higher than the minimum required for the welding of a IACS W22 R4 Grade steel, and particularly good impact properties, which indicates that the correct control of the chemical composition, particularly, of Mn-Ni balance, makes possible to achieve an adequate strength/toughness relationship for high strength steel weld metals, where the PWH is mandatory. In addition, it was verified that the increase in the time of PWHT did not promote substantial impairment on mechanical properties.

Abstract: The main objective of this work is to propose an optimization of welding parameters through the process of resistance spot welding, applied to Interstitial Free (IF) steel sheets in order to increase the life of conventional copper electrodes. The methodology consisted of using a weld tester in order to ensure the accuracy of values. The main parameters selected through a survey of the weldability were: force, time and welding current. To evaluate the resistance of the weld and the electrode wear, mechanical tests were performed like tensile-shear, cross-tension, peel test and microhardness. A microstructural evaluation by optical microscopy (OM) and scanning electron microscopy (SEM) was made. The results show reducing the thickness of the coating in the weld region and macro and microstructures were observed. Through the tensile-shear and cross-tension tests were verified using electrodes caps up to 1250 points with acceptable welds. The microhardness test results indicated significant hardness increase in weld nugget elucidated by differences in microconstituents evaluated by OM and SEM. The methodology used for parameter selection has highlighted an optimal combination with 200kgf electrodes force, 9cy welding time and 7.8 kA welding current, in overlap IF steel sheets with 0.75mm of thickness, thereby increasing the lifetime of the electrode and ensures a better quality of welds and the consequent reduction of energy applied to the weld process.

Abstract: In harsh operational conditions, the low-alloy steels need to be protected from the environment. Thus, against corrosion and wear, an ordinary choice is metallic cladding. In this sense, the present study aimed to evaluate the properties of cobalt base superalloy coating deposited by gas tungsten welding process (GTAW) on steel SAE 4140. A circumferential weld was chosen due to its critical restraint. Four coating conditions were studied varying the welding currents. A microstructural evaluation was done using optical and scanning electron microscopy. The physical properties of coatings were additionally evaluated by microhardness measurement and dilution quantification. The results obtained indicated, for all conditions, a uniformity of layers. However, the deposited weld characteristics are strongly dependent on welding parameters. For the welding parameters studied, the maximum dilution of 60.8% was observed in coatings with austenitic and dendrite microstructures welded with 110 A current. Moreover, the metallographic analysis and microhardness tests showed, for some cases, the presence of partially diluted zone, a microstructural layer in the transition region of base metal and coating. The welding performed with current of 90 A showed the best combination of microhardness and dilution aspects, without defects in coating.

Abstract: This work appears as an experimental attempt to verify the validity of thetheoretical model used to interface ratio proposed by Chicot and Lesage. For this, wepresent a study on the mechanical properties of interface from the instrumentedpenetration tests. One of the important points of the study is related to the characteristicsof materials with micro deformations and influence of defects on the mechanicalproperties. Discussions are developed concerning the intrinsic properties, particularlythe behavior of the modulus of elasticity for different situations in which the materialsare. Were used for development work, samples taken from sucker rods, which are usedin oil production equipment. These rods were coated with two types of materials, thebasis of a NiCr alloy and the other with Al, both obtained by thermal spray process (arcspray). With the use of instrumented penetration testing was carried out indentations inthe coatings, substrate and interface of each sample. With these results, the model wasapplied and theoretical interfacial later tried to verify its validity.

Abstract: The quantification methodology used to predict the residual life of Cr-Mo steel was microstructure/life fraction correlation under creep conditions. Microstructural evolution has been correlated with accelerated creep testing progress during testing at constant load and a temperature of 600°C, as well as with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. Experimental results from TEM revealed six distinct stages, which were directly related to the life fraction values obtained. However, in the case of MEV, five stages were found. This difference is likely due to the better analysis possible via TEM of both microstructural evolution and types of carbides formed during the creep process. In addition, an evolution map is proposed to allow for easy interpretation of the relationship between microstructural characteristics and life fraction.

Abstract: The goal of the present paper is to propose a simple methodology to estimate the failure pressure of thin-walled metallic pipelines with arbitrary localized corrosion damage. This methodology is conceived as a preliminary tool for a quick analysis of the structural integrity of real corroded pipelines. Due to the different possible geometries of the corroded region, the exact analysis of this kind of problem can be very complex (in general using an elasto-plastic finite element simulation). The idea is to obtain an approximate exact analytical solution of the problem for any arbitrary geometry of the corroded region considering elasto-plastic constitutive equations and a factor that accounts for the stress concentration due to the metal loss caused by corrosion. With a simple expression, a reasonable lower limit for the failure pressure can be obtained.

Abstract: The new offshore areas being explored in Brazil presents higher concentration of CO₂ compared with most existing offshore fields. The presence of these more aggressive environments has led to the development of new technologies. Due to the construction characteristics of flexible pipes, any increase in CO₂ concentration in the conveyed fluid will, in turn, increase the CO₂ concentration in the pipe annulus, subjecting the metallic armor layers to a more aggressive environment. Evaluation of the CO₂ effects of corrosion fatigue behavior in tensile armor wires is therefore of vital importance. A comprehensive corrosion fatigue experiment for tensile armor wires in environments up to 10 bar of CO₂, has been established and the experimental results have shown a fatigue life reduction in the tensile amour wires due to higher levels of CO₂.