Applied Mechanics and Materials Vol. 750

Abstract: The upper and lower courses of sea oil and gas exploitationare connected by submarine pipeline which is called life line project. Free span often occurs because of the unevenness and scour of seabed, and fatigue is one of the main failure modes.In this paper, with the finite element numerical simulation method, based on the harmonic response analysis, the research on the structural response of free span under the vibration induced by vortex was investigated, and the effect of the factors such as flow velocity, length of free span. According to the analysis results,the fatigue life of free span was evaluated.

Abstract: This paper reports the results of an ongoing research which was set up to investigate the effect of microstructures on the activity of different deformation mechanisms and the resulting mechanical behavior of the Ti-10V-2Fe-3Al alloy. Various microstructures were established by different heat treatments. Depending on the microstructures, obvious differences in the mechanical properties of this alloy were obtained. It is shown that in α+β phase field solution treatment conditions, with the increase of heating time, the fraction of globular α phase decreases, the compression strength increases. In β+(α+β) phase field solution treatment conditions, with the increase of heating time, alloys have opposite trends. In aging conditions, the microstructure and mechanical property do not change apparently with the aging time. The microstructure of the α phase and β phase has a great influence on the fractographic morphology.

Abstract: The variable cross-section H-type structure, which is used in the new type of heat exchanger with longitudinal flow of shell side, could reduce the scour action of imports fluid on the tube bundle and prevent vibration of the tube bundle. It could also improve the state of the shell side fluid flow, reducing the flow dead zone, allowing for a more efficient use of the heat transfer area and improving the energy efficiency. The new structure will make the temperature and stress distribution in the heat distribution more complex, so it is necessary to analyze the stress intensity of the variable cross-section H-type structure. A three-dimensional finite element model of the variable cross-section H-type structure is established in this paper, and the surface temperature of the various parts of the heat exchanger are determined through temperature analysis. Using ANSYS Workbench software, thermal-stress analysis of the H -type structure with different structural parameters is tested, and the temperature and stress field are obtained. The results show that a Ring plate of H-type structure has a larger temperature gradient along the thickness direction. The maximum stress of the heat exchanger is 203.13 MPa, which occurred on the connections of the ring plate and jacket in the lower temperature side. The ring plate thickness of the H-type structure has a significant influence on its maximum stress. Therefore, a reasonable selection of ring plate thickness is important for the safety of the heat exchanger.

Abstract: In this paper, the influence of double wire submerged arc welding parameters on the mechanical properties of high strength low alloy has been investigated. The 20mm steel plate has been welded by double wire submerged arc welding process using different welding parameters. The Charpy absorbed energy of specimens is assessed using impact test at the temperature of -50°C. Testing results show that high heat input parameters will lead to low strength of welded joint. Impact toughness of fusion line is lower than that of other areas of welded joint.

Abstract: The reheat cracking, also known as stress relief cracking, has occurred many times in the welding coarse grained heat affected zones (CGHAZ) of Vanadium-modified (V-mod) 2.25Cr1Mo steel, but seldom researches on this problem have been conducted until now. In this paper, reheat cracking in welding CGHAZ of V-mod 2.25Cr1Mo steel was studied by the thermo-mechanical simulation methods. A screening test was carried out for simulating similar material to the real CGHAZ. The high temperature ductility of simulated CGHAZ was measured in the range of 600~705°C for determining the most sensitive temperature to reheat cracking. And then, the reheat cracking phenomena at the sensitive temperature was reproduced by Isothermal stress relaxation test and the micro-morphology of cracks was observed and analyzed. The results show that the thermal simulation parameters for acquiring the most similar materials to real CGHAZ are heat input of 35kJ/cm, heating rate of 1000°C/s and holding time at 1320°C of 1s. The reduction of area (RoA) of the simulated CGHAZ specimen first decreases and then increases with increasing temperature. It exhibits a minimum value at 675°C, corresponding to the most sensitive temperature to reheat cracking. In the stress relaxation test, the sample failure typically occurs with very low stress relaxation proportion (less than 15%) and extremely poor ductility (less than 5% in RoA). Both microvoid coalescence along the PAGB and intergranular wedge type cracks were observed in the fractured sample, proposing a mixed cracking mode of W-type and R-type in stress relaxation.

Abstract: Magnetic memory signals and hardness of 35CrMo tempered and quenched steel were acquired during tensile testing. The magnetic signals of 35CrMo steel increased monotonously with the increase of tensile stress before yielding, followed by sudden decrease with further increase of stress after yielding. The zero-cross point of H_p(y) varied during tensile testing, indicating varied position of stress concentration zones with the process of tensile testing. Vickers hardness of the stress concentration zones of 35CrMo steel was lower than that around it, which may be attributed to the residual stress. The relationship between zero-cross point of H_p(y) and Vickers hardness was considered.

Abstract: Cr25Ni35Nb and Cr35Ni45Nb austenitic steels, used as furnace tube material in the ethylene pyrolysis furnace, generally suffer from coking during the long operation period. In the present paper, for coked Cr25Ni35Nb and Cr35Ni45Nb austenitic steel, using the finite element ABAQUS code, a sequentially coupled thermal stress procedure was developed to calculate distribution of the temperature and thermal stress field. The results show that thermal diffusion property of Cr25Ni35Nb and Cr35Ni45Nb steel deteriorates obviously with coking layer increasing. Radial bulge and rupture at local field would generate due to excess temperature for applied subsequent measurement. Maximum stress distribute along the cross-section of coking layer and metal matrix. Finally, the critical depth of coking layer of Cr25Ni35Nb and Cr35Ni45Nb austenitic steel are predicted when decoking measurement must be executed.

Abstract: The local wall thinning(LWT) is a kind of common volume defect in pressure pipe. The limit loads of elbows with LWT under pressure, bending moment, torque and their combined loads have been studied in detail by orthogonal experimental design and finite element method. The results have shown that the influence of depth and circumferential length of LWT on the limit load is more obvious compared to that of axial length when an elbow is under pressure, bending moment or torque. The change of limit bending moment and torque with the depth of LWT and circumferential length is significant for an elbow under combined bending moment and torque. At last, the safety assessment equations for elbow under combined in-plane closing bending moment and torque were proposed by regression analysis.

Abstract: An ultrasonic method was developed to measure the residual stress in a welded joint based on the modified acoustic-elasticity theory. Phase frequency analysis method was introduced by incombination with shear wave and longitudinal wave. The acoustic-elasticity parameters of joint material was derived by ultrasonic inspection during loading experiment. Transverse and longitudinal residual stress was determined by precisely measuring of short-distance acoustic travel time. The ultrasonic measurement results was confirmed by H&K theory and XRD methods with good repeatability and reliability.

Abstract: Ni-W-Si intermetallic composite coatings consisting of primary tungsten dendrites and eutectic W/WSi₂ were fabricated on 45 steel by laser cladding process using Ni-W-Si powder. The microstructure and composition of the coatings were characterized by SEM, XRD and EDS. The effect of the W content on the hardness and wear resistance of coatings was investigated. Results indicate that attributed to the high hardness and toughness of tungsten dendrites and fine and compact eutectic W/WSi₂, coatings had high hardness being 950HV in maximum and the wear resistance was elevated to 4-8 times higher.