International Journal of Pressure Vessels and Piping最新文献

In this research, microstructural characterization and mechanical properties of 9%Cr, 0.4%Ni (including W and Mo) steel weld metal with alloyed cobalt were investigated. Due to their strong creep resistance, oxidation resistance, and low thermal expansion, 9%Cr steels are used in nuclear power plants, petrochemical industries, and fossil fuel-powered power plants that operate in high temperature conditions. In this context, weld metals comprising 0.5 %, 1 % and 1.5 % cobalt and cobalt-free weld metal were produced by SMAW (shielded metal arc welding) technique. Microstructure of the weld metals were characterized with optical microscope (OM), scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Also, XRD analysis was performed on the bulk samples and precipitated carbide/nitride phases extracted from the weld metals. In addition, differential scanning calorimetry (DSC) was used to determine the phase transformations. Thermo-Calc modeling study was also performed. Hardness, tensile and Charpy-V impact tests were carried out to determine the mechanical properties. The hardness did not change significantly when cobalt up to 1.5 % in the weld metal. However, with the increase of cobalt, the yield and tensile strength increased without affecting the elongation value too much. In the Charpy impact tests performed at different temperatures (−40 °C, −20 °C, +20 °C, +40 °C, +60 °C), the amount of cobalt increased toughness, especially at +40 and + 60^oC temperatures. Ductile brittle transformation temperature (DBTT) of the weld metal with 1.5 % Co decreased from 29 °C to 15 °C compared to cobalt free weld metal. It is thought that this may be caused by the further separation of C, Cr and W from the matrix through forming precipitate by the cobalt effect. Besides the mechanical properties, microstructure was also affected by adding Co with inhibition of delta ferrite formation which decrease the toughness. Curie temperature increased with increasing cobalt content detected by DSC.

This present study is aimed to produce high-strength dissimilar joints of AA5083-O and AA7075-T6 alloys through optimization in the friction stir welding (FSW) process parameters. In this work, we have employed the response surface methodology (RSM) iteratively to generate 20 different welding process parameter combinations. These parameters are used to manufacture dissimilar weld joints, and the impact of these process parameters on the mechanical properties of the weld joints is evaluated by performing a series of mechanical tests such as tensile and microhardness. The microstructural evolution of the welded joints is studied using optical and SEM techniques. EBSD analysis is performed to understand the grain size, orientation, and changes in microstructure. The weld nugget zone reveals the presence of finer grains measured as 0.3 μm–0.7 μm. The specimen manufactured using FSW optimized process parameters of rotational speed 1100 rpm, welding speed 50 mm/min, and tool tilt angle 2° shows the ultimate tensile strength of 417 MPa, weld nugget hardness of 189 HV_0.1, and weld efficiency 80.65 %. The tensile fractured surface of specimens is also inspected using SEM to determine the mode of failure reached in the welded joints.

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Vessel steel Q345R is widely used in pressure applications in China due to its martensitic structure, often operating in high-temperature, high-stress environments. This study investigates its susceptibility to creep strain, which can lead to significant deformation and structural damage, compromising safety. Thermal creep tests for Q345R steel were conducted within the temperature range of 400 °C–600 °C under various stress ratios. Results indicate that, below 500 °C, stages I and II predominate creep curves, with stage II evolving slowly. However, as temperature rises, stage II becomes predominant, with stage III emerging earlier, especially under high stress, leading to accelerated creep and potential fracture. Temperature significantly influences creep rates, with even minor stress increments at elevated temperatures inducing substantial creep effects. Through fitting by creep strain curves, parameters for three different time hardening creep models were determined. Subsequently, material CREEP subroutines based on these models were developed and integrated into ABAQUS to simulate the entire creep test process. The simulation results validate the ability of calibrated creep models to accurately characterize Q345R steel's creep properties, facilitating the evaluation of safety in high-temperature service structures.

Continuous damage mechanics (CDM) is based on the theories of continuous medium mechanics and continuous medium thermodynamics, which considers damage is an irreversible dissipative process within the materials, and employs the field-theoretic approach of image-only science to study the internal macroscopic damage evolution law of the materials and its influence on the deterioration of the macroscopic mechanical properties of the materials. Hence, within the framework of CDM, a damage-coupled cyclic plasticity constitutive model is proposed based on combined isotropic and Chen-Jiao-Kim (CJK) kinematic hardening rule for evaluating the ratcheting-fatigue behavior of 90° elbow pipes under strong cyclic loading in this paper. Stress return mapping and numerical solution procedures of the proposed model are formulated based on the backward finite difference method. Formulation of the consistent tangent operator is presented for the Finite Element implementation of the plasticity model in ABAQUS UMAT. The FE analysis of non-pressurized and pressurized elbow pipes under cyclic displacement-controlled loading is respectively performed using the implemented constitutive model. The results reveal that the predicted results of the damage-coupled cyclic plasticity constitutive model are better than combined isotropic-kinematic hardening model, and well consistent with the experimental data.

For safety-related systems fluid loads due to fluid transients have to be quantified for subsequent structural analyses to ensure their integrity or function, as required. Usually transient fluid loads in pipe systems are determined with one-dimensional water hammer software. For single-phase liquid flow, the method of characteristics (MOC) is often used that gives in this case appropriate results. For the consideration of local vapor bubbles, the MOC is combined with the discrete vapor cavity model (DVC). The DVC model may generate unrealistic pressure spikes due to the calculation of the collapse of multi-cavities in scenarios, where only one vapor bubble should actually occur. The application of a two-phase code may improve the calculation results. One requirement for the latter codes is the ability to calculate the propagation of steep gradients without suffering from numerical diffusion to exclude the underestimation of fluid loads. This is commonly attained by applying higher-order numerical schemes. However, the application of a numerical method of pure 2nd order leads to the calculation of unphysical oscillations at steep gradients causing severe problems during the solution. To exclude this, numerical methods with flux limiters can be used. With their application, the calculation of unrealistically high loads due to numerical deficiencies can be minimized. In addition, the consideration of further physical effects, that lead to the reduction of loads during transient flow processes, allows for a more realistic calculation of the loads. These are unsteady friction, widening of the pipe caused by pressure increase, fluid-structure interaction at junctions and due to friction, degassing of gas that is initially dissolved physically in a liquid and thermodynamic non-equilibrium during vapor bubble collapse. The in-house code DYVRO applies a second-order accurate scheme with flux limiters based on the Godunov method and can account for the above-described physical phenomena. It is shown by comparison of calculation results obtained by DYVRO with experimental data from literature that with modeling of these physical effects the loads can be calculated more realistically. Generally, these loads are lower than the results calculated by simplified models, which do not account for these effects. Considering that these loads are applied in subsequent structural analyses, cost-intensive oversizing of pipes and their supports can be avoided, by ensuring the necessary safety.

The use of thin-walled steel shells is rapidly becoming widespread. Depending on their purpose, they can be susceptible to corrosion in the environments they are used in. This leads to metal loss and consequently a decrease in buckling strength. In this study, the effect of corrosion and CFRP reinforcement on thin-walled cylindrical steel tanks was experimentally investigated. Six samples with a length and diameter of 400 mm were tested; two samples were not subjected to corrosion, two samples were subjected to 2.5 % HCl corrosion, and two samples were subjected to 5 % HCl corrosion. The effect of CFRP usage was examined in both corroded and non-corroded samples. The results show that as the corrosion rate increases, the buckling strength decreases, and CFRP reinforcement recruitments the loss in buckling strength.

A previous study proposed a passive safety structure to mitigate severe damage to crucial components under the beyond design basis event (BDBE). To examine the applicability of this concept to the piping system, the effect of the elastic-plastic behavior of the piping support structure on the seismic response of the piping system was investigated through elastic-plastic time history response analyses on a piping system model with multiple support structures. The numerical analyses considered the elastic-plastic behavior of supports and/or the inelastic characteristics of pipe material. The analysis results confirmed that the response of the piping system can be suppressed by adequately introducing the inelastic behavior of support structures. The results show the possibility of realizing of passive safety structure in piping systems and addressing some issues, such as changing vibration modes due to the elastic-plastic deformation of the support or the order of generation of inelastic behavior of pipe material and supports.

A comprehensive fracture mechanics test program called CAMERA was carried out for seven RPV base and weld materials in unirradiated and irradiated conditions. The objective was to establish an application-oriented completion of the fracture mechanics approach for the RPV safety assessment over the entire relevant temperature range up to operating temperature. For this purpose, fracture toughness tests in the ductile-brittle transition region with and without warm pre-stress (WPS), and in the ductile region (upper shelf) were performed in order to complete the fracture toughness curves of the material concerned. The test program was completed by various analytical and numerical calculations and microstructural analyses. The benefit of the WPS effect was confirmed for both the material (higher apparent fracture toughness values) and the load path (no initiation after maximum loading). By fracture toughness tests in the ductile regime the fracture toughness curve could be completed up to the operating temperature and the T₀ based criterion temperature T_US above which no brittle fracture occurs was determined and successfully verified. Based on the results obtained by the Master Curve, WPS and crack resistance tests it was demonstrated how to integrate the criterion temperature T_US in the fracture toughness-temperature diagram including the loading transient to an application window for the RPV safety assessment that is based on T₀ only. For three out of nineteen data sets the homogeneity screening procedure described in ASTM E1921 did indicate a macroscopically inhomogeneous material for that a generally conservative reference temperature T_0IN was calculated that is on average 11 K higher than the reference temperature T₀. A higher susceptibility of weld materials for macroscopic material inhomogeneity compared to base materials was found. The suitability of the applied SE(B) and C(T) specimens for the specific fracture mechanics tests (WPS and/or crack resistance in ductile region) was proven. Micromechanical Local Approach damage models (Bordet and Gurson) were successfully applied for test design and prediction of fracture toughness. The overall results reveal even so some open gaps remaining for future work that are addressed as well.

The current paper presents the development and experimental validation of a thermo-metallurgical-mechanical model for a multi-pass Laser Metal Deposition (LMD) process, with the objective of accurately predicting the resultant constituent phases and residual stresses. The thermal model is calibrated using temperature readings and is shown to accurately capture the transient temperature field and extent of the fusion zone associated with the multi-pass LMD process. The metallurgical model incorporates the kinetics of ongoing solid-state phase transformations (SSPTs) and tempering reactions. The predictions are validated via hardness measurements, demonstrating a very good agreement with the predictions. The developed mechanical model predicts the residual stress field, which is validated using X-ray diffraction measurements, and the comparison also shows a very good agreement between the predictions and measurements. The validated numerical model is then used to explore alternative LMD strategies showing that the choice of deposition strategy can significantly impact resultant constituent phases and residual stresses.

In our previous study, a unified parameter λ based on crack-tip plastic strain energy was proposed which using BS1501-224 28B steel specimens at a low temperature showed promise to quantify both in-plane and out-of-plane constraints concurrently. In this study, a large number of A516 Gr70 steel specimens with wide-ranging changes in thicknesses and crack lengths were tested at the materials’ upper shelf (−15^oC) to further validate the applicability of λ. A series of numerical models were conducted based on the experimental data and the applicability and effectiveness of parameter λ were studied. A comparison between parameter λ and another unified constraint parameter A_p was discussed to justify using this new method. It is found that parameter λ can be used for specimens with large plasticity and it performs better than parameter A_p. What is more, an application method of using parameter λ to predict the fracture toughness of nonstandard specimens was proposed which sets out a method to incorporate it into engineering assessments.