Thermal and stress analyses of a novel coated steam dual pipe system for use in advanced ultra-supercritical power plant

Improving the energy efficiency of advanced ultra-supercritical power plants, by increasing steam operating temperature up to 700℃, can be achieved, at reduced cost, by using novel engineering design concepts, such as coated steam pipe systems manufactured from high temperature materials commonly used in current operational power plants. This paper describes a preliminary feasibility analysis of the design concept of a novel coated dual pipe system under steady-state operation, using analytical and finite element models to evaluate the possible thermal gradients and stresses generated. The results show that the protective coating layer contributes to the effective reduction in the surface temperature of the primary steel pipe. Thermal stresses generated due to the significant difference in the thermal and mechanical properties of the coating and substrate pipe are larger than the mechanical stresses generated by the combined effects of the internal steam pressure in the primary steam pipe and external pressure from the counter-flow cooling steam during steady-state operation. Compared with the stress relaxation of the coating and substrate pipe, creep has a significant impact on the stress distribution within the coating layer. Several key factors have been identified, such as the coating thickness, conductivity, thermal expansion, heat transfer coefficient of cooling steam, cooling steam temperature and cooling steam pressure, which are found to govern thermal and stress distributions during steady-state operation

Double-sided incremental forming: a review

Incremental sheet forming (ISF) or single point incremental forming (SPIF) processes have been developed rapidly in the past three decades. Its high flexibility and easy operability have a significant appeal for industrial applications and substantial progress has been made in fundamental understanding and demonstration of practical implementation. However, there are a number of obstacles including achievable accuracy and instability in material deformation which are considered as a main contributing factor for preventing the ISF process to be widely used in industry. As a variant of the general ISF process, Double-Sided Incremental Forming (DSIF) uses an additional supporting tool in the opposite side of the workpiece, maintains the flexibility and at the same time improves the material deformation stability and reduces material thinning. In recent years, there has been increased research interest in looking into DSIF specific material deformation mechanisms and DISF investigation. This paper aims to provide a technical review of the DSIF process as benchmarked with SPIF. It starts with a brief overview of the current state of the art of both SPIF and DSIF. This is followed by a comparative study between SPIF and DSIF with the key research challenges identified. This leads to a recommendation of key research challenges for DSIF focused research

Analysis of shape and location effects of closely spaced metal loss defects in pressurised pipes

Metal loss due to corrosion is a serious threat to the integrity of pressurised oil and gas transmission pipes. Pipe metal loss defects are found in either single form or in groups (clusters). One of the critical situations arises when two or more defects are spaced close enough to act as a single lengthier defect with respect to the axial direction, causing pipe ruptures rather than leaks, and impacting on the pressure containing capacity of a pipe. There have been few studies conducted to determine the distance needed for defects to interact leading to a failure pressure lower than that when the defects are treated as single defects and not interacting. Despite such efforts, there is no universally agreed defect interaction rule and pipe operators around the world have various rules to pick and choose from. In this work, the effects of defect shape and location on closely spaced defects are analysed using finite element analysis. The numerical results showed that defect shapes and locations have a great influence on the peak stress and its location as well as the failure pressure of pipes containing interacting defects

Simulation of Cyclic Plastic Behavior of 304L Steel Using the Crystal Plasticity Finite Element Method

AbstractThe stabilised cyclic plasticity behaviour of 304L austenitic stainless steel at room temperature is studied by using the multiscale crystal plasticity finite element method within the software ABAQUS. The physical-based material constitutive equations are coded in the UMAT user-subroutine. A polycrystal model is constructed and is shown to be able to approximate the macroscale cyclic plasticity behaviour. The distributions of stress, strain and plastic dissipation energy are examined locally to investigate their relationship with the possible crack initiation sites, and the effects of grain orientation. The effects of grain boundaries are also studied by studying the distribution of dislocation density and the number of active slip systems

Numerical simulation of residual stresses induced by weld repair in a stainless steel pipe considering the influence of an initial fabrication weld

This work presents the application of a finite element (FE) model developed to simulate the repair process in the case of components with a pre-existing stress state. The approach is tested in the case of a repair of a laser beam weld in a stainless steel pipe with the region of repair located in the heat affected zone of the original weld. The area of the repair is removed and refilled testing different approaches in terms of the number, and direction of the repair passes. The comparison between the refilling procedures is presented with the aim of evaluating the effects on the final residual stress distribution

Numerical simulation of residual stresses induced by weld repair in a stainless steel pipe considering the influence of an initial fabrication weld

This work presents the application of a finite element (FE) model developed to simulate the repair process in the case of components with a pre-existing stress state. The approach is tested in the case of a repair of a laser beam weld in a stainless steel pipe with the region of repair located in the heat affected zone of the original weld. The area of the repair is removed and refilled testing different approaches in terms of the number, and direction of the repair passes. The comparison between the refilling procedures is presented with the aim of evaluating the effects on the final residual stress distribution

FE modelling strategies of weld repair in pre-stressed thin components

Two computational procedures have been developed in the commercial finite element (FE) software codes Sysweld and Abaqus to analyse and predict the residual stress state after the repair of small weld defects in thin structural components. The numerical models allow the effects of the repair to be studied when a pre-existing residual stress field is present in the fabricated part and cannot be relieved by a thermal treatment. In this work the modelling strategies are presented and tested by simulating a repair of longitudinal welds in thin sheets of Inconel 718 (IN718). Although the numerical strategies in the two codes are intrinsically different, the results show a significant agreement, predicting a notable effect imposed by the initial residual stress

Investigation of short-term creep deformation mechanisms in MarBN steel at elevated temperatures

This paper reports the short-term creep behavior at elevated temperatures of a MarBN steel variant. Creep tests were performed at 3 different temperatures (625oC, 650oC and 675oC) with applied stresses ranging from 160 MPa to 300 MPa, and failure times from 1 to 350 hours. Analysis of the macroscopic creep data indicates that the steady-state creep exhibits a power-law stress dependence with an exponent of 7 and an activation energy of 307 kJ.mol-1, suggesting that dislocation climb is the dominant rate-controlling creep mechanism for MarBN steel. Macroscopic plastic instability has also been observed, highlighted by an obvious necking at the rupture region. All the macroscopic predictions have been combined with microstructural data, inferred from an examination of creep ruptured samples, to build up relations between macroscopic features (necking, damage, etc.) and underlying microstructural mechanisms. Analysis of the rupture surfaces has revealed a ductile fracture mode. Electron Backscatter Diffraction (EBSD) analysis near to the rupture surface has indicated significant distortion and refinement of the original martensitic substructure, which is evidence of long-range plastic flow. Dislocation pile-ups and tangles from TEM were also observed near substructure boundaries and precipitate particles. All of these microstructural observations suggest that creep is influenced by a complex interaction between several elements of the microstructure, such as dislocations, precipitates and structure boundaries. The calculated stress exponent and activation energy have been found to agree quantitatively with the highlighted microstructural features, bearing some relationships to the true observed creep microstructures