Creep crack growth of high temperature weldments

The experimental programme of the EC supported project (SMT 2070) SOTA aimed at addressing a technical and industrial need to provide guidelines for creep crack growth (CCG) testing and data analysis of weldments. Mechanical and creep properties were determined on two pressure vessel steels of P22 (2.25Cr1Mo) and P91 (9Cr1MoVNb). The specimens were taken from pipe welds for weld metal (WM) tests, and simulated heat affected zone (HAZ) material for the HAZ tests.The CCG tests were carried out on cross-weld compact tension (CT) specimens machined out from weldment of pipes. The tests were done at 550 and 600°C on P22 and P91 materials, respectively. The CT specimens were notched using electrical discharge method, for a sharp starter crack. This method of initiating sharp starter crack was chosen to make sure that all partners will test specimens with starter crack location as specified in the work programme to study crack initiation and growth in WM and HAZ (both in the centre and type IV region). The CCG tests were carried out following the ASTM E1457-92 [ASTM E1457-92, Standard test method for measurement of creep crack growth rates in metals, ASTM, Philadelphia, PA 19103, USA]. The partners assessed their data and sent both assessed and their raw data to be further assessed centrally. All the data from partners were analysed and compared with those of partners' own assessed data.The present paper reports on the analyses of CCG data obtained in the programme including six laboratories from six European countries. The programme addresses the differences and difficulties in testing and the assessment of weldments, and provides guidelines for harmonisation of testing procedures for reliable data production for remanent life assessment of plants with welded components.     

Creep damage in small punch creep specimens of Type 304 stainless steel

Small punch creep tests on Type 304 stainless steel have been performed at 650 °C. Based on these tests, a finite element model, with modified Kachanov–Rabotnov creep damage constitutive equations, was established. The variation of central deflection and creep strain with time and the evolution of creep damage under constant loads were analysed by using the finite element model. The central creep deflection curves in the specimens were obtained at different loads in both tests and simulations and have three different stages, similar to conventional creep tests. There is good agreement between experimental results and simulation data. The creep damage at the central part is high, and localization of damage is obvious. Initial failure occurs at the bottom surface, about 0.8 mm away from the centre which agrees well with the finite element mode observation.     

3D cohesive modelling of hydrogen embrittlement in the heat affected zone of an X70 pipeline steel

A three-dimensional finite cohesive element approach has been developed and applied in order to simulate the crack initiation of hydrogen-induced fracture. A single edge notched tension specimen of an X70 weld heat affected zone was simulated. The results were compared to similar two-dimensional plane strain model and the cohesive parameters were calibrated to fit the experimental results. The three dimensional simulations gave higher values in terms of opening stress at the stress peak, plastic strain levels at the crack tip and hydrogen lattice concentration when compared with two-dimensional simulations under the same global net section stress levels. Nevertheless a higher cohesive strength was needed for the 2D model for the onset of crack propagation. The best fit to the experimental data were obtained for a cohesive strength of 1840 MPa and 1620 MPa for the 2D and 3D simulation respectively. The critical opening was assigned to 0.3 mm for both models. The threshold stress intensities K_IC,HE were 142 MPa√m and 146 MPa√m for the 2D and 3D models, respectively.     

Evaluation of creep damage in heat affected zone of thick welded joint for Mod.9Cr–1Mo steel

Mod.9Cr–1Mo steel has been used for boiler components in ultra-supercritical (USC) thermal power plants. The creep strength of welded joint of this steel decreases due to the formation of Type IV cracking in heat affected zone (HAZ) at higher temperatures. The present paper aims to clarify the damage processes and mechanisms of the welded joint for Mod.9Cr–1Mo steel. Long-term creep tests of base metal, welded joint and simulated fine- grained HAZ were conducted at 550, 600 and 650 °C. Creep tests using thick plate welded joint specimen were interrupted at several time steps, and evolutions and distributions of creep damages were measured quantitatively using laser microscope. It is found that creep voids initiate at early stage of creep life (0.2 of life), the number of creep voids increases until 0.7 of life, and then voids coalesced into the macro crack at the later stage of life (0.8 of life). Creep damages concentrate mostly at a quarter depths of the plate thickness within the fine-grained HAZ of the present welded joint. The experimental creep damage distributions were compared with the computed results by using the FEM analysis. Both creep strain concentration and high stress triaxiality in fine-grained HAZ of welded joint are considered to accelerate the creep void formation and growth.     

3D cohesive modelling of hydrogen embrittlement in the heat affected zone of an X70 pipeline steel – Part II

A three steps 3D cohesive element based modelling procedure for hydrogen embrittlement susceptibility prediction of an X70 weld thermal simulated Heat Affected Zone has been developed. Fracture mechanics testing both under electrochemical charging and pressurized hydrogen gas are used for simulations validation. A new approach for the cohesive parameter's choice, based on the initial cohesive stiffness entity, has been adopted. The best fit to the experimental data were obtained for cohesive strength of 14,450 MPa and for opening of 0.005 mm, yielding a reduced threshold stress intensity K_IC,HE of 49 MPa√m, for test in CP. Hydrogen boundary condition for simulations of test under hydrogen gas obtained by applying Sievert's law provided far too low concentrations for any appreciable influence on the cohesive energy. Toward the end of the paper, comparison between the two hydrogen sources is made by imposing the same initial hydrogen boundary conditions for the simulations. Results indicate that crack initiation is located at the hydrostatic stress peak and at the crack tip for cathodic protection and 0.6 MPa H₂ gas conditions, respectively. Simulations suggest that the hydrogen in lattice interstitials is mainly responsible for the embrittlement.     

A numerical creep analysis on the interaction of twin semi-elliptical cracks

The interaction and coalescence of multiple flaws will significantly influence the service life of components. In this paper, the interaction of two identical semi-elliptical cracks in a finite thickness plate subjected to the remote tension is investigated. The results indicated that interaction of multiple cracks is different between the time-dependent fracture characterized by C^*-integral and linear elastic fracture noted by SIF. The magnifying factors of creep fracture are obviously larger than that of the linear elastic fracture cases. Therefore, the current interaction and coalescence rule developed from linear elastic fracture analysis may lead to a non-conservative result when it is used in the assessment of creep crack. At the end, an empirical equation is developed based on the numerical results.     

Numerical studies on Vibro-creep of PEM fuel cell elements used in mobile applications

The wide use of fuel cells in mobile applications carries significant risks caused by accompanying vibrations, the origin of which can be found in the interaction of the ground or other vibration-generating components. The classical approach treated vibration and creep as two separate phenomena. One generated pulses, and the other caused durable deformations under the influence of temperature and static loads. This work combines both phenomena, describing the impact of vibrations on the material's structure and estimating the magnitude of durable deformations caused by the aggregation of alloy inclusions and dislocations in the material mesh. The Norton creep model and the failure model described by Kachanov were used for this. In addition, Stobyriev's reduced stresses were used to reflect the failure spectrum of the material: from brittle to ductile.     

Effects of geometry and combined loading on steady-state creep stresses in welded branches

In our previous paper, it was found that the mis-match effect in creep on steady-state stresses within the weld metal for a large bore branch junction could be uniquely quantified by the mis-match factor defined as a function of the creep stress exponent and the ratio of creep constants for the base and weld materials. Furthermore ratios of section-averaged (effective and maximum principal) stresses for the mis-matched case to those for the even-matched case were linearly dependent on the mis-match factor. Above results were obtained for a specific branch geometry under single loading. This paper extends our previous analysis to other branch geometries and to combined loading. It is found that above conclusions can be applied to general branch components under combined loading.     

Effect of the welding heat input on residual stresses in butt-welds of dissimilar pipe joints

This study used finite element techniques to analyse the thermo-mechanical behaviour and residual stresses in dissimilar butt-welded pipes. The residual stresses at the surface of some weld specimens were measured experimentally by using the hole-drilling method. The results of the finite element analysis were compared with experimentally measured data to evaluate the accuracy of the finite element modelling. Based on this study, a modelling procedure with reasonable accuracy was developed. The developed finite element modelling was used to study the effects of welding heat input on magnitude and distribution of welding residual stresses in butt-welded pipes made of ferritic and austenitic steels. The hoop and axial residual stresses in dissimilar pipe joints of 8 mm thick for V-groove shape were studied. It is shown that the welding heat input has a significant effect on magnitude and distribution of residual stresses in the stainless steel side of the studied joints.     

Numerical investigation on the single phase forced convection heat transfer characteristics of TiO2 nanofluids in a double-tube counter flow heat exchanger

In this study, forced convection flows of nanofluids consisting of water with TiO₂ and Al₂O₃ nanoparticles in a horizontal tube with constant wall temperature are investigated numerically. The horizontal test section is modeled and solved using a CFD program. Palm et al.'s correlations are used to determine the nanofluid properties. A single-phase model having two-dimensional equations is employed with either constant or temperature dependent properties to study the hydrodynamics and thermal behaviors of the nanofluid flow. The numerical investigation is performed for a constant particle size of Al₂O₃ as a case study after the validation of its model by means of the experimental data of Duangthongsuk and Wongwises with TiO₂ nanoparticles. The velocity and temperature vectors are presented in the entrance and fully developed region. The variations of the fluid temperature, local heat transfer coefficient and pressure drop along tube length are shown in the paper. Effects of nanoparticles concentration and Reynolds number on the wall shear stress, Nusselt number, heat transfer coefficient and pressure drop are presented. Numerical results show the heat transfer enhancement due to presence of the nanoparticles in the fluid in accordance with the results of the experimental study used for the validation process of the numerical model.     

渐变孔隙率对泡沫铝矩形通道热沉流动和换热性能的影响

风冷是最普遍的电子器件散热方式。对填充渐变孔隙率泡沫铝的热沉的流动与换热性能进行CFD模拟,考察了孔隙率分别沿流动方向和高度方向发生梯度变化对热沉性能的影响。结果表明:与均匀孔隙率热沉相比,孔隙率沿高度方向渐变的热沉压力损失减小,最佳的孔隙率渐变方式为沿高度方向由0.963 0递减至0.700 0,与当量孔隙率0.831 5的均匀孔隙率热沉相比,明显提高了泡沫铝热沉的综合性能。当进口速度高于3.0 m/s后,渐变孔隙率热沉的综合性能甚至优于高孔隙率(0.963 0)的均匀孔隙率热沉。     

DNS study by a bilayer model on the mechanism of heat transfer reduction in drag-reduced flow induced by surfactant

A bilayer model proposed in [1] is used to investigate the mechanism of heat transfer reduction of surfactant-induced drag-reducing channel flow with a constant heat flux imposed on both walls by direct numerical simulation. In the bilayer model, Newtonian fluid and viscoelastic fluid are assumed to coexist with shear stress balance satisfied between the two fluid layers. A Giesekus model is used to model the viscoelastic fluid induced by the addition of surfactant additives. High-order compact difference schemes are applied to discretize the convective and diffusion terms whereas MINMOD scheme is used to discretize the convective terms in the Giesekus constitutive equations to enhance numerical stability. The effectiveness of the surfactant additives at different flow region on heat transfer reduction is investigated.