The process of magnetic flux penetration into superconductors

In the present paper the magnetic flux penetration dynamics of type-II superconductors in the flux creep regime is studied by analytically solving the nonlinear diffusion equation for the magnetic flux induction, assuming that an applied field parallel to the surface of the sample and using a power-law dependence of the differential resistivity on the magnetic field induction. An exact solution of nonlinear diffusion equation for the magnetic induction B(r, t) is obtained by using a well-known self-similar technique. We study the problem in the framework of a macroscopic approach, in which all length scales are larger than the flux-line spacing; thus, the superconductor is considered as a uniform medium.

316L(N) Creep Modeling with Phenomenological Approach and Artificial Intelligence Based Methods

A model that describes creep behavior is essential in the design or life assessment of components and systems that operate at high temperatures. Using the RCC-MRx data and the LCSP (logistic creep strain prediction) model, processed design data were generated over the whole creep regime of 316L(N) steel—i.e., primary, secondary, and tertiary creep. The processed design data were used to develop three models with different approaches for the creep rate: a phenomenological approach; an artificial neural network; and an artificial intelligence method based on symbolic regression and genetic programming. It was shown that all three models are capable of describing the true creep rate as a function of true creep strain and true stress over a wide range of engineering stresses and temperatures without the need of additional micro-structural information. Furthermore, the results of finite element simulations reproduce the trends of experimental data from the literature.

Effect of Different Precipitation Routes of Fe2Hf Laves Phase on the Creep Rate of 9Cr-Based Ferritic Alloys

We performed creep tests for three types of Fe-9Cr-Hf alloys with a ferritic matrix w/o Fe2Hf Laves phase particles formed by two precipitation routes: (1) with fine Fe2Hf particles formed by the conventional precipitation route (hereafter the particles are called CP particles), namely formed in the α-ferrite matrix after γ-austenite ® α-ferrite phase transformation; (2) with fine Fe2Hf particles formed by interphase precipitation (hereafter called IP particles) during δ-ferrite ® γ-austenite phase transformation before γ ® α phase transformation and (3) without Laves phase particles. CP particles were found to be effective in reducing the creep rates from the transient creep regime to the early stage of a slowly accelerating creep regime but were coarsened after the creep tests. IP particles were less effective in reducing the creep rate in the early creep stages but showed a higher stability against particle coarsening than CP particles in the creep tests, suggesting their effectiveness in delaying the recovery and recrystallization processes in the matrix and thereby retarding the onset of a rapid creep acceleration and creep rupture. The effects of the different precipitation routes are discussed based on the results obtained.

Toward the Better Understanding of Shear Localization in the Lower Mantle Caused by the Strength Contrast between Bridgmanite and Ferropericlase: the Role of Stress/Strain Rate Heterogeneity in Diffusion Creep Regime

<p>      Localized deformation is a possible scenario that may explain the preservation of geochemical heterogeneity in the lower mantle. Recent experimental studies (e.g., Girard et al., 2016) showed that Fp (ferropericlase), which is a weak and volumetrically minor phase (~20 %), accommodates a large fraction of strain of its mixture with Br (bridgmanite), which is a stronger (approximately order of 2 - 3) and volumetrically major phase (~60-70 %). Localized deformation of the Fp phase within this two-phase mixture may provide an important insight to the long-standing question of the mechanical differentiation process between the weak boundary layer and the relatively unmixed volume in the lower mantle. Since the dominant deformation mechanism in the lower mantle is thought to be the diffusion creep, and the deformation state is mostly simple shear, it is important to understand how the deformation by diffusion creep occurs in a mixture of Fp-Br under the simple shear.</p><p>In the context of multiscale modeling methodology, we approach a grain’s length scale deformation where a single 2D elliptic Fp grain is embedded in the infinite Br matrix medium. These two phases are treated as linear viscous incompressible materials where deformation occurs by the fluxes of atoms (vacancies) caused by the applied boundary normal stress gradient. We conducted a theoretical analysis to investigate the nature of the diffusion-induced deformation of the Fp grain under the simple shear. We focused on the following issues: (i) when the two-phase mixture deforms under the far-field simple shear, what the local stress and strain rate fields within the Fp inclusion are, (ii) how the local stress gradients induce the diffusion fluxes of vacancies of the Fp grain, and (iii) the dependences of diffusion creep of the Fp to its shape (changing with the strain) and its viscosity contrast against the Br.</p><p>      To investigate the internal stress states, we used the Eshelby’s inhomogeneous inclusion theory translating its elastic formulations to the linear viscous ones using the Hoff analogy. This approach provides the stress, strain rate, and vorticity within a 2D elliptic Fp grain embedded in Br (3 orders of magnitude greater viscosity than Fp) matrix subjected to the far-field simple shear. From these mechanical states, the lattice diffusion within Fp grain and its influences on the rheology were found by using the Finite Element method solving the Fick’s laws of diffusion. This study shows that the diffusion creep rate increases as the ellipse elongates and rotates. As the Fp ellipse elongates (i.e., its aspect ratio increases), the local shear stress in the Fp increases, and the stress is somewhat concentrated near the small radius tips, which induces the strong diffusion fluxes due to the high normal stress gradients. These theoretical and numerical results support that the strain localization under diffusion creep regime can occur and be a possible mechanism that created the localized mantle flow particularly where the shear deformation is dominantly applied.</p><p>   </p>

Simplified Procedure to Assess Hot Spots in Pressure Vessels

In many instances, pressure vessels and piping system designed for high temperature applications are exposed to localized hot spots. Hot spots usually occur as the refractory lining degrades over time during operation or process changes causing the surface temperature of the localized region to exceed Code allowable metal temperature. These localized overheating can reduce the overall structural integrity of the pressurized components due to lower yield and or ultimate tensile strength of the damaged region. If hot spots are left undetected, they can lead to catastrophic failure of the components. This paper provides a simplified procedure to assess the effect of the hot spots on the pressure strength of the vessel. The procedure presented in this paper is applicable for hot spot temperatures in non-creep regime.

Modified 9Cr Steel: Coke Resistant Tubes/Pipes Developed by Vallourec for Refinery Furnaces

A new modified 9 wt% Cr steel has been developed in order to improve the resistance against coke deposition on the internal surface of refinery tube furnaces, in comparison to widely used grades, such as Grade 5 (5 wt% Cr) or Grade 9 (9 wt% Cr). The new grade has an improved composition, based on Cr and Si and further additions of Cu and Ni. This optimal chemical analysis has been specified after extensive laboratory testing on different laboratory and industrial heats. Thermogravimetric analyses have been performed to benchmark various materials (ferritic and austenitic grades) in terms of coking rate. Specimens of these alloys have been exposed to this coking atmosphere in a wide temperature range. The new modified 9Cr steel exhibits an almost 10 times lower coking rates than typical Grade 9 steel. The new 9Cr steel shows allowable stress levels up to 90% higher than Grade 9 at temperatures below 500°C (time independent regime) and up to 7% higher stress levels at temperatures above 500°C (creep regime). The industrial feasibility of production of elbows has been successfully implemented and a welding solution using a commercially available filler material has been established.

The effect of internal pressure and thickness on the creep strain of the superheater pipes

Superheater pipes in turbines commonly are used to produce superheated steam. Internal pressure is critical for steam superheater elements. The pipes in such applications are vulnerable to temperature environments, which can bring the component to enter the creep regime, creep deformation, or even creep fracture. In general, most of the failures in boilers are caused by creep. Creep-resistant materials used in facilities operated at high temperatures must, therefore, be able to withstand the highest possible temperature loads. This study aims to investigate the creep behaviour of a 617 alloys steel steam pipe, which operated within 100,000 hours. The temperature of steam was set at 700?C, and the pressure in the pipe was 35 MPa. Abaqus software based on the finite element method was used in the study. The effect of internal pressure and pipe thickness on the creep strains was observed. The variation of the internal pressure was 35, 37.5, 40, 42.5, and 45 MPa. Whereas, the thickness variations were 30, 35, 40, 45, and 50 mm. The simulation results revealed that an increase in the internal pressure and the decrease of the pipe thickness increase the creep strain. This study can be used to predict the possibility of creep damaged for the superheater pipes operated at high temperatures, which have different thicknesses.

Microstructural evolution during high-temperature tensile creep at 1,500°C of a MoSiBTiC alloy

Microstructural evolution in the TiC-reinforced Mo–Si–B-based alloy during tensile creep deformation at 1,500°C and 137 MPa was investigated via scanning electron microscope-backscattered electron diffraction (SEM-EBSD) observations. The creep curve of this alloy displayed no clear steady state but was dominated by the tertiary creep regime. The grain size of the Moss phase increased in the primary creep regime. However, the grain size of the Moss phase was found to remarkably decrease to <10 µm with increasing creep strain in the tertiary creep regime. The EBSD observations revealed that the refinement of the Moss phase occurred by continuous dynamic recrystallization including the transformation of low-angle grain boundaries to high-angle grain boundaries. Accordingly, the deformation of this alloy is most likely to be governed by the grain boundary sliding and the rearrangement of Moss grains such as superplasticity in the tertiary creep regime. In addition, the refinement of the Moss grains surrounding large plate-like T2 grains caused the rotation of their surfaces parallel to the loading axis and consequently the cavitation preferentially occurred at the interphases between the end of the rotated T2 grains and the Moss grains.