A methodology to distinguish martensite formed in the first quench (M1) from martensite formed in the second quench (M2) of the Quenching and Partitioning process is presented, enabling the study of ...the structural characteristics of both microstructural constituents. Investigations show that M1 displays larger block size and less lattice imperfections than M2, differences that can be related to their respective carbon contents.
•An approach to distinguish “old” from “new” martensite in Q&P steels is presented•Methodology allows separate characterization of microstructure and crystallography “Old” martensite has larger block size and more perfect lattice than the “new” one•The differences between the old and new martensite depend on their carbon contents
The aim of this study was to evaluate the association between actigraphy-based measures of sleep and prevalent hypertension in a sample of US Latinos.
We analyzed data from 2,148 participants of the ...Sueño Sleep Ancillary Study of the Hispanic Community Health Study/Study of Latinos (HCHS/SOL), who underwent 1 week of wrist actigraphy to characterize sleep duration, sleep efficiency, sleep fragmentation index, and daytime naps. Insomnia was defined as an Insomnia Severity Index ≥ 15. Hypertension was defined based on self-reported physician diagnosis. Survey linear regression was used to evaluate the association of sleep measures with hypertension prevalence. Sensitivity analyses excluded participants with an apnea-hypopnea index (AHI) ≥ 15 events/h.
The mean age was 46.3 ± 11.6 years, and 65% of the sample consisted of women. The mean sleep duration was 6.7 ± 1.1 hours. Thirty-two percent of the sample had hypertension. After adjusting for age, sex, ethnic background, site, and AHI, each 10% reduction in sleep efficiency was associated with a 7.5% (95% CI, –12.9 to –2.2; P = .0061) greater hypertension prevalence, each 10% increase in sleep fragmentation index was associated with a 5.2% (95% CI, 1.4-8.9; P = .0071) greater hypertension prevalence, and frequent napping was associated with a 11.6% greater hypertension prevalence (95% CI, 5.5-17.7; P = .0002). In contrast, actigraphy-defined sleep duration (P = .20) and insomnia (P = .17) were not associated with hypertension. These findings persisted after excluding participants with an AHI ≥ 15 events/h.
Independent of sleep-disordered breathing, we observed associations between reduced sleep continuity and daytime napping, but not short sleep duration, and prevalent hypertension.
In this work, the application of the Quenching and Partitioning (Q&P) process to two low-carbon steels has led to the development of a new kind of steel microstructure formed by laths of martensite ...separated by films of intercritical ferrite and retained austenite. The chemical compositions of the steels have been specially designed for this process, containing 3.5
wt.% Mn to retard the formation of bainite and combinations of Si and Al to avoid cementite precipitation. The microstructural changes occurring during the application of the heat treatments are discussed in terms of the current knowledge of the Q&P process and the experimental observations. A significant amount of retained austenite has been obtained in both steels after application of appropriate heat treatments, especially in the steel alloyed with higher amount of Si, in which the volume fraction of retained austenite reached values up to 0.19. Tensile tests in some selected specimens of both materials have shown outstanding combinations of strength and ductility, indicating that the designed Q&P steels are a promising candidate for the development of a new generation of advanced high strength steels.
Rolling Contact Fatigue (RCF) is very high-cycle fatigue process, leading to the formation of so-called butterfly cracks around non-metallic inclusions. The purpose of this study is to describe the ...crack initiation and microstructural changes around butterfly cracks in the hardened and tempered bearing steel with artificially introduced Al
2O
3 inclusions. This paper presents results from investigations using state-of-art electron imaging techniques such as Transmission Electron Microscopy (TEM), Electron Backscattered Diffraction (EBSD) and Focused Ion Beam (FIB). EBSD measurements showed high level of local grain misorientation at Al
2O
3/steel matrix interface which suggests possible locations for microcracks initiation. The TEM samples, containing the cracks, were selected from specific locations using precise FIB preparation process, allowing the TEM analyses of the large microstructural changes between the butterfly crack (formation of ultra fine nano-crystalline ferrite) and the steel matrix (tempered martensite). It was found that the butterfly crack growth and microstructural changes (formation of nano-crystalline ferrite) are simultaneous processes as a result of low-temperature recrystallization.
The C, N, and W content in EUROFER97, a 9CrWVTa reduced-activation ferritic/martensitic (RAFM) steel, was varied to obtain an experimental assessment of the main effects of the compositional ...variation on the mechanical properties and microstructural characteristics of six different experimental grades. Light optical microscopy (LOM) and electron back-scattered diffraction (EBSD) revealed in almost all cases a fine tempered lath martensite structure. Analyses of transmission electron micrographs, together with inductively coupled plasma mass spectrometry (ICPMS) and energy-dispersive x-ray spectroscopy (EDS) data, shows the precipitation state and spatial distribution of MxCy (M = Cr, W and Fe) and MX (M = V and/or Ta, X = C or N) carbonitrides within the matrix. The mechanical characterization of the six different steel grades was carried out by means of A50 tensile testing and Charpy tests on standard specimens (55 × 10 × 10 mm3). Lowering the carbon content and keeping the nitrogen content higher than 0.02 wt%, leads to a reduction of the ductile-to-brittle-transition-temperature (DBTT) in comparison with EUROFER97-2. The addition of tungsten further reduces the DBTT to - 94 °C, while maintaining good tensile strength and elongation.
Heating experiments in a wide range of heating rates from 10°C/s to 1200°C/s and subsequent quenching without isothermal soaking have been carried out on a low carbon steel. The thermal cycles were ...run on two different cold rolled microstructures, namely ferrite+pearlite and ferrite+martensite. It is shown that the average ferritic grain size, the ferrite grain size distribution, the phase volume fractions and the corresponding mechanical properties (ultimate tensile strength and ductility) after quenching are strongly influenced by the heating rates and the initial microstructure. The ferrite grain size distribution is significantly modified by the heating rate, showing a markedly bimodal distribution after fast annealing. The rise of the heating rate has produced a change in the relative intensities of texture components, favouring those of the cold-deformed structure (RD fibre) over the recrystallization components (ND fibre).
ABSTRACT
Understanding the geometry and kinematics of the broad line region (BLR) of active galactic nuclei (AGN) is important to estimate black hole masses in AGN and study the accretion process. ...The technique of reverberation mapping (RM) has provided estimates of BLR size for more than 100 AGN now; however, the structure of the BLR has been studied for only a handful number of objects. Towards this, we investigated the geometry of the BLR for a large sample of 57 AGN using archival RM data. We performed systematic modelling of the continuum and emission line light curves using a Markov chain Monte Carlo method based on Bayesian statistics implemented in PBMAP (Parallel Bayesian code for reverberation−MAPping data) code to constrain BLR geometrical parameters and recover velocity integrated transfer function. We found that the recovered transfer functions have various shapes such as single-peaked, double-peaked, and top-hat suggesting that AGN have very different BLR geometries. Our model lags are in general consistent with that estimated using the conventional cross-correlation methods. The BLR sizes obtained from our modelling approach is related to the luminosity with a slope of 0.583 ± 0.026 and 0.471 ± 0.084 based on H β and H α lines, respectively. We found a non-linear response of emission line fluxes to the ionizing optical continuum for 93 ${{\ \rm per\ cent}}$ objects. The estimated virial factors for the AGN studied in this work range from 0.79 to 4.94 having a mean at 1.78 ± 1.77 consistent with the values found in the literature.
Display omitted
•The mathematical modeling of the coked Cr-Mg catalyst regeneration is studied.•The parameters of the model were determined from the lab-scale experiments.•The inlet gas temperature ...higher than catalyst one leads to the adiabatic bed heating.•The low inlet gas temperature leads to the overheating and catalyst damage.
On the base of two-dimensional (bed length and pellet radius coordinates) two-temperature (gas and catalyst phases) mathematical model the coked CrF3/MgF2 catalyst regeneration process in the adiabatic reactor of gas-phase perchloroethylene hydrofluorination to pentafluoroethane (Freon 125) is studied. The parameters of the mathematical model such as coke burning rate constant, temperature of the coke burning beginning T* (ignition point), heat of the reaction and activation energy were determined from experiments in lab-scale reactor and from the thermal analysis data. The influence of process parameters, e.g. oxygen and coke concentrations, initial gas and catalyst temperatures and pellet size was investigated. The simulation was carried out under conditions of low oxygen concentration (oxygen shortage) and showed that if the inlet gas temperature Tgo is lower than initial catalyst temperature Tp0 (Tgo<Tp0, Tpo≥T*), the coke burning occurs only near the outer surface of catalyst pellet, because the bed is “cooled” by the inlet gas flow. This leads to the dramatic rise of temperature in a narrow “peak burning wave” and to an unexpected bed overheating that exceeds the adiabatic heating. This effect was not discussed in publications but was detected in industrial reactor during the coked chromium–magnesium catalyst regeneration. This result is very significant for industrial applications since it helps to avoid the catalyst overheating and the following catalyst activity decreasing during the regeneration.
The influence of the heating rates from 10 to 1000 °C/s and annealing temperatures on the microstructure and mechanical properties of two 0.2%C, 1.9%Mn, 1.4%Si cold-rolled steels with and without the ...addition of carbide-forming elements (Mo, Nb, and Ti) have been investigated. Results show that the increase of the heating rate above 100 °C/s refines the parent austenitic grains in both alloys. The increment of the heating rate led to carbon heterogeneities in the austenite, which after subsequent cooling promoted the formation of a complex mixture of fine-grained constituents. As expected, at the lower heating rates the presence of Nb and Ti-rich carbides and carbonitrides controls the austenite grain growth during the annealing treatment. The tensile test results reveal that high heating rates do not have a significant influence on the tensile strength of the alloy with carbide-forming elements. On the other hand, both the ultimate tensile strength (UTS) and total elongation of the alloy without carbide-forming elements decrease, due to the formation of bands of ferrite and high carbon martensite. However, samples treated at heating rates above 100 °C/s show a combination of UTS in the range of 1400–1600 MPa, and 12–18% of total elongation. The results suggest that the microstructure heterogeneity obtained after high heating rates, especially the ferrite content, has the major effect on the mechanical behavior of the studied steels.
PDF
