Showing papers on "Quenching published in 2016"

Evolution of density profiles in high-z galaxies: compaction and quenching inside-out

Abstract: Using cosmological simulations, we address the interplay between structure and star formation in high-redshift galaxies via the evolution of surface density profiles. Our sample consists of 26 galaxies evolving in the redshift range $z=7-1$, spanning the stellar mass range $(0.2-6.4)\times 10^{10}M_\odot$ at $z=2$. We recover the main trends by stacking the profiles in accordance to their evolution phases. Following a wet compaction event that typically occurs when the stellar mass is $\sim10^{9.5}~M_{\odot}$ at $z\sim2-4$, the gas develops a cusp inside the effective radius, associated with a peak in star-formation rate (SFR). The SFR peak and the associated feedback, in the absence of further gas inflow to the centre, marks the onset of gas depletion from the central 1 kpc, leading to quenching of the central SFR. An extended, star-forming ring that forms by fresh gas during the central quenching process shows as a rising specific SFR (sSFR) profile, which is interpreted as inside-out quenching. Before quenching, the stellar density profile grows self-similarly, maintaining its log-log shape because the sSFR is similar at all radii. During the quenching process, the stellar density saturates to a constant value, especially in the inner 1 kpc. The stellar mass and SFR profiles deduced from observations show very similar shapes, consistent with the scenario of wet compaction leading to inside-out quenching and the subsequent saturation of a dense stellar core. We predict a cuspy gas profile during the blue nugget phase, and a gas-depleted core, sometimes surrounded by a ring, in the post-blue nugget phase.

Evolution of phases in P91 steel in various heat treatment conditions and their effect on microstructure stability and mechanical properties

Abstract: To achieve high thermal efficiency, modern day thermal power plants operate at higher operating temperature and pressure which necessitates use of steels with high creep rupture strength such as modified 9Cr-1Mo steels. In the present study, the evolution of phases in modified 9Cr-1Mo P91 steel and their effects on microstructural stability and mechanical properties have been studied for specimens that were subjected to different thermal heat treatment conditions. The main focus has been to study the effect of heat treatment temperature ranging from 623 K to 1033 K (350–760 °C) on P91 steel. Further, the effect of furnace cooling, water quenching, tempering at 1273 K (1000 °C) and austenitizing on the mechanical properties and microstructure has been studied. The techniques used for material characterization were scanning electron microscopy (SEM), optical microscopy (OM) and X-ray diffraction. For low tempering temperature, i.e. 623 K (350 °C), M 23 C 6 , M 3 C, M 7 C 3, and MX precipitates have been observed with high yield strength (YS), tensile strength (UTS), hardness and low toughness. In the high temperature range, 923–1033 K (650–760 °C), fine MX, M 7 C 3 , M 23 C 6 , M 2 X, and M 3 C precipitates have been observed with low YS, UTS, hardness and high toughness. The steel tempered at 1033 K (760 °C) was observed to be having best combination of YS, UTS, hardness, toughness and ductility.

Effect of Prior Austenite Grain Size Refinement by Thermal Cycling on the Microstructural Features of As-Quenched Lath Martensite

Abstract: Current trends in steels are focusing on refined martensitic microstructures to obtain high strength and toughness. An interesting manner to reduce the size of martensitic substructure is by reducing the size of the prior austenite grain (PAG). This work analyzes the effect of PAGS refinement by thermal cycling on different microstructural features of as-quenched lath martensite in a 0.3C-1.6Si-3.5Mn (wt pct) steel. The application of thermal cycling is found to lead to a refinement of the martensitic microstructures and to an increase of the density of high misorientation angle boundaries after quenching; these are commonly discussed to be key structural parameters affecting strength. Moreover, results show that as the PAGS is reduced, the volume fraction of retained austenite increases, carbides are refined and the concentration of carbon in solid solution as well as the dislocation density in martensite increase. All these microstructural modifications are related with the manner in which martensite forms from different prior austenite conditions, influenced by the PAGS.

Effect of quenching temperature on martensite multi-level microstructures and properties of strength and toughness in 20CrNi2Mo steel

Abstract: The martensite multi-level microstructures of 20CrNi2Mo steel, which were quenched at the different temperatures of 900–1200 °C and tempered at 200 °C, were investigated by optical microscope (OM), scanning electron microscopy (SEM), electron backscattering diffraction (EBSD) and transmission electron microscopy (TEM), and the relationship between the microstructures and properties of strength and toughness was discussed by the classic formula of Hall–Petch. The results show that the size of prior austenite grain ( d r ), martensite packet ( d p ) and block ( d b ) increase with increasing of the quenching temperature, while the martensite lath ( d l ) size is opposite. On another hand, the confusion degree of the martensite packets changes from disorder to order. The boundaries of prior austenite grain, packet, block and the martensite lath are high angle boundaries (HBs) and low angle boundaries (LBs), respectively, and the ratio of the low angle boundaries increase with the quenching temperature by calculating to the multi-level microstructure size with the mathematical model established by myself. In addition, the relationship between the packet/block and strength follows the classical formula of Hall–Petch, and the size of d b is far lower than the size of d p , d b is the effective control unit of the strength. Meanwhile, d l is the effective control unit of toughness because it strongly impacts the crack initiation and propagation and follows also the Hall-Petch with toughness in 20CrNi2Mo steel.

Enhancing the Mechanical Properties and Formability of Low Carbon Steel with Dual-Phase Microstructures

Abstract: In the present study, a special heat treatment cycle (step quenching) was used to produce a dual-phase (DP) microstructure in low carbon steel. By producing this DP microstructure, the mechanical properties of the investigated steel such as yield stress, tensile strength, and Vickers hardness were increased 14, 55, and 38%, respectively. In order to investigate the effect of heat treatment on formability of the steel, Nakazima forming test was applied and subsequently finite element base modeling was used to predict the outcome on forming limit diagrams. The results show that the DP microstructure also has a positive effect on formability. The results of finite element simulations are in a good agreement with those obtained by the experimental test.

Influence of martensite-austenite (MA) on impact toughness of X80 line pipe steels

Abstract: Varying cooling rates are used to investigate the influence of martensite-austenite (MA) microconstituent on the mechanical properties of X80 linepipe steel after intercritical reheating. It is shown that air and water cooling forms MA, while furnace cooling does not. The increase in cooling rate decreases carbon diffusion to promote MA during air or water cooling. Faster cooling using water results in a change of MA morphology from slender to blocky. Both water and air cooled samples exhibit poor impact toughness in comparison to furnace cooling, with lower Charpy impact toughness occurring with a high percentage of blocky MA following water quenching. It is shown that MA deteriorates toughness by facilitating debonding, cracking and crack initiation during impact testing.

Effect of retained austenite stability and morphology on the hydrogen embrittlement susceptibility in quenching and partitioning treated steels

Abstract: The effect of retained austenite (RA) stability and morphology on the hydrogen embrittlement (HE) susceptibility were investigated in a high strength steel subjected to three different heat treatments, i.e., the intercritical annealing quenching and partitioning (IAQP), quenching and partitioning (QP) and quenching and tempering (QT). IAQP treatment results in the coexistence of blocky and filmy morphologies and both QP and QT treatments lead to only filmy RA. No martensitic transformation occurs in QT steel during deformation, while the QP and IAQP undergo the transformation with the same extent. It is shown that the HE susceptibility increases in the following order: QT, QP and IAQP. Despite of the highest strength level and the highest hydrogen diffusion rate, the QT steel is relative immune to HE, suggesting that the metastable RA which transforms to martensite during deformation is detrimental to the HE resistance. The improved resistance to HE by QP treatment compared with IAQP steel is mainly attributed to the morphology effect of RA. Massive hydrogen-induced cracking (HIC) cracks are found to initiate in the blocky RA of IAQP steel, while only isolate voids are observed in QP steel. It is thus deduced that filmy RA is less susceptible to HE than the blocky RA.

Microstructure, mechanical property and metal release of As-SLM CoCrW alloy under different solution treatment conditions.

Abstract: In the study, the microstructure, mechanical property and metal release behavior of selective laser melted CoCrW alloys under different solution treatment conditions were systemically investigated to assess their potential use in orthopedic implants. The effects of the solution treatment on the microstructure, mechanical properties and metal release were systematically studied by OM, SEM, XRD, tensile test, and ICP-AES, respectively. The XRD indicated that during the solution treatment the alloy underwent the transformation of γ-fcc to e-hcp phase; the e-hcp phase nearly dominated in the alloy when treated at 1200°C following the water quenching; the results from OM, SEM showed that the microstructural change was occurred under different solution treatments; solution at 1150°C with furnace cooling contributed to the formation of larger precipitates at the grain boundary regions, while the size and number of the precipitates was decreased as heated above 1100°C with the water quenching; moreover, the diamond-like structure was invisible at higher solution temperature over 1150°C following water quenching; compared with the furnace cooling, the alloy quenched by water showed excellent mechanical properties and low amount of metal release; as the alloy heated at 1200°C, the mechanical properties of the alloy reached their optimum combination at UTS=1113.6MPa, 0.2%YS=639.5MPa, and E%=20.1%, whilst showed the lower total quantity of metal release. It is suggested that a proper solution treatment is an efficient strategy for improving the mechanical properties and corrosion resistance of As-SLM CoCrW alloy that show acceptable tensile ductility.

Boiling Heat Transfer of Alumina Nano-Fluids: Role of Nanoparticle Deposition on the Boiling Heat Transfer Coefficient

Abstract: This paper focuses on the thermal performance of alumina nano-fluids during the quenching process of a surface at the boiling condition, which can be a good answer to the controversial results available in the nano-fluid related literature. For this purpose, an experimental study is conducted to investigate the potential application of alumina/water nano-fluid for cooling a stainless steel rod under the flow boiling heat transfer mechanism. Nano-fluids are prepared by dispersing the 5, 50 and 80nm alumina nanoparticles into the deionized water. The experimental facility provides conditions to quantify the heat transfer coefficient in forced convection and nucleate boiling heat transfer domains at different operating conditions. In terms of operating time, the experiments are divided into two domains namely short time study and extended time study. For the short time study (0-60 minutes of study with neglecting the role of time on the deposition of nanoparticles) enhancement of heat transfer coefficient is reported for all nano-fluids, however for nano-fluid with smaller nanoparticle size, higher thermal performance is registered. In extended time study (60-1000 minutes) heat transfer coefficient is found to be considerably deteriorated for all nano-fluids. This work demonstrates that the reason for deterioration of heat transfer coefficient is referred to the surface roughness, nanoparticle size, static contact angle and thermal fouling resistance parameters. These four parameters are simultaneously determinative factors, which strongly control the thermal behaviour of nano-fluids over the extended time and are the exact reasons for the controversies raised in the literature.

Deformation-Induced Martensite: A New Paradigm for Exceptional Steels.

Abstract: Martensite steel is induced from pearlitic steel by a newly discovered method, which is completely different from the traditional route of quenching austenitic steel. Both experimental and theoretical studies demonstrate that Fe-C martensite forms by severe deformation at room temperature. The new mechanism identified here opens a paths to material-design strategies based on deformation-driven nanoscale phase transformations.

Quenching effects for piezoelectric properties on lead-free (Bi1/2Na1/2)TiO3 ceramics

Abstract: Lead-free ferroelectric and piezoelectric ceramics, (Bi0.5Na0.5)TiO3 (BNT), were fabricated by a quenching procedure after sintering, and then their electrical properties were investigated with the aim to increase their depolarization temperature T d. From the measurement of the temperature dependence of dielectric properties, T d increased with increasing quench temperature. The T d of a BNT sample quenched from 1100 °C was 223 °C, which was almost 50 °C higher than that prepared by the ordinary cooling process. From the measurement of P–E hysteresis loops, both the remanent polarization P r and the coercive field E c of BNT samples prepared by ordinary firing were almost the same as those quenched from 1100 °C. Additionally, from the measurements by a resonance–antiresonance method, the electromechanical coupling factor k 33 of ordinarily fired BNT was 0.45, and that of the quenched BNT was 0.46. From these results, it is clarified that the quenching procedure is an effective way to increase the T d of BNT ceramics without deteriorating ferroelectric and piezoelectric properties.

Effect of retained austenite on the hydrogen embrittlement of a medium carbon quenching and partitioning steel with refined microstructure

Abstract: The effect of retained austenite on the hydrogen embrittlement of a medium carbon quenching and partitioning steel was investigated by comparison to a traditional quenching and tempering steel with an identical chemical composition. Electrochemical precharging reduces the plasticity, including the elongation and reduction in area, of both steels, and the embrittlement phenomenon is more severe in the quenching and tempering steel based on a slow strain rate tensile test. As a result, the ultimate tensile strength decreases, as well. The fracture mode was dominated by intergranular features in the areas containing hydrogen, suggesting the weakening of boundary cohesion. Retained austenite, which retards diffusion and increases the solubility of hydrogen, is stable under the attack of hydrogen, contributing to the high hydrogen embrittlement resistance of quenching and partitioning steel. Refining the microstructure further improves plasticity due to the lower hydrogen content per area. In general, the quenching and partitioning steel with a refined microstructure exhibits the lowest hydrogen embrittlement susceptibility.

Defect-dominated fatigue behavior in type 630 stainless steel fabricated by selective laser melting

Abstract: Fatigue behavior of type 630 (17–4 pH) stainless steel specimens fabricated by selective laser melting (SLM) was studied. The results obtained were compared with those of the conventionally-melted (CM) type 630. In the microstructure of SLM type 630, martensite-poor and ferrite-rich areas were recognized, while in the CM type 630, acicular martensite uniformly distributed in a matrix. Relatively high temperature was held due to the additive lamination of melted powder during SLM process, which prevented full-martensite structure formation. In the SLM-quenched type 630, the microstructure consisted of acicular martensite due to the post heat treatment at 1050 °C followed by water quenching. The hardness was the highest in the CM type 630, and followed by SLM-quenched and SLM type 630 in decreasing order. The microstructural observation revealed that some defects (

Influence of Mg-doping, calcium pyrophosphate impurities and cooling rate on the allotropic α ↔ β-tricalcium phosphate phase transformations

Abstract: Alpha and beta-tricalcium phosphates are allotropic phases which play a very important role as bone graft substitutes, namely in calcium phosphate cements. Despite extensive research efforts, contradictory reports exist on the importance of quenching for maintaining α-TCP purity. The role of calcium pyrophosphate impurities derived from a certain calcium-deficiency, hydroxyapatite impurities derived from calcium excess, and various ionic substitutions on thermal stability of these phases was not yet fully disclosed. The present work reports on the kinetics of α ↔ β-TCP phase transformations of calcium-deficient TCP powders with different Mg-doping extents (0–5 mol%) prepared by precipitation. Mg clearly enhanced the thermal stability of β-TCP. The effect of cooling rate was more complex and interdependent on the Mg content and the heat treatment schedule. High α-TCP contents were retained upon cooling at 5 °C min −1 for Mg ≤ 1 mol% or upon quenching from 1550 °C for Mg contents ≤2 mol%.

On-stack two-dimensional conversion of MoS2 into MoO3

Abstract: Chemical transformation of existing two-dimensional (2D) materials can be crucial in further expanding the 2D crystal palette required to realize various functional heterostructures. In this work, we demonstrate a 2D 'on-stack' chemical conversion of single-layer crystalline MoS2 into MoO3 with a precise layer control that enables truly 2D MoO3 and MoO3/MoS2 heterostructures. To minimize perturbation of the 2D morphology, a nonthermal oxidation using O2 plasma was employed. The early stage of the reaction was characterized by a defect-induced Raman peak, drastic quenching of photoluminescence (PL) signals and sub-nm protrusions in atomic force microscopy images. As the reaction proceeded from the uppermost layer to the buried layers, PL and optical second harmonic generation signals showed characteristic modulations revealing a layer-by-layer conversion. The plasma-generated 2D oxides, confirmed as MoO3 by x-ray photoelectron spectroscopy, were found to be amorphous but extremely flat with a surface roughness of 0.18 nm, comparable to that of 1L MoS2. The rate of oxidation quantified by Raman spectroscopy decreased very rapidly for buried sulfide layers due to protection by the surface 2D oxides, exhibiting a pseudo-self-limiting behavior. As exemplified in this work, various on-stack chemical transformations can be applied to other 2D materials in forming otherwise unobtainable materials and complex heterostructures, thus expanding the palette of 2D material building blocks.