Precipitation Start Time Research Articles

Distinct impacts on precipitation by aerosol radiative effect over three different megacity regions of eastern China

Abstract. Many studies have investigated the impacts of aerosol on the intensity and amount of precipitation, but few have been done so regarding the impacts of aerosol on the start and peak times of precipitation. Using the high-resolution precipitation, aerosol, and meteorological data in the warm season of June–August from 2015 to 2020, this study investigates the influence of aerosol on the start and peak times of precipitation over three different regions, the North China Plain (NCP), the Yangtze River Delta (YRD), and the Pearl River Delta (PRD). It shows that the period with the highest frequency of precipitation start time, defined as the frequent period (FP) of precipitation start time, is delayed and prolonged by aerosols in NCP, contributing to the similar durations of precipitation in NCP, YRD, and PRD. This study also shows that different types of aerosol (absorbing versus scattering) have caused different influences on the start and peak times of precipitation over the three study regions. The precipitation start time is 3 h advanced in NCP but 2 h delayed in PRD by aerosols during precipitation FP and shows no response to aerosol in YRD. Compared to stratiform precipitation, the convective precipitation is more sensitive to aerosol. The start and peak times of convective precipitation show similar responses to aerosols. This study further shows that the aerosol impacts on precipitation can vary with meteorological conditions. Humidity is beneficial to precipitation, which can advance the precipitation start and peak times and prolong the precipitation duration time. Correspondingly, the impacts of aerosol on start time of precipitation are significant under low humidity or weak low tropospheric stability conditions. The impacts of vertical wind shear (WS) on the start and peak times of precipitation are contrary to that of aerosols, resulting in the fact that WS inhibits the aerosol effects on precipitation.

Modelling of transition from upper to lower bainite in multi-component system

A model for estimating the upper to lower bainite transition has been developed for the iron–carbon–manganese–chromium–silicon alloy system by comparing the time required to decarburise a supersaturated bainitic ferrite platelet and that needed for the start of cementite precipitation in the ferrite. The problem is treated as a competition between the decarburisation time and the kinetics of cementite precipitation. Lower bainite is induced when the latter process is faster. The time for forming a volume fraction of 0.01 of the equilibrium amount of cementite is taken as the precipitation start time. The model was calibrated using experimental data from the iron–carbon system, and verified with the iron–carbon–manganese–molybdenum system and an experimental steel currently being developed for high-strength applications.

From heterogeneous to homogeneous nucleation for precipitation in austenite of microalloyed steels

This paper studies the influence of strain on precipitate nucleation in austenite for three microalloyed steels with different microalloying element (Nb, V) contents. Precipitation start–time–temperature diagrams were determined by means of hot torsion tests and nucleation periods were measured at strains of 0.20 and 0.35. The increase in the dislocation density caused by the strain was calculated for both strains, and the driving forces for precipitation were also calculated. The results show that the influence of the strain on the nucleation time (t0.05) is dependent not only on the strain magnitude but also on the driving force for precipitation. When the driving force is high, or low in absolute terms, the influence of the strain, i.e. the increase in the dislocation density, gives rise to a notable reduction in the t0.05 value due to heterogeneous nucleation on the dislocation nodes. In contrast, when the driving force is low, or high in absolute terms, the influence of the strain on t0.05 decreases considerably and the nucleation of precipitates becomes preponderantly homogeneous. Therefore, the driving force value is responsible for the transition from heterogeneous nucleation to homogenous nucleation.

Model for Strain-Induced Precipitation Kinetics in Microalloyed Steels

Based on Dutta and Sellars’s expression for the start of strain-induced precipitation in microalloyed steels, a new model has been constructed which takes into account the influence of variables such as microalloying element percentages, strain, temperature, strain rate, and grain size. Although the equation given by these authors reproduces the typical “C” shape of the precipitation start time (P s) curve well, the expression is not reliable for all cases. Recrystallization–precipitation–time–temperature diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels with different Nb, V, and Ti contents. Mathematical analysis of the results recommends the modification of some parameters such as the supersaturation ratio (K s) and constant B, which is no longer a constant, but a function of K s when the latter is calculated at the nose temperature (T N) of the P s curve. The value of parameter B is deduced from the minimum point or nose of the P s curve, where ∂t 0.05/∂T is equal to zero, and it can be demonstrated that B cannot be a constant. The new expressions for these parameters are derived from the latest studies undertaken by the authors and this work represents an attempt to improve the model. The expressions are now more consistent and predict the precipitation–time–temperature curves with remarkable accuracy. The model for strain-induced precipitation kinetics is completed by means of Avrami’s equation.

Strain-induced precipitation and softening behaviors of high Nb microalloyed steels

The effects of Nb on strain-induced precipitation and softening were quantified by means of the Gleeble-3500 thermo-mechanical simulator and inductively coupled plasma-atomic emission spectrometry (ICP-AES). The results show that Nb cannot completely dissolve during the reheating processes in the high Nb steels. The undissolved precipitates act as heterogenous nucleation sites for the precipitates induced by the strain, and prolong the strain-induced precipitation start time shown in the stress relaxation curves. The strain-induced precipitation start time is linearly related to the content of supersaturated Nb (the difference between dissolved Nb content after soaking and dissolved Nb content in equilibrium at corresponding deformation temperature). Increasing the content of supersaturated Nb can shift the nose of the precipitation C-curve to shorter time and higher temperatures. The effect of content of supersaturated Nb on precipitation increases along with the decrease of temperature. The softening ratio is linear with dissolved Nb content before the strain-induced precipitation and with precipitable Nb content after the strain-induced precipitation. The precipitates induced by strain have more strong retarding effect on the static recrystallization than the solid solution of Nb atoms. However, with the increase of the average precipitate radius, the retarding effect of precipitate rapidly decreases.

Influence of Temperature on Strain Induced Precipitation Kinetics in Microalloyed Steels

Starting from the expression of Dutta and Sellars for the beginning of strain induced precipitation in microalloyed steels, the influence of temperature on t0.05 parameter has been studied. Although the equation given by these authors reproduces well the typical “C” shape of the curve of precipitation start time Ps, the expression is not reliable for all cases. The precipitation-time-temperature (PTT) diagrams have been plotted thanks to a new experimental study carried out by means of hot torsion tests on approximately twenty microalloyed steels having different contents of Nb, V and Ti. Mathematical analysis of results recommends the modification of some parameters such as supersaturation ratio (Ks) and constant B, which is no longer a constant but a function of Ks when the latter is calculated at the nose temperature (TN) of curve Ps. The value of parameter B is deduced from the minimum point or nose of the Ps curve, where ∂t0.05/∂T is equal to zero, and it can be demonstrated that B cannot be a constant. The new expressions for these parameters derive from the latest studies developed by the authors and this work supposes an attempt to improve the model. The expressions are now more consistent and predict with remarkable accuracy the PTT curves.

Effects of Nb on Strain-Induced Precipitation of NbC and Static Recrystallization for High Nb Pipeline Steels

Strain-induced precipitation of complex carbonitrides and recrystallization for three high-Nb pipeline steels with different Nb and C content have been studied by using a stress relaxation technique and two-passes interrupted compression test. Sequentially, the PTT diagrams were obtained, and static recrystallization activation energy was calculated. Furthermore, the effects of Nb and C content on strain-induced NbC precipitation and static recrystallization were discussed. The results confirm the faster kinetics of precipitation and its retarded recrystallization in the case of higher Nb pipeline steel, and that the recrystallization is easier in low Nb pipeline steel in comparison to the case of high Nb steel. However, the effects of Nb on strain-induced precipitation and static recrystallization were associated with the Nb/C ratio. The precipitation start time (Ps) of strain-induced NbC is delayed in lower Nb/C ratio pipeline steel. It is suggested that the reduced supersaturation of Nb can result in the delay of precipitation of strain-induced NbC carbides forming in the low Nb/C ratio steel.

Time of flight analysis of pulsating aurora electrons, considering wave‐particle interactions with propagating whistler mode waves

We propose a model for the energy dispersion of electron precipitation associated with pulsating auroras, considering the wave‐particle interactions with propagating whistler mode waves from the equator. Since the resonant energy depends on the magnetic latitude, the pitch angle scattering of different energy electrons can occur continuously along the field line. Considering the energy‐dependent path length and the precipitation start time of the precipitating electrons, the transit time of whistler mode waves, and the frequency drift, we calculated the precipitation of electrons observed at the topside ionosphere. Note that higher energy electrons precipitate into the ionosphere of the opposite hemisphere earlier than lower energy electrons. As a result, an energy dispersion of precipitating electrons is observed at the topside ionosphere, even though the modulation of low energy electrons occurs prior to that of high energy electrons. Using the model, we conducted a time‐of‐flight (TOF) analysis of precipitating electrons observed by the REIMEI satellite, assuming an interaction with the whistler mode chorus rising tone. Our TOF analysis suggests that the modulation region of the pitch angle scattering is near the magnetic equator, whereas previous models expected that the modulation region is far from the magnetic equator. The estimated parameters, such as wave‐frequency and latitudinal distribution of the modulation region, are consistent with previous statistical studies of whistler waves at the magnetosphere.

Effect of Austenite Deformation on Recrystallisation Behaviour in an X-70 Microalloyed Steel

In the present study, the effect of austenite deformation on the recrystallisation behaviour in terms of recrystallisation-stop and recrystallisation-limit temperatures (T5% and T95%) of an X70 niobium microalloyed pipeline steel have been investigated by interrupted plane strain compression tests. The extents of recrystallisation are calculated using a modified fractional softening parameter. And the 20% and 60% of fractional softening were correlated to T5% and T95%. Quantitative optical metallography indicates that this method provides for a convenient and reliable experimental measurement of the critical temperatures associated with the recrystallisation of austenite. The recrystallisation kinetics and the precipitation kinetics of Nb(CN) were calculated using two widely applied models. The experimental results from this study suggest that the current model of precipitation kinetics might overestimate the precipitation start time.

Comparison of Experimental Methods to Measure the Isothermal Precipitation Kinetics of Nb(C,N) in Austenite of a Nb-Microalloyed Steel

The isothermal precipitation kinetics of Nb(C,N) in strained austenite of a Nb-microalloyed steel was quantitatively investigated using electrical resistivity (ER) and interrupted compression methods and validated with transmission electron microscopy (TEM). The difference in precipitation start time (P s ) between the two methods is too small to compare the accuracy of the methods. The precipitation finish time (P f ) exhibits the larger difference between the two methods. The interrupted compression method gives too early P f because of particle growth with holding time.

Strain-induced precipitation of NbC in Nb and Nb–Ti microalloyed HSLA steels

The precipitation start time ( P s) of strain-induced NbC carbides is delayed in Nb–Ti steel in comparison to the case of Nb steel. The delay of precipitation of strain-induced NbC carbides is attributed to both the insufficient solution of Nb during a reheating treatment, and the heterogeneous nucleation of (Nb,Ti)C carbides.

Effect of alloying elements on metadynamic recrystallization in HSLA steels

By means of interrupted torsion tests, the kinetics of metadynamic recrystallization (MDRX) were studied in a Mo, a Nb, and a Ti microalloyed steel at temperatures ranging from 850 °C to 1000 °C and strain rates from 0.02 to 2 s1. Quenches were also performed after full MDRX. In contrast to the case of static recrystallization (SRX), the kinetics of MDRX are shown to be highly sensitive to a change of an order of magnitude in strain rate and are relatively insensitive to temperature changes within the range of values applicable to industrial hot-rolling practice. A similar algebraic dependence of the MDRX grain size on strain rate and temperature was found in the three steels. The kinetics of MDRX were slower in the Nb than in the Mo steel, and those of the Ti steel were slower than in the Nb and Mo steels. Above 900 °C and 950 °C, the retardation of MDRX in the Nb and Ti steels, respectively, is due to solute drag. Models predicting the start time for Nb and Ti carbonitride precipitation showed that MDRX is delayed below these temperatures by this mechanism. Comparison of the MDRX and precipitation start times in the Nb steel indicated that a temperature of “no-MDRX” could not be defined, in contrast to the well-definedT nr (no recrystallization temperature) of SRX. By means of torsion simulations composed of multiple interruptions, it is shown that MDRX is retarded decreasingly as the accumulated strain is increased. This appears to be due to the promotion of precipitate coarsening by the continuing deformation.

Influence of dynamic precipitation on grain boundary sliding during high temperature creep

The influence of dynamic or strain-induced precipitation on grain boundary sliding has been investigated. For this purpose, the boundary sliding rate during the high temperature creep of electrical steels was measured. The progress of MnS precipitation in the course of the creep was also determined. The experimental results reveal that the sliding of grain boundaries is arrested when precipitation takes place. During the interval between P s (precipitation start time) and P f (precipitation finish time), the blocking effect exerted on sliding by the precipitates becomes more and more significant because both the size and density of the particles increase. Beyond P f, however, this effect is weakened due to the dissolution of the smaller particles, which results in an increase in the interparticle spacing. On the basis of the experimental results, a quantitative relationship between the sliding rate of a particle-free and of a precipitate-containing boundary has been derived. This relationship illustrates the interaction between precipitation and the rate of grain boundary sliding.

Computer Simulation of VC Precipitation at Moving γ/α Interfaces

A computer simulation technique was developed on the basis of the interaction between carbide precipitation and moving γ/α interfaces for predicting both the interphase precipitation kinetics and the microstructural evolution during austenite-to-ferrite transformation. Theoretical models for the calculation of the driving and pinning forces exerted on a moving interface boundary are described. The variations of the two forces lead to a phenomenon of periodic pinning and unpinning of the interface and, in turn, the characteristic microstructure of parallel sheet particles which is often associated with the interphase precipitation. The experimental data reported for a series of V-bearing steels were analyzed using the computer simulation technique. Three un- known physical parameters,i.e., the thickness of an incoherent interface or the height of the ledge of a coherent interface, the diffusion coefficient of V at the γ/α interface, and the co- herence loss parameter of a VC nucleus, were determined. The calculated intersheet spacing, precipitate size, and precipitation start time all show a good correlation with the experimental observations.