Validation of thermal-kinetic-diffusion model for arc plasma surface hardening of steel

Realistic analysis of the surface hardening pocesses during an arc plasma torch application indicates that the events taking place during a plasma torch pass should be properly looked at as a thermal-kinetic-diffusion problem. Arbitrary separation of these components may lead to erroneous evaluation of the variables and parameters that characterise the process. Consequently, a complex finite element model, which describes elementary processes of gas-solid surface interaction by the method of molecular gas dynamics, connects the transfer processes between gaseous and solid phases and takes into consideration thermal and diffusion events in the steel, has been developed. The adequacy of the mathematical model was substantiated by comparing model predictions to measurements made during surface hardening using an arc nitrogen plasma torch. It was proved that the approach gives the possibility to compute with sufficient accuracy the transient distributions of temperature and concentration fields in the steel surface layer during surface hardening using high-enthalpy gas flux.     

高、低喷涂功率下内送粉等离子喷涂YSZ粒子熔化状态研究

等离子喷枪采用内送粉方式后,喷涂功率对YSZ热障涂层微观结构和性能有着独特的作用,高功率下制备的涂层结合强度反而较低。本文研究了内送粉等离子喷枪在高功率(42.5 kW)和低功率(33.6 kW)时距喷嘴出口90 mm处的粒子熔化状态(粒子温度、粒子飞行速度、粒度分布及扁平粒子形貌)。研究结果表明,采用高喷涂功率时,由于等离子射流较高的热焓值使粒子迅速熔化并细化成粒径较小的熔滴,熔滴在撞击基体前发生再凝固降低了扁平粒子间的粘结。涂层出现横向裂纹,结合强度平均值仅为22.58 MPa。采用小喷涂功率时,既能保证粒子熔化良好又不会导致粒子发生明显细化。涂层结合强度平均值为37.25 MPa,随试片弯曲180(°)后仅出现一处剥落,1100℃炉中保温10 min取出后迅速置于水中淬冷130次后,试片中部涂层完好,显示出良好的抗热震性能。     

Nitrogen transfer into plasma heated steel melts as a function of arc polarity

Heating liquid steel in the tundish with argon stabilized plasma arcs is one example of the increasing importance of mobile and inert heating systems in metallurgy. Unwanted nitrogen pick-up caused by infiltrated air, and possibly aggravated by the activating effect of plasma, should be minimized by selecting the optimum current mode and torch polarity. With this aim in mind, the nitrogen transfer kinetics have been investigated on 150-kg low carbon steel melts at different nitrogen partial pressures as a function of arc polarity. The experimental plasma plant was equipped with a 2000-A torch, housing a thoria stabilized tungsten electrode, and a counter electrode located at the bottom of the crucible. The torch could be operated with DC of either polarity or with AC. Kinetic models of nitrogen transfer have been developed, which show good agreement with experimental results. Arc polarity has a significant effect both on the rate of nitrogen pick-up in the arc area (pumping effect) and on the mass transport in the melt (stirring effect). Of all configurations tested, the torch as cathode is the most suitable arc configuration for inert heating, since the particularly active N⁺ ions in the plasma are repelled by the melt surface. Furthermore, the relatively strong stirring effect of the plasma jet leads to the highest heat transfer efficiency. On the other hand, where controlled nitrogen pick-up is required, the highest nitrogen transfer rates are obtained with the torch poled as anode.     

氮等离子体弧加热中间包用保护渣

为防止氮等离子体弧加热加热钢液而引起的钢液吸氮，用１００ｋＷ直流等离子体炉研究了ＴｉｘＯ－Ｂ２Ｏ３－Ｎａ２Ｏ系保护渣对钢液吸氮的影响，结果表明，保护渣尤其是渣中的ＴｉｘＯ及Ｂ２Ｏ３能有效地减缓氮等离子体弧作用下钢液吸氮，另外还对熔渣的作用及成分的选择作了热力学及动力学分析，所得参数对保护渣的选择具有一定意义。     

超低压等离子喷涂与沉积技术的发展动态

超低压等离子喷涂沉积技术是近几年来正在发展的一项崭新技术。该技术特点是在原低压等离子喷涂基础上(通常喷涂工作压力为5000～8000Pa),采用大流量真空泵,使容器内动态工作压力达到100Pa以下,配置150kw大功率等离子喷枪,使从喷枪喷出的高温粉体在加热过程中达到一定比率的气化。该技术结合了传统等离子喷涂或者低压等离子喷涂以凝固为主形成喷涂涂层和物理气相沉积以气/固方式形成沉积薄膜的特征。利用该技术可以制备不同与传统等离子喷涂和物理气相沉积能够获得的涂层组织结构,为材料表面改性提供了新的技术途径。本文将介绍这一技术的特点、发展动态和最近大连海事大学自行研制的超低压等离子喷涂与沉积设备。     

Evaporation of reaction-zone components in the low-temperature plasma treatment of chromium-bearing melts

The evaporation of iron, Fe–Cr, Fe–Ni, and Fe–Cr–Ni melts at the plasma spot in treatment by low-temperature argon plasma is studied. Experiments with different masses of metal and with variation in arc power of the plasmatron are conducted so as to determine the conditions corresponding to stable surface temperature of the metal. The results show that, for experiments in which the plasma flux completely exposes the surface of the metal droplet, arc power no less than 2.0 kW is optimal; the mass of the metal should be 5?10 g. The evaporation process is studied as function of the melt composition, and the evaporation rates are determined. Of the alloys considered, Fe–Cr–Ni melt is characterized by the highest evaporation rate in the neutral atmosphere of a laboratory plasma furnace. The surface temperature of the melt is determined indirectly, on the basis of the evaporation rate. The surface temperature of the plasma-treated melt is found to vary in the range 1950–2100 K with variation in arc power from 1.6 to 2.4 kW.     

Plasma-particle interactions in plasma spraying systems

A mathematical formulation is presented to represent the interactions between the plasma jet exiting a nontransferred arc plasma torch and injected solid particles. This is a generic problem in plasma spraying operations. The calculations are based on the solution of the two-dimensional equation of motion and the thermal energy balance for the particles. Additionally, the plasma temperatures and velocities in the torch and plume are calculated using a mathematical model based on a simplified set of conservation equations. In the formulation, we allow for departure from continuum conditions, particle vaporization, and temperature gradients within the particles. The calculations are compared with previously published experimental measurements of alumina particles injected into a room-temperature, turbulent air jet and into the plume of a commercial plasma torch operating in a turbulent mode. The second set of experiments provides simultaneous measurement of particle temperature, size, and velocity and so form an excellent basis for testing our model. The comparison of the model and the measurements brings new insight into the behavior of particles in plasma jets. This article is based on a presentation made in the symposium “Spray Processing Fundamentals: Coating and Deposition” presented as part of the 1990 TMS Fall Meeting, October 9, 1990, in Detroit, MI, under the auspices of the TMS Synthesis and Analysis in Materials Processing Committee.     

Influence of In Situ Factors on Dynamic Response of Piedmont Residual Soils

The in situ chemical and physical weathering of igneous and metamorphic rocks, indentified as the process of formation of Piedmont residual soils, is a fairly well understood phenomenon. However, the effect this weathering has on the physical, mechanical, and dynamic properties of the rock∕soil is not understood fully. This study focuses on the dynamic shear modulus, G, and material damping ratio, D, of this soil formation for low- to mid-level amplitudes of vibration. The paper presents laboratory test results and correlations that demonstrate the effects that the degree of weathering has on these properties for various levels of confining pressure and shear strain amplitude. A total of 12 specimens of Piedmont residual soils from different depths were tested in a Resonant Column (RC) device. The specimens tested were SM and ML soils according to the USCS classification. The low-amplitude shear modulus and damping values were found to be similar to those reported in the literature from laboratory and in situ tests on the same type of soils. It was found that weathering, void ratio, and apparent overconsolidation ratio exert a noticeable influence on the dynamic response as a result of variations in confining pressure. The understanding of these effects will allow for a better prediction of phenomena such as soil amplification, which may result in damage to existing civil infrastructure founded on these soil deposits. The response in free field soil deposits compared with that of soils experiencing added confining stresses due to foundation loading appears to vary significantly in these geologic formations. Threshold strain and the variation of damping, D, with the normalized shear moduli, G∕Gmax, fall within the same range as those recently reported by other authors in similar soils.     

Xenopus Zic-related-1 and Sox-2, two factors induced by chordin, have distinct activities in the initiation of neural induction

Broad-scale differences in soil microbial community composition were analyzed in two contrasting soils using DNA reassociation and % G + C profiles for analysis on the community-level, and filter- and whole cell hybridization techniques for a coarse-level characterization of larger phylogenetic groups of bacteria. Reassociation analysis of DNA from bacterial fractions extracted from the organic soil Seim and the mineral soil Hau revealed similar complexity of the communities with 5700 and 4900 different bacterial genomes (g soil [dry wt])-1, respectively. Thermal denaturation studies showed wide % G + C distributions in DNA from bacteria of both soils. Differences in the median % G + C with 55 to 61% for the bacterial community in soil Seim and 61 to 66% for that in soil Hau indicated a higher proportion of bacteria with a high DNA G + C content in soil Hau. In situ hybridization with fluorescent (Cy3-labeled) probes targeting larger phylogenetic groups showed minor differences between both soils, and between direct detection of bacteria in dispersed soil slurries and in bacterial fractions extracted from soils through about 90% of the total bacteria were lost during extraction. In dispersed slurries of both soils, only probes ALF1b, SRB385, and PLA46 hybridized to cells accounting for more than 1% of the DAPI-stained cells, while numbers obtained after hybridization with probes ARCH915, BET42a, GAM42a, HGC69a, and CF319a were below the detection limit set at < 1%. These results were confirmed by in situ hybridization with horseradish peroxidase (HRP)-labeled probes and subsequent Cy3-tyramide signal amplification. In contrast, dot blot hybridization with probe HGC69a indicated significant amounts of Gram-positive bacteria with a high DNA G + C content in both soils. These could subsequently be visualized in non-dispersed soil slurries by in situ hybridization with HRP-labeled probe HGC69a and Cy3-tyramide signal amplification. Filamentous Gram-positive bacteria with a high DNA G + C content, likely actinomycetes, which are present in soil Hau in significant numbers are obviously destroyed by procedures used for soil dispersion.     

喷涂参数对内送粉等离子喷枪制备YSZ涂层微观结构和性能的影响

为了进一步提高大气等离子喷涂YSZ热障涂层性能并降低其成产成本,开发了内送粉等离子喷枪.本文采用Spray Watch 2i在线监测系统测量了YSZ粉末粒子温度和飞行速度,采用实验室手段表征了涂层的微观结构、结合强度、弯曲性能和抗热震性能.研究结果表明,内送粉方式下喷涂距离对涂层微观结构和性能的影响规律与外送粉相似,而电流和主气流量表现出了独特的作用效果：随电流的增大,涂层综合性能先增加后降低,高电流下制备的涂层结合强度较低;在30～50 L/min范围内,增大主气流量可显著提高涂层结合强度.与外送粉最优参数制备YSZ涂层相比,内送粉时的功率消耗大幅降低,但粒子熔化效果明显增强,相同送粉速率下粉末沉积效率略有增加,涂层热循环寿命有所提高.     

超低压等离子喷涂喷嘴设计与喷枪特性

相比大气等离子喷涂（APS）,超低压等离子喷涂（VLPPS）在喷嘴出口处的自由区域对粉末的加热能力较低。目前,大多数超低压等离子喷涂设备采用大功率等离子喷枪,电弧电流高达2500A,以补偿加热能力较低的缺点,然而这样对等离子喷枪及其辅助设备要求较高。根据低压环境下的等离子体特性,设计了内送粉变径阳极等离子喷枪。本研究使...     

Power increase of plasma heating systems by CO2 addition into the furnace atmosphere: Investigations in view of the plasma ladle furnace

For certain steel grades the treatment in conventional ladle furnaces equipped with graphite electrodes may not be advisable because of the risk of carbon pick-up. In these cases metallic plasma torch systems operated with argon as plasma gas may be preferable. However, the maximum active power available from a 3 torch AC argon plasma system is at present limited to about 4 MW. Tests were carried out with 200 kg heats in a pilot furnace equipped with 2 AC argon plasma torches to investigate the arc voltage increase by adding CO₂ to the furnace atmosphere. With 20% CO₂, the voltage of a 30 cm arc was raised from 100 to 173 V. The transfer rate of oxygen from the CO₂ in the atmosphere via the plasma arc to the steel melt was measured by means of steel and off-gas analyses. After formation of a slag layer, about 10% of the oxygen supply injected into the furnace as CO₂ was transferred to the melt. The measured values concerning power increase and oxygen transfer were extrapolated to ladle capacities of 50 to 150 t and 3-torch 12 kA AC plasma heating systems. In a furnace atmosphere containing 60% CO₂, the active power available from a plasma system would be 10 MW as compared to 3.5 MW in pure argon. The oxygen transfer rates proved to be relatively small due to the low gas flow rates of such systems and to the low mass transfer efficiency. The addition of 40% CO₂ would raise the power to heat 110 t of steel at a heating rate of 3 K/min, while the oxygen level would increase within 30 min by less than 10 ppm. The specific CO₂ demand would in that case be 0.12 m³(STP)/t.     

Hydraulic Conductivity of Soil Sorptive Zones Created by In Situ Injection of a Cationic Surfactant

The sorptive capabilities of soils for organic contaminants can be greatly enhanced by treatment with cationic surfactants, and this has been suggested as a potential in situ approach for contaminant plume management. The hydraulic properties of soils modified by injection of hexadecyltrimethylammonium (HDTMA) were investigated using soil columns and a fixed-ring consolidometer. Oshtemo soil (87% sand, 10.5% clay, 2.5% silt) under two different effective stresses, was equilibrated with 1?mM NaCl and treated by recirculation of two different HDTMA soil concentrations, one above and one below the cation exchange capacity. No statistically significant changes in hydraulic conductivity occurred as a result of HDTMA treatment at any of the experimental conditions studied. These results suggest that sorptive zones created in situ with HDTMA may be hydraulically feasible.     

Estimating Compaction of Cohesive Soils from Machine Drive Power

To evaluate roller-integrated machine drive power (MDP) technology for predicting the compaction parameters of cohesive soils considering the influences of soil type, moisture content, and lift thickness on machine power response, a field study was conducted with 15-m test strips using three cohesive soils and several nominal moisture contents. Test strips were compacted using a prototype CP-533 static padfoot roller with integrated MDP technology and tested using various in situ compaction measurement devices. To characterize the roller machine-soil interaction, soil testing focused on measuring compaction parameters for the compaction layer. Variation in both MDP and in situ measurements was observed and attributed to inherent variability of the compaction layer and measurement errors. Considering the controlled operations to create relatively uniform conditions of the test strips, measurement variability observed in this study establishes a baseline for acceptable variation in production operations using MDP technology in cohesive soils. Predictions of in situ compaction measurements from MDP were found to be highly correlated when moisture content and MDP-moisture interaction terms were incorporated into regression models.     

DC PJ-CVD等离子体炬通道中的电弧行为

运用电磁学和热流体学的方法研究了直流大阳极喷嘴长电弧通道等离子体炬中电弧的行为,并通过实验证明了温度梯度与电场梯度作用于流体时所产生的特殊现象;结果表明:电弧弧柱的高温效应剧烈排斥冷气流的混入,使CVD过程受到抑制.新产生的胶体粒子在热泳现象作用下逐渐沉积于通道内壁上.改变不同气体的进气位置以及减小温度与电场梯度并增设径向旋转磁场,可有效地解决上述问题.     

Effects of particle loading on a reduced pressure inductively coupled radio frequency plasma torch

Our previous attempts in the modeling of the heat transfer and fluid flow in radio frequency (RF) plasma torches considered dilute particle-loading conditions. It was assumed that the injected particles have no effect on the plasma temperature and velocity profiles. However, in practice, the plasma deposition process is carried out under fairly high loading conditions to achieve high energy efficiency. The plasma gas experiences significant local cooling and deceleration due to high particle injection rates. To this end, a numerical model has been developed which considers the coupling effects between the plasma temperature and velocity fields and injected particles. In this study, effort has been focused on the particle-loading effect in an inductively coupled RF plasma torch under a reduced pressure environment. The temperature and flow fields in an inductively coupled RF plasma torch are solved using an axisymmetric, variable property formulation of the Navier-Stokes equations. Pseudo two-dimensional electrical and magnetic field equations were considered. In addition, an integral constraint condition is used to maintain a specified discharge power in the plasma torch. The interaction between the plasma gas and injected particles is considered using the well-known “Particle-Source-In-Cell” (PSI-Cell) method. The exchanged mass, momentum, and energy between the plasma gas and injected particles are accounted for through additional source terms in the governing equations. The effect of particle loading on the resulting torch flow, thermal profiles, and particle-melting characteristics are presented and discussed.     

Accounting for Soil Aging When Assessing Liquefaction Potential

It has been recognized that liquefaction resistance of sand increases with age due to processes such as cementation at particle contacts and increasing frictional resistance resulting from particle rearrangement and interlocking. As such, the currently available empirical correlations derived from liquefaction of young Holocene sand deposits, and used to determine liquefaction resistance of sand deposits from in situ soil indices [standard penetration test (SPT), cone penetration test (CPT), shear wave velocity test (Vs)], are not applicable for old sand deposits. To overcome this limitation, a methodology was developed to account for the effect of aging on the liquefaction resistance of old sand deposits. The methodology is based upon the currently existing empirical boundary curves for Holocene age soils and utilizes correction factors presented in the literature that comprise the effect of aging on the in situ soil indices as well as on the field cyclic strength (CRR). This paper describes how to combine currently recorded SPT, CPT, and Vs values with corresponding CRR values derived for aged soil deposits to generate new empirical boundary curves for aged soils. The method is illustrated using existing geotechnical data from four sites in the South Carolina Coastal Plain (SCCP) where sand boils associated with prehistoric earthquakes have been found. These sites involve sand deposits that are 200,000?to?450,000?years in age. This work shows that accounting for aging of soils in the SCCP yields less conservative results regarding the current liquefaction potential than when age is not considered. The modified boundary curves indicate that old sand deposits are more resistant to liquefaction than indicated by the existing empirical curves and can be used to evaluate the liquefaction potential at a specific site directly from the current in situ properties of the soil.     

Effect of Soil Type on Electrokinetic Removal of Phenanthrene Using Surfactants and Cosolvents

Numerous sites have been contaminated with polycyclic aromatic hydrocarbons (PAHs), and these sites pose a significant risk to public health and the environment because PAHs are often toxic, mutagenic, and/or carcinogenic. Furthermore, these sites are often difficult or costly to remediate because PAHs are hydrophobic and highly resistant to degradation. The in situ flushing process using surfactants and/or cosolvents has shown great promise for sites possessing uniform and high-permeability soils, but it is generally ineffective for sites containing heterogeneous and/or low-permeability soils. Thus, for difficult soil conditions, electrokinetics can be integrated with the in situ flushing process to improve soil-solution-contaminant interaction. This investigation was conducted to evaluate the effects of two different low-permeability soils, kaolin and glacial till, on electrokinetically enhanced flushing. Each soil type was used in three bench-scale electrokinetic experiments, where each test employed a different flushing solution, deionized water, a surfactant, or a cosolvent. The results indicated that the contaminant was more strongly bound to the glacial till than the kaolin, and this was attributed to its higher-organic content. The glacial till also generated a greater electrical current and electro-osmotic flow, and this was probably a result of its higher-carbonate content and more diverse mineralogy. Based on the contaminant mass remaining in the soil, it was apparent that the surfactant or cosolvent solution caused contaminant desorption, solubilization, and/or migration in both soils, but additional research is required to improve PAH removal efficiency.     

Some characteristics of hydroxylapatite powder particles after plasma spraying

We have studied the particles of hydroxylapatite (HA) powder, the particles after plasma spraying, their distribution on substrate surface and their condition after transfer through the plasma torch. Mean particle size of HA powders was as follows: HA-A: 3.8 microm, HA-B 88.2 microm. The area of HA coating after plasma spraying, when the torch had a constant position against the substrate surface, shows two characteristic zones: the central part of coating formed mainly from deformed particles and the marginal part of coating with small non-deformed particles. These small non-deformed particles can be found in all zones of the coating and together with greater non-deformed particles and partially deformed particles will unfavourably affect the adhesive and cohesive strength of the coating and its porosity. The maximum diameter of the molten (spherical) particles in the conditions of Ar + H2 plasma, output P = 24 kW was: DA = 25 microm (HA-A) and DB = 65 microm (HA-B). The intervals of dimensions in which most of molten particles occurred were HA-A: 0-15 microm (98%), HA-B: 5-35 microm (84%). From comparison of HA-A and HA-B powders it can be concluded that the transport of HA-A powder was not continuous, the amount of molten HA-A particles was considerably greater (90%) than that of HA-B powder (63%). Phase decomposition and also solubility of HA-A powder (at in vitro tests) was greater. If we consider transport of particles, their melting, splitting and spraying efficiency, the suitable size of HA powder particles for the given spraying conditions is somewhere between the size of HA-A and HA-B particles--let us say--in the interval from 20 to 60 microm.     

Modeling Transport of Gaseous Ozone in Unsaturated Soils

The use of in-situ ozone venting is an effective and economic technology to remediate soils contaminated with organic chemicals. A model was developed in this study to describe the transport of gaseous ozone in unsaturated soils. Mass balance equations for soil organic matter, phenanthrene, and ozone were incorporated into the model. The model was found to fit the experimental data obtained from one-dimensional columns, using the previously published rate expressions for the reactions of ozone with soil organic matter and phenanthrene. However, it was observed that the initial unsteady-state conditions for the gas pressure resulted in minor deviations between the simulated and experimental results. Based on the simulated results, the reactions of ozone with phenanthrene and organic matter can be modeled as either parallel or serial reactions. As the initial distribution of contaminant would be nonuniform and some contaminant would be sorbed or trapped in immobile regions, it is recommended that in situ ozonation be ceased no sooner than when the effluent ozone concentrations begins to stabilize.