Metallurgical investigation of prematurely failed hot-strip mill work-rolls: Some microstructural observations

A metallurgical investigation of failed samples of hot-strip mill work-rolls used in an integrated steel plant was made to determine the influence of microstructural characteristics on failure susceptibility and roll life. The samples investigated pertained to prematurely failed indefinite chill double-poured (ICDP) iron work-rolls, which exhibited varying roll lives under similar mill operating environments. Although microstructures of all the investigated rolls showed similar graphite morphologies irrespective of their mill performance, discernible differences in carbide characteristics could be observed between high and low life rolls. Microstructural observation of nital-etched roll specimens revealed that lower life rolls were characterized by carbide microcracking. The propensity for cracking was particularly high in carbides exhibiting microhardness greater than 1020 VPN. Electron-probe microanalysis (EPMA) indicated that carbides in the spalled rolls were mostly of M₃C type, where M was Fe and Cr. Quantitative image analysis of phases in the investigated rolls revealed that while graphite volume fraction in the range of 4.0 to 6.4% did not significantly affect roll life, carbide content higher than 28.5 vol% was found detrimental. In fact, a carbide content in the range of 24.0 to 28.50 vol% was found to be desirable for higher roll life. The study thus revealed that although carbides are indispensable for high hardness, resistance to wear, and thermal cracking, an excessive volume fraction (>30 vol%) of high hardness (microhardness > 1020 VPN) carbides accentuated microcracking, which ultimately induced premature spalling of hot-strip mill work-rolls.     

Premature failure of work-rolls in tandem mill: Some microstructural revelations

The microstructural features of prematurely spalled tandem mill work-rolls were examined in an attempt to correlate microstructure with spalling behavior and roll performance. Spalled samples were collected from work-rolls that had shown variations in roll life under similar conditions of mill usage. Optical microscopy revealed that a fine dispersion of spheroidal carbides in a matrix of tempered martensite was conducive to superior performance in terms of roll life (i.e., tonnage rolled), and that coarse angular and irregular shape carbides were detrimental to roll life. Image analysis of roll microstructures indicated that small carbide size, large carbide volume fraction, and high carbide count were characteristic of higher-life rolls, and that large carbide size, low carbide volume fraction, and less carbide density were typical of lower-life rolls. The carbides in both types of microstructure were M₇C₃ type.     

Wear-related failures of nitrocarburized steels: Some microstructural and morphological observations

This paper describes some characteristic microstructural and morphological features of nitrocarburized tool and die steels after wear testing (standard pin-on-disk technique). The steels under study are used for the fabrication of dies and molds for metal working (cold- and hot-work) and plastic injection molding. Surface spalling by interface failure and/or intralayer fracture was observed for the two classes of tool steels (AISI D6 and a CrMoV steel), and uniform surface layer removal by micropolishing and microcutting was observed on the precipitation-hardened (PH) stainless steel.     

Corrosion of a copper U-shaped heating element: Some morphological and microstructural observations

In this work, observations of the corrosion on a failed copper U-shaped tube, of 1 mm nominal thickness, are presented. This tube was subjected to severe corrosion and subsequent cracking after a month of operation as the heating element of an electrical boiler. Morphological and microstructural observations of the failure area as well as cross sections of the corroded copper tube are presented. Moreover, the chemical composition of the material and the principal physicochemical characteristics of the tap water used for boiler operation were determined in the context of the failure investigation. The quality of the water used for boiler operation was a significant factor influencing the heating element efficiency and reliability. Hard water with high electrical conductivity accelerated salt precipitation and led to the corrosion and premature failure of the components.     

Fire resistance of ultra-high performance strain hardening cementitious composite: Residual mechanical properties and spalling resistance

Ultra high performance strain hardening cementitious composites (UHP-SHCC) is a special type of cement-based composite material with outstanding mechanical and protective performance at room temperature. But its fire performance is unknown and there is a lack of research in this aspect. This study presents an experimental program to study fire resistance of UHP-SHCC under two aspects, viz. high-temperature explosive spalling resistance and residual mechanical performance after a fire. Both compressive strength and tensile strength of UHP-SHCC were found to deteriorate with increasing exposure temperature. Tensile strain-hardening feature of UHP-SHCC would be lost at 200 °C and above. It was found that PE fibers are found not effective in mitigating explosive spalling, although they start to melt at 144 °C. FE-SEM (Field Emission Scanning Electron Microscopy) and EDX (Energy Dispersive X-ray) techniques were used to study the state of fiber, fiber/matrix interaction, and microcracks development. Microscopic study found that melted PE fibers were still present in the cementitious matrix, and the melting did not introduce more microcracks. Furthermore, it was difficult for melted PE fibers to diffuse through the matrix, thus providing the reason that PE fibers did not mitigate explosive spalling in UHP-SHCC.     

Experimental and simulation study of solid flows in beads mill

Nanoparticles (NPs) are widely used in various applications. NPs agglomeration will alter its physical and chemical properties. To overcome this, dispersion of NPs by beads mill is desirable to achieve good dispersion stability. Experimental works to investigate the dynamics of the system is complicated and high cost. On the other side, numerical models offer alternative method inexpensively. In this research, the effect of impeller rotation speed on the NPs by experimental and simulation study dispersion state was discussed. Experimental results showed that rotational speed of 2400 rpm could introduce a better reduction size and dispersion state than that of 1200 and 1800 rpm. Numerical modelling via Discrete Element Method (DEM) simulation was carried out to study the solid velocity distribution profile during the dispersion process. Experimental and simulation results were correlated to investigate the relation between the particle size distribution and the particle velocity distribution profile.     

Quantification of evolving moisture profiles in concrete samples subjected to temperature gradient by means of rapid neutron tomography: Influence of boundary conditions,hygro-thermal loading history and spalling mitigation additives

Concrete has a tendency to spall, that is, to eject layers when subjected to high temperatures. This is an erratic phenomenon, and our understanding of the underlying physical process is still limited. A driving process is moisture transfer, whose experimental investigation has so far mostly been limited to macroscopic or point-wise observations, limiting both our understanding and the validation of the proposed models. In this paper, a non-contact technique, neutron imaging, is used to extract a the full-field distribution of moisture in 3D and in real time, while the concrete is heated at high temperatures. This reveals a number of processes often underestimated or ignored in the traditional experimental approaches reported in the literature. Notably, the effect on the evolving moisture profiles of varying heating rates for multiple insulation techniques as well the strong influence of the addition of spalling-mitigating additives is presented. The first ever example of neutron tomography of a spalled sample is also reported, and some preliminary analyses of the effect that moisture clog formation and heating rates have on it are revealed.     

Reduction of fire spalling in high-performance concrete by means of superabsorbent polymers and polypropylene fibers: Small scale fire tests of carbon fiber reinforced plastic-prestressed self-compacting concrete

High-performance concrete (HPC) is prone to explosive spalling when exposed to fire, which may lead to failure of the concrete elements. Polypropylene fibers (PP) are often added to HPC, as upon their melting they create channels through which water vapor is evacuated, preventing the build-up of high vapor pressures. In self-compacting HPC (HPSCC), the amount of PP fibers needs to be limited in order to keep the self-compacting properties, which may reduce the fire resistance.In this paper, a novel strategy to reduce fire spalling in HPSCC is illustrated, based on adding small particles of superabsorbent polymers (SAP) during mixing. The SAP end up as empty macropores, similar to air voids, in the HPSCC matrix. The PP fibers-SAP voids system percolates at a lower fiber loading than the fibers alone, allowing maintenance of the self-compacting properties while reducing substantially the fire spalling. In particular, in this paper it is shown how addition of SAP is able to reduce fire spalling in thin-walled HPSCC slabs prestressed with carbon fibre reinforced plastic reinforcement.     

Analysis of laminar Newtonian fluid flowing around two spheres in a tandem arrangement

Abstract

The steady state laminar flow of a Newtonian fluid from a two‐spheres in tandem arrangement is analyzed by the finite element method. The computational strategy is first investigated by solving a simple flow system and the computational results are compared with previous published data. The effects of the Reynolds number of the flow field to the position of separating point, the location and size of vorticity, the shear stress acting on the surface of the spheres and the pressure distribution of the fluid flow are also studied.     

Grinding kinetics of red grape seed residue in stirred media mill

The main objective of the present paper was to experimentally investigate the grinding kinetics of red grape seed which is the by-product of winery and juice industry. Stirred media mill was used as a high energy density mill to improve the raw grape seed fineness, i.e. mean particle size approx. 10 µm using various rotor circumferential velocities under dry condition. The effect of stress intensity and stress number on the particle size distribution of ground grape seed was investigated. Optimum conditions (rotor velocity and residence time) were determined, mean particle size close to 10 µm and 5000 cm²/g geometric specific surface area were reached within the studied variables. Additionally, concerning the material structure, FTIR measurements of the ground grape seed samples were carried out which demonstrated that no structural changes were detected. Furthermore, the specific grinding work was measured for each test, in this way energy utilization, efficiency was determined.     

Modeling breakage and motion of black pepper seeds in cryogenic mill

The present investigation aims to represent three-dimensional motion and breakage phenomena of black pepper seeds in the cryogenic mill (hammer mill) using discrete element method (DEM). In DEM modeling, bonded particle model was coupled with Hertz-Mindlin contact model. Calibration method was used to select appropriate model (bond) parameters. The calibrated set of bond parameters includes 3.12?×?10¹¹?Pa?m^?1 normal stiffness; 1.56?×?10¹¹?Pa?m^?1 shear stiffness; 3.88?×?10⁸?Pa critical normal stiffness; 1.94?×?10⁸?Pa critical shear stiffness. Besides, the validity of calibrated parameters was tested in the hammer mill. The observed qualitative and quantitative results (breakage and flow pattern) of numerical and experimental approaches were in good agreement. Based on these results, a few prefatory suggestions were provided to improve the design aspects of the mill. Overall, DEM modeling offered a better understanding of particle breakage and flow pattern in the mill.     

Analysis of grinding kinetics in a laboratory ball mill using population-balance-model and discrete-element-method

Grinding processing consumes a lot of energy in mineral processing, but it is a low-efficiency process in which only approximately 1% of the total energy is used to reduce the actual particle size. Therefore, an efficient operation in the grinding process increases the competitiveness of the production and is an essential for enhancing the energy efficiency of the entire mineral processing procedure.Therefore, the study will focus on to finding a different method to predicting the particle size distribution of the ball mill, by using the PBM which reflects the actual size distributions of ground product and the DEM which can understand the internal particle behavior in the mill chamber. First, the grinding parameters were calculated by applying size distributions of ground product under various conditions to PBM and the behaviors of the particles inside the ball mill obtained through DEM were analyzed to predict the distribution of the impact energy used for grinding. Next, the relational expression between the grinding rate parameter and the normal force applied to the grinding materials was derived. Using the relational expression derived from this study, it was confirmed that the size distributions in other conditions can be predicted.     

Some comparisons in the behaviour of materials at elevated temperatures under uniaxial and under multiaxial stress

Most data on the mechanical behaviour of materials at elevated temperature concerns the influence of a uniaxial stress. For many purposes such information may suffice but there is increasing awareness of its inadequacy when used in the design or service life estimation of engineering components operating under more complex stress systems. Such stress systems do not only arise from the external stresses that are applied but may also result from thermal effects, particularly at interfaces and joints, or from the special geometrical features of a component. Some experimental techniques to provide information on creep behaviour under multiaxial stresses are described together with a discussion and evaluation of the results obtained. It is noted that data from uniaxial stress tests can be used to predict such behaviour when the material is isotropic and is not subject to volume changes, microstructural instability or creep damage. Frequently, materials do not fulfill these conditions and information is presented on the influence of some of these complicating features both on creep rate and on fracture.     

搅拌磨制备煤直接液化用黄铁矿催化剂的研究

通过条件实验,系统地研究了介质充填率、搅拌器转速、原料粒度、研磨介质尺寸及研磨时间等主要操作参数对搅拌磨研磨黄铁矿效果的影响规律,分析了产生这些影响的原因.在优化操作条件下研磨10h后,获得了d50=0.239μm的黄铁矿超细粉体,可作为煤直接液化用催化剂.     

Investigation of optimum design for nanoparticle dispersion in centrifugal bead mill using DEM-CFD simulation

A bead mill is commonly used to produce nanomaterials. The design of the bead mill rotor is an important factor in efficient nanomaterial production to avoid re-agglomeration. We investigated the effect of bead-mill rotor shape on the dispersion state using experimental tests and the discrete-element method (DEM) coupled with computational fluid dynamics (CFD) simulations. Experimental results using TiO₂ in the bead mill showed that the high rotor rotation speed caused TiO₂ particles re-agglomeration, and a sharp particle-size distribution was obtained by dispersion with a mill with a wide gap between the rotor and the chamber. To evaluate the dispersion performance, bead collisions were analyzed using the DEM-CFD simulation. The simulation results indicated that an increase in bead-collision energy lead to damage of the TiO₂ primary particles and re-agglomeration at a high rotation speed. A uniform dispersion was achieved when the frequency of high-energy collision between the particle and wall decreased and a small standard deviation of the collision energy frequency was obtained by the mill with a wide gap. These simulation results correlate with the experimental results. Therefore, this study shows that the DEM-CFD simulation could contribute to an appropriate rotor design for uniform dispersion.     

ICP-AES法测定氧化铁皮中铅、铬、镉

建立了电感耦合等离子体原子发射光谱法测定氧化铁皮中铅、铬、镉的新方法。样品采用盐酸分解,以高纯铁进行基体匹配。铅、铬、镉的检出限分别0.0008%、0.00012%、0.00008%,最大相对标准偏差为8.33%,方法回收率介于92.0%-110%之间。     

The effects of friction characteristic of particle on milling process in a horizontal rice mill

The physical and mechanical properties of rice are of significant change during milling from brown rice to white rice, especially the friction characteristics. In order to clarify the effects of roughness of rice surface on the milling process and mechanism, in this work, motion of spherical particle in a horizontal rice mill under different static friction coefficients (i.e., between particle and cylinder sieve wall μ_s,pt and between particles μ_s,pp) was simulated using the discrete element method. The uniformity of axial motion and circular motion were qualitatively and quantitatively analyzed, which are characterized by introducing the axial dispersion coefficient and uniformity index, respectively. Then, the effects of static fiction coefficients on residence time and collision energy among particles were discussed. The results indicated that the μ_s,pt mainly affects the axial motion while the μ_s,pp primarily influences on the circular motion. The residence time is strongly affected by the uniformity of axial motion while the collision energy is significantly influenced by the uniformity of circular motion. Finally, the relationship between friction characteristics and milling performance can be described based on the method of polynomial fitting. This work is useful for providing essential references to control the quality of milled rice.     

Crack cause analysis of pulverizing wheel in fan mill of 600 MW steam turbine unit

Compared with other types of coal mills, fan mill presents advantages of compact structure and less construction cost. Therefore, fan mill is widely applied in thermal plants. This work carried out root cause analysis of crack failures occurred in pulverizing wheels of fan mills, which are applied in 600 MW steam turbine units of a power plant in China. The cracks are observed in dovetail grooves with beaters installed, and they are with substantial depths and critical to the safety of the fan mills. To find out the failure cause, Finite Element Analysis (FEA) is carried out including: (I) modal analysis and transient analysis of the assembled pulverizing wheel to determine the dynamic characteristics; (II) stress and strain computation of the local structure to obtain the concentrated stress and strain, where operational loads and contact behavior between different components are taken into account. The results indicate that the initial stress and strain exceed the yield limit under the normal operational condition, which reduces the margin of safety. Moreover, the stress and strain increase significantly when the pulverizing wheel is worn, which greatly increased the risk of failure. To avoid crack failures caused by wearing, analysis with different wearing quantities is carried out to determine the optimal repair strategy.     

复合型超声马达纵-扭振动固有频率简并研究

纵、扭振动固有频率简并是提高纵-扭复合型超声马达输出力矩的关键问题。然而同一弹性体内的纵振固有频率远高于扭振固有频率．目前对两种振动固有频率的简并缺乏深入的理论研究。纵一扭复合型超声马达振动分析模型都基于一维理论。在此提出了一种纵、扭振动固有频率简并的新方法。通过将纵一扭复合型超声马达设计成双定子对称结构．在定子上附加一个调整环改变定子的力学边界条件。实现纵、扭振动同频共振。应用Hamilton原理建立了考虑泊松效应的定子纵、扭振动理论模型，分析了定子的纵振与扭振第一阶固有频率随调整环质量和位置的变化规律，通过优化马达的几何结构参数获得了纵振与扭振的同频谐振点。