改善高强度船板断口性能的试验研究

通过对高强度船板拉伸断口分层的检测分析,认为断口分层的主要原因是板厚中心存在严重的偏析现象和马氏体、贝氏体组织,影响心部组织性能。为改善断口形貌提高高强船板的正检合格率,试验研究降低锰含量以减小钢板中心锰的偏析,严格控制生产工艺在保证力学性能的条件下提高高强度船板的断口合格率。     

高强船板拉伸试验断口分层的原因分析

高强船板拉伸试验后出现断口分层现象。利用低倍检测、扫描电镜能谱仪和金相显微镜,对断口以及平行于拉伸方向、垂直于钢板表面的剖面进行了观察、检测和分析,指出分层现象与板厚中心偏析处的硫化物夹杂、带状组织、未轧合孔洞有关。     

高强船板AH36/DH36的成分优化

采用铸坯低倍检验、原位分析、扫描电镜等方法,分别对高强船板成分优化前后的低倍、偏析和拉伸断口进行对比分析。试验结果表明:优化后的高强船板铸坯低倍及元素偏析明显好于优化前,同时拉伸断口分层也得到明显改善,合格率显著提高。     

高强船板AH36／DH36成分优化研究

采用铸坯低倍、原位分析、扫描电镜和统计等方法,对高强船板降锰前后的低倍、锰元素偏析和拉伸断口合格率进行了研究。通过降锰,高强船板的铸坯低倍和锰元素偏析明显好于降锰前的情况,同时拉伸断口没有以前的分层现象、合格率显著提高。     

微合金化钢板断口分层开裂原因分析

针对微合金化钢板断口分层的问题，以高强船板钢A36为研究对象，对炼钢-精炼-连铸-加热-轧制工艺进行了全面跟踪调查。使用金相显微镜、扫描电镜及能谱分析等设备研究了断口分层的原因。认为带状组织对船板钢的分层有较大影响，带状组织越严重，船板钢拉伸时分层越严重；夹杂物也是导致船板钢分层的原因之一，夹杂物仅为氧化物时断口正常，夹杂物为铝酸盐的复合夹杂物时，断口容易分层；带状组织的出现与化学成分偏析有关。通过降低Mn和S含量，提高合金含量，延长均热时间，控轧控冷等措施，船钢板的分层缺陷率由2020年的5.82%降低至2021年的0.56%,解决了船板钢分层缺陷问题。     

AH36船板钢拉伸试样断口分层缺陷的分析

AH36船板钢拉伸试验后试样断口出现分层现象,本研究采用扫描电镜与能谱仪、金相显微镜等手段对断口以及断口附近的截面组织进行了观察、检测和分析,并对拉伸后的船板钢试样进行了酸腐蚀低倍组织观察,指出分层现象与碳、硫和锰等元素偏析从而引起组织中出现贝氏体、马氏体有关,钢板厚度越大,分层情况越严重。     

船板拉伸断口分层原因分析

利用扫描电镜对船板拉伸断口形貌、夹杂物分布和显微组织进行了观察和分析。用能谱仪对断口分层处的夹杂物进行了成分分析。结果表明,钢板拉伸断口分层与心部成分偏析、偏析带过冷组织、硫化物夹杂有关。提出了减缓钢板中心分层的措施。     

船板钢拉伸断口分层原因分析及工艺改进

通过金相显微镜、扫描电镜对船板钢拉伸试样断口的金相组织、低倍组织、形貌进行分析,确定船板钢拉伸试样断口缺陷是由铸坯中的夹杂物、中心偏析和轧制工艺中形成的带状组织引起的。结合生产实际,提出了连铸坯二冷区凝固末端强制冷却措施,使得柱状晶生长速度变慢,等轴晶增多,减少了钢水静压力,减轻了中心偏析,船板拉伸断口分层缺陷得到有效控制。     

高强船板拉伸试样断口缺陷分析研究

某钢厂近期生产的36kg级以上船板出现断口分层现象,通过金相显微镜、扫描电镜对断口进行了分析,发现断口处有大型夹杂物、硫化物带状组织。结合炼钢生产实际,提出了改进冶炼和提高钢水纯净度的措施,通过试验跟踪,船板延伸率大大提高,拉伸断口分层现象得到了解决。     

高强度船板拉伸断口分层原因分析

针对高强度船板拉伸试样断口出现分层的问题,对典型断口分层试样和断口合格试样取样进行金相检验及扫描电镜能谱分析,结果表明,异常断口试样夹杂物级别高,试样心部组织晶粒粗大,带状组织明显且有裂纹,分析认为,2.0级以上的硅酸盐夹杂和MnS夹杂、28μm宽的珠光体带及Ti、Nb元素的富集是导致拉伸裂纹进而出现断口分层的主要原因。     

Q235B热轧带钢焊接后热影响区分层原因分析及措施

对冷水江钢铁有限责任公司2014年5月生产的出现焊接后分层质量问题的Q235B热轧带钢取样进行了理化检测、组织分析。分析表明:Q235B热轧带钢发生焊接后分层的主要原因是钢中非金属夹杂物含量偏高,而尤以塑性非金属夹杂物--硫化物和硅酸盐等造成的危害最大。采取优化冶炼工艺,降低钢中硫含量、硅含量、氧含量的方法成功解决了分层问题。     

Density Stratification Effects in Sand-Bed Rivers

In this paper the effects of density stratification in sand-bed rivers are studied by the application of a model of vertical velocity and concentration profiles, coupled through the use of a turbulence closure that retains the buoyancy terms. By making the governing equations dimensionless, it is revealed that the slope is the additional dimensionless parameter introduced by inclusion of the buoyancy terms. The primary new finding is that in general density stratification effects tend to be greater in large, low-slope rivers than in their smaller, steeper brethren. Under high flow conditions the total suspended load and size distribution of suspended sediment can be significantly affected by density stratification, and should be accounted for in any general theory of suspended transport.     

Stratification in Laboratory Simulations of Water Hyacinth Ponds

The technical and economical feasibility of using water hyacinth ponds for the treatment of wastewater is well established. However, various purification mechanisms operative in a pond have not been understood fully. In the present work, an attempt has been made to study stratification in four laboratory scale models of different depths operated continuously over a period of 4 months. It was concluded from this study that stratification did not occur in ponds. An attempt also was made to study the possible reasons for it. First, dense water hyacinth canopy stops radiation in excess of 90%. This prevents the growth of algae and subsequent increase in dissolved oxygen. Second, based on temperature profiles, thermal and density stratification also were found to be absent. Third, the secondary circulation in the ponds, induced because of influent and effluent being at different levels, also assists in checking the stratification. All these effects make a water hyacinth pond unstratified, which helps in uniform treatment throughout the depth and maintenance of designed hydraulic detention time.     

Influence of Sediment Structure on Erosional Strength and Density of Kaolinite Sediment Beds

The role of sediment pore-water chemistry and the resulting particle structure in determining the erosional stability of settled cohesive sediment beds in rivers, lakes, and estuaries is examined. Kaolinite sediment is used as the surrogate sediment in this experimental investigation with the beds settled from concentrated suspensions. The bed stability with respect to erosion or resuspension is measured in a laboratory flume as a function of sediment pore-water chemistry. The chemical properties varied are sediment pH, ionic strength, and natural organic matter. The remolded bed sample is prepared from a sediment suspension having controlled chemical properties that is allowed to settle into the flume bed where its erosional strength and density are determined with depth in the sample. Different structures of settled beds are observed with changes in chemical parameters. Under low pH and low organic content conditions, the initial suspension before settling is flocculated. The resulting settled beds show strong stratification with respect to erosional strength but weak stratification of bulk density with depth. On the other hand, under high pH or high organic content conditions at low ionic strength, the initial suspension is dispersed. The resulting settled beds have lower erosional strength and weak stratification of erosional strength with depth but strong stratification of bulk density with depth. This research shows that the relationship between erosional strength and bulk density of a settled bed depends strongly on the structure of the sediment particle associations as determined by pore-water chemistry.     

Warfarin and aspirin as thromboprophylaxis in atrial fibrillation

1. Anticoagulant therapy with warfarin is now established as effective thromboprophylaxis against stroke in atrial fibrillation, in high-risk persons. Aspirin is indicated in moderate-risk persons or if warfarin is contraindicated. 2. Risk stratification is suggested, using clinical factors supplemented by echocardiography, to aid choice of prophylaxis. 3. Further studies are required to establish how best to identify undiagnosed patients who have atrial fibrillation; to develop new therapeutic strategies; and to refine risk stratification to define which patients with atrial fibrillation are at the highest risk of stroke.     

Reducing Numerical Diffusion Effects with Pycnocline Filter

Numerical or artificial diffusion is the unintentional smoothing of gradients associated with the discretization of the transport equations. In lakes and reservoirs where through-flow is small, the effects of numerical diffusion of mass are cumulative, leading to a progressive weakening of vertical density stratification. This density field misrepresentation precludes accurate, long-term, three-dimensional (3D), hydrodynamic simulations on fixed grids in closed basins with an active thermocline. An ad hoc technique to limit the destratifying effects of numerical diffusion of mass is presented and tested for a 3D, hydrostatic, Z-coordinate numerical model. The technique quantifies the domain-integrated numerical diffusion by assessing the change in the background potential energy Eb. At each time step, the change in Eb associated with numerical diffusion is calculated, then removed using a sharpening filter applied to each water column. In idealized test cases, the filtering technique is effective in maintaining density stratification over one year while undergoing periodic, large-amplitude forcing by internal waves. Forty-day simulations of Lake Kinneret compared to field measurements demonstrate improved representation of density stratification using the filtering technique.     

Eutrophication Model for a Coastal Bay in Hong Kong

Flow and transport in a natural water body commonly interact with density stratification and in some cases the stratification may be characterized as a two-layered system. A rigorous, two-layered, two-dimensional (2D) finite difference numerical model for eutrophication dynamics in coastal waters, based on the numerically generated, boundary-fitted, orthogonal curvilinear grid system as well as a grid “block” technique, is proposed here. The model simulates the transport and transformation of up to nine water quality constituents associated with eutrophication. The structure of the model is based on a generally accepted framework with the exception of the interaction between the two layers via vertical advection and turbulent diffusion. Some kinetic coefficients are calibrated with field data specifically for the scenario in Tolo Harbour, Hong Kong. The pollution sources are unsteady and hourly solar radiation is imposed. Sediment oxygen demand (SOD) and nutrient releases from sediment are incorporated in the model based on the relevant in-situ sampling analysis. The hydrodynamic variables are predicted simultaneously with a hydrodynamic model previously developed. The computed results show that the present model successfully reproduces the stratification tendency in all the water quality constituents, showing an obvious bottom water anoxic condition during the summer, which is consistent with the density stratification and the unsteady layer-averaged 2D eutrophication processes in Tolo Harbour.     

Risk stratification and prediction of sudden death following myocardial infarction

Accurate and reliable identification of those survivors of acute myocardial infarction who are at high risk of sudden death remain an important and challenging problem. This review summarises the current state-of-the-art of the risk stratification techniques and lists achievements in this field. The review comments in detail on individual factors used in risk stratification. Residual ischemia may be considered as one of the main triggering factors of post-infarction arrhythmia. Depressed left ventricular ejection fraction indicates deterioration of ventricular function. Electrical instability of the myocardium reflects the potential substrate of arrhythmia. Frequent ventricular ectopic activity provides triggers of ventricular tachycardia and/or fibrillation when acting on a suitable substrate. Impaired autonomic status of the heart may lead to the loss of vagal antiarrhythmic protection. Further, the tests used for risk stratification are discussed. Ventriculography provides estimates of left ventricular ejection fraction. Holter monitoring is used for the assessment of ventricular ectopic activity and heart rate variability. Exercise testing is used to address residual ischemia. Programmed ventricular stimulation and the analysis of signal averaged electrocardiograms estimate electrical instability of the myocardium. Baroreflex sensitivity is a measure of cardiac parasympathetic reflexes. The design and results of different experimental and clinical studies which utilised these tests are also discussed.     

Classroom environments and the stratification of pupils' ability perceptions.

Rosenholtz and Simpson have suggested that the task structures, ability grouping practices, and evaluation practices present in a classroom are important determinants of the degree to which children's ability perceptions become stratified within the classroom. The effects of classroom practices on stratification of pupils' self-perceptions of math ability were estimated in 67 upper elementary math classes from schools in predominantly White, middle-income communities. The results suggest that the effects of task structure depend on the "talent dispersion" (actual dispersion of ability levels) in the class. In classes in which talent dispersion was high, differentiated task structures were associated with lower levels of stratification than were undifferentiated task structures. This effect disappeared in classes in which talent dispersion was low. The dispersion of report cards grades, but not the frequency and emphasis of grading, was positively related to stratification. The implications of the results for educational theory and practice are discussed. (PsycINFO Database Record (c) 2010 APA, all rights reserved)