Erosion–corrosion behaviour of Ni–20Cr plasma coating in actual boiler environment

Abstract

Erosion–corrosion is encountered in a large variety of engineering industries. In such environments, protective coatings are used. In this investigation, erosion–corrosion of the Ni–20Cr coating on nickel and iron based superalloys has been investigated by subjecting them to the boiler of coal fired thermal power plant at the temperature zone of 540°C for 1000 h duration. The erosion–corrosion kinetics of the plasma sprayed Ni–20Cr coating on different superalloys has been investigated. XRD, SEM with EDS and EPMA have been used to analyse the eroded–corroded products along the surface and cross-section. Main phases identified in all the Ni–20Cr coated superalloys after exposure are NiO, Cr₂O₃, Al₂O₃ and SiO₂. Aluminium has penetrated from the bond coat to the top coat along the splat boundaries. Oxides of chromium, nickel and aluminium are recognized as protective oxides for boiler environment. The probable mechanism of attack for Ni–20Cr coating in the given boiler environment is discussed.     

Evaluation of novel Ti-doped 3D carbon-carbon composites under transient thermal loads

3D Ti-doped and undoped carbon-carbon composites (CFCs) were exposed to transient thermal loads to simulate plasma disruptions, in the electron beam test facility JUDITH at different power densities and multiple shots in order to study the evolution in the behavior of the material. The thermal shock response of the undoped and Ti-doped materials was compared in order to study the influence of titanium carbide as dopant. The erosion itself is driven during the first shots by macroscopic erosion (brittle destruction), which is a result of thermally induced stresses. With increasing number of shots, no more brittle destruction is observed and the main erosion mechanism is sublimation due to local overheating. This is also confirmed by the decrease of the erosion rate with increasing the number of shots. The pitch fibers are hardly affected by the applied heat loads and they show almost no erosion, especially in the Ti-doped composite.     

水煤浆喷咀耐磨材料的试验研究

本文根据水煤浆喷咀的工作条件以及不同材料在冲蚀磨损时的行为,对硬质合金,陶瓷材料及超细晶粒硬质材料等进行了试验研究。实验室冲蚀磨损试验和现场运行试验结果表明:超细晶粒硬质材料具有良好的抗冲蚀磨损性能,并可预期热震性得到改善的高致密、高硬度陶瓷材料将是一种新型的优良的水煤浆喷咀材料。     

软磨粒冲刷下材料的行为及磨损机理

研究了具有不同硬度的材料在不同硬度的磨粒冲刷下的磨损特性。结果表明,在相对较软磨粒冲刷下,即使磨粒硬度低于材料硬度,也能使材料发生磨损。且韧性材料的冲刷磨损率随冲击角的增大而增大。对材料的磨损形貌进行了研究,其软、硬磨粒造成的磨损形貌是不相同的。在低角度下,较软磨粒的冲刷使材料发生犁沟及小片脱落,而不像通常硬磨粒冲刷下材料呈现微切削及锻和挤压。研究了热处理硬化后钢在较软磨粒冲刷下的磨损行为,建立了硬度影响磨损率的规律。在高硬度的磨粒冲刷下(H_p/H_m>3)*,材料的硬度变化对冲刷磨损率影响很小,即以热处理硬化来改善材料抗冲刷磨损作用并不大。而在较软磨粒冲刷下(H_p/H_m<3)材料硬度对冲刷磨损率有影响,热处理强化可提高材料抗冲刷磨损性能。     

Scour Downstream of an Apron Due to Submerged Horizontal Jets

The results of an experimental investigation on scour of noncohesive sediment beds (uniform and nonuniform sediments) downstream of an apron due to a submerged horizontal jet issuing from a sluice opening are presented. Attempts are made to explain the similarity existing in the scour process and profiles (including dune in the downstream of the scour hole). The scour profiles at different times follow a particular geometrical similarity and can be expressed by the combination of two polynomials. Using experimental scour depth at different times, the time variation of scour depth is scaled by an exponential law, where time scale increases linearly with densimetric Froude number. The equilibrium scour depth, related to the sediment size relative to the sluice opening, decreases with increase in sediment size and sluice opening. On the other hand, the equilibrium scour depth increases with increase in densimetric Froude number. The variation of equilibrium scour depth with tailwater depth indicates a critical tailwater depth corresponding to a minimum equilibrium scour depth. The effect of sediment gradation on scour depth is pronounced for nonuniform sediments, which reduce scour depth significantly due to formation of an armor layer, and therefore prompted study of the reduction of scour depth by a launching apron placed downstream of the rigid apron. The results show that the average reduction of scour depth by placing a launching apron was 39%, having a maximum of 57.3% and a minimum of 16.2%. The characteristic parameters affecting maximum equilibrium scour depth are identified based on the physical reasoning and dimensional analysis. Equation of maximum equilibrium scour depth obtained empirically agrees well with the experimental data.     

Flow-Induced Failure of Cable-Tied Blocks

An experimental study of flow-induced failure of cable-tied blocks is presented. The particular failure mechanism studied is overturning and rolling up of the leading edge of a cable-tied block mat. Individual blocks were investigated also. The block size, flow depth, and block (mat) protrusion above the surrounding bed were systematically varied. The results are presented in terms of the critical dimensionless shear stress θc for block (mat) failure. A relationship between θc, block size and block protrusion, and flow depth is given.     

数学形态学在灰度图像处理中的理论和应用

研究了数学形态学在灰度图像处理中的基本理论和应用；提出了一种新的基于灰度图像形态学的图像过滤算法。该算法由一个连续交替的开运算和闭运算序列组成，相应的用了一个逐次递增的结构元素序列，以实现逐层过滤，试验表明该算法对灰度图像的过滤有明显的改进。     

三峡地区的水土流失与水土保持

三峡地区沟壑纵横、地形破碎，交通不便、生态环境恶化，经济和社会发展一直比较缓慢，三峡工程的兴建，给三峡地区的经济和社会发展带来了良好的机遇，了对这一地区的农业生产和生态保护提出出更高的要求。１９８８年，国务院批准将长江上游列为国家水土保持重点防治区，这是增强三峡地区的农业基础设施改善生态环境，实现该地区经济可持续发展的基础工程。     

Scour below a High Vertical Drop

Experimental results on scour below a high vertical drop (drop height/critical depth >1) in uniform sands and gravels are presented. The experimental results are used to describe the effects of important parameters, identified from the dimensional analysis, on equilibrium scour depth. The important observations are that the equilibrium scour depth increases with increase in densimetric Froude number, whereas the scour depth decreases with increase in sediment size and tailwater depth. The time scale of scour depth that follows an exponential law is determined. The nondimensional time scale decreases with increase in densimetric Froude number.     

Relation between shear banding and penetration characteristics of conventional tungsten alloys

The dynamic compression failure and ballistic penetration characteristics of conventional tungsten alloys similar in strength were investigated. Dynamic compression failure properties were generated with a symmetric Taylor test technique and penetration characteristics were obtained with 44 mm kinetic penetrators against an 300 HB hardness steel target at 1400 m/s. From shear crack length data generated with Taylor specimens impacted at different impact speeds a critical speed characterizing shear band initiation was deduced. The critical equivalent plastic strain at shear band initiation sites, obtained from the numerical simulation of the Taylor test at the critical impact speed, was found to decrease with the increase of the penetration performance. These results reinforce the argument that shear band formation is a failure mechanism associated with the erosion process for conventional tungsten alloys.